Vanya Darakchieva, Linköping University
Hou-Tong Chen, Los Alamos National Laboratory
Charles Schmuttenmaer, Yale University
Ingrid Wilke, Rensselaer Polytechnic Institute
NM5.1: Terahertz Metamaterials and Devices
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 103 A
9:00 AM - *NM5.1.01
THz Ellipsometry Characterization of Spatially Confined Carrier Systems and Metamaterials
Tino Hofmann 1 2 3 Show Abstract
1 , University of North Carolina at Charlotte, Charlotte, North Carolina, United States, 2 , Linköping University, Linköping Sweden, 3 , University of Nebraska-Lincoln, Lincoln, Nebraska, United States
The precise measurement of electromagnetic material properties at THz frequencies is essential for the development of increasingly advanced THz optical systems and a prerequisite for the design and manufacturing of optical elements for this spectral range. The accurate knowledge of material THz dielectric functions furthermore provides new insights into fascinating excitation mechanisms such as spin-transitions, collective modes of biological molecules, local free-charge carrier oscillations and may allow exploration of novel physical phenomena as observed in artificially structured metamaterials. Over the last decade THz ellipsometry has been demonstrated as a powerful tool to measure accurate material THz dielectric function data including anisotropy and THz ellipsometers are starting to become more widely available .
In this talk, I will give an overview of applications where THz ellipsometry contributed valuable material parameters and insights over the last years and I will sketch an outlook of possible future directions. The combination of THz ellipsometry with external magnetic fields which allows the accurate measurement of the optical Hall effect will be discussed in detail. The optical Hall effect gives contact-free, optical access to the free charge carrier properties effective mass, mobility, and density in semiconductor heterostructures at THz frequencies providing crucial parameters for the design of future THz electronic devices. Recent developments where cavity-enhancement effects were used to facilitate accurate optical Hall effect measurements at small magnetic fields will be emphasized. This novel approach may present a new avenue to allow a more widespread use of this powerful technique which otherwise predominantly relies on rather expensive high-field superconducting electromagnets . In the second part of my talk I will focus on surfaces with self-organized, spatially coherent arrangements of nanostructures which have unique optical, mechanical, and electrical properties that differ dramatically from the host material. The optical and transport properties of such sculptured thin films have attracted recent interest because of their potential to achieve novel optical sensing and separation mechanisms. Although being orders of magnitude smaller than the probing wavelength, metamaterials composed of highly-ordered 3-dimensional metal nanostructures exhibit a strong anisotropic optical response at THz frequencies. My presentation will be concluded with examples showing that such THz metamaterials may provide an interesting new pathway for the design of optical devices and sensors .
 P. Kühne, M. Schubert, T. Hofmann, et al., Rev. Sci. Instrum. 85, 071301 (2014).
 T. Hofmann, P. Kühne, M. Schubert, and V. Darakchieva, et al., Appl. Phys. Lett. 101, 192102 (2012).
 S. Knight, V. Darakchieva, T. Hofmann, et al., Opt. Lett. 40, 2688 (2015).
 T. Hofmann, M. Schubert, et al., Appl. Phys. Lett. 99, 081903 (2011).
9:30 AM - NM5.1.02
High-Performance Terahertz Metasurface Flat Lenses
Chun-Chieh Chang 1 , Daniel Headland 2 , Withawat Withayachumnankul 2 , Derek Abbott 2 , Abul Azad 1 , Hou-Tong Chen 1 Show Abstract
1 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 School of Electrical & Electronic Engineering, The University of Adelaide, Adelaide, South Australia, Australia
Conventional optical lenses focus electromagnetic waves by imparting position dependent phase delay through shaping their geometry. This poses difficulties in eliminating the geometric aberrations in high numerical aperture lenses, in addition to the fabrication challenges when operating at short wavelengths (e.g. visible light), and bulky devices operating at long wavelengths (e.g. microwaves). In contrast, metasurfaces  realize full control of phase through tailoring the subwavelength resonant structures, allowing for the demonstration of ultrathin flat lens  without suffering from geometric aberrations. However, the efficiency of metasurface lenses using single-layer metasurfaces is still rather low. Here we report the demonstration of high-performance flat lenses in the terahertz (THz) frequency range using three-layer metasurfaces. It has been shown that the three-layer metasurface structure  is capable of rotating the incident linear polarization by 90° with a very high efficiency over a bandwidth of two octaves. Moreover, the phase of the output light can be tuned over the entire 2π range with subwavelength resolution by simply tailoring the structure geometry of the basic building blocks. Based on this success, we design, fabricate, and characterize a metasurface lens operating at 0.4 THz. The basic building blocks of the metasurface are sixteen subwavelength resonant elements with various dimensions and geometries, creating a linear phase variation of the cross-polarized transmission spanning the entire 2π range. All sixteen elements exhibit transmission amplitude larger than 82% (i.e., > 60 % power efficiency) to ensure a very high efficiency of our lens. With a lens diameter and focal length of both 5 cm, the fabricated metasurface lens possesses a high numerical aperture of 0.44, and achieves a diffraction-limited focal spot of 2 mm. Terahertz time-domain spectroscopy measurements also show that the metasurface lens is capable of achieving the same signal intensity as compared to a TPX lens of the same diameter and focal length, demonstrating the high efficiency of our metasurface lens.
 N. Yu, P. Genevet, M. A. Kats, F. Aieta, J.-P. Tetienne, F. Capasso, and Z. Gaburro, Light propagation with phase discontinuities: generalized laws of reflection and refraction, Science, 334 (6054), 333-337 (2011).
 Francesco Aieta, Patrice Geneve, Mikhail A. Kats, Nanfang Yu, Romain Blanchard, Zeno Gaburro, and Federico Capasso, Aberration-Free Ultrathin Flat Lenses and Axicons at Telecom Wavelengths Based on Plasmonic Metasurfaces, Nano Lett., 12, 4932 (2012).
 N. K. Grady, J. E. Heyes, D. R. Chowdhury, Y. Zeng, M. T. Reiten, A. K. Azad, A. J. Taylor, D. A. R. Dalvit, and H.-T. Chen, Terahertz metamaterials for linear polarization conversion and anomalous refraction, Science, 340 (6138), 1304-1307 (2013).
9:45 AM - NM5.1.03
Metasurfaces for Terahertz Antireflection Coatings
Beibei Zeng 1 , Chun-Chieh Chang 1 , Li Huang 2 , Hou-Tong Chen 1 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , Harbin Institute of Technology, Harbin, Heilongjiang, China
In the terahertz frequency range, the large refractive index of typically used materials results in high Fresnel reflection loss. Considering the difficulties in efficient generation and sensitive detection of terahertz radiation, antireflection coating becomes critical for performance improvement in free space THz photonic systems. In principle, antireflection coatings in the THz frequency range can be accomplished following the same approaches adopted in the optical regime, such as quarter-wave antireflection for narrow-band operation using a single-layer dielectric film with refractive index matched with the substrate, or broadband antireflection using multi-layer dielectric films with carefully arranged refractive indices and thicknesses. However, in the THz frequency range, it is difficult to find low-loss dielectric materials with particular refractive index values and, at the same time, suitable for coating with tens of micrometer thickness. Here we show a class of simple metasurface structures that enable narrow- and broad-band antireflection and dramatically enhanced transmission in the THz frequency range. These structures only involve low-loss metals and the substrate material with no need of additional materials, and are easy to fabricate by photolithographic patterning, reactive ion etching, and directional metal coating. Additionally, the operation frequency of the metasurface antireflection is scalable to mid- and near-infrared.
10:00 AM - *NM5.1.04
Advanced Modulation Techniques with Metamaterials for Single Pixel Terahertz Imaging
Willie Padilla 1 , Christian Nadell 1 Show Abstract
1 Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina, United States
Electromagnetic metamaterials have been shown to be able to absorb narrow-band radiation in the terahertz range. Arrays of metamaterial absorbers may be fashioned into all-electronic fast and high performance spatial light modulators (SLMs). The utilization of metamaterials SLMs offers a new route to achieve reconfigurable single pixel imaging systems. Here we demonstrate a metamaterial SLM that permits high fidelity imaging at terahertz frequencies using techniques from the field of communications engineering. The all-electronic nature of the metamaterial SLM permits implementation of a quadrature amplitude modulation (QAM) scheme thus doubling the imaging frame rate. An alternative approach, termed frequency diverse imaging, utilizes some number of distinct carrier frequencies to enable parallelization of the imaging process resulting in an increase in imaging speed. Our results are not limited to the terahertz band, but may be scaled to nearly any sub-optical range of the electromagnetic spectrum, and verify the potential of metamaterials to operate as reconfigurable multifunctional devices which will give rise to next generation imaging systems.
10:30 AM - *NM5.1.05
Generic Graphene Based Components and Circuits for Millimeter Wave High Datarate Communication Systems
Herbert Zirath 1 Show Abstract
1 , Chalmers University of Technology, Gothenburg Sweden
The transmission rate of wireless data in the mobile networks is doubling every year due to the increased usage of mobile multimedia services like streaming video, music, television, data transfer in smartphones and laptop-computers etc. This tendency will require continuously improved telecom infrastructure regarding both base-stations and the backhaul communication links. Today, the E-band (71-76, 81-86, 92-95 GHz) is employed increasingly in the networks, allowing multi Gbps data rate. In a near future however, the E-band will be crowded, and novel, higher frequency bands can to be employed as well. Several hundred Gigahertz bandwidth is available for new communication and sensing applications just waiting to be exploited at frequencies above 100 GHz. For these application we have developed a process technology for the fabrication of graphene field-effect transistors (G-FETs) for achieving high values of fT and fmax suitable for circuit applications. The process is scalable and based on the large area bilayer intercalated epitaxial graphene on SiC substrate. In addition, the ongoing work developing an MMIC fabrication process for analog high frequency applications is described. Based on a compact model for G-FETs, generic circuits to be used in future wireless transceiver frontend demonstrators including modulators, demodulators, power detectors and frequency multipliers are designed for the W-band (70-110 GHz) and fabricated in the newly developed MMIC process. Due to the symmetry in the Id-Vds characteristic, modulators, demodulators, and power detectors becomes very linear which is important for communication systems using complex modulation like high order QAM and multicarrier signals. Our initial experiments using OOK, BPSK, QPSK and 16-QAM signals show measured data rates up to 16 Gbps. A conversion loss of 16 dB is achieved in our first generation of linear modulators/demodulators at 90 GHz.
NM5.2: Materials for Terahertz Technologies
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 103 A
11:30 AM - *NM5.2.01
Recent Progress on THz Technologies Based on 2D Electron Systems
Huili Xing 1 , Jimy Encomendero 1 , Rusen Yan 1 , SM Islam 1 , Hugo Condori 2 , Shubhendu Bhardwaj 3 , Lina Cao 4 , Kubilay Sertel 3 , John Volakis 3 , Patrick Fay 4 , Berardi Sensale-Rodriguez 2 , Debdeep Jena 1 Show Abstract
1 , Cornell University, Ithaca, New York, United States, 2 , University of Utah, Salt Lake City, Utah, United States, 3 , Ohio State University, Columbus, Ohio, United States, 4 , University of Notre Dame, Notre Dame, Indiana, United States
In two-dimensional electron systems with mobility on the order of 1,000 – 10,000 cm2/Vs, the electron scattering time is about 1 ps. For the THz window of 0.3 – 3 THz, the THz photon energy is in the neighborhood of 1 meV, substantially smaller than the optical phonon energy of solids where these 2D electron systems resides. These properties make the 2D electron systems interesting as a platform to realize THz devices. In this paper, I will review 3 approaches investigated in the past few years in my group toward THz devices. The first approach is the conventional high electron mobility transistor based on GaN toward THz amplifiers. The second approach is to employ the tunable intraband absorption in 2D electron systems to realize THz modulators, where I will use graphene as a model material system. The third approach is to exploit plasma wave in these 2D electron systems that can be coupled with a negative differential conductance element for THz amplifiers/sources/detectors.
12:00 PM - NM5.2.02
Terahertz Cavity-Enhanced Optical Hall Effect Reveals Anisotropic Mobility in Epitaxial Graphene
Nerijus Armakavicius 1 , Philipp Kuehne 1 , Chamseddine Bouhafs 1 , Vallery Stanishev 1 , Tino Hofmann 1 2 , Mathias Schubert 1 2 , Rositsa Yakimova 3 , Camilla Coletti 4 , Vanya Darakchieva 1 Show Abstract
1 THz Materials Analysis Center, Linköping University, Linköping Sweden, 2 Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 3 Semiconductor Materials, IFM, Linköping University, Linköping Sweden, 4 Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Pisa Italy
Graphene is a well-known two-dimensional material with unique electronic properties resulting from a linear dispersion relation of its electronic band structure. Because of the two-dimensional nature and extremely high free charge carrier (FCC) mobility, graphene is a very promising candidate for future high frequency electronics and sensing devices. In particular, epitaxial graphene (EG) grown on silicon carbide (SiC) by silicon sublimation is a good candidate for large-scale production, since it provides large-area homogeneity and good thickness control that opens new pathways for its integration in current device fabrication technologies. In order to push EG technology forward, the growth conditions and subsequent processing have to be optimized. It is very important to understand how different processes affect FCC properties of EG. Thus, development of analytical techniques able to probe FCC properties of EG in a simple, non-destructive and contactless manner is highly needed.
In this work, we present a study of the effect of hydrogen intercalation on the FCC properties of EG using a new home-built terahertz (THz) ellipsometer . THz cavity-enhanced optical Hall effect (OHE) measurements are employed to determine FCC properties of as-grown monolayer EG, hydrogen intercalated monolayer EG and buffer layer samples on 4H-SiC (0001). The THz OHE is a generalized ellipsometry based technique that measures the magnetic field induced optical anisotropy of conductive layers . Experimental data analysis using an optical model allows us to extract the FCC sheet density and mobility. THz cavity-enhanced OHE measurements  were performed in the range of 700-950 GHz at ±0.5 T magnetic fields using a Mueller matrix approach. Results show an increase in the FCC mobility due to the hydrogen intercalation. Furthermore, after hydrogen intercalation, the graphene samples show an azimuth angle dependence of the optical response that is a clear fingerprint of structural in-plane anisotropy of the FCC properties. Based on model dielectric function analysis, this dependence can be attributed to an anisotropy of the FCC mobility. Combining OHE results with surface morphology investigations, we show that there is a clear correlation between the mobility anisotropy and the direction of the steps on the SiC substrate, with a higher mobility along the terraces than across them. Such anisotropy of the mobility parameters is presented only in the intercalated monolayer EG samples, but not in the as-grown or hydrogen intercalated buffer layers. Different mechanisms that could induce the observed anisotropic mobility will be discussed and conclusion about the dominant scattering mechanisms will be presented.
 N. Armakavicius et al., Appl. Surf. Sci. (2016) http://dx.doi.org/10.1016/j.apsusc.2016.10.023.
 M. Schubert et al., J. Opt. Soc. Am. A, 33 (8), 1553 (2016).
 S. Knight et al., Opt. Lett. 40, 2688 (2015).
12:15 PM - NM5.2.03
Terahertz Quantum Well Infrared Photodetectors Based on Semi-Polar GaN/AlGaN Heterostructures
Habibe Durmaz 1 2 , Denis Nothern 1 , Gordie Brummer 1 , Theodore Moustakas 1 , Roberto Paiella 1 Show Abstract
1 , Boston University, Boston, Massachusetts, United States, 2 , Recep Tayyip Erdogan University, Rize Turkey
The THz spectral region is attracting considerable attention due to potential applications in biomedical sensing, security screening, and industrial process control. However, existing THz optoelectronic devices based on intersubband (ISB) transitions in arsenide quantum wells (QWs) are fundamentally limited to incomplete coverage of the THz spectrum and to cryogenic temperatures. Recently, it has been suggested that these limitations can be overcome using III-nitride heterostructures, by virtue of their substantially larger LO-phonon energies.1,2 The initial demonstrations of THz ISB photodetection3 and electroluminescence4 with GaN/AlGaN QWs have already been reported.
An important challenge in the design of these devices is provided by the strong polarization-induced internal electric fields that exist in GaN/AlGaN QWs grown along the polar crystallographic c axis (their most common growth direction).3 To address this issue, here we use QWs grown on a free-standing semi-polar GaN substrate, where the internal electric fields are almost completely eliminated. With this approach, a high-performance bound-to-quasi-bound quantum well infrared photodetector (QWIP) has been developed, providing ISB photodetection peaked near 10 THz.5 The responsivity spectrum of this device covers the entire frequency range that has so far been completely inaccessible to ISB optoelectronics due to reststrahlen absorption in arsenide semiconductors. The same materials platform is also promising for the development of III-nitride quantum cascade lasers potentially capable of room-temperature operation.
1. V. D. Jovanović et al., Appl. Phys. Lett. 84, 2995 (2004).
2. E. Bellotti et al., Appl. Phys. Lett. 92, 101112 (2008)..
3. F. F. Sudradjat et al., Appl. Phys. Lett. 100, 241113 (2012).
4. W. Terashima et al., Phys. Status Solidi C 8, 2302 (2011).
5. H. Durmaz et al., Appl. Phys. Lett. 108, 201102 (2016).
12:30 PM - *NM5.2.04
III-V MOSFETs for THz Applications
Lars-Erik Wernersson 1 Show Abstract
1 Electrical and Information Technology, Lund University, Lund Sweden
III-V MOSFETs are considered one candidate to extend the CMOS scaling roadmap. The advantageous transport properties of the materials allow for an increase in the drive current, what has driven development of advanced transistor technologies. Besides logic applications, III-V MOSFETs also show promise for further gate length scaling what will improve the transistor high-frequency properties.
Coherent wavelets, that is short pulses at a defined carrier frequency, can be used for radar and imaging techniques. The time-of-flight can be used to accurately determine distance whereas the waveform will contain information about the reflector materials properties including permittivity and frequency dispersion. In the first part of the talk, a coherent wavelet generator technology will be presented where a III-V MOSFFET is used to switch the oscillations generated by a resonant tunneling diode. The implementation is extremely power efficient as current only is consumed during wavelet generation. Examples of measurements include imaging with 1 mm accuracy at 60 GHz and detection studies of dispersive media.
In the second part of the talk, I will review present efforts to integrate III-V nanowire MOSFETs on Si substrates for millimeter and THZ applications. This approach may allow for a cost reduction of advanced systems and facilitate production for a wider market. The nanowires facilitate the integration on the Si substrates as they are less prone to defect formation. Besides, they provide better scaling opportunities due to the improved electrostatic control. Currently state-of-the-art includes transistors with fmax>400 GHz.
This work is supported by the Swedish Foundation for Strategic Research and the EU H2020 program INSIGHT (grant agreement 688784).
NM5.3: Terahertz Spectroscopy and Material Properties
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 103 A
2:30 PM - *NM5.3.01
Long-Range and High-Speed Electron and Spin Transport at GaAs/AlGaAs Interface
Petr Kuzel 1 , Lukas Nadvornik 1 , Tomas Jungwirth 1 Show Abstract
1 , Institute of Physics, Czech Academy of Sciences, Prague Czech Republic
We combine optical pump–probe (time and spatially resolved Kerr effect measurements) and optical pump–terahertz probe (ultrafast THz photoconductivity measurements) techniques to determine fundamental spin and charge transport parameters in a semiconductor heterostructure. Unprecedented combination of long-range and high-speed spin transport is achieved by suppression of processes limiting the carrier lifetime and mobility. Our undoped MBE-grown GaAs/AlGaAs heterostructure enables an efficient spatial separation of photoexcited electrons and holes due to the built-in electric field in the GaAs layer. Electrons accumulated close to the GaAs/AlGaAs interface exhibit a high mobility (>105 cm2V-1s-1) and long lifetime at 10 K. Highly mobile electrons are detected by THz probing even at times >200 microseconds after photoexcitation. Since the spin decay channel due to the electron-hole recombination is suppressed, spin lifetimes of 20 ns and spin diffusion length of 13 micrometers are observed.
3:00 PM - NM5.3.02
Charge Transport in TiO2 Nanotubes Studied by Terahertz Spectroscopy
Jiri Kucharik 1 , Hanna Sopha 2 , Milos Krbal 2 , Ivan Rychetsky 1 , Petr Kuzel 1 , Jan Macak 2 , Hynek Nemec 1 Show Abstract
1 , Czech Academy of Sciences, Prague 8 Czech Republic, 2 , University of Pardubice, Pardubice Czech Republic
Titanium dioxide nanostructures are promising for a wide range of applications. Many of them like photovoltaics or photochemistry [Chem. Rev. 114, 9281 (2014)] require an efficient charge transport. Nanotubes naturally combine a high surface to volume ratio (similarly as nanoparticles) with a continuous path at least theoretically enabling fast long-distance charge transport. Indeed, the performance of e.g. Grätzel cells based on nanotubes is nowadays approaching that of nanoparticles-based devices, and there is a prospect for further improvement.
Here we perform a detailed study of charge transport in TiO2 nanotubes using terahertz spectroscopy: this non-contact method is capable of identifying various obstacles for charge transport on nanometer length scale [J. Phys. Chem. C 119, 19485 (2015)]. The investigated TiO2 nanotubes with various lengths were prepared by anodization of Ti substrates [Curr. Opin. Solid State Mater. Sci. 11, 3 (2007)]. Some of the samples were annealed, which transformed their originally amorphous structure to crystalline anatase. Transient terahertz (THz) transmittance upon above band-gap photoexcitation (266 nm) was measured using conventional time-resolved THz spectroscopy based on amplified femtosecond laser system. The spectra were carefully analyzed in order to correctly account for Fabry-Pérot interferences and for the impact of depolarization fields [J. Phys. D: Appl. Phys. 47, 374005 (2014)]. This permitted a reliable determination of the lateral THz conductivity of the nanotubes, which was subsequently correlated with the results of Monte-Carlo calculations of conductivity of confined charges [Phys. Rev. B 79, 115309 (2009)].
The THz conductivity spectrum contains fingerprints of charge transport mechanism in the nanotubes. Band-like transport has been observed in all crystalline samples. For 1 μm long nanotubes, the spectra were interpreted in terms of a very low mobility (short charge scattering times) and a weak confinement solely by the nanotube surfaces; for these short nanotubes, almost identical response was also seen in amorphous nanotube layers [Phys. Status Solidi: Rapid Res. Lett. 10, 691 (2016)]. Conversely, the spectra of 30 μm long nanotubes show that the mobility of charges is close to that of crystalline anatase; however, the charges are confined on the length scale of about 5 nm which is much smaller than the wall thickness. The photoconductivity of long amorphous nanotubes was much lower, close to the sensitivity of our experimental setup. For the synthesis of these results, we are developing a model considering band-bending close to the nanotubes surfaces and obstacles for charge transport due to impurities adsorbed on the surfaces.
3:15 PM - NM5.3.03
Advances in Terahertz Spectroscopy of Clay Minerals
Ingrid Wilke 1 , Michael Aldersley 2 , Prakash Joshi 2 Show Abstract
1 Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York, United States
Clays are among the most common minerals on Earth. Large quantities of clay are sold as consumer products or employed in industrial manufacturing and processing. In technology, clays are valued for both their mechanical and chemical properties. Among clay minerals, montmorillonites are known to catalyze numerous chemical reactions relevant to the chemical and pharmaceutical industries. Since montmorillonites are a natural resource abundantly present on Earth with no reported toxicity to humans, animals and plants, they are regarded as a promising candidate for the development of chemical processes that reduce or eliminate the generation of hazardous waste.
Synthesis of organic molecules using montmorillonite as catalysts is empirically known to work but not clearly understood at the molecular level. This is a major problem for the application of montmorillonite as an efficient and green catalyst for the industrial production of organic chemicals. Catalysis by montmorillonite is the result of the layered structure of the material. In montmorillonite, two tetrahedral sheets of silicon oxides sandwich a central octahedral sheet of aluminum oxides. The spacing between the sheets is a nanometer. Naturally, the space is filled with exchangeable cations (e.g. Na+) and water molecules. For catalysis, the molecules of interest are “inserted” into the space between the layers. This is the location where the chemical reaction occurs and the reaction products are formed.
Spectroscopic probes for the investigation of interlayer adsorbate molecular structure are well established in surface chemistry. State-of-the-art methods are for example x-ray absorption, infrared spectroscopy, electron spin resonance spectroscopy and nuclear magnetic resonance spectroscopy.
Terahertz (THz) spectroscopy is currently tested as an experimental method to study the dynamics of polar molecules intercalated in clay minerals. THz spectroscopy probes energetically ( ~ meV) intermolecular vibrations and operates intrinsically on a sub-picosecond timescale. Therefore, THz spectroscopy provides unique complementary information about the adsorbate molecular dynamics in comparison with state-of-the art spectroscopic probes of adsorbate molecular structure.
In this presentation, the THz frequency absorption coefficient of a monolayer of water intercalated in montmorillonites is reported. This experimental result demonstrates that THz spectroscopy meets the key physical criteria for accurate spectroscopic identification of adsorbate molecular dynamics. Firstly, the approach is selective, i.e. the octa- and tetrahedral sheets of the montmorillonite absorb THz radiation very weakly whereas polar molecules, e.g. water, absorb strongly in the THz frequency band. Secondly, THz spectroscopy is sufficiently sensitive to detect adsorbed molecules at relevant concentrations, e.g. monomolecular layers. Thirdly, the method is non-invasive.
3:30 PM - *NM5.3.04
Understanding Ultrafast Charge Carrier Dynamics in Solar Cell Materials Using Time Resolved Terahertz Spectroscopy
Carlito Ponseca 1 Show Abstract
1 Division of Chemical Physics, Lund University, Lund Sweden
The need for developing highly efficient solar cell devices have never been so pressing until recently when the urgency of using renewable energy sources becomes more evident. There are several promising technologies being explored by many groups with the sole purpose of optimizing harvesting sunlight and converting it to useful electricity. These include, but not limited to, dye- and quantum dot-sensitized, bulk heterojunction organic, inorganic nanowires, and very recently perovskite-based solar cells. In this talk, charge carrier dynamics of an assortment of solar cell technologies probed using time-resolved terahertz spectroscopy will be presented. Electron injection, mobility, charge carrier lifetime and recombination dynamics will be discussed.
NM5.4: New Terahertz Methods
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 103 A
4:30 PM - NM5.4.01
THz Nano-Spectroscopy with 25 nm Spatial and 10 fs Time Resolution
Tobias Gokus 1 , Andreas Huber 1 , Max Eisele 1 Show Abstract
1 , neaspec GmbH, Martinsried Germany
THz spectroscopy is a powerful tool for studying and controlling dynamics in solids on sub-cycle timescales, providing a better understanding of a diverse array of low-energy elementary excitations in solid-state systems, e.g. phonons, plasmons and excitons. However, the spatial resolution of far-field THz studies is limited by diffraction, rendering it impossible to extract intrinsic, local material characteristics of individual nanoparticles, nanocrystals or nanodomains.
Scattering-type scanning near-field optical microscopy (s-SNOM) bypasses this diffraction limit, enabling optical measurements with extreme subwavelength spatial resolution down to 20nm. s-SNOM employs an externally-illuminated sharp metallic AFM tip to generate a nanoscale hot-spot at its apex. The optical tip-sample near-field interaction is determined by the local dielectric properties (refractive index) of the sample. Detection of the elastically tip-scattered light yields nanoscale resolved near-field images and spectra simultaneous to topography.
By extending the concept of broadband s-SNOM based spectroscopy to the THz-spectral range, we demonstrate optical near-field imaging and spectroscopy of various nanostructured materials such as SiO2 and graphene at THz-frequencies between 0.5-2.5 THz. This is achieved by coupling the free space output of a modified THz-TDS system to a commercially available s-SNOM microscope.
The time-dependent change of the local dielectric function upon photoexcitation of nanostructures can by studied by s-SNOM based near-field pump-probe measurements employing intensity resolved or field-resolved optical detection schemes. Such s-SNOM microscopes can achieve a spatial resolution of 10 nm well beyond the diffraction limit of the multi-terahertz probe pulses and an ultimate, sub-cycle temporal resolution of 10 fs. In combination with pump-probe studies, the microscope is capable of recording ultrafast photoinduced dynamics with an unprecedented temporal and spatial resolution, opening the door to a new dimension in experimental solid-state physics in which the local electric near-field of ultrafast low-energy elementary excitations can directly be traced in space and time. In this presentation the capabilities of the s-SNOM system is demonstrated on single photoexcited InAs nanowires [1,2] and VO2 nanobeams  tracing the time-dependent dielectric function at the surface in all three spatial dimensions.
 M. Eisele et al., “Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution”, Nat. Phot. 8, 841 (2014).
 M. Wagner et al., “Ultrafast dynamics of surface plasmons in InAs by time-resolved infrared nanospectroscopy”, Nano Letters 14, 4529 (2014).
 M. A. Huber et al., “Ultrafast mid-infrared nanoscopy of strained vanadium dioxide nanobeams”, Nano Letters, published online: doi: 10.1021/acs.nanolett.5b04988.
4:45 PM - *NM5.4.02
Using Ultrafast Terahertz Spectroscopy to Study Low Energy Excitations in Quantum Materials
Rohit Prasankumar 1 Show Abstract
1 , Los Alamos National Lab, Los Alamos, New Mexico, United States
The term “quantum materials” covers a broad range of materials that exhibit exotic phenomena, often due to unique broken symmetry states. Low energy excitations can shed light on the interplay between different degrees of freedom in these materials, particularly by using ultrashort terahertz (THz) pulses to both drive and probe these excitations. Here, we will describe the use of ultrafast THz spectroscopy to study low energy excitations in several different quantum materials, particularly multiferroics, two-dimensional electron gases, and topological insulators.
Although techniques for measuring spin dynamics in ferromagnets are relatively well developed, it has been challenging to directly measure antiferromagnetic (AFM) spin dynamics. One approach is to use THz pulses to probe magnon resonances after optical photoexcitation. We used femtosecond optical pulses to photoexcite the multiferroic AFM manganites HoMnO3 and TbMnO3 along with time-delayed THz pulses to probe the low energy magnon and electromagnon dynamics. This reveals a photoinduced change in the magnon line shape that we link to spin-lattice thermalization. More generally, our results reveal fundamental difference in spin-lattice thermalization between FM and AFM manganites.
We used terahertz magneto-optical spectroscopy to study two-dimensional (2D) electron and hole gases in semiconductor quantum wells. Our measurements reveal a nonlinear scaling of the cyclotron resonance frequency with magnetic field in 2D hole gases, due to the non-parabolicity of the valence band structure. Furthermore, optical photoexcitation of a 2D electron gas reveals not only photoinduced changes at the cyclotron frequency, but also a higher frequency mode that varies linearly with magnetic field.
Intense THz pulses can also be used to drive low energy excitations in Dirac materials. We used high field THz pulses to resonantly drive an IR active phonon; the resulting ultrafast changes in surface symmetry were probed using optical second harmonic generation (SHG). Our measurements reveal time-dependent oscillations in the SHG signal not only at the phonon frequency, but also at twice the phonon frequency. This enables us to separate the photoinduced structural dynamics at the surface from transient inversion symmetry breaking in the bulk. Overall, our experiments demonstrate the power of ultrafast THz spectroscopy for both exciting and probing low energy excitations in quantum materials.
5:15 PM - NM5.4.03
In Situ Terahertz Optical Hall Effect Measurements of Ambient Doping Effects in Epitaxial Graphene
Sean Knight 1 , Chamseddine Bouhafs 2 , Nerijus Armakavicius 2 , Philipp Kuehne 2 , Vallery Stanishev 2 , Ivan Ivanov 3 , Rositsa Yakimova 3 , Shawn Wimer 1 , Mathias Schubert 1 2 4 , Vanya Darakchieva 2 , Tino Hofmann 5 2 1 Show Abstract
1 Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 THz Materials Analysis Center, Linköping University, Linköping Sweden, 3 Department of Physics, Chemistry, and Biology, Linköping University, Linköping Sweden, 4 , Leibniz Institute of Polymer Research Dresden eV, Dresden Germany, 5 Department of Physics and Optical Science, University of North Carolina, Charlotte, North Carolina, United States
Recently, the cavity-enhanced THz optical Hall effect (THz-OHE) has been demonstrated as non-contact method to obtain free charge carrier properties using low-field permanent magnets [1,2]. A tunable, externally-coupled cavity is used to enhance the THz-OHE signal which allows accurate determination of sample's free charge carrier properties even at low magnetic fields. In this work we take advantage of this approach by integrating the permanent magnet into a gas flow cell. We demonstrate for the first time the application of the cavity-enhanced THz-OHE for the in-situ characterization of free charge carrier properties of monolayer epitaxial graphene on Si-face (0001) 4H-SiC as a function of various inert and environmental gases. The experiments were performed using a new rotating-analyzer THz ellipsometer at Linköping University. Upon changing the gas type in the cell, large variations in both free charge carrier sheet density Ns and mobility μ are observed for the n-type graphene. The significant changes are attributed to an ambient doping redox reaction involving CO2, H2O, and O2 at the graphene surface which results in the extraction of electrons from graphene . The change in μ as a function of Ns is examined to gain insight to the scattering mechanism due to ambient gas doping. In conclusion, we demonstrate the in-situ THz-OHE as a new and powerful technique to determine ambient-dependent doping and scattering mechanisms which is illustrated here using graphene on Si-face 4H-SiC.
 S. Knight, S. Schöche, V. Darakchieva, P. Kühne, J.-F. Carlin, N. Grandjean, C. M. Herzinger, M. Schubert, and T. Hofmann, Opt. Lett. 40, 2688 (2015).
 P. Kühne, C.M. Herzinger, M. Schubert, J.A. Woollam, and T. Hofmann, Rev. Sci. Instrum. 85, 071301 (2014).
 A.N. Sidorov, K. Gaskill, M.B. Nardelli, J.L. Tedesco, R.L. Myers-Ward, C.R. Eddy Jr., T. Jayasekera, K.W. Kim, R. Jayasingha, A. Sherehiy, R. Stallard, and G.U. Sumanasekera, J. Appl. Phys. 111, 113706 (2012).
5:30 PM - *NM5.4.04
Implications of High-Repetition Rate Single-Shot Terahertz Diagnostics for Materials Research
Erik Bruendermann 1 Show Abstract
1 Institute for Beam Physics and Technology (IBPT), Dept. Accelerator Research, Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen Germany
Laser-based and electron accelerator sources provide very short and intense pulses of radiation over substantial ranges of the electromagnetic spectrum covering ten to hundreds of terahertz [Bründermann et al., “Terahertz Techniques”, Springer Series in Optical Sciences, vol. 151 (2012)]. Typically, the pulses are emitted up to high repetition rates well into the megahertz and gigahertz range leading to frequency combs [Steinmann et al., “Frequency-comb spectrum of periodic-patterned signals,” Phys. Rev. Lett. 117, 174802 (2016)]. The presentation will focus on methods for terahertz diagnostics of electron density near-fields and radiation far-fields with a special emphasis on measuring short pulses with wide terahertz frequency coverage at high repetition rates in the megahertz to gigahertz range. These continuous, high-repetition measurement methods of single-shots without interruption are readily transferable to materials research studies. Recording each single-shot avoids averaging methods and enables statistical methods used for ensembles promoting data processing techniques. Intense electric fields emitted from electron accelerator-based sources approaching GV/m also open new windows to materials research.
NM5.5: Poster Session: Terahertz Materials and Technology
Friday AM, April 21, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NM5.5.01
The Influence of Stoichiometry on the Index of Refraction of Cobalt Ferrite Samples at Terahertz Frequencies
Alan Boss 1 , Ingrid Wilke 2 , Antonio Migliano 3 1 Show Abstract
1 , Instituto Tecnológico de Aeronáutica, São José dos Campos, São Paulo, Brazil, 2 , Rensselaer Polytechnic Institute, Troy, New York, United States, 3 , Instituto de Estudos Avançados, São José dos Campos, São Paulo, Brazil
Ferrimagnetic materials play an important role in devices that operate at microwave frequencies. They can be applied e.g. in filters, absorbers, and metamaterials. In order to advance microwave ferrite device technology to the terahertz frequency band, it is crucial to characterize ferrites at terahertz frequencies and understand their behavior.
Here we report an experimental study on the terahertz frequency dielectric properties of manganese cobalt ferrites (MnxCo1-xFe2O4) and nickel cobalt ferrites (NixCo1-xFe2O4) with three different stoichiometries each, x=0.3, x=0.5 and 0.7. Particularly, we present a comparison and discussion of the terahertz frequency indices of refraction of these two ferrites compositions.
MnxCo1-xFe2O4 and NixCo1-xFe2O4 pellets with different Ni/Co ratios (x=0.3, x=0.5 and x=0.7) were prepared by state-of-the-art ceramic processing. All samples were sandpapered to become flat and then submitted to heat treatment in order to release mechanical stress caused by machining. In addition to time-domain terahertz spectroscopy, the morphology and chemical homogeneity of the cobalt ferrite samples were characterized by optical microscopy and energy dispersive x-ray spectroscopy.
We observed that the indices of refraction for manganese cobalt ferrite are 3.22, 3.71 and 3.67 for ratios of 0.3, 0.5 and 0.7 of manganese to cobalt. We notice that there is a substantial difference in the index of refraction for sample with ratio 0.3 to samples with ratios 0.5 and 0.7. In the case of nickel cobalt ferrite, the indexes of refraction are 3.53, 3.57 and 3.47 for ratios of 0.3, 0.5 and 0.7 of nickel to cobalt. For nickel cobalt ferrites the difference in the indexes of refraction are much smaller than for manganese cobalt ferrites. The influence of the sample stoichiometry, morphology and chemical composition on the evolution of the terahertz frequency index of refraction of cobalt ferrites is discussed.
9:00 PM - NM5.5.02
A Mechanical Analogy for Heat Transfer in Surface Films with Transformations
Rahul Basu 1 2 Show Abstract
1 , VTU, Bangalore India, 2 Mechanical, Adarsha Institute of Technology, Bangalore, Ka, India
A model for describing effects of a variable frequency heat source on the surface of a film is developed. The effect of material parameters is analysed to describe possible attenuation of the thermal fluctuations and sustaining these through coupling with the surrounding matrix. Application to amorphous alloy formation with phase field concepts is described. An analogy with a damped oscillator driven by surface fluctuations gives the relation of non dimensional parameters llike the Stefan, Fourier and Biot numbers to surface heating with convection. Recent discoveries of high temperature super-conductivity in the femto second regimes are included. Impurities and stress fields caused by large oxygen atoms and interaction with the phonon field may be responsible for such effects especially in the "crust" of oxide films. Phonon interactions with driven oscillators in the film may be possible and design of suitable materials to give sustained longer duration high temperature effects is outlined.
9:00 PM - NM5.5.03
Ultra-Broadband, Solid State Terahertz Phase Modulator Based on Graphene
Chen Zefeng 1 , Xuequan Chen 1 , Xudong Liu 1 , Edward Parrott 1 , Emma MacPherson 1 , Jian-Bin Xu 1 Show Abstract
1 , CUHK, Hong Kong China
Terahertz technology promises a myriad of applications including imaging, spectroscopy and communications. Recent research on THz phase modulators is largely based on resonance structures of electromagnetic waves making it challenging to achieve a broadband modulator with a high modulation range. Here, we theoretically and experimentally demonstrated a solid-state, graphene-based phase modulator with a wide modulation range and ultralbroad bandwidth.
Theoretically, the phase of a (p-polarized) electromagnetic wave reflected from a media is zero or π, depending on the incident angle being smaller or larger than the Brewster angle. We show that the Brewster angle of a media can be controlled by manipulating the conductivity of graphene placed on the media, as a result the incident angle of a THz wave can be switched from larger to smaller than the Brewster angle. With this principle in mind, we designed a solid-state, graphene-based phase modulator. Experimental results show that the phase of the reflected wave can be tuned from 0 to π/4 in a frequency range from 0.1THz to 0.8THz. We believe our findings provide new strategies for designing highly tunable and broadband THz phase modulators.
9:00 PM - NM5.5.04
Planer Metal-Insulator-Metal Diode Based on NiO Using Langmuir-Blodgett Technique for Infrared Detection
Ibrahim Azad 1 , Apoorv Kaushal 2 , Manoj Ram 1 , Yogi Goswami 3 , Elias Stefanakos 1 Show Abstract
1 Electrical Engineering, University of South Florida, Tampa, Florida, United States, 2 Mechanical Engineering, University of South Florida, Tampa, Florida, United States, 3 Chemical Engineering, University of South Florida, Tampa, Florida, United States
Metal-insulator-metal (MIM) diode plays important role in infrared detection and energy harvesting. Quantum tunneling conduction mechanism in MIM diode helps to rectify the signal at infrared (IR) frequency range. An effort to produce a MIM tunnel diode, thin nickel oxide (NiO) insulating nanolayer was synthesized from nickel (Ni) based organic precursor using Langmuir-Blodgett (LB) technique. The nickel sulfate was used as a subphase where nickel stearate monolayer was formed at the air-water interface in LB trough with spread of stearic acid dissolved in chloroform. The nickel stearate monolayers were deposited on different substrates (silicon, glass, gold (Au) coated silicon). Initially, the nickel stearate LB films were exposed to UV light and subsequently heated at 350 °C in air. The NiO nanolayer was formed on various substrates. Energy-dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM) were used to analyze the deposition and formation of the NiO layer. The average surface roughness of NiO film was measured to be 1.402 nm using AFM technique. The constituent elements of the nickel stearate LB film and NiO layer films were studied using EDS technique. MIM diode was fabricated by sandwiching NiO thin film between thin layers of gold (Au) and Ni. The current – voltage (I-V) characteristics of the diode was studied to realize electron tunneling conduction mechanism. The highest measured sensitivity around 22 (A/W) and the rectification ratio ~11 at ±100 mV of the fabricated MIM diode suggests its possible application in infrared detection.