GaN electronics with its sub-micron critical device dimensions requires the ability for thermal device characterization with nanometer spatial resolution. I will review our latest developments in the development of solid immersion lenses for enabling ultra-high spatial resolution thermal analysis of electronic devices, also very recent developments in optimizing the heat extraction pathways in GaN electronics by integration of GaN with diamond.

We report an experimental and theoretical investigation on the effects of phonon-isotopic impurity scattering on the thermal conductivity of ultrathin graphite (UG) over the temperature range between 20 and 420 K. The material system for this investigation is highly oriented ultrathin graphite foam (UGF) synthesized by methane chemical vapor deposition on reticulated nickel foam and thermally annealed at 3000 ^oC. At densities of about 26 mg cm^-3, the annealed UGF synthesized with 99.2 at.% 13C exhibits an effective thermal conductivity (κ_UGF) of ~3.5 W m^-1 K^-1 at room temperature, corresponding to a solid thermal conductivity (κ_UG) of ~890 W m^-1 K^-1 in the UG constituents. The effect of isotopic impurity scattering on thermal transport was most pronounced near the peak in κ_UG, which occurred at ~150 K. The experimental results agree well with a theoretical model incorporating isotopic effects on the phonon dispersion and scattering.

Recent progress in research on nanoscale heat transport has stimulated a renewed interest in the classic work of Casimir [1] on the thermal conductivity of thin rods in which phonon scattering at the surfaces dominates over bulk scattering. For many years this regime has been studied at low temperatures, where the thermal conductivity of a rod varies as the cube of the temperature, as determined by the Debye specific heat and an effective mean free path on the order of the diameter of the rod. Present day access to nanostructures allows probing of the Casimir regime even at room temperature, where the specific heat has plateaued out. In anisotropic materials, the fact that the phonon group velocity deviates from the phase velocity introduces an additional consideration. McCurdy et al. [2] modified Casimir&’s analysis to take this effect into account and concluded that it leads to a moderate anisotropy in the thermal conductivity of rods made of cubic crystal materials. The question not addressed in their study is what happens when the axis of the rod lies along a phonon focusing caustic direction, so that many more phonons, compared to the isotropic case, have their group velocity direction close to the axis. We show that for one specific kind of a caustic, i.e., external conical refraction, encountered in hexagonal crystals, the thermal conductivity in the Casimir limit would tend to diverge, and be limited mainly by the remnant bulk scatterings.
Returning to the original Casimir&’s formulation for isotropic solids, we examine the validity of the assumption that the boundaries are either perfectly diffuse or have a constant specularity. It is well known that diffuse surfaces tend to become specular at grazing incidence angles. We show that the angular dependent specularity model due to Soffer [3] leads, again, to a diverging thermal conductivity of infinitely long rods. However, for large enough surface roughness, the dependence of the thermal conductivity on the length of the rod will have a plateau at the Casimir limit value. In this case, experiments with very high aspect ratio rods may be needed to detect the divergence.
[1] H. B. G. Casimir, Physica 5, 495 (1938).
[2] A. K. McCurdy, H. J. Maris, and C. Elbaum, Phys. Rev. B 2, 4077 (1970).
[3] S.B. Soffer, J. Appl. Phys. 38, 1710 (1967).

Research on reducing thermal conductivity of materials via phonon-surface scattering in nanostructures has important implications related to thermoelectric conversion and thermal insulation. One of the unresolved questions about the nanoscale phonon-surface scattering is the frequency-dependence of phonons scattered by boundaries. Recently, our group implemented superconducting tunnel junction (STJ) based phonon spectrometry in micro-scale and successfully measured frequency dependent transport of non-equilibrium phonons through single crystalline nano-structures at low temperatures [1, 2]. Although often overlooked in thermal transport experiments, at low temperatures directions of phonon group velocity vectors are significantly influenced by the elastic anisotropy of the single crystalline transport medium leading to a ‘phonon focusing&’ effect. Phonon focusing preferentially depletes phonons along certain crystallographic directions and concentrates them along other directions. In this work, phonon transport through single-crystal silicon nanosheets is modeled by Monte Carlo (MC) simulations taking into account phonon focusing effects. Only phonon-boundary scattering is taken into account as the source of deviation in the phonon path because phonon-phonon scattering is negligible at low temperatures. After each phonon-surface interaction the phonons are either reflected specularly or scattered diffusively from the surfaces depending on the specularity of the surfaces. The specularity of the nanosheet surfaces are determined based on phonon frequency and surface roughness using the well-known Ziman theory. Upon emission or a diffusive scattering event, the phonon&’s wavevector is randomized according to a Lambertian distribution, and its group velocity vector is determined based on the elastic constants of the silicon. The phonon frequency range for the simulations is 80 GHz to 400 GHz (~75 nm to ~15 nm in phonon wavelengths), which is an achievable range in our experimental work. Comparison of the MC simulations with the experimental results indicate that phonon-surface interactions reach purely diffusive limit at much lower frequencies than that predicted by Ziman theory. Our result implies that for a nanowire with ~ 1 nm surface roughness the surface-scattering contribution to thermal conductance at T asymp; 7 K should be 4 times lower than is typically assumed based on Casimir-Ziman theory.
This work is supported by the NSF DMR-1149036 and the DOE Office of Basic Energy Science under Award Number DE-SC0001086.
[1] J. B. Hertzberg, O. O. Otelaja, N. J. Yoshida, and R. D. Robinson. "Non-equilibrium phonon generation and detection in microstructure devices," Rev. Sci. Inst. 82, 104905 (2011).
[2] Obafemi O. Otelaja, Jared B. Hertzberg, Mahmut Aksit, Richard D. Robinson, "Design and operation of a microfabricated phonon spectrometer utilizing superconducting tunnel junctions as phonon transducers," New J. Physics 15, 43018 (2013).

Understanding phonon transport is important for many technological developments, examples are thermoelectric energy conversion, for which phonon heat conduction should be minimized, and thermal management of microelectronics, photonic devices, and batteries, for which heat conduction should be maximized. In this talk, I will start with a discussion of first-principles simulation on phonon heat conduction in bulk crystals, which reveals details on phonon scattering and mean free path distributions, and explains why III-V semiconductors have higher thermal conductivity than IV-VI and V-VI compounds. I will then present a recently developed thermal conductivity spectroscopy technique to measure phonon mean free distribution and discuss experimental evidence on coherent contribution of phonons to heat conduction in superlattices, supported by detailed simulations.
This material is based upon work supported as part of the “Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.

Thermal management plays a critical role in the functionality and reliability of modern microelectronic and other nanoscale devices. As the characteristic device lengths move towards the nanoscale, an important challenge in device design is increasing power density. This has drawn attention to the need to understand thermal transport at the nanoscale, and to develop techniques able to accurately measure thermal properties of materials with high spatial resolution. We have developed a thermal property imaging technique based on frequency domain thermoreflectance (FDTR). FDTR utilizes a modulated pump laser beam to provide periodic localized heating while a probe laser beam monitors the thermal response of the sample via surface reflectivity. By simultaneously fitting the thermal phase at different frequency regimes, FDTR can reliably extract multiple combinations of geometrical and thermal properties of interest, such as the thickness of surface and sub-surface films, heat capacity, in-plane and cross-plane thermal conductivity, and thermal interface conductance. We present our experimental setup based on two continuous-wave lasers as well as a sensitivity analysis of the technique to different thermal properties. We demonstrate quantitative thermal property imaging of a multilayer stack of nanoscale thin films. Both interface conductance and substrate thermal conductivity maps are obtained simultaneously. By carefully selecting frequency points based on sensitivity analysis, other thermal property images can also be reconstructed. These results confirm the potential of our technique in quantifying and imaging thermal properties of buried layers and therefore can be an important tool in studying phonon interactions and transport in nanostructures and bulk materials.

The nature of bonding affects the details of both elastic response and phonon dynamics. Layered substances like WSe2 can be turned into superlow thermal conductors [Science 315, 251 (2007)] by introducing disorder that increases the layer separation. While density functional theory in the local density approximation (LDA) does describe the equilibrium lattice parameters and the phonons of ideal crystalline WSe2 surprising well there is clear evidence that a pronounced contribution to bonding stems from van der Waals (vdW) interactions. The lattice expansions in disordered WSe2 further increases the relative importance of vdW coupling and such engineering of the WSe2 bonding nature impacts especially the shear resistance. We will present a characterization of the expansion effects on shear resistance and phonon dynamics. The results demonstrate the importance of vdW contributions for capturing the behavior of the material under effective expansion (as a first approximation to disorder). We also detail consequences of the changes in bonding nature on the phonon band structure. Finally, we discuss the implications for modeling thermal transport and for using pressure to control the thermal contact resistance.

We have employed a semi-continuum model to investigate the effect of tensile strain on thermal properties of graphene. Analytical expressions derived by Nihira and Iwata [1] for phonon dispersion relations and vibrational density of states are employed, based on the semicontinuum model proposed by Komatsu and Nagamiya [2]. The thermal conductivities in graphene, in the graphite basal planes, and along the graphite c -axis are computed within the framework of Callaway&’s effective relaxation time theory [3]. The theory successfully explains available experimental measurements for these systems. The conductivity of graphene is predicted to be higher than the in-plane conductivity of graphite for all temperatures. Incorporation of the 13C isotope produced significant reduction in the conductivity of graphene in the temperature range 50-300 K. The relationship among phonon dispersions, density of states, velocity and strain is analysed. It is found that thermal conductivity of graphene is very sensitive to tensile strain. In the presence of tensile strain the specific heat increases but the lattice conductivity can decrease or increase depending on the level of the purity and temperature of the graphene sample. It is also found that upon increased compression frequencies of the flextural modes become imaginary, indicating instability of graphene. Our results are supportive of the experimental investigations [4] of the effect of strain on the thermal conductivity of graphene.
[1] T. Nihira and T. Iwata, Phys. Rev. B 68, 134305 (2003).
[2] K. Komatsu and T. Nagamiya, J. Phys. Soc. Jpn. 6, 438 (1951).
[3] G. P. Srivastava, The Physics of Phonons (Taylor and Francis, 1990).
[4] N. Wei, L. Xu, H.Wang, and J. Zheng, Nanotechnology 22, 105705 (2011).

Characterisation of metal-semiconductor thermal interface conductances heavily relies on pulsed laser metrology. For these experiments, a thin film of the metal is deposited onto the semiconductor surface. A modulated 'pump' beam heats up the sample while lock-in detection of the reflected 'probe' beam records the thermal transient of the metal surface. Fitting the measured signals to a diffusive thermal model provides the metal-semiconductor interface resistivity (rms) and semiconductor conductivity (ks).
The typical penetration lengths of the induced thermal field make laser techniques also very suitable to study high-speed heat dynamics in the semiconductor over length scales comparable to the phonon mean free paths. Several room temperature observations of quasi-ballistic thermal transport in technologically important materials including SiGe, InGaAs and Si have been reported over the past years.
Here, we show that the presence of such anomalous transport modes inside the semiconductor has a severe impact on thermal interface metrology. Even when allowing both rms and ks to vary with the laser modulation frequency, the state of the art models are unable to properly capture the rich physics of the thermal dynamics inside the sample. Unresolved fitting discrepancies remain over portions of the data where the sensitivity to the extracted thermal parameters is actually the highest, raising some questions regarding the confidence of the identification.
We demonstrate that these issues can be overcome by incorporating ballistic transport into the theoretical analysis. We have developed a novel formalism that encompasses the natural transition of non-Fourier thermal transport at short length/time scales to regular diffusion inside the semiconductor. Without requiring any modulation frequency dependent parameters, our theory can explain the experimental transients with superior fitting quality. The interface is found to be up to 250% more conductive than believed thus far. The reason is that diffusive models mistakenly interpret part of the ballistic heat flow suppression in the upper semiconductor region as a seemingly higher resistivity of the adjacent interface. Conventional identification of simulated data with a perfect metal-semiconductor interface (rms = 0 in the non-Fourier model) perceives nonzero resistivities that strongly depend on the severity and spatial extent of the ballistic heat component. For Al/SiGe and Al/InGaAs interfaces, we found that respectively 40% and 70% of the conventionally extracted rms value are in fact identification artifacts induced by quasi-ballistic thermal effects.
For semiconductor alloys, and any other circumstances where the experimental thermal length scale overlaps with the phonon spectrum, existing models assuming frequency-dependent thermal conductivity are insufficient. Properly accounting for non-Fourier heat conduction is needed to ensure adequate characterisation of metal-semiconductor interfaces.

Thermal transport across molecular layers is of high relevance for current and future nanoelectronic devices. Molecular layers are used today mostly as passive layers like adhesion promoters or photoresists with the goal to use them as active materials, e.g. as electrical conductors or insulators or as thermal interface materials. Their thermal transport properties are predicted to show signatures of ballistic phonon propagation with effects of localization, interference and coupling strength.
Here, we present thermal transport measurements conducted with a vacuum-operated scanning thermal microscope (SThM) to study the thermal conductance of monolayers of nine different alkane thiols self-assembled on gold Au(111) surfaces and four alkane silanes coupled to silicon oxide surfaces as a function of the length and bonding conditions of the alkane chains. The key element of our microscope is a microfabricated resistive silicon probe tip which allows probing local molecular thermal conductance at a lateral resolution down to 10 nm with a relative sensitivity well below 10-12 W/K/molecule. We found the thermal conductance of molecular layers highly dominated by the thermal interface and developed a new method to quantify the thermal conductance by tuning the contact area as well as the coupling strength of the thermal interface between the probe and sample. This was achieved by subsequently contacting the sample with the tip and varying the tip&’s load during the scanning of the sample. With a fundamental understanding of the contact mechanics between the tip and the sample, we are able to extract the thermal conductance of the sample. Finally, we discuss the results in terms of the apparent thermal conductance of the molecules and their boundaries to the thermal contacts at different coupling strengths. Moreover, the dependence of the thermal conductance on the chain length is compared to theoretical predictions.

We have used inelastic neutron scattering to measure phonon properties of UO₂, the most widespread used nuclear fuel, to investigate the underlying mechanisms of anharmonic phonon-phonon scatterings that limit its thermal transport. High-resolution phonon lifetime measurements in UO₂ showed that pure oxygen phonons actually transport a large amount of heat despite their highly anharmonic nature and short lifetimes, contrasting result from existing theories. We have performed the first ab-initio simulations of phonon lifetimes for the strongly-correlated UO₂ that correctly predict the relative importance of the low-velocity uranium and pure oxygen phonon modes, but anharmonic phonon-phonon scattering magnitudes are off by a factor of two. Comparisons of the simulated microscopic Grüneisen parameters with values from the phonon dispersion measurements also show discrepancies. Facilitating with the phonon linewidth measurements, we have further devised a new approach to compare the phonon density of states measurements with the first-principles quasi harmonic simulations. This helps identifying the differences between simulated and measured zone boundary energies and phonon dispersion gradients from the phonon density of states. We have also used phonon density of states to investigate the effect of fission product impurity on polycrystalline U(5%)CeO₂ and the effect of point defects on hyper-stoichiometric UO_2.1. The measurements have shown that doping strongly impacts zone boundary phonons with pure uranium and pure oxygen vibrations, representing the two strongest heat carrying phonon branches. The collective phonon measurements provide unprecedented new insights into anharmonicity and lattice dynamics of UO₂ and form new benchmark to guide ab-initio electronic structure modeling for UO₂ that will impact simulations of all its physical properties including thermal transport.
This research was supported as part of the Center for Materials Science of Nuclear Fuel, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.

Radiative heat transfer at small separations can be enhanced by orders of magnitude via the use of surface phonon polariton or plasmon polariton waves. This enhancement has potential applications in different devices, such as thermal emitters, thermal rectifiers, thermophotovoltaic and thermoelectric energy conversion systems. In this work, we explore the tunable optical properties of ferroelectric materials to manipulate the near-field radiative heat transfer between two surfaces, aiming at the active control of near-field radiation heat transfer. Soft mode hardening of ferroelectric thin films induced by environmental changes, such as temperature and electric field, is widely used as a basis for tunable and switchable electrical and optical devices. However, to the best of our knowledge, this mechanism has not yet been examined for heat transfer applications. Using the fluctuation-dissipation theorem and the Dyadic Green&’s function method, we show via simulation that the magnitude and spectral characteristics of radiative heat transfer can be tuned via an externally applied electric field and temperature. We explore ways to maximize the tuning contrast and discuss the trade-off between maximizing tunablility and heat transfer. Our results suggest that ferroelectrics can be used to develop new types of tunable nano-scale devices for thermal and energy conversion applications.
This work is support by DOE BES (DE-FG02-02ER45977) and AFOSR MURI via UIUC FA9550-08-1-0407.

Recent advances in the fabrication of inorganic nanostructures have led to the opportunity to create novel vibrational phenomena that will ultimately yield unique and controllable thermal and electronic phenomena. A fundamental understanding of the phonon dispersion in nanoscale systems is critical in understanding the functionality of these structures. Theoretical studies have predicted that there will be an order-of-magnitude reduction in the thermal conductivity in silicon quantum wells in comparison with bulk Si. Already, Si/SiGe superlattices exhibit an enhanced thermoelectric figure of merit with respect to their bulk counterparts. The contributions made by large-wavevector phonons to the material properties are expected to be larger in nanostructured systems than in equivalent bulk systems. However, these large-wavevector modes have not been experimentally observed in nanostructures due to fundamental limitations of conventional probes. As a result, theoretical predictions for the behavior of confined phonon modes have only been experimentally investigated for small-wavevector modes near the center of the Brillouin zone.
Synchrotron x-ray thermal diffuse scattering (TDS) allows thermally populated large-wavevector phonon modes to be probed with high momentum resolution and with particular sensitivity to low-energy acoustic phonons. TDS measurements were performed at station 26 ID-C of the Advanced Photon Source at Argonne National Laboratory, using flat, suspended silicon nanomembranes with thicknesses as small as 4 nm. TDS measurements show that there is an excess in the scattered intensity at large-wavevectors in nanomembranes in comparison with bulk samples. The TDS technique can be used to probe the entire Brillouin zone by varying the orientation of the sample relative to the incident beam and detector location. Doing so yields measurements of acoustic phonons in silicon nanomembranes from throughout the Brillouin zone. This allows the extraction of TDS profiles along any arbitrary path in reciprocal space, including the important high-symmetry directions. Results of this experiment indicate deviations from bulk-like phonon dispersions in a 21 nm silicon membrane.

Confined acoustic phonons have attracted attention due to their role as thermal energy carriers in sol-ids and in particular in semiconductors. Initial experiments in a Si thin film1 and membranes2,3 sug-gested the existence and confined acoustic modes and the dispersions relations of these were finally observed by Cuffe et al4 in a set of high crystalline quality ultra-thin Si membranes5. The dispersion relations showed unambiguous evidence of the flattening of the acoustic phonon bands interpreted as a reduction of the phase (group) velocity of the fundamental flexural mode, responsible for much of the heat transport specially at low temperature regime. A reduction in the group velocity is expected to have an impact on the thermal conductivity and thus, Chavez et al6 simulated the specific heat ca-pacity in Si membranes using a modified dispersion relation in the context of the elastic continuum approach. The phonon lifetimes were measured directly up to 0.5 THz and evidence was obtained for the significant decrease of the first-order dilatational mode from ~ 4.7 ns to 5 ps with decreasing membrane thickness from ~ 194 to 8 nm. The results were compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wave-length-dependent specularity. At this stage we can state that for membranes thicker than 30-50 nm the lifetime is controlled by intrinsic phonon-phonon scattering processes, while for thinner membranes the role of surfaces becomes increasingly important7. Based on the above, COMSOL and FDTD simu-lations using the band structures of confined acoustic phonons, designs for thermal rectifiers and pho-non storage devices have been carried out and will be discussed.

While the origin of LO-TO splitting in the phonon dispersion of polar materials is well known, the effect of this splitting on the thermal conductivity has still not been quantified. Given that optical branches have small group velocities, at first thought, one might expect the effect of this splitting to be minimal. However, previous works have demonstrated that optical branches provide an important source for phonon scattering [1,2]. In high thermal conductivity materials, LO-TO splitting should increase the available phase space for phonon-phonon scattering and could play a role in determining the overall material thermal conductivity. In this work, we examine the thermal conductivity of cubic boron nitride, a high thermal conductivity material with significant LO-TO splitting, and we compare it with the thermal conductivity of diamond. Given that diamond and cubic boron nitride (cBN) share many similar traits (crystal structure, comparable lattice constant, and identical average atomic mass), this comparison should isolate the role of polarity effects. The thermal conductivity is calculated using an iterative solution of the phonon Boltzmann transport equation (BTE) that integrates harmonic and anharmonic force constants calculated using density functional perturbation theory[2,3].
Our predicted thermal conductivity for isotopically pure cBN is much higher than the estimations based on Callaway models. In addition, we use pressure as a tool to manipulate the LO-TO splitting and resolve its impact on the available phonon-phonon scattering phase space and the thermal conductivity. Finally, we will discuss how these polar effects could be used to engineer the thermal conductivity of materials.
[1] G. P. Srivastava, The Physics of Phonons (Adam Hilger Press, Philadelphia, 1990)
[2] A. Ward et al., Phys. Rev. B 80, 125203 (2009)
[3] D. A. Broido et al. Appl. Phys. Lett., 91, 231922 (2007)
This research was performed under support by NSF CBET Grant through contract number 1066406 and 1066634. Calculations for this work were performed using simulation resources available at the Cornell Nanoscale Facility and the Boston College Research Services.

Here we propose a new approach for constructing a first-principles interatomic potential based upon a Taylor series expansion in clusters of the atomic displacements. Our expansion is based upon interatomic displacement differences which are used to construct polynomials with the symmetry of the space group, satisfying all symmetry constraints by construction. Cluster coefficients up to fourth order and within next-nearest-neighbor are then obtained by finite difference using a large dataset precisely computed with Density Functional Theory. The appliction of this method to PbTe is presented and agreement is achieved within a large range of atomic displacements in addition to lattice strains, including the dependence of the phonons under strain.

Phonons have long been believed to play an important role in the initiation of secondary explosives. These explosives are molecular crystals for which the initial stages of chemical decomposition are endothermic, making them less sensitive to external stimuli. Understanding the mechanisms that allow initiation—the initial reaction stages that can lead to detonation—will aid in enhancing the safety and efficiency of explosives. Phonons are believed to play an important role in shock-induced initiation, with molecular bonds being excited by high-frequency internal phonons[1]. We have recently developed a new computational technique for quantifying the fraction of energy carried by phonons that excites different bonds[2]. This is done through a decomposition of dynamical matrix based on the bond types of the material. We found that a greater fraction of energy is transferred to the key bonds in the chemical decomposition pathway of the explosive RDX by low-frequency lattice phonons, rather than the expected internal phonons. This bond excitation was enabled by the flexible nature of the RDX molecule, due partly to its relatively large size of twenty-one atoms per molecule. In the current paper we extend this work to solid nitromethane, which has seven atoms per molecule and is less flexible. We find that in nitromethane the excitation of the bonds in the decomposition pathway requires internal phonons.
1. D. D. Dlott, Ann. Rev. Phys. Chem. 62:575—597 (2011).
2. B. Kraczek and Peter W. Chung, J. Chem. Phys. 138:074505 (2013).

The direct method has been unable to accurately calculate the phonon properties of a polar material until the mixed-space approach is developed. The mixed-space approach makes full use of the accuracy of the force constants calculated in the real space and the dipole-dipole interactions in the reciprocal space, making the accurate phonon calculation possible with the direct method for polar materials. After three years of extensive tests we finally decide to distribute the computer code of the mixed-space approach. An efficient C++ implementation of the mixed-space approach, YPHON, is provided as open source, including demos and Linux scripts for extracting input data to YPHON from the output of VASP.5.

Fourier law has an infinite speed of propagation so many non-Fourier models have been proposed to describe heat propagation at finite speeds. Surprisingly, Fourier equation with an effective thermal conductivity/diffusivity parameter has been sufficient to describe most experimental results of ultrafast heat transport in nanoscale materials and devices. In this talk we describe some of the recent studies of ballistic heat and energy transport. Prediction of the energy oscillations at very small scales based on Shastry&’s formalism is presented as well as its relation to the Cattaneo (hyperbolic) heat equation and the Jeffrey&’s type equation for second sound. Finally, we discuss about the stochastic nature of energy diffusion and if ballistic effects can be captured while one could still define a local temperature. We present experimental results showing that non-Fourier diffusion equation can better describe heat propagation in submicron semiconductor alloys. This has important implications in the design of high power and high speed electronic and optoelectronic devices.

Understanding phonon-mediated heat transport in nanostructured materials requires detailed knowledge of the phonon mean free path (MFP). Of particular importance is the interplay between phonon-phonon scattering and boundary/interface scattering in nanostructures. In recent years significant progress has been made in first-principles calculations of phonon lifetimes [1]. Typically, theoretical models are compared to thermal conductivity measurements that provide information integrated over the phonon spectrum. Laser-based generation and detection of coherent phonons offers a direct way to measure the lifetime of specific phonon modes. Harnessing this potential requires understanding of different factors affecting the apparent coherent phonon lifetime in laser pump-probe experiments. We present measurements of sub-THz coherent phonons in GaAs-AlAs superlattices (SLs) [2,3] and thin Si membranes and focus our attention on the separation of contributions due to phonon-phonon scattering, boundary/interface scattering and inhomogeneous broadening caused by the variations of the structure parameters. The “intrinsic” room temperature lifetimes measured in 8 nm - 8 nm and 14 nm - 2 nm GaAs-AlAs SLs will be compared with three-phonon scattering calculations based on DFT potentials. At ~0.5 THz frequency we observe a transition from predominantly intrinsic (i.e. due to lattice anharmonicity) to predominantly extrinsic (i.e. caused by scattering due to defects and interface roughness) lifetimes. We will show that this finding sheds light on the discrepancy between the calculated and measured thermal conductivity values in SL structures. The lifetime of high-order thickness resonances in 0.4 - 1.7 mu;m thick Si membranes is found to be dominated by surface roughness. This result can be used to validate the diffuse boundary scattering model for thermal transport in thin films. This work was supported as part of the S3TEC Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award DE-FG02-09ER46577.
[1] A. Ward and D. A. Broido, Phys. Rev. B 81, 085205 (2010).
[2] A. A. Maznev, F. Hofmann, A. Jandl, K. Esfarjani, M. T. Bulsara, E. A. Fitzgerald, G. Chen, and K. A. Nelson, Appl. Phys. Lett. 102, 041901 (2013).
[3] F. Hofmann, J. Garg, A. A. Maznev, A. Jandl, M. T. Bulsara, E. A. Fitzgerald, G. Chen, and K. A. Nelson, Journal of Physics: Condensed Matter, in press, arXiv:1303.4413

Phonons are the dominant heat carriers in insulators and dielectrics and their behavior in nanoscale geometries is not fully understood. In nanostructures, the interaction of phonons with surfaces and interfaces is believed to result in markedly different phonon behavior compared to bulk materials. To better understand how nanostructure geometries affect phonon propagation, we have developed a microfabricated phonon spectrometer that is capable of spectrally resolving acoustic phonons. [1,2] Our spectrometer, which utilizes superconducting tunnel junctions as phonon transducers, emits and detects tunable, non-thermal acoustic phonons with frequencies ranging from ~100 to ~870 GHz. [2] We utilize the spectrometer to probe phonon surface scattering in silicon nanosheets of varying lengths and with experimentally determined root mean square roughness amplitude. Our measurements indicate that the well known Ziman theory on phonon surface scattering, which takes into account the roughness of the surface, is not sufficient for completely describing the surface interactions in nanostructures. In addition, we present studies of how nanoscale geometrical designs could potentially enhance or reduce phonon propagation by testing transmission through bent nanosheets and nanosheets with “echo” cavities. Our spectroscopic phonon studies will help elucidate the contribution of phonon scattering to nanoscale thermal transport.
References:
[1] J.B. Hertzberg et al, Rev. Sci. Instrum. 82, 104905 (2011).
[2] O.O. Otelaja et al, New J. Phys. 15 (2013), 043018

Ultra-thin single-crystal silicon nanomembranes are of interest in transferrable and flexible electronics applications. These semiconducting nanomembranes have been shown to be readily transferrable resulting in robust alignment, positioning, and stacking. The extreme thinness of nanomembranes means that the interfaces between crystal regions play an extremely important role when compared to bulk samples, and means we can bring these interfaces closer to the surface of semiconductor samples for better measurement. We use a thermal release tape transfer technique to transfer silicon nanomembranes onto silicon substrates. We investigate the dependence of thermal resistivity on crystal separation at the resultant interfaces. Van der Waals bonding theory predicts an increase of thermal resistivity with separation distance. We use the 3-omega method to measure thermal resistances of relevant interfaces. The experimental measurements show a one-sigma increase in thermal resistance over Si-Si interfaces when a hexamethyldisilazane (HMDS) monolayer is used as an interfacial layer. Supported by DOE (DE-FG02-03ER46028).

The reduction of the size of nano-objects or nano-materials down to the nanoscale leads to strong modifications of its transport properties depending then on its size, shape, structure and obviously on its environment. Carrier confinement combined to interface effects gives rise to new transport properties. That is the case in absorption and emission of light where the new properties are given by electromagnetic near field coupling between the nano-objets included in the material. Concerning phonon transport, a frequency dependence of thermal conductivity can be observed.
All these processes occurring at time scales from femtoseconds up to nanoseconds are routinely accessible with ultrafast pump-probe techniques. i.e heterodyne optical sampling allows to access to the energy transfer and understand the heat propagation into nano-objects themselves. The comprehension of energy transport mechanisms had been initiated by the study of a collection of nano-objects in solution without any coupling between them.
We will describe different situations where the energy deposited by a femtosecond flash can be converted into phonons or plasmons traveling respectively at the speed of sound and speed of light in nanomaterials. Our ultrafast imaging technique enables to film at 20 Tera image per second.
[1] G. Pernot et al., Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers, Nature Materials 9, 491 (2010).
[2] S. Maier et al., Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides, Nature Materials 2, 229 (2003).
[3] S. Shen et al., Surface phonon polaritons mediated energy transfer between nanoscale gaps, Nano Lett. 9, 2909 (2009).
[4] E. Rousseau et al., Radiative heat transfer at the nanoscale, Nature Photonics 3, 514 (2009).

Gallium nitride (GaN) is often considered the most technologically pertinent semiconductor after the discovery of silicon. Understanding the optical and thermal properties of GaN surfaces is imperative in determining the utility and applicability of this class of materials to devices. In this work, we present preliminary results of spectroscopic ellipsometry measurements, Raman spectroscopy as well as time-domain thermoreflectance (TDTR) as a function of surface root mean square (RMS) roughness. We used commercially available 5mm x 5mm, one side polished GaN (3-7 µm)/Sapphire (430 µm) substrates that have a wurtzite crystal structure and they are slightly n-type doped. The GaN substrates were cleaned with Acetone (20 min)/Isopropanol (20 min)/DI water (20 min) before they were annealed in the open atmosphere for 10 minutes ( for a range of temperature of 900-1050 °C). This high-temperature treatment produced RMS values from 1-30 nm as measured with an atomic force microscope. Qualitative ellipsometric measurements show that the complex refractive index and the complex dielectric function monotonically decrease towards lower wavelengths with an increase of RMS. We also show a dependency of the thermal boundary conductance across metal/GaN interfaces with RMS roughness. In addition, the Raman spectra taken of each film show a dependence of surface localized stress and increased phonon lifetimes with roughness.

The interaction between electrons and phonons can reveal and control important properties, for example limiting mobility via scattering, and the strength of this interaction can be described by the electron - phonon coupling (EPC) parameter [1]. In graphene the electrons are treated as massless Dirac fermions due to the linear dispersion. The EPC in graphene is responsible for violation of the Born - Oppenheimer approximation [2], where the carbon nuclei would be treated as frozen, which means that the electronic properties are intrinsically linked to the phonon properties. Based on the linear electron dispersions, one might expect similar massless electrons from silicene or germanene therefore the contribution to the electronic properties from the EPC is a pressing matter of concern. Results presented here make use of density functional perturbation theory and show that the EPC in graphene, silicene and germanene varies by at least an order of magnitude (5 - 65 (eV/Å)^2). Data is presented for flat and buckled geometries and the EPC variation is shown to be a consequence of the difference in mass and geometry.
1. Piscanec, S., et al., Kohn Anomalies and Electron-Phonon Interactions in Graphite. Physical Review Letters, 2004. 93(18): p. 1-4.
2. Castro Neto, A.H., Graphene: Phonons behaving badly. Nat Mater, 2007. 6(3): p. 176-177.

In the process of understanding and developing a structured material permitting the modulation of phonon transport, we investigated how thickness and microstructure of monolayer and multilayer aluminum nitride (AlN) thin-films influence thermal conductivity. We present experimental results on the cross-plane thermal conductivity of AlN thin-films of different thicknesses. The films were deposited by reactive DC magnetron sputtering on single-crystal silicon substrates (100), and their thermal conductivities were determined using the 3 omega method. To prepare the multilayer films, the deposition process was interrupted periodically in order to grow multiple AlN layers, one on top of the other. This interruption aimed to demonstrate the creation of an interface between two layers of the same material, and to slightly alter the crystalline quality of the monolayer versus multilayer films. Thermal conductivity values for monolayer and multilayer configurations with identical thicknesses were compared. The thermal conductivity was found to increase with thickness in both synthesis configurations depending on the microstructure and growing conditions. However, significant overall reduction in thermal conductivity is observed for multilayer films. These results suggest that an interface between AlN layers is created by the interruption of the deposition process thereby increasing the interface scattering of phonons.

Conventional materials for thermal insulation possess a high percentage of trapped gas, either in form of bubbles (foam glass, polystyrene) or as an air layer (glass wool). Aerogels as the nanoscale variant offer the so far highest insulation values. We show that the use of at minimum two solid constituents of a powder compact will turn this system to being a good thermal insulator despite containing good thermal conductors. Composites of anthracite coal, granite, and alumina powders are chosen as an example of a low cost easily available material combination potentially easily applicable in building insulation. The mix of different materials and their phonon properties will be discussed for systems at the nano- and micron scales. Thermal conductivity, scanning electron microscopy (SEM), and porosity measurements are shown for this simple material system. The resultant low thermal conductivity values are discussed in the framework of phonon spectra and interface properties.

Understanding phonon mean free path distributions is one of the key challenges in understanding and engineering thermal transport in nanoscale materials, ranging from thermoelectric to nanoelectronic devices. While recent theoretical work has predicted these distributions from first-principles, there is a lack of experimental measurements with the accuracy or range of transport length scales required to test these predictions. The current work presents an all-optical study of the thermal conductivity free-standing, ultrathin silicon membranes ranging from 15 nm to 1.5 um in thickness. The membrane boundaries serve as diffusive filters to limit the phonon mean free path and define a length scale which can be converted to mean free path using a suppression function. Using a wide range of thicknesses then allows an accurate reconstruction of the phonon mean free path distribution for Si. The measurements were performed using the transient thermal grating technique. An ~ 85 % reduction of the bulk thermal conductivity was measured in the 15 nm membrane. The results from all membrane thicknesses were used to reconstruct the bulk phonon mean free path distribution for silicon. The reconstructed distribution was found to agree well with that predicted from first principles calculations.
This work is supported by S3TEC, a DOE BES Energy Frontier Research Center and the EU projects NANOPOWER, MERGING and NANOTHERM, the ENIAC project NANOTEG and the Spanish projects Plan Nacional TAPHOR and Consolider nanoTHERM.

The conventional upper and lower bounds on phonon thermal conductivity in materials are well known: the highest room temperature thermal conductivity is found in isotopically pure diamond and the lowest thermal conductivities are found in amorphous polymers. In this talk I will emphasize recent examples of extremes of thermal conductivity—and conduction under extreme conditions—in polymers and molecular solids. Our measurements of heat conduction in novel materials are enabled by variety of ultrafast optical pump-probe metrology tools developed over the past decade. At the low end of the thermal conductivity spectrum, fullerene derivatives display the lowest thermal conductivity ever observed in a fully dense solid, comparable to the conductivity of disordered layered WSe2 and only twice that of air. Extremes of high pressures (up to 60 GPa) allow us to continuous change the strength of molecular interactions in glassy polymers and test theoretical descriptions of the mechanisms for heat conduction based on the model of the minimum thermal conductivity. The thermal conductivity of aligned, crystalline and liquid crystalline polymer fibers can be surprisingly high, comparable to that of stainless-steel. Near room temperature, the thermal conductivity decreases with 1/T. The dominate carriers of heat appear to be longitudinal acoustic modes with lifetimes controlled by anharmonic processes.

Thermal conductivity of nuclear fuel, critically related to reactor efficiency and safety, is significantly influenced by irradiation defects. In this presentation we investigate the influence of irradiation defects on thermal transport in surrogate nuclear fuel materials. Depleted uranium oxide and cerium oxide samples were irradiated with hydrogen and helium ions. The irradiation damage, limited by the penetration depth of the energetic ions, was confined to a surface layer several microns in thickness. The microstructure of damaged layer was characterized using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Low temperature and low dose irradiation resulted in isolated point defects inferred from lattice expansion measured using XRD. On the other hand, high temperature and high dose irradiation provided a microstructure containing large defect clusters primarily in the form of dislocation loops as revealed by TEM characterization. The influence of isolated point defects and dislocation loops on thermal transport was investigated using modulated thermoreflectance microscopy. It was found that the influence of dislocation loops is stronger than was expected using standard models for straight dislocation lines.

Over the last decade, there has been a continued interest in Si-based thermoelectric (TE) materials because they are cheap and easy to work with and have good modifiable electrical properties for thermoelectric applications. In order to develop Si-based TE materials, much research has been directed towards finding ways to reduce the intrinsic high thermal conductivity of Si to increase the thermoelectric efficiency. While heat conduction in Si is mainly governed by phonon transport, many attempts have been made to suppress the thermal conductivity by introducing phonon scattering through nanostructuring and alloying. Likewise, introducing dopants results significant suppression of the thermal transport in Si-based nanostructures. However, only a very limited amount of effort has been undertaken to address explicitly the roles played by dopants in the phonon-impurity scattering. Most of the earlier theoretical investigations have focused on explaining the variation of thermal conductivity with temperature and dopant concentration, not the specific contributions of different dopants. In this talk, we will present the underlying mechanisms for thermal conductivity suppression in crystalline Si by substitutional doping with different elements (X= boron, aluminum, phosphorus, and arsenic), particularly the relative contributions of doping-induced mass disorder, bond disorder, and lattice strain effects. Moreover, we will also discuss the impact of dopant agglomeration on thermal conductivity suppression. The improved understanding provides important insight into how to modify Si-based materials to enhance their thermoelectric properties through doping and/or alloying. For nonequilibrium molecular dynamics (NEMD) simulations used in this work, we have optimized Stillinger-Weber potential parameters for Si-X interatomic interactions by fitting to relevant atomic forces from first-principles calculations.

We study thermal transport properties by conduction using molecular dynamics simulations. In our approach, two portions are delimited and heated at two different temperatures before the approach-to-equilibrium in the whole structure is monitored. The observed decay of the temperature difference is interpreted and used to extract thermal properties of systems ranging from bulk materials, interfaces and nanoconstrictions. First we study the case of bulk. The numerical results are compared to the corresponding solution of the heat equation and a relation is found between the decay time and the bulk conductivity [1]. The method is applied to bulk silicon modeled with Tersoff potential [2]. Systems longer than one micrometer are studied thanks to the reduced computational cost of the method. The bulk conductivity is extrapolated and an excellent agreement with previous calculations [3] is obtained. The approach is used afterwards to predict the thermal conductivity of germanium and alpha-quartz[4]. The method is also applied to the case of different materials in the two heated portions. The lump capacitance assumption is extended to extract the boundary conductance. The application is made on the crystalline silicon/amorphous silicon or silica [5] interface. The method is shown to be sensitive enough to enable the determination of the low interface resistance [6] despite the presence of a poor conductor on one side of the interface. Finnaly, current investigation dedicated to nanoconstrictions are presented. These multiple applications illustrate that the AEMD method is ideally suited for studying atomic-scale systems including complex features, such as nanostructures, disordered materials and lightly to strongly resistive interfaces.
[1] E. Lampin, P. L. Palla, P.-A- Francioso and F. Cleri, submitted to J. Appl. Phys.
[2] J. Tersoff, Phys. Rev. B 38, 9902 (1988)
[3] P. C. Howell, J. Chem. Phys. 137, 224111 (2012)
[4] B. W. H. van Beest et al, Phys. Rev. Lett. 64, 1955 (1990)
[5] S. Munetoh et al, Comput. Mater. Sci. 39, 334 (2007)
[6] E. Lampin et al, Appl. Phys. Lett. 100, 131906 (2012)

Semiconducting clathrates are being investigated intensively due to their high application potential as thermoelectric materials for converting waste heat to electric energy. Due to their guest-framework structure, the (doped) semiconducting clathrates are typical representatives of "Phonon Glass - Electron Crystal" compounds, where the rattling guest atoms are considered to decrease the lattice thermal conductivity via phonon scattering. Recent studies have also pointed out that other phenomena such as the framework disorder and the unit cell complexity alone should also decrease the lattice thermal conductivity of the clathrates [1, 2]. We have recently investigated the structural characteristics of a novel family of anisotropic clathrate frameworks, which also show highly anisotropic thermoelectric properties [3]. To investigate the effect of the structural anisotropy on the lattice thermal conductivity, we analyse the phonon transport properties of the clathrates by combining density functional perturbation theory calculations with anharmonic elastic continuum theory [4, 5]. In the case of anisotropic silicon clathrate frameworks, the c-axis of the studied hexagonal structural variants varies from 17 to 85 Å, which has a distinct effect on the phonon transport properties in this direction.
[1] M. Christensen, S. Johnsen, B. B. Iversen, Dalton Trans. 2010, 39, 978-992.
[2] E. S. Toberer, A. F. May, G. J. Snyder, Chem. Mater. 2010, 22, 624-634.
[3] A. J. Karttunen, T. F. Fässler, ChemPhysChem 2013, 14, 1807-1817.
[4] G.P. Srivastava, The Physics of Phonons, Adam Hilger, Bristol, 1990.
[5] Hepplestone, S.P.; Srivastava, G.P., Phys. Rev. B, 2006, 74, 165420.

Most thermal transport studies have explored rather conventional conditions of temperature, pressure and irradiation. Extreme environments place materials under conditions not usually encountered; the thermal response under these conditions allows the better elucidation of the physics of the various phenomena. While the conditions of high temperature, high pressure and irradiation are interesting from a fundamental perspective, they are also relevant technologically. For example, the high pressure and temperature conditions present in the Earth&’s interior greatly affect thermal conductivity of the mantle and influence its heat balance and consequently evolution. As a second example, irradiation effects are crucial for the understanding thermal transport and therefore performance characteristics of nuclear fuels. In this presentation, we will discuss how these conditions influence thermal conductivity and demonstrate insights obtained with the combination of the atomistic simulations techniques such as molecular dynamics and lattice dynamics on the basis of classical potentials and first principles methods. This work is supported by the Center for the Materials Science of Nuclear Fuel, a DOE-BES Energy Frontiers Research Center, as well as NSF Materials World Network Project (DMR-0710523), National Natural Science Foundation of China (Grant 11005070) and Shandong Natural Science Foundation (ZR2010EM030).

PbTe is of great interest both due to its thermoelectric properties and highly non-linear lattice dynamics. Inelastic neutron scattering experiments reveal a signature of strong anharmonicity as evidenced by the emergence of a new peak in the vibrational spectra with increasing temperature. Novel approaches based on first-principles calculations have been developed for including phonon interactions at elevated temperatures, though none of these techniques are able to predict the experimentally observed anomalies in the case of PbTe. Here we measure the vibrational spectrum as a function of temperature using molecular dynamics based upon a first-principles polynomial potential. Our calculations successfully predict the emergence of the anomalous mode in agreement with experiment. The origin and implications of the anomalous mode are discussed.

The thermal properties of nanoscale systems often critically depend on internal interfaces where differing vibrational properties result in phonon scattering. Scattering and its effects at such interfaces is often quantified by measuring or calculating the Kapitza resistance. The ability to design systems with a prescribed or minimal Kapitza resistance requires a deeper understanding of how interfacial properties effect this thermal resistance. Experimental challenges with measurements of thermal properties at the nanoscale along with the need for predictive models to aid in systems design invites computer simulation to play a prominent role in the study of phonon-mediated thermal transport. An approach to enhancing thermal transport across an interface is to include an adhesive layer with vibrational properties intermediate to those on either side of the interface. We focus on the effects of adhesion layers by modeling bicrystalline systems with a diamond lattice structure and the Stillinger-Weber interatomic potential. We consider 001 and 111 interfacial normals and vary the thickness of the interfacial region along with its bonding to the bicrystal. The systems are characterized using non-equilibrium molecular dynamics via the direct method to along with interfacial Green-Kubo simulations and local density of states (LDOS) analysis of the spatial variation of phonon populations in the system. The multiple wave-packet method is also used to investigate scattering in these systems at prescribed frequencies. Taken together, this work provides a detailed characterization of such model systems, and the implications for design of actual systems with prescribed thermal properties is discussed.

Energy transfer across nanometer spaced surfaces plays a key role in several technologies such as thermophotovoltaics, heat assisted magnetic recording and interfacial thermal conductance. Heat transfer between two surfaces at contacts is described by interfacial resistance, and for two surfaces separated by a small distance in vacuum is by near-field radiation. To this day, there is no well developed, unified model that can describe the power transferred between two bodies as a function of their separation that accurately describes the correct physical behavior as the gap approaches zero. Fluctuating electrodynamics techniques, following the methods developed by Rytov, are able to describe the energy transfer for larger distances, but is flawed in that it uses the macroscopic Maxwell equations and continuum descriptions such as dielectric functions. We develop a different approach using lattice dynamics and the microscopic Maxwell equations to bridge the theories of conduction and radiation. This is achieved by coupling two ionic slabs through both long range Coulomb and short range chemical bonding forces and using an atomistic Green's function approach to compute the energy transmission. Finally, comparison with Rytov's theory provides insight into the validity of the developed approach and also helps understand the limitations of the continuum theory in the near-field regime.

Many established methods for measuring the thermal conductivity of solid or soft materials evolves making the radiation and convection negligible. In this work, a steady-state method is introduced to measure the thermal conductivity of a sample with non-negligible radiation and convection. In this method, the sample is suspended between a heater and a thermoelectric cooler (TEC). By manipulating the temperatures of the heater and cooler around the ambient temperature, both the thermal conductivity of the sample and the effective heat transfer coefficient can be obtained. Measurements on polymer sheet, glass sheet, aluminum foil and porous zeolite disk are demonstrated. Sensitivity, uncertainty, advantages and limitations are discussed.
This work is supported By DOE DE-EE0005756.

The knowledge of phonon-phonon interactions in bulk systems is generally inferred from the measurement of macroscopic data such as the thermal conductivity. We are today still lacking precise and detailed information on phonon meanfree paths in very used materials such as silicon and gallium arsenide. Recent advances in the field of high frequency phonons transduction give the opportunity to fill these gaps. Thus, nanostructured materials such as superlattices proved to be excellent high frequency coherent phonons generators and detectors for more than a decade [1-4]. We report on a series of experiments performed with coherent longitudinal acoustic waves with different frequencies ranging from 0.3 up to 1THz. The dependence of the inverse mean free path has been accurately determined in terms of temperature from 4K to 60 - 80K. The sound attenuation first increases with frequency but, surprisingly, a plateau can be observed between 0.7 and 1THz on a large temperature range.
We performed a detailed theoretical analysis of phonon-phonon interactions in GaAs limited to 3 phonons interactions. We derived calculations of two-phonon density of states deduced from ab initio calculations data for scattering and fission processes and show that the behaviour we observed can be reasonably taken into account within the framework of this first approach.
1) P. Hawker, A. J. Kent, L. J. Challis, A. Bartels, T. Dekorsy, H. Kurtz, K. Khöler, Appl. Phys. Lett. 77, 3209 (2000)
2) A. Huynh, B. Perrin, N. Lanzillotti-Kimura, B. Jusserand, A. Fainstein and A. Lemaître, Phys. Rev. B 78, 233302 (2008)
3) A Huynh, B. Perrin, B. Jusserand, A. Lemaître, Appl. Phys. Lett. 99, 191908 (2011)
4) M. F. Pascual-Winter, A. Fainstein, B. Jusserand, B. Perrin, A. Lemaître , Phys. Rev. B85, 235443 (2012)

Previously it has been shown that Schottky diodes can be used to detect high frequency coherent acoustic phonons [1]. A picosecond-duration acoustic phonon packet crossing the edge of the depletion layer causes the current through the device to change. It is already well established that Schottky diodes can also be used to detect electromagnetic radiation as they are widely used in THz spectroscopy [2]. By heterodyne mixing of sub-terahertz coherent acoustic phonons with electromagnetic radiation of a similar frequency it should be possible to detect the phonons electrically.
We describe the use of a microwave (W-band) beam-lead Schottky diode for heterodyne detection of coherent acoustic phonons with frequencies up to about 100 GHz. In the experiments, quasi-monochromatic acoustic waves were generated by excitation of the GaAs substrate on which the Schottky is fabricated with a train of femtosecond optical pulses. The pulse train, with variable pulse separations in the 10-100 picosecond range, was formed using an adjustable Fabry-Perot cavity at the output of a femtosecond amplified Ti:Sapphire laser. The W-band reference frequency for the detector was from a 94 GHz Gunn diode oscillator coupled to the Schottky diode via a waveguide. We detect the electrical output from the Schottky diode at the difference frequency between the Gunn oscillator and the acoustic waves, which was in the 0 - 12.5 GHz range, using a high-bandwidth digitizing oscilloscope. We propose that this scheme could find applications in high-resolution acoustic phonon spectroscopy.
[1] D. M. Moss, A. V. Akimov, B. A. Glavin, M. Henini and A. J. Kent, Phys. Rev. Lett. 106, 066602 (2011)
[2] A. Maestrini, B. Thomas, H. Wang, C. Jung, J. Treuttel, Y. Jin, G. Chattopadhyay, I. Mehdi and G. Beaudin, C. R. Phys. 11, 480-495 (2010)

We present the measurement of the coherent acoustic vibrations (phonons) in different hypersonic (GHz) phononic crystals, comprised of silica or porous silica spheres, by using ultrafast pump-probe spectroscopy. The coherent phonons are generated by pulse laser heating, and the resulting propagation of the phonon waves are monitored by a time-delayed probe laser. The transient reflection spectra indicate that the acoustic vibration is heavily damped in the porous silica specimen and the coherent oscillation lasts for a much shorter period of time, due to the nanoporous structures (1-6 nm). Although it is known that defects or porous structures can enhance the scattering of phonons resulting in a reduced thermal conductivity, this is the first direct experimental observation of how coherent phonon transport can be affected by nanoporous structure. The observation of hypersonic frequency phonons by using ultrafast optical technique could be useful not only to understand the transport properties of energy carriers (phonons) that are lost at macroscopic scales, but also to investigate how the transport of coherent phonons can be manipulated by nanophononic crystals.

In this work, we explore coherent phonon transport in superlattice (SL) using molecular dynamics and Green&’s function-based simulations. From non-equilibrium molecular dynamics (NEMD) simulations, contributions of coherent phonon transport to heat conduction are observed in both periodic and aperiodic SL&’s. First-principles based atomic green&’s function (AGF) calculations are used to further identify phonon spectral transmission functions and coherence in these superlattice structures.
This material is based on work supported as part of the Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001299/DE-FG02-09ER46577.

One of the major challenges for integrated photonics, as compared with electronics, is the ability of controlling and manipulating photons within small dimensions. It is, therefore, of paramount importance to search for devices that are, at the same time fast, compact, efficient, low-cost, externally controlled, and compatible with integration technology. A promising approach consists of using a strong population of coherent surface acoustic phonons (in the form of surface acoustic waves, SAWs) to modulate multiple optical waveguides (WGs) through the acousto-optical effect [1]. In this way, it is possible to address multiple devices using only one SAW, due to its very large phase coherence. This provides an excellent compromise between speed and size.
In this contribution, we will review recent developments in the area of SAW based-acoustooptic devices. Starting from an ultra-compact SAW-driven Mach-Zehnder interferometer [2], and then, increasing the complexity, we will discuss the modulation of multiple WG devices [3,4]. Special emphasis will be given to multiple channel, SAW-synchronized photonic modulators as well as to SAW-controlled arrayed waveguide devices (AWGs) [5]. The latter is a very important tool for multiplexing and demultiplexing optical signals. With our approach, the otherwise static AWG response varies periodically. Thus, a given AWG output channel receives distinct light wavelengths at different SAW phases. Finally, we will illustrate how SAW-driven AWGs can be employed for the fabrication of a very compact spectrum analyzer. The majority of the devices discussed have been fabricated in the (Ga,Al)As material platform. These concepts, however, can be extended for virtually any material system with processing steps easily implemented in any modest cleanroom.
[1] M. M. de Lima, Jr. and P. V. Santos, Rep. Prog. Phys. 68, 1639 (2005).
[2] M. M. de Lima, Jr., M. Beck, R. Hey, and P. V. Santos, Appl. Phys. Lett. 89, 121104 (2006).
[3] M. Beck, M. M. de Lima, Jr., P. V. Santos, J. Appl. Phys. 103, 014505 (2008).
[4] E. C. S. Barreto, and J. M. Hvam, Proc. of SPIE 7719, 771920 (2010).
[5] M. M. de Lima, Jr., P. Muñoz, J. Capmany, P. V. Santos, Patent Application WO 2012/152977 A1 (2012).

We investigate the propagation of ultrashort strain pulses in oxide materials such as the ferroelectric PbZr0.2Ti0.8O3 (PZT) and the quantum-paraelectric SrTiO3 (STO). These strain pulses are pruduced by an optical excitation of a metallic nanolayer such as SrRuO3 (SRO) or LaSrMnO3 (LSMO) epitaxially connected to STO or PZT. These materials are very susceptible to the formation of domains or structural defects. PZT films tend to form dislocations influencing the strain propagation and piezoelectricity even if all domains have the same c-axis orientation and ferroelectric polarization below the Curie temperature Tc=530K. The acoustic properties of STO show subtle effects in the anti-ferrodistortive phase below 105K which are associated with domain-wall motion and the orientation of domains mediated by epitaxial strain..
We apply Ultrafast X-ray Diffraction (UXRD) and Time-Domain Brillouin Scattering as direct structural probes of the strain propagation. Both methods allow a determination of the frequency and damping rate of phonons with wavelengths of about 100 nm. UXRD not only serves for calibration of the transient strain amplitude of the GHz waves.[1] We have improved our tabletop UXRD towards a reciprocal space mapping technique with femtosecond time resolution which can quantify in-plane lattice motion mediated by structural defects.[2] These direct observations show that out-of-plane expansion waves are damped due to a coupling to in-plane lattice dynamics in PZT on a picosecond time scale, whereas this coupling is absent for out-of-plane compression waves. Time-Domain Brillouin Scattering of STO with strain amplitudes calibrated by UXRD directly proves that for strain amplitudes around 0.2% the sound velocity for compressive strain exceeds the tensile strain velocity by 3%. Below 105K a giant slowing down of the sound velocity by 12% is interpreted as a superelastic response by previously inconceivable coupling of GHz phonons to ferroelastic domain walls.
We believe that these measurements are very helpful for a direct observation of ultrafast dynamics in thin films and structures which are either artificially tailored on the nanoscale or intrinsically form nanodomains.
[1] Bojahr et al, Phys. Rev. B, 86, 144306 (2012)
[2] D. Schick et al, Phys. Rev. Lett., 110, 095502 (2013)

We use inelastic scattering of ultrashort visible and hard X-ray pulses as a probe of the transient occupation of phonon modes constituting large amplitude strain-wavepackets. These time-domain Brillouin scattering experiments permit the time-resolved observation of the phonon dynamics including phonon damping and nonlinear interaction. High strain fields lead to nonlinear phenomena like sum and difference frequency mixing, which depend strictly on the excited phonon spectra.
We excite large amplitude single cycle strain pulses in SrTiO3 by optical pumping a nanometric metal layer to observe the change of the phonon spectra associated with the self-steepening of the pulse fronts.[1] The evolution of spectrally narrow phonon wavepackets excited by multipulse excitation of a metal layer can be understood by difference- and sum-frequency generation. The changes in the spectrally narrow phonon distributions can be directly observed by Ultrafast X-ray Diffraction, e.g. after multipulse excitation [2] or by pumping a superlattice structure with a single laser pulse.[3] Measurements of the phonons generated in such superlattice structures show a large damping rate of the excited phonon k-vectors and a shift of the maximum of the phonon distribution to higher k-vectors when they propagate through bulk SrTiO3, which is attributed to the different sound velocities of large amplitude tensile and compressive strain. Optical time-domain Brillouin scattering with even higher excitation fluences provide additional insight. The higher time and k-vector resolution and the advantage of directly measuring the coherent phonon spectrum yield a precise time resolved view of the occupation of phonon modes. The measured phonon dynamics is quantitatively modeled by coupled anharmonic oscillators and roughly explained by a parametric down-conversion process of GHz strain waves.

The rapid decay is understood by the pronounced down-conversion and sum-frequency generation processes which remove energy from the excited modes. The damping of coherent phonons to the thermal bath increases with k^2. In total, the coexistence of sum and difference frequency conversion processes leads to a faster damping of the coherent phonon wavepacket.
We think our steps towards “nonlinear phononics” could help to establish new ultrafast experiments in which the direct optical excitation of a material by an ultrashort laser pulse is replaced by a highly intense ultrashort pressure pulse which excites the material to trigger the dynamics without electronic excitation.
[1] Bojahr et al., PRB 86, 144306 (2012)
[2] Herzog et al., APL 100, 094101 (2012)
[3] Shayduk et al., PRB 87, 184301 (2013)

We report two types of Si nanobeam transducers based on optomechanical effect in photonic crystal cavities to realize single-Dalton-level order mass spectrometry. One of them is based on a slotted nanobeam cavity (SNC) which can obtain both ultrahigh optical Q of ~10^6 and high optomechanical coefficient gOM of 450 nm/GHz for the fundamental in-plane mechanical mode when the slot width s = 40 nm. Therefore, it is suitable for “High efficient phonon emitter” since their mechanical modes can be excited and amplified remotely by coupling a laser light whose wavelength is slightly detuned from the cavity resonant mode to the shorter wavelength (higher energy) side.
Another one is based on coupled nanobeam cavity (CNC) which is intended to realize “Low threshold phonon lasers (SASERs)”. It has several-order of even modes and odd modes, and typical optical Q of both fundamental modes are very high (10^5 - 10^7), and even mode has a high gOM of 200 nm/GHz when the gap width g between two beams is 40 nm. When g becomes narrower, only the resonant wavelengths of the even modes are largely red-shifted and make crossing points with higher order odd modes since only even modes have strong modal localization in the gap. If the wavelength difference of these two modes is corresponding to the energy of the nanobeam&’s mechanical frequency fm, we can obtain the mechanical gain G based on the phonon laser theory in coupled optical cavities when we input the light for a higher energy optical mode. In this case, G can be enhanced by both optical Q of two modes. It is expected to realize more robust phonon lasing operations which overcomes the viscosity dumping even in air and liquid sample. We also found multiple (triple or quadruple) CNCs are much easier to obtain G since they have so many crossing points for various g.
In the device fabrication, we employed SOI wafer which consists of 220 nm thick silicon layer, 2 mu;m thick BOX and silicon substrate. In the e-beam lithography, we used 495 PMMA C2 resist to obtain higher resolution. As a result, we could obtain the suspended SNC and CNC with very narrow gap width of ~ 40 nm without any damages through ICP dry etching and HF wet etching. By controlling the wet etching time, we could also obtain the various length of the suspended region which is very useful to control fm and mechanical stress at the boundaries.
In the measurement, the devices are set in the vacuum chamber with an optical window. We irradiated the tunable laser light on the suspended device from the vertical direction through a free space optics consists of several mirrors, prisms, and x20 objective lens. The reflected light was collected and analyzed by using same optics, polarizers, a photo detector, and a spectrum analyzer to see the mechanical oscillations. We observed the mechanical oscillations whose fm = 1 - 10 MHz, and mechanical Q of 5,000. Further results will be reported at the conference.

Phonon meanfree path in amorphous systems is a long term issue.
Density fluctuations at a length scale of a few nanometers is responsible
for a drastic decrease of phonon mean free path for acoustic waves in the
subterahertz range. A crossover between propagative waves and localized
excitations is expected between 0.5 and 1 THz where measurements are rather
difficult. Because of the diversity of the possible contributions to sound absorption
combined with the spasity of observations, sound attenuation in amorphous system is still controversial.
We present in this work an experimental determination of sound absorption
for longitudinal waves in thin fused silica films obtained by the picosecond
laser ultrasonics technique over a large temperature range (20K-300K).
These results are discussed and compared with previous measurements.

Second Harmonic Generation (SHG) is an optical technique that is known to be sensitive to weak electrical fields at hetero-interfaces. The fields and resulting SHG signals have been shown to be dependent on parameters of interest including strain, trapped charge, contamination, and interface roughness [1,2].
Coherent acoustic phonon (CAP) interferometry is an ultrafast pump-and-probe technique, wherein a strong optical pump pulse perturbs a system. The propagating strain pulse generated represents a distortion of the ordinary lattice spacing of the host material, and modulates the local material properties [3].
The combination of the two analytical techniques: CAP and SHG gives us opportunity to observe time-dependent the transient nonzero second-order susceptibility caused by coherent acoustic phonons. Simultaneous observation of SHG and CAP spectra provides a new look at buried interface properties.
In this study, we examine the impact of the well-defined strain produced by the CAP wave and its influence on a SiO2/Si interface and on bulk Si substrate. SHG spectra reveal oscillating electric field strength near the interface as a result of interference of the incident probe beam and its reflection by the moving compressional wave surface. At the same time, CAP spectra exhibit long lasting, high amplitude oscillations. For CAP reflectivity experiments, the property of interest is the complex index of refraction, N = n + iκ. The strain- induced modulation of the refractive index produces an optical interface in the host material that is particularly strong at the sharp transition from compressive to tensile strain.
Combining CAP and SHG studies provide a wealth of new information on the second-order susceptibility and as well as the third-order susceptibility from pulsed, phonon induced deformation caused by the coherent acoustic wave. We demonstrate that the CAP/SHG technique will prove to be extremely useful in the study of SiO2/Si interface properties.
[1] H. Park, J. Qi, Y. Xu, G. Lupke, N. Tolk, “Polarization-dependent temporal behavior of second harmonic generation in Si/SiO2 systems,” J. Opt. 12, 055202, 2012
[2] H. Park, J. Qi, Y. Xu, K. Varga, S. M. Weiss, B. R. Rogers, G. Lüpke, and N. Tolk, “Boron induced charge traps near the interface of Si/SiO2 probed by second harmonic generation,” phys. status solidi (b), vol. 247, no. 8, pp. 1997-2001, 2010.
[3] A. Steigerwald, Y. Xu, J. Qi, J. Gregory, X. Liu, J. K. Furdyna, K. Varga, A. B. Hmelo, G. Lupke, L. C. Feldman, and N. Tolk, “Semiconductor point defect concentration profiles measured using coherent acoustic phonon waves,” Appl. Phys. Lett., vol. 94, no. 11, p. 111910, 2009.

Polymers usually have low thermal conductivity values that are unfavorable for thermal management of microelectronics. Filler loaded polymers are used as interface materials for enhanced thermal conductivity. The thermal conductivity of the base polymers plays a critical role in determining the effective thermal conductivity of such interface materials. The goal of this study is to investigate thermal conductivity and interface resistance of polymers with different molecular structures and molecular weights. Several well characterized polymer standards were analyzed by laser flash to determine their thermal conductivity and interface resistance, including poly(vinyl acetate), poly(bisphenol A carbonate), polystyrene, and polydimethyl siloxane. A three-layer model corresponding to the experimental Si-polymer-Si configuration is used to analyze the laser flash results. Experimental results obtained provide insights on factors determining heat conduction in polymeric materials and directions for developing high performance thermal interface materials.

Thermoelectric materials have seen large increases in their performance due to nanostructuring on various length scales. When the nanostructures are on the order of the phonon mean free path, thermal conductivity is greatly reduced due to enhanced scattering. The relaxation time approximation, which assumes a directionally independent and isotropic scattering process, lies at the heart of thermal conductivity calculations using the Boltzmann Transport Equation. With the advent of first-principles calculations from Density Functional Perturbation Theory, phonon dispersion relations can be determined without adjustable parameters. Green&’s function methods derived from the dispersion relation allow for higher-order corrections to scattering amplitudes due to nanostructured impurities in a crystal lattice. Using full-order time-dependent perturbation theory, angular and frequency dependent scattering rates are calculated for nanostructures. These results show great deviations from first-order isotropic calculations from Born&’s approximation. At mid-range frequencies still in the non-dispersive regime of the acoustic branch, phonon scattering becomes very directionally dependent. Additionally, the total cross-section calculations capture the transition from the long-wavelength (Rayleigh) limit to the geometric limit. The deviation of the nanostructure cross-section from the single impurity limit is calculated as a function of frequency and average particle spacing. This divergence quantifies the validity of approximating impurities as dilute independent scatterers in the relaxation time approximation. The alloy limit is further investigated by computing the Green&’s function of a configurationally averaged finite system and comparing it to the Virtual Crystal Approximation.

Understanding thermal transport at the nanoscale is central to advancing thermoelectric materials and nanoelectronic devices. The major reason for reduced thermal conductivity in nanoscale objects is phonon scattering from the boundaries, which reduces the phonon mean free path. However, a deep understanding of these scattering processes has yet to be achieved. This is largely due to difficulties that arise when trying to consider the individual contributions from the wide range of phonon wavelengths and phonon mean free paths that contribute to thermal transport. Ultra-thin single-crystal silicon membranes provide an ideal platform to address this problem as they have well-defined characteristics and can be fabricated with a wide range of thicknesses. We use the transient thermal grating technique to study the effect of boundary scattering in these membranes with thicknesses ranging from 15 nm to 1.5 mu;m. In addition to providing thermal conductivity values for a large range of thicknesses and temperatures, the temperature studies down to 80 K allow the relative contributions of intrinsic and extrinsic scattering process to be varied. An increase in thermal diffusivity was found with decreasing temperature, along with observations of non-diffusive transport. These results are compared to theoretical models for the reduction in conductivity due to the thickness as well as temperature. These measurements provide valuable insight into how phonons with different mean free paths and wavelengths contribute to thermal transport at the nanoscale.
This work is supported by S3TEC, a DOE BES Energy Frontier Research Center and the EU projects NANOPOWER, MERGING and NANOTHERM, the ENIAC project NANOTEG and the Spanish projects Plan Nacional TAPHOR and Consolider nanoTHERM.

We have previously demonstrated that a semiconductor superlattice (SL) in the Wannier-Stark electron transport regime amplifies sub-THz sound due to the stimulated emission of acoustic phonons as electrons cascade through the SL1. Experimental evidence for saser action in such a SL was provided by observation of a sasing threshold2 and acoustic spectral line narrowing3 in a distributed feedback arrangement using a single SL as both the gain medium and phonon confining element. In the work described here, we incorporate the sound amplifying SL into an acoustic cavity between a pair of SL acoustic Bragg mirrors to form a vertical cavity saser structure, and investigate the properties of this structure.
Using the femtosecond optical pump-probe technique, we measure the acoustic properties of the cavity and show that the losses are low enough that the threshold for saser oscillation is theoretically achievable. We then measure the phonon emission dynamics of the structure following turn-on of the electrical pumping to the gain SL. We observe that the emitted phonon intensity builds up to a steady state in The advantage of this vertical cavity device compared to the distributed feedback structure previously investigated is that the frequency of operation can be engineered by changing the parameters of the acoustic Bragg mirrors, and thus can be matched with the requirements of an application. In the distributed feedback saser, the frequency of operation was fixed by the parameters of the gain SL necessary to achieve the optimum amplification. However, these results show that, due to the increased length of the cavity in the vertical cavity structure, the short coherence time and long build up time of the phonon oscillations need to be taken into account. The latter is particularly important when considering applications requiring short (<~ ns) acoustical pulses.
1. Beardsley, R.P., Campion, R.P., Glavin, B.A., Kent, A.J. New J. Phys. 13, 073007 (2011).
2. Kent, A. J., et al. Phys. Rev. Lett. 96, 215504 (2006).
3. Beardsley, R. P., et al. Phys. Rev. Lett. 104, 085501 (2010).
4. Moss, D., et al. Phys. Rev. B 80, 113306 (2009).

Scalability is presently a major barrier for the integration of different functionalities in opto-electronic quantum devices. In this contribution, we show that scalability in excitonic devices [1] can be achieved via exciton control by electrostatic gates and long-range transport by surface acoustic waves (SAWs). In this concept, information bits transported by photons are converted into excitons for processing using interconnected solid-state devices and then reconverted to photons for further transmission. Spatially indirect excitons (IX) - a bound state of an electron and a hole localized in a double quantum well structure - are particularly suitable for information storage due to their long lifetimes and strong non-linearities. In this talk, we demonstrated that IX systems and devices [1] can be integrated in a single chip using the long-range acoustic transport. The experiments are carried out in GaAs quantum well structures with interdigital transducers for the generation of GHz SAW beams. In these structures, the moving band-gap modulation induced by the SAW strain field traps and transports the long-living IXs over several hundreds of micrometers [2]. Furthermore, IXs can be exchanged between SAW beams using moving dots formed at the beam intersection. Using this approach, we demonstrate an acoustic optical multiplexer, a device capable of interconnecting a scalable number of IXs ports via an array of configurable transport channels defined by SAW beams. The multiplexer thus provide a pathway for the realization of scalable devices using the conventional planar technology.
[1] A. A. High et al., Science 321, 229-231 (2008)
[2] J. Rudolph, R. Hey and P. V Santos, Phys. Rev. Lett. 99, 047602[4] (2007)

The present work offers a study of the transport coefficients of n-type PbTe, which is one of the most important mid temperature range (400-800 K) thermoelectric materials. The isotropic-nearly-free-electron approximation [1,2] was applied to study the electronic transport coefficients, including the effect of band non-parabolicity on electron-phonon scattering. The lattice thermal transport coefficient was computed by employing the isotropic continuum model for the dispersion relation for acoustic as well as optical phonon branches, an isotropic anharmonic continuum model for crystal anharmonicity [3], and the single-mode relaxation time scheme. The role of transverse optical (TO) phonon modes in anharmonic interactions will be discussed in detail. After a successful reproduction of experimental measurements for the temperature variation of the power factor and the total thermal conductivity [4], we will present an assessment of the range of thermoelectric figure of merit ZT for different donor concentrations.
[1] Drabble, J. R., Goldsmid, H. J., &’Thermal Conduction in Semiconductors&’, Pergamon Press, (1961).
[2] Balkanski,M. and Wallis, R. F., &’Semiconductor Physics and Application&’, Oxford University Press, (2000).
[3] Srivastava, G. P., ‘The Physics of Phonons&’, Adam Hilger, Bristol, (1990).
[4] Yan, L.P., and Yong L., Journal of Alloys and components , 514, 40-44 (2012).

Phase-changing materials can provide functionality in nanophotonic devices as the interplay among its microscopic degrees of freedom conspires to generate macroscopic quantum phenomena. Vanadium dioxide (VO2), exhibiting a metal-to-insulator transition, has recently been shown to switch on a timescale that is even shorter than its intrinsic single phonon period. The ultrafast photo-induced phase transition in VO2 is promising for data storage and sensing applications.
Here, we describe the ability to trigger the ultrafast phase transition on an even faster timescale by injecting ballistic electrons from plasmonically resonant gold nanoparticles on nanostructured VO2 using a 50 fs ultrafast laser pulse. The optical switching threshold of the hybrid material is lower than that of pristine VO2 by a factor of five. First-principles density-functional calculations show that the collapse of a 6 THz optical phonon, corresponding to a twisting motion of V atoms, is responsible for the ultrafast phase transition. We find that a critical density of injected electrons from Au couple to the lattice and cause collapse of the VO2 phonon, which stimulates the monoclinic-to-rutile structural phase transition. We also show that hole-doping can induce the same effect. The abrupt change of the critical phonon results from the weakening of the V-V bonds induced by the combined flux of injected electrons and holes. Molecular dynamics simulations at finite temperature show temperature-dependence of the required density of electrons/holes for the structural phase transition, that is, at higher temperature, fewer free carriers are required.
Thus, the results explain the experimental finding of plasmonic-electron-driven ultrafast phase transition and represent a step towards manipulating the photo-induced phase transition by surface modification and interfacial engineering. This demonstration of a sub-picosecond light induced phase transformation paves the way to optically modulated electronics by tailoring the electronic decay pathways of hybrid quantum nanomaterials.
Support was provided by the DTRA Grant HDTRA1-10-1-0047, NSF Grant DMR-1207241, and McMinn Endowment at Vanderbilt University. DFT calculations were performed at the DoD AFRL.

Since the first demonstration of the THz quantum cascade laser (QCL) in 2002 that operated at 50K,1 the prospect of a THz source that is compact, mass producible, and has high output power (>1 mW) has stimulated much effort in pushing THz QCLs to operate at room temperature. A decade of research has managed to raise the maximum temperature to about 200K in 2012.2 As a matter of fact, chances of continuing to push the temperature much higher are very slim because of the fundamental limitations of the THz QCLs based on the GaAs/AlGaAs material system. These THz QCLs based on intersubband transitions all relied on near-optical-phonon-resonance for depopulation of the lower laser state subband to the ground-state subband, in GaAs the optical phonon energy is roughly 36 meV, not much higher than the room temperature thermal energy k_B Tasymp;26 meV. The result is that a rather significant percentage of electrons reside in the lower laser state subband due to thermal excitation at room temperature, effectively making population inversion needed for achieving optical gain nearly impossible to achieve. We have previously proposed to use the GaN/AlGaN-based material system to realize room temperature THz QCLs,3 taking advantage of its large optical phonon energy of ~90meV, which can be used to suppress thermal excitation of electrons into the lower lasing state subband from the ground state subband under the near-optical-phonon-resonance depopulation scheme. However, the GaN-based material system has not reached the same maturity level as GaAs and InP systems in terms of its complexity in structural growth, it is therefore highly desirable to grow as few periods as possible in implementing GaN/AlGaN THz QCLs, much less than those typical in GaAs THz QCLs (a few tens of periods). If the total active region thickness of the GaN QCL is to remain around 0.1µm, conventional surface plasmon (SP) waveguide constructed with two metal layers confining the active QCL does not work because the loss would be too large for the QCL to overcome. We propose to use spoof SP waveguide formed with two corrugated metal surfaces to provide the optical confinement for this ultrathin GaN QCL region, and the waveguide can be designed to reduce its loss to a level that can be compensated for by the active region.
[1] R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, Rita C. Iotti, and Fausto Rossi, Nature 417, 156 (2002).
[2] S. Fathololoumi, E. Dupont, C.W.I. Chan, Z.R. Wasilewski, S.R. Laframboise, D. Ban, A. Mátyás, C. Jirauschek, Q. Hu, and H. C. Liu Opt. Express 20, 3866 (2012).
[3] G. Sun and R. A. Soref, J. B. Khurgin, Superlattices and Microstructures 37, 107 (2005).

The viability of thermoelectric energy conversion as an alternative to more traditional technologies depends on the availability of materials with a high thermoelectric figure of merit (ZT). Such materials would be characterized by low thermal conductivities and high electrical conductivities and Seebeck coefficients. The intricacy of this materials optimization problem caused a stagnation of the field for several decades until the advent of modern, powerful nanostructuring techniques. Among them, the synthesis of nanograined materials has several advantages, such as relying only on intrinsic properties and being able to produce bulk samples directly. Still, experimental resources are scarce and so far researchers have focused their efforts on improving known compounds with high bulk ZT, or alternatively on cheap and readily available materials like silicon.
This presentation is devoted to the results of a fully ab-initio high-throughput screening of a large library of nanograined thermoelectrics. All the phenomenological coefficients involved in ZT were directly calculated for each material without experimental input. The first class explored comprised the 78,768 ternary compounds with half-Heusler prototype available in the aflowlib.org repository. Comparison shows that many half-Heusler candidates can have higher performance than those from elementary group-IV and binary III-V semiconductors. More specifically, values of ZT near 3 are plausible at high temperatures. Good candidates can be found for use either as type-p or as type-n thermoelectrics.
The factors underlying the advantages of half Heuslers in this context are analyzed, and the distribution of ZT over this class of materials studied in terms of their constituent elements, leading to practical recipes that can help guide experimental studies. For instance, good candidates are more likely to appear if two of their elements come from the first columns of the periodic table. Comparison with experimental data for several well-known half Heuslers shows that thermoelectric performance in the bulk and in nanograined form do not necessarily go hand in hand, underlining the importance of looking for the right material for each regime.

Focusing of high-amplitude surface acoustic waves leading to material damage is visualized in an all-optical experiment. The optical set-up includes an axicon that focuses an intense picosecond excitation pulse into a ring-shaped pattern at the surface of a gold coated glass substrate. Optical excitation induces a surface acoustic wave (SAW) that propagates in the plane of the sample and converges toward the center. The evolution of SAW profile is monitored using interferometry with a femtosecond probe pulse at variable delays. A series of images is obtained tracing the converging wave as it collapses at the focal point. The quantitative analysis of the full-field images provides direct information about the surface displacement profiles, which are compared to calculations. The high strain amplitude at the focal point leads to the removal of the gold coating and, at higher energies, to damage of the glass substrate. The results open the prospect for testing material strength on the microscale using laser-generated SAWs.

Thermal management through cooling is essential for the safety and the daily operations of modern micro- and nanoelectronic devices, as well as energy systems such as batteries and solar thermal cells, which dissipate more heat as they become more powerful. Graphene/organic nanocomposites are emerging as a novel thermal interfacial material in which heat can transfer very efficiently across device interfaces. However, the intrinsic interfacial thermal resistance at graphene/organic interfaces, as a result of mismatches in the phonon vibrational spectra of the two materials, diminishes the overall heat transfer performance of the graphene/organic nanocomposites. In this paper, we use molecular dynamics (MD) simulations to design alkyl-pyrene molecules that can non-covalently functionalize graphene surfaces in contact with a model organic phase composed of octane. The alkyl-pyrene molecules possess phonon spectra features of both graphene and octane and therefore, can serve as phonon-spectra linkers to bridge the vibrational mismatch at the graphene/octane interface. In support of this hypothesis, we find that the best linker candidate can enhance the out-of-plane graphene/organic interfacial thermal conductance by ~ 22%, attributed to its capability to compensate the low-frequency phonon mode of graphene. We also find that the length of the alkyl chain indirectly affects the interfacial thermal conductance through different orientations of these chains because they dictate the contribution of the out-of-plane high frequency carbon-hydrogen bond vibrations to the overall phonon transport. This study advances our understanding of the less destructive non-covalent functionalization method and design principles of suitable linker molecules to enhance the thermal performance of graphene/organic nanocomposites while retaining the intrinsic chemical, thermal, and mechanical properties of pristine graphene.

An overview of recent work on the interaction of elastic waves with dislocations is given. The perspective is provided by the wish to develop nonintrusive tools to probe plastic behavior in materials. For simplicity, ideas and methods are first worked out in two dimensions, and the results in three dimensions are then described. These results explain a number of recent, hitherto unexplained, experimental findings. The latter include the frequency dependence of ultrasound attenuation in copper, the visualization of the scattering of surface elastic waves by isolated dislocations in LiNbO3, and the ratio of longitudinal to transverse wave attenuation in a number of materials. Specific results reviewed include the scattering amplitude for the scattering of an elastic wave by a screw, as well as an edge, dislocation in two dimensions, the scattering amplitudes for an elastic wave by a pinned dislocation segment in an infinite elastic medium, and the wave scattering by a sub-surface dislocation in a semi-infinite medium. Also, using a multiple scattering formalism, expressions are given for the attenuation coefficient and the effective speed for coherent wave propagation in the cases of anti-plane waves propagating in a medium filled with many, randomly placed screw dislocations; in-plane waves in a medium similarly filled with randomly placed edge dislocations with randomly oriented Burgers vectors; elastic waves in a three-dimensional medium filled with randomly placed and oriented dislocation line segments, also with randomly oriented Burgers vectors; and elastic waves in a model three-dimensional polycrystal, with only low angle grain boundaries modeled as arrays of dislocation line segments. The theory suggests a non-intrusive way of measuring dislocation density in materials, which is confirmed with Resonant Ultrasound Spectroscopy (RUS) experiments using aluminum.

Electron-phonon coupling in nanoscale copper attracted recent research interest for the further miniaturization of electronic devices. In this presentation, we report the nanoscale confinement effect on electron-phonon interaction in epitaxial copper films using ultrafast transient reflectivity measurement. Electron dynamics in metals following the ultrafast laser excitation is subjected to both of electron-electron, and electron-phonon scattering processes. Electron-phonon coupling is often determined by analyzing the decay of the transient reflectivity signal in the time period after the electron temperature is established. However, the contribution of initial non-thermal relaxation process makes the analysis ambiguous. We studied probe wavelength dependent response of the ultrafast transient reflectivity near the optical transition from d-band to Fermi surface, and we could selectively observe the initial non-thermal relaxation process, and electron-phonon relaxation processes by choosing the probe wavelength. The samples were epitaxially grown copper thin films with the thickness from 5 nm -1 mu;m. These films are grown on a high resistivity silicon substrate by e-beam lithography. Electrons near the Fermi surface were excited with optical pump pulses with wavelength of 800 nm to above the Fermi surface. The time profile of the transient reflectivity showed strong dependence on the probe wavelength. The time profile was analyzed using phenomenological decay model.[1] The model predicts the ratio of two components, the initial non-equilibrium relaxation contribution and the thermal decay, has strong dependence on the wavelength. The model and the experimental data showed very good agreement, and the wavelength dependence can be classified into 4 regions. Size dependent electron-phonon coupling showed the rapid increase below 10 nm. In a separate experiment, the electrical resistivity was determined using THz transmission spectroscopy, and the resistivity of single crystal copper nanofilm showed sharp increase of the resistivity below 10 nm, and this increase is much steeper then expected from the surface scattering process.
1. C. K. Sun, F. Vallee, L. H. Acioli, E. P. Ippen, and J. G. Fujimoto, "Femtosecond-Tunable Measurement of Electron Thermalization in Gold," Phys. Rev. B 50, 15337-15348 (1994).

Colloidal semiconductor nanocrystal (NCs) quantum dots of materials such as CdSe offer size-controlled bandgaps, intense absorption features, and substantial photoluminescence quantum yields. Such desirable light-absorbing and emitting properties, in addition to facile solution processing, make these materials attractive for solid-state lighting, photovoltaics, bio-labeling, and optical amplification applications. Here, we describe detailed studies of radiative recombination in CdSe NCs wherein we uncover signatures of NC thermalization and subsequent phonon transport in the scenario that the phonon wavelength exceeds the nanostructure. Specifically, measurements of spectrally and temporally-resolved photoluminescence as a function of temperature reveal signatures of phonon dissipation with clear NC size-dependence.
Furthermore, whereas known manipulations of semiconductor thermal transport properties rely upon higher-order material organization, we demonstrate a “bottom-up” means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. In particular, growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap, thus decoupling the thermal and electronic properties of the material.

In this work, we use coarse grained and atomistic models to predict the elastic and vibrational properties of nanocrystal superlattices (NCSLs). NCSLs are hierarchical materials formed by assembly of monodisperse nanocrystal building blocks that are tunable in composition, size, shape, and surface functionalization. We have found that existing coarse grained interaction potentials for CdSe NCSLs predict elastic properties two orders of magnitude smaller than experimental results for CdSe. By making physically justifiable modifications to these coarse grained interaction potentials and recalculating the elastic properties, we obtain predictions that are in much better agreement with experiment. Atomistic models provide even better agreement with experiment, but are computationally much more costly. With regard to vibrational spectra, the coarse grained models only capture the low frequency (GHz range) behavior of NCSLs, and atomistic models are needed to deal with higher frequencies arising from internal vibrations of nanocrystal building blocks.

The electron phonon coupling (EPC) is of primary concern for several fundamental properties of electronic materials and devices and can affect electron mobility, Fermi velocity, superconductivity and heat transfer. Graphene exhibits several interesting electronic and phonon properties including massless electrons and violation of the Born - Oppenheimer approximation, linked to the EPC. There has also been significant interest in bilayer graphene due to a band structure that can be manipulated and a possible band gap [1]. Interactions between graphene layers can significantly affect vibrational modes but it is dependent on the interlayer configurations. Quantifiable values for the EPC can be calculated from the electron and phonon dispersion relations [2], which can in turn be obtained from ab initio density functional perturbation theory. Presented here are the changes to the EPC for bilayer graphene in the AB and AA stacking configuration, which is shown to vary by more than 50%. The interlayer effects on the phonon properties has major implications for bilayer graphene but also provides insights into the thermal transport and mobility differences observed in suspended, twisted and decoupled graphene sheets.
1. Ohta, T., et al., Controlling the electronic structure of bilayer graphene. Science (New York, N.Y.), 2006. 313(5789): p. 951-4.
2. Piscanec, S., et al., Kohn Anomalies and Electron-Phonon Interactions in Graphite. Physical Review Letters, 2004. 93(18): p. 1-4.

We report a pump-probe infrared photothermal spectroscopy (PTS) method for an ultrasensitive optical system that could rival current techniques such as fluorescence spectroscopy. Using high power quantum cascade lasers (QCL) we have developed a high-resolution nonlinear absorption spectroscopy method that may increase both the sensitivity and the spectral resolution for identifying weakly absorbing molecules. The photothermal pump-probe excitation technique is based on mode-locked QCL pump (tunable near 6 mu;m) and Erbium-doped fiber laser probe (1.6 mu;m) and was used to investigate vibrational resonances by the nonlinear photothermal signal. We have used an 8CB liquid crystal sample to investigate the spectral sharpening effect in the smectic, nematic and isotropic phases. Photothermal technique has led to rapid developments in spectroscopy and imaging nanoparticles and organelles with high signal-to-noise ratio. It is a nondestructive and contactless method where the measurements come directly from the physical and thermodynamic property changes based on direct absorption of light. Compared to conventional spectroscopic methods such as Fourier Transform Infrared Spectroscopy (FTIR) that require cryogenically cooled detectors for best performance, PTS detects a short wavelength probe beam with conventional visible or near-infrared photodetectors and imaging systems. Extension of the photothermal technique to the mid-infrared region is particularly attractive because the presence of a large number of characteristic normal modes of molecules in the “fingerprint” region of the electromagnetic spectrum that allows for spectroscopy and imaging without the need of perturbing label.

Femtosecond pump-probe measurements provide a powerful means by which to study zone centre coherent optical phonons (COP) that provide a fingerprint of the underlying crystallographic structure, and which may appear as a precursor to a structural phase transition when excited with sufficient amplitude. When such measurements are made on epitaxial thin film samples, the reflectance (R) and anisotropic reflectance (AR) signals depend upon the polarization of the pump and probe beams relative to the crystallographic axes. In a recent study of a cubic crystalline
Ge2Sb2Te5/GaSb(001) thin film [1], the amplitude of a COP with frequency of 3.4 THz observed in the AR signal was found to exhibit a four-fold dependence upon the polarization of the probe beam. A theory due to Merlin [2], which considers the symmetry of the Raman tensor for a particular mode, was used to interpret the results. The appearance of the mode in the AR but not the R signal, and the dependence upon probe polarization, both suggest a three-dimensional mode character. The lack of a clear dependence upon pump polarization was attributed to the action of an
Interface space charge field. Confirmation that this mode indeed has three-dimensional character, similar to the Raman inactive T2 mode in the pristine rocksalt structure, is highly important in understanding the structure of the crystalline phase of Ge2Sb2Te5 that has important applications within data storage technology. The theory of Merlin [2] has now been applied to films of different crystallographic orientatiions. The T2 mode is predicted to appear in both the R and AR signals for both the (110) and (111) orientations, again with a characteristic dependence upon pump and probe polarization that will be described. The results will motivate further experimental studies to clarify the structure of cubic Ge2Sb2Te5.
[1] A. Shalini et. al. (submitted for publication).
[2] R. Merlin, Solid State Commun., 102, 207 (1997).