Oleg Mitrofanov Bell Laboratories, Lucent Technologies
Xi-Cheng Zhang Rensselaer Polytechnic Institute
Richard Averitt Los Alamos National Laboratory
Kazuhiko Hirakawa University of Tokyo
Alessandro Tredicucci NEST-INFM
K1: THz Spectroscopy I
Thursday PM, April 20, 2006
Room 3016 (Moscone West)
9:30 AM - **K1.1
Broadband Pulsed Terahertz Sources.
Giles Davies 1 , Edmund Linfield 1 , Yao-Chun Shen 2 , Prashanth Upadhya 1 Show Abstract
1 Electronic and Electrical Engineering, University of Leeds, Leeds United Kingdom, 2 TeraView Limited, Platinum Building, St John's Innovation Park, Cambridge United Kingdom
Since the first use of femtosecond lasers to generate coherent THz frequency radiation, there has been a drive to develop higher power sources and systems of broader bandwidth. Of the various methods pursued, photoconductive emitters have proven to be an effective technique for converting visible/near-IR pulses to THz radiation, and have been widely used for THz spectroscopy and imaging. Electron-hole pairs generated in a semiconductor crystal (such as GaAs) by an above-bandgap femtosecond pulse, are accelerated by an applied electric field. Their physical separation forms a macroscopic space-charge field oriented opposite to the biasing field, and thus, the externally applied field is screened. The fast temporal change in field produces a transient current, which generates a pulse of electromagnetic radiation in the THz frequency range. Theoretical simulations suggest that sub-100 fs electrical pulse can be obtained. Until recently, however, 200 fs electric pulses and 350 fs free-space THz pulses are amongst the shortest realized experimentally for GaAs emitters, giving a useful bandwidth of about 4 THz. We will discuss the development of an ultra-broadband terahertz (THz) spectrometer with bandwidth > 20 THz, which significantly extends the capability of the THz pulsed imaging technique. In contrast to previous experiments, where the THz radiation was collected forwards from the photoconductive emitter (after transmission through the GaAs substrate), we collect the THz radiation backwards (in the direction of the reflected pump laser beam). As a result, the absorption and dispersion of the THz pulses in the GaAs substrate are minimized. We will illustrate the use of this ultra-broadband spectrometer in number of applications, and highlight the influence of phonon absorptions in the emitter and detector semiconductor crystals on the spectra obtained. When an energetically excited molecule returns to its ground state, a damped oscillating electric field is emitted with a characteristic resonance frequency and damping constant that is unique to each transition. The direct observation of such a field, however, requires experimental techniques with a temporal resolution better than the oscillating period, and hence such measurements are not possible for transitions in the visible and ultra-violet. However, for far-infrared emission, THz time-domain spectroscopy, which can measure the electric field with a sub-ps resolution, can allow this to be achieved. We will show how time-partitioned Fourier transforms of the measured THz time-domain signal after transmission through a sample allows the progressive observation of absorption and subsequent emission of THz radiation at the frequency of the vibrational mode.
10:00 AM - K1.2
Dielectric Properties of [Fe(NH2-trz)3]Br2H2O Thermal Spin Crossover Compound in Terahertz Wavelength.
Patrick Mounaix 1 , Jerome Degert 1 , Eric Freysz 1 , Nathalie Daro 2 , Jean francois LETARD 2 Show Abstract
1 , CPMOH, TALENCE France, 2 , ICMCB, PESSAC France
10:15 AM - K1.3
Protein Characterization using Terahertz Dielectric Response
Andrea Markelz 2 , Jing-Yin Chen 2 , Joseph Knab 2 , Shuji Ye 2 Show Abstract
2 Physics, University at Buffalo, SUNY, Buffalo, New York, United States
10:30 AM - K1: Spect1
11:00 AM - **K1.4
Materials Contrast for Electronic Terahertz Imaging.
Daniel van der Weide 1 Show Abstract
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Extremely short electrical impulses (1-100 ps) have correspondingly broad frequency spectra, which can range from the radio-frequency (RF) through the microwave and millimeter-wave regime. Ultrawideband radiation of this range is difficult to generate and radiate with good fidelity, but has great potential in spectroscopic imaging for target identification; most chemical compounds show strong and specific absorption and dispersion in the 1–1000 GHz range. Gaining spatial resolution with arrays of these sources and detectors and spectral resolution from their broadband characteristics would ultimately enable new and more informative images to be formed.The technology we are using—the integrated-circuit nonlinear transmission line (NLTL)—can essentially trade power for speed, producing pico- or even sub-picosecond pulses with peak powers less than one watt and average powers in the low microwatt regime, which are sufficient for driving transient digitizers (samplers) given high-power UWB (ultrawideband) generators. These power levels are non-ionizing and biologically inconsequential, but because we can employ coherent generation using a unique dual-source interferometer, rejecting noise outside the frequencies of interest, we can perform target identification while using room-temperature detectors optimized for both sensitivity and low cost. Furthermore, we can extend the sensitivity of conventional UWB systems (normally having frequency limits below 10 GHz) to higher frequencies; we examine chemical and material contrast available in the 10-500 GHz regime to detect the presence of concealed threats.
11:30 AM - K1.5
Hydration Dynamics of Water Solution Revealed by THz Time-domain Attenuated Total Reflection Technique.
Takeshi Arikawa 1 , Masaya Nagai 1 , Koichiro Tanaka 1 Show Abstract
1 Department of Physics, Kyoto University, Kyoto Japan
Hydration water plays a key role on the functions of biomolecules such as DNA and proteins, because it strongly influences the stabilization of their three dimensional structures. Hydrated water molecules in biomolecular solution are roughly classified into three layers A to C by their relaxation times. Layer A includes water molecules strongly bounded to the biomolecule with hydrogen bonding. The relaxation time is as slow as in the order of 10-7 s, whereas water molecules in layer C show fast relaxation with 10-12 s. Layer B is an intermediate region. Water molecules in layer A and B is called as structured water. However, it has been quite difficult to determine the whole number of structured water. For example, the hydration number of sucrose (C12H22O11) was determined experimentally by calorimetric, viscosity, and ultrasound measurement. The obtained hydration numbers are 6.3, 11.2, and 13.8, respectively. The different results come from the time scale of the experimental perturbation method. These experimental values are also smaller than the number estimated by MD simulation, 20.9. This means that one cannot measure whole number of structured water molecules using these conventional experimental methods. In this study, we propose a novel method to evaluate whole number of hydrated water molecules. This method is based on the fact that the relaxation time of structured water increases by hydration from picoseconds to nanoseconds. We measured precisely the concentration dependence of complex dielectric constant of sucrose solution with THz time-domain attenuated total reflection technique (TD-ATR). Local field correction was made with Onsager’s theory to obtain orientational polarizability of sucrose in water solution. Hydration number was determined to be satisfied that all polarizabilities for different concentrations should be the same. Obtained hydration number is 19.3 for H2O and 24.7 for D2O, which is in good agreement with the results of MD simulation. This method will open a new possibility to resolve the hydration dynamics and related functional properties of biomolecules.
11:45 AM - K1.6
Terahertz (THz) Spectroscopy of Freon-11 (CCl3F)
Hakan Altan 1 , Baolong Yu 1 , Robert Alfano 1 , Scott Alfano 2 Show Abstract
1 Institute for Ultrafast Spectroscopy and Lasers, Department of Physics, City College of New York, New York, New York, United States, 2 Union College , 807 Union Street, Box 0018 , Schenectady, New York, United States
Trichlorofluoromethane (Freon-11, CFC-11, CCl3F) is the second most widely used of the fully halogenated Chlorofluorocarbons (CFCs). Since it is a chemically inert and non-toxic compound it has been extensively used as an aerosol propellant and refrigerant; however, its effects on the ozone layer in the stratosphere has caused concern. The measurement of its concentration in the atmosphere is of considerable interest.In this presentation, the lowest vibrational mode for CCl3F has been measured in the 0.1 to 1.5THz frequency range. The observed absorption profile was modeled with rotational and vapor pressure parameters. The vibrational spectra of CCl3F in the gaseous and liquid states have been extensively studied by both Raman and infrared techniques, and the normal modes have been well documented. The rotational spectrum due to the ground and low-lying vibrational states has also been studied. Typically, in these studies, harmonic-generation from klystrons was used to generate the far-infrared light which was sent through meter-long pathlengths of the vapor to obtain sufficient sensitivity to resolve rotational linewidths. In contrast, THz-Time Domain Spectroscopy (THz-TDS) methods have proven to obtain as good or even better sensitivity over shorter gas pathlengths. In addition, this technique allows for gas analysis above a few hundred gigahertz, crucial in studying the low-lying vibrational-rotational transitions in a variety of gases. The THz absorption spectrum of Freon-11 was measured through a pathlength of ~10cm and vapor pressure of ~10hPa. At T = 300K, the spectrum showed a broad absorption feature centered below 0.5THz. The broad absorption feature at low frequencies is due to the rotational spectrum of the ground state of CCl3F, since the 1st excited vibrational state lies at 241cm-1(above the limit of our frequency range). For CCl3F, a symmetric top molecule, excitation of the molecular vapor by THz radiation induces transitions between pairs of J, K levels of the lowest vibrational state. This broad absorption was well modeled assuming a maximum occupational level J max = 143. While individual transitions between pairs of rotational states can not be resolved, THz-TDS of rotational transitions as spectral markers can still be useful since it allows for gas characterization by identifying the entire rotational spectrum for each vibrational state.
12:00 PM - K1.7
Phase Transitions in Crystalline Materials Studied by Terahertz Pulsed Spectroscopy.
J. Axel Zeitler 1 2 3 , David Newnham 3 , Philip Taday 3 , Terry Threlfall 4 , Robert Lancaster 5 , Michael Pepper 2 3 , Keith Gordon 6 , Thomas Rades 1 Show Abstract
1 School of Pharmacy, University of Otago, Otago New Zealand, 2 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 3 Applications Group, TeraView Limited, Cambridge United Kingdom, 4 School of Chemistry, University of Southampton, Southampton United Kingdom, 5 Department of Chemistry, University College London, London United Kingdom, 6 Department of Chemistry, University of Otago, Dunedin New Zealand
12:15 PM - **K1.8
Integration Technologies for THz Biomolecular Sensors.
Peter Haring Bolivar 1 Show Abstract
1 , University of Siegen, Siegen Germany
12:45 PM - K1.9
Metal-Insulator Phase Transition in VO2: A Look from the Far Infrared Side
Peter Jepsen 1 , Bernd Fischer 2 , Andreas Thoman 2 , Hanspeter Helm 2 , J. Suh 3 , Rene Lopez 3 , Richard Haglund 3 Show Abstract
1 COM.DTU, Technical University of Denmark, Kongens Lyngby Denmark, 2 Freiburg Materials Research Center, University of Freiburg, Freiburg Germany, 3 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee, United States
Vanadium dioxide (VO2) displays a well-known metal-insulator (MI) transition at a temperature of 68oC. The MI transition in VO2 has been studied by a wide range of optical, electrical, structural, and magnetic measurements. In spite of this there is still some controversy about the nature of the phase transition. Mid-IR studies  indicate that a model based on growth of metallic domains accounts for the phase transition whereas other studies  argue that a homogeneous increase of the carrier concentration is responsible for the change in properties across the phase transition. In this study we use Terahertz Time-Domain Spectroscopy (THz-TDS) to investigate the optical properties of VO2 in the vicinity of the MI transition in the 0.1-2 THz frequency range. At frequencies below the phonon resonances of the material we can obtain a clean spectroscopic signature of the carrier dynamics of the material. Radiation in the THz region interacts with an electron gas in a characteristic way, often well described by the Drude model. It turns out that the capability of THz-TDS to measure both amplitude and phase of the transmission is crucial for the interpretation of the results.The 160-nm thick VO2 film was was grown on a 0.15-mm thick glass substrate. A vanadium metal target was ablated in an oxygen ambient (5 mTorr) by a KrF excimer laser (248 nm) at a fluence of 4 mJ/cm2. Approximately 105 laser pulses were required to deposit this film. The as-deposited film is amorphous and exhibits an incomplete stoichiometry close to VO1.7 as determined by Rutherford backscattering (RBS). After conversion to VO2 by thermal oxidation at a temperature of 450oC, phase and crystallinity were confirmed by X-ray diffraction (XRD), while stoichiometry was verified by RBS. Half of the substrate area was left without VO2-film, enabling a precise determination of the phase delay of the THz signal transmitted through the thin film.We observe the typical temperature hysteresis of the far-infrared transmission through the thin film with temperature. Interestingly the temperature-dependent transmission amplitude shows a markedly different switching temperature than the transmission phase. This effect has not been observed previously, and is very important for the interpretation of the results.We have performed simulations of the temperature-dependent transmission amplitude and –phase based on two different models of the phase transition, namely the domain growth model and a model based on homogenous increase of the carrier concentration as the temperature is increased across the transition point. We show that the scenario where the carrier concentration increases homogeneously throughout the thin film is consistent with our experimental data, and reproduces the difference in the observed transition temperature for the transmission amplitude and –phase. H. S. Choi et al, Phys. Rev. B 54, 4621 (1996) A. Zylbersztejn and N. F. Mott, Phys. Rev. B 11, 4383 (1975)
K2: THz Spectroscopy and Applications
Thursday PM, April 20, 2006
Room 3016 (Moscone West)
2:30 PM - **K2.1
THz-wave Parametric Sources and Imaging Applications.
Kodo Kawase 1 2 3 , Masatsugu Yamashita 2 , Chiko Otani 2 , Yuichi Ogawa 3 Show Abstract
1 Engineering Dept., Nagoya University, Nagoya Japan, 2 , RIKEN, Wako Japan, 3 Agricultural Science Dept., Tohoku University, Sendai Japan
After more than a dozen years of basic research into the submillimeter and far infrared range, terahertz wave research has finally come into its own, and is recognized by the world scientific community as a new frontier. While femtosecond laser pumped THz wave sources have opened up a new vista in applied research, the ideal THz wave source will likely require high coherence and wide tunability. When this level of quality is finally made available in a user-friendly device, there is little doubt that applied research efforts into the THz region will enjoy a true renaissance. In this direction we have developed a widely tunable injection seeded THz-wave parametric generator (is-TPG) that operates at room temperature. The spectral resolution is the Fourier transform limit of the nanosecond THz wave pulses. In our laboratory, THz-waves continue to broaden their range of applications as following.We have recently developed two palmtop THz-wave parametric generators (TPG) with different characteristics. One generates high energy and broadband THz waves, being suitable for detecting the transmission of highly absorptive or diffusive samples, and the other has a potential of wide tunability and narrow linewidth, useful for spectroscopic measurements. The characteristics of these THz-wave generators depend on their pump sources.One TPG includes a 20 cm long Nd:YAG laser with a top-hat beam profile, which generates high energy, broad-band THz waves. The THz and the idler waves begin to be generated at a pump energy of around 25 mJ/pulse. The highest values obtained are 105 pJ/pulse for the THz wave and 15 mJ/pulse for the idler wave when the pump energy is 66 mJ/pulse, which corresponds to a pump intensity of 820 MW/cm2. We also observed about 70 % of pump-depletion. The other is a narrow-linewidth injection-seeded TPG, based on an LD-pumped single-mode microchip Nd:YAG laser. This laser enables the low order axial and transverse mode laser oscillation, so that the linewidth is below 0.009 nm. The laser delivers 1.8 MW peak power pulses (750 µJ/pulse) with 420 ps pulse width at 100 Hz repetition rate with a M2 factor of 1.09. As the pump energy is increased above the generation threshold, about 400 µJ/pulse, the energy of the idler wave increases linearly. The maximum energy of the idler wave is 110 µJ/pulse, obtained at a pump energy of 750 µJ/pulse, which corresponds to a pump intensity of 2.2 GW/cm2. We are also studying several novel steps toward real-life applications of terahertz waves, such as, i) printable terahertz sensors using metal mesh, ii) noninvasive detection of concealed drugs using terahertz wave scattering, iii) observation of MOSFET chips using an infrared laser THz emission microscope. In my presentation, I will introduce our recent activities.
3:00 PM - K2.2
Teraherz Magnetooptic Generalized Mueller-matrix Ellipsometry.
Tino Hofmann 1 , Ullrich Schade 2 , Craig Herzinger 3 , Mathias Schubert 1 Show Abstract
1 Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 , BESSY mbH, Berlin Germany, 3 , J. A. Woollam Co., Inc., Lincoln, Nebraska, United States
3:15 PM - K2.3
Time-Resolved THz Studies of Polaron Dynamics in Quasi-One-Dimensional Molecular Materials
Susan Dexheimer 1 , A. V. Nampoothiri 1 Show Abstract
1 Physics, Washington State University, Pullman, Washington, United States
The localization of electronic excitations via electron-lattice interactions is an important fundamental process that has a direct impact on the transport properties of molecular-based electronic materials. In this work, we use time-resolved THz techniques to follow the dynamics of charge carriers in quasi-one-dimensional molecular materials, including the formation and subsequent evolution of polaron states. The studies are carried out on crystalline inorganic polymers, specifically mixed-valence halide-bridged transition metal linear chain (or MX) complexes. These materials have attracted interest as model systems for the physics of low-dimensional materials owing to their structural tunability, in that the relative strengths of the electron-electron interaction and the electron-phonon coupling, the fundamental physical parameters that determine properties of the excitations, can be systematically tuned by chemical substitutions in the chain structure. Theoretical studies of these materials have predicted the possibility of the formation and evolution of a number of types of metastable nonlinear excitations, including self-trapped excitons, solitons, and polarons, and our previous work on ultrafast dynamics in these materials focused on the dynamics of excitonic self-trapping using femtosecond optical impulsive excitation techniques to time resolve the lattice dynamics associated with exciton localization. In this work, we present time-resolved THz studies in which optical pulses 35 fs in duration centered at 800 nm are used to excite charge carriers in oriented crystals of the complex [Pt(en)2][Pt(en)2I2].(ClO4)4 (en = ethylenediamine, C2H8N2), and single-cycle THz pulses are used to probe the photoinduced response. The measurements reveal a biphasic induced absorbance, with a large amplitude component that decays rapidly with a time constant of ~ 400 fs, followed by a much more slowly decaying, smaller amplitude component. The initial response is assigned to photogenerated free carriers with high mobility and correspondingly strong THz induced absorption. As the carriers interact with the lattice, they are expected to form lower mobility polaron states, consisting of a carrier localized in a distorted region of the lattice. Theory predicts that in a one-dimensional lattice, the trapping process should be barrierless, so that polaron formation should take place on a time scale of a single vibrational period of the lattice, consistent with the observed rapid decrease in the THz induced absorption. We find that the subsequent response decays slowly as a power law ~ t-1/2, the characteristic time dependence for diffusive motion in one dimension, reflecting loss of the polaron population via diffusion-limited recombination.This work is supported by the National Science Foundation.
3:30 PM - K2.4
THz Pump-Probe Spectroscopy of Single-Wall Carbon Nanotubes
Yang Wu 1 , Feng Wang 1 , Gordana Dukovic 2 , Louis Brus 2 , Tony Heinz 1 Show Abstract
1 Depts. of Physics and Electrical Engineering, Columbia University, New York, New York, United States, 2 Dept. of Chemistry, Columbia University, New York, New York, United States
In this paper we present recent results on charge transport in single-wall carbon nanotubes (SWCNTs) using THz time-domain spectroscopy. THz spectroscopy presents an excellent complement to the large body of conventional transport measurements in which electrical leads must be attached to the SWCNTs. THz spectroscopy provides unique capability in two respects. First, it permits a determination of the complex conductivity up to THz frequencies. This information that can be related to carrier scattering rates in these one-dimensional systems. Second, THz TDS allows us to follow the temporal evolution of the nanotube conductivity following photo-excitation with picosecond resolution. We will emphasize data on the time-dependent conductivity measured for an ensemble of SWCNTs following photoexcitation. The observed behavior will be discussed in terms of fundamental aspects of the carrier dynamics, electron-phonon scattering processes, and carrier trapping in SWCNTs. The results will be compared with time-resolved probes based on purely optical measurement techniques.
3:45 PM - K2.5
Electric Field-Controlled Dielectric Properties of SrTiO3 in the Terahertz Range.
Petr Kuzel 1 , Filip Kadlec 1 , Hynek Nemec 1 , Roland Ott 2 , E Hollmann 2 , Norbert Klein 2 Show Abstract
1 , Institute of Physics, Czech. Acad. Sci., Prague 8 Czech Republic, 2 cni - Center of Nanoelectronics and Information Technology, Forschungszentrum Juelich GmbH, Juelich Germany
The investigation of dielectric voltage-controlled thin film structures is important for the development of fast-acting frequency agile microwave and sub-THz integrated devices . SrTiO3 (STO) is an incipient ferroelectric material, which means that, on the one hand, its dielectric behavior is fully controlled by the soft mode dynamics, and on the other hand, it remains paraelectric down to the lowest temperatures due to quantum fluctuations. These properties along with the possibility to prepare high quality thin films make STO a material of choice for applications in tunable microwave and THz components. Recently, a highly tunable photonic crystal filter operating in the sub-THz range has been demonstrated based on the temperature tunability of the dielectric properties of STO .In this contribution we demonstrate an electric-field induced variation of the permittivity of an STO thin film from the MHz up to the THz frequency range at room temperature. We have prepared an interdigited electrode structure for applying a static (or a low-frequency ac) electric field to a ferroelectric thin film, which enables nearly full transmission of a linearly polarized terahertz wave for the polarization perpendicular to the electrode pattern. This approach has been used to determine with high accuracy the electric field dependence of the complex permittivity of a STO thin film on a sapphire substrate up to about 2 THz employing time-domain terahertz spectroscopy. We demonstrate up to 10% variation of the film permittivity at 300 GHz at room temperature induced by an applied electric field of 100 kV/cm). No dielectric dispersion is observed between 1 MHz and about 500 GHz. The field-induced changes are attributed to soft mode hardening. A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N. Setter, J. Electroceram., 11, 5 (2003). H. Němec, P. Kuzel, L. Duvillaret, A. Pashkin, M. Dressel, and M. Sebastian, Opt. Lett., 30, 549 (2005).
4:00 PM - K2: Spect2
4:30 PM - **K2.6
THz near-field Optics and Microscopy.
Paul Planken 1 , Aurele Adam 1 Show Abstract
1 , University of Technology Delft, Delft Netherlands
5:00 PM - K2.7
Submicron Material Characterization Using Terahertz Scanning Near-field Microscopy
Hou-Tong Chen 1 3 , Federico Buersgens 2 , Roland Kersting 2 3 , Antoinette Taylor 1 , Richard Averitt 1 Show Abstract
1 MST-CINT, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Department of Physics, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Physics, University of Munich, Munich Germany
Terahertz (THz) radiation is a powerful technique for material characterization ranging from superconductors to semiconductor heterostructures to biomedical imaging, all in a non-contact geometry. Such non-contact conductivity measurements are especially important in the characterization of individual quantum electronic building blocks whose response can be significantly influenced by electric leads. However, the spatial resolution of conventional THz imaging techniques is limited to hundreds of micrometers by diffraction. The recent development of the apertureless terahertz scanning near-field optical microscope (THz-SNOM) allows for submicron spatial resolution  and enables a broad variety of novel applications in material characterization. The basic mechanism is that a metallic probe allows for mapping of the THz permittivity of a surface. In this contribution, we will report on measurements of microscopic scale charge carrier distributions and dielectric contrast with sub-micrometer resolution in various material systems and structures using THz-SNOM. We have identified a novel imaging mechanism in terms of a configurational resonance , which contrasts the widely accepted scattering model at visible and near-infrared frequencies. H.-T. Chen, et al., Appl. Phys. Lett. 83, 3009 (2003) H.-T. Chen, et al., Phys. Rev. Lett. 93. 267401 (2004)
5:15 PM - K2.8
Persistent Photoconductivity Induced by Terahertz Radiation in Pb1-xSnxTe(In)
Aleksander Kozhanov 1 , Dmitry Dolzhenko 1 , Ludmila Ryabova 1 , Dan Watson 2 , Dmitry Khokhlov 1 Show Abstract
1 Physics Department, Moscow State University, Moscow Russian Federation, 2 Department of Physics and Astronomy, University of Rochester, Rochester, New York, United States
Doping of the lead telluride and related alloys with some of the group III impurities (In, Ga) results in appearance unusual effects such as the Fermi level pinning and the persistent photoconductivity at low temperatures. The spectra of the persistent photoresponse have not been measured so far because of the difficulties with screening the background radiation. We report on the observation of strong persistent photoconductivity in Pb0.75Sn0.25Te(In) under the action of monochromatic Terahertz radiation at wavelengths of 176 and 241 microns. The sample temperature was 4.2 K, the background radiation was completely screened out. The sample was initially in the semiinsulating state providing dark resistance of more than 100 GOhm. The responsivity of the photodetector is by several orders of magnitude higher than in the state of the art Ge(Ga). The red cut-off wavelength exceeds the upper limit of 220 microns observed so far for the quantum photodetectors in the uniaxially stressed Ge(Ga). It is possible that the photoconductivity spectrum of Pb1-xSnxTe(In)covers all the Terahertz frequency range.
5:30 PM - K2.9
Sources and Receivers for Frequency Domain Terahertz Spectroscopy
Thomas Crowe 1 , David Kurtz 1 , Jeffrey Hesler 1 , David Porterfield 1 , Kai Hui 1 Show Abstract
1 , Vriginia Diodes, Inc., CHARLOTTESVILLE, Virginia, United States
5:45 PM - K2.10
Nanostructured ultrathin NbN film as a Terahartz Hot-Electron Bolometer Mixer
Grigory Gol'tsman 1 , Sergey Maslennikov 1 , Matvey Finkel 1 , Sergey Antipov 1 , Natalia Kaurova 1 , Elisaveta Grishina 1 , Stanislav Polyakov 1 , Yury Vachtomin 1 , Sergey Svechnikov 1 , Konstantin Smirnov 1 , Boris Voronov 1 Show Abstract
1 , MSPU, Moscow Russian Federation
Currently, NbN hot-electron bolometer (HEB) mixers have no acceptable alternative at the frequencies higher than 1.3 THz. At the same time, noise temperature of state-of-the-art receivers employing NbN HEB mixers is still 8 - 10 times of the quantum limit while the gain bandwidth of NbN HEB mixers is not sufficient for up to date applications.We have investigated the structures with additional buffer layers between the superconducting film and Si substrate. The buffer layer enabled us to increase the gain bandwidth due to better acoustic transparency. The structure and composition of NbN films were investigated by high resolution TEM and X-ray spectroscopy methods. The data obtained showed that the processes of surface oxidization caused significant degradation of the film properties and consequently mixer performance. The technique of in-situ Au deposition was suggested and implemented. This way fabricated nanostructures based on ultrathin (2.5 - 3 nm) superconducting NbN films demonstrated much better values of critical current density (up to 5 x 106 A/cm2), critical temperature (11 K), and superconducting transition width (0.2 K). The mixers based on in-situ NbN/Au had significantly lower contact resistance. The decrease of this resistance allowed us to reduce the mixer noise temperature down to 1300 K at 2.5 THz and to 3100 K at 3.8 THz, and diminish required LO power to 100 nW for a structure with in-plane dimensions of 1.5 x 0.15 μm2.
K3: Poster Session: Properties of Materials at THz Frequencies
Friday AM, April 21, 2006
Salons 8-15 (Marriott)
9:00 PM - K3.1
The Terahertz Dielectric Constant of BaxSr1-xTiO3
David Hilton 1 , John O'Hara 1 , Bo Soo Kang 2 , Richard Averitt 1 , Quanxi Jia 2 , Antoinette Taylor 1 Show Abstract
1 MS G756, Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 MS K763, MST-STC, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
9:00 PM - K3.10
InAs-GaSb Bbased Nanostructure for THz Lasing: The Way to Overcome Principal Gain Limitations.
Leonid Shvartsman 1 , Boris Laikhtman 1 Show Abstract
1 Racah Institute of Physics, Hebrew University, Jerusalem Israel
9:00 PM - K3.11
Coupling Effects in Symmetric and Asymmetric InAs/GaAs Quantum Dot Molecules.
Anup Pancholi 1 , Valeria Stoleru 1 Show Abstract
1 Department of Materials Science and Engineering, University of Delaware, Newark, Delaware, United States
9:00 PM - K3.12
Emission and Detection of Terahertz Pulses from a Metal-tip Antenna.
Markus Walther 1 , Geoffrey Chambers 1 , Zhigang Liu 1 , Mark Freeman 1 , Frank Hegmann 1 Show Abstract
1 Department of Physics, University of Alberta, Edmonton, Alberta, Canada
9:00 PM - K3.13
Terahertz Photo-Hall Measurements of Carrier Mobility.
James Heyman 1 , Daniel Bell 1 , Thamsanqa Khumalo 1 Show Abstract
1 Physics and Astronomy, Macalester College, Saint Paul, Minnesota, United States
We have developed a sensitive ultrafast technique for measuring the mobility of photocarriers in semiconductors. Samples are photoexcited with a femtosecond laser, and carrier mobilities are determined by polarization-sensitive THz emission measurements in a magnetic field. For example, measurements on a semi-insulating GaAs sample at T=280K yielded electron mobility 4400±600 cm2/Vs and hole mobility = 850±400 cm2/Vs within 0.6 ps of excitation with 800nm radiation. Photoexcitation of GaAs with 400nm radiation produced THz emission consistent with hot carrier relaxation into the gamma conduction band valley with a time constant of 1.2±0.3 ps and a gamma valley mobility = 2600±500 cm2/Vs. In GaAs, this zero-background technique requires ~10 pJ/pulse photoexcitation and can be easily implemented with an unamplified Ti:S laser oscillator.
9:00 PM - K3.14
Active Optical Control of Reflectivity in the Terahertz Range
Ladislav Fekete 1 , Petr Kuzel 1 , Filip Kadlec 1 , Patrick Mounaix 2 Show Abstract
1 Dielectrics, Academy of sciences, Prague Czech Republic, 2 , CPMOH, Universite de Bordeaux I, Talence France
Generation and control of pulsed and continuous-wave terahertz radiation have received considerable attention during last years. As more efficient THz sensors and sources become available, there will be increasingly more research emphasis placed on manipulation of freely propagating THz beams for future technology. An area of particular interest is that of all-optical devices allowing transfer of information from the optical spectral band to the THz one: opto-THz switches, modulators and phase-shifters.High-resistivity semiconductors show a great potential for the opto-THz coupling. On one hand, in their ground state, they are transparent and virtually dispersion-free for the THz radiation. On the other hand, optically excited semiconductors exhibit a strong interaction with the THz light mediated by photo-excited carriers. A fine tuning of the strength of interaction by the intensity of optical excitation then may lead to a number of interesting phenomena exploitable for the THz-light modulation and switching.Illumination of a high-resistivity semiconductor wafer by a laser pulse leads to the generation of photo-carriers within the optical penetration depth d. For a low incident optical fluence, d is much smaller than the skin depth of the photo-excited sheet in the THz range: κ << λ/(4πd); n + iκ is the THz refractive index of the sheet and depends on the illumination conditions, λ is the probing THz wavelength. In this case the photo-excited layer acts as a shunting impedance between the bulk semiconductor and the air . Antireflective properties for the THz probing wave on the semiconductor / conductive sheet / air interface are then achieved for a specific photoexcited layer thickness and carrier density.If the incident optical fluence is high enough, such that κ > λ/(4πd), the photo-excited layer behaves like a metallic mirror and fully reflects the THz probing field with a phase shift equal to π.The experiments were performed on a semi-insulating GaAs and Si samples using an optical pump–THz probe scheme (see ref. 2 for details). Both amplitude and phase of the THz wave reflected on the structure under investigation are found to be strongly dependent on the intensity and wavelength of the optical pump beam. The wave exhibits a π/2-shift upon reflection close to an antireflecting regime and a change of the sign (π-shift) in the high-reflectivity regime. The thickness of the photo-excited layer is tuned by the wavelength of the optical illumination. The proposed structure can act as an optically controlled THz switch or as a phase shifter. J. Kroll, J. Darmo, and K. Unterrainer, Electron. Lett. 40, 763 (2004). L. Fekete, J. Y. Hlinka, F. Kadlec, P. Kuzel and P. Mounaix Opt. Lett. 30, 1992 (2005).
9:00 PM - K3.15
Anomalous Transmission through a Periodic Subwavelength Hole Array in Heavily Doped Conducting Polymer Films.
Tatsunosuke Matsui 1 , Z. Valy Vardeny 1 , Amit Agrawal 2 , Ajay Nahata 2 , Reghu Menon 3 Show Abstract
1 Physics Department, University of Utah, Salt Lake City, Utah, United States, 2 Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States, 3 Department of Physics, Indian Institute of Science, Bangalore India
Since Ebbesen et al. reported the phenomenon of “anomalous transmission” through optically thick metallic films perforated with two-dimensional (2D) subwavelength hole arrays, numerous studies have been carried out to explore both fundamental issues and potential device applications. So far, studies on “anomalous transmission” were carried out using metals and semiconductors. We report here the observation of “anomalous transmission” of terahertz (THz) radiation in 2D subwavelength hole array perforated on films of another, more exotic class of conductors, namely heavily-doped organic conducting polymers. This opens another exciting research and applications route. Using conducting polymers with properties that can be widely varied by chemical synthesis and doping, it would be possible then to fabricate a variety of novel optical devices with different characteristic properties and response compared to hole arrays in more traditional metallic and semiconductor films.For the metallic film we used a 25 μm thick free-standing film of heavily doped polypyrrole [PPy] with PF6 [PPy(PF6)] that was electrochemically polymerized; such a film is believed to be one of the most highly conducting polymers. The 2D hole array was fabricated on the film using a pulsed excimer laser machining system in a square lattice geometry. The film transmission spectrum was studied in the THz spectral range using a conventional THz time-domain spectroscopy (THz-TDS) technique from 50 GHz to 1 THz. With the perforated film we observed several transmission bands that were not present in the transmission spectrum of the film without the 2D hole array. The obtained transmission bands are in good agreement with a model involving surface plasmon polariton (SPP) excitations on the film surfaces. It has been postulated that heavily doped PPy(PF6) has two plasma frequencies, where the lower frequency seems to be caused by a Drude free electron dielectric response with a plasma frequency in the THz spectral range. Our findings show that SPP excitations can be photogenerated on the surfaces of metallic conducting polymers, and thus the metallic polymer films behave like regular metals. However when comparing the “anomalous transmission” spectra of the PPy(PF6) films with those of regular metals such as Ag, we found that the SPP in PPy(PF6) have much larger attenuation.From the viewpoint of device application our findings are quite exciting because of the possibility of in situ tunability. The doping process in conducting polymers is completely different from that of conventional semiconductors. Specifically, doping may proceed in situ, and when using an electrochemical process may be easily controlled by external field, biasing voltage, etc. Therefore we could control the conductivity of doped conducting polymers and consequently also the SPP transmission bands. With this goal in mind we will report the “anomalous transmission” spectra of PPy(PF6) films at various doping levels.
9:00 PM - K3.16
Experimental Study of Depolarization Effects in Polymer Granular Materials Using mm-wave Incoherent Signals.
Boris Kapilevich 1 , Boris Litvak 1 , Vladimir Vainstein 1 Show Abstract
1 Electrical and Electronics Eng., The College of J@S, Israeli Center for Radiation Sources and Applications, Ariel Israel
9:00 PM - K3.18
Fabrication and Characterization of Thin-Film Metal-Insulator-Metal Diode for use in Rectenna as Infrared Detector
Subramanian Krishnan 1 , Shekhar Bhansali 1 , Kenneth Buckle 1 , Elias Stefanakos 1 Show Abstract
1 Clean Energy Research Center, Dept. of Electrical Engineering, University of South Florida, Tampa, Florida, United States
Uncooled Infrared detectors with high sensitivity and shorter response times are preferred as through-the-wall detection device. An alternate approach for making these sensors, being pursued by us is to use the concept of rectenna with tunnel diodes. Successful fabrication of such high frequency switching diodes with antenna as detectors, offer a much faster response time than existing bolometer. This paper presents the fabrication and characterization of thin-film MIM diode for use in rectenna as an Infrared detector. MIM diodes operate on the basis of quantum mechanical phenomenon, i.e., when a sufficiently thin barrier (2-3nm) is sandwiched between two electrodes, current can flow between them by means of tunnelling. This tunnelling probability increases with a decrease in the dielectric barrier height and the separation distance. The MIM diodes were fabricated with asymmetric electrodes with 1µm2 contact areas with cut-off frequency ~0.1THz. The electrodes of the Ni-NiO-Cr MIM diodes have been fabricated through Photolithography, e-beam lithography, followed by conventional lift-off process. The dielectric layer (NiO) was deposited through plasma oxidation to obtain 2nm thin films. The composition and the thickness of the insulator layer are characterized by metrological tools like SEM, TEM, XPS, and Spectroscopic Ellipsometer. The diode characteristics presented in this paper have been found to be stable and reproducible with the established fabrication conditions. Electrical behaviour (I-V) of the MIM junctions were investigated and compared with the theoretical tunnelling characteristics of the Ni-NiO-Cr MIM diodes. For devices with such non-linear electrodes, excellent agreement is obtained between the measured and the calculated result with the forward bias current as 25.3mA at 0.5V and the reverse bias current as -0.707mA at -0.5V. The C-V characteristics of the device and its frequency roll-off will also be presented in the paper.
9:00 PM - K3.2
Characterization of the Dielectric Losses of BST Ferroelectric Thin Materialsby Terahertz Time Domain Spectroscopy.
Patrick Mounaix 1 , Mario Maglione 2 , Dominique Michau 2 , Filip Kadlec 3 , Petr Kuzel 3 Show Abstract
1 , CPMOH, talence cedex France, 2 , ICMCB, PESSAC France, 3 , Institute of Physics,Academy of Sciences of the Czech Republic, Prague Czech Republic
As a consequence of the presence of a soft mode, ferroelectric materials display a very high dielectric permittivity and strong dielectric losses in the sub-phonon frequency range. In the mm-and submm wavelength many discussions arouse as to the exact dispersion of dielectric parameters. Such debate is presented by comparing results obtained by several experimental set up in the case of BaTiO3 and (Ba,Sr)TiO3 (BST) thin films. To overcome the spurious contributions of interfaces and metallic electrodes, contact less techniques such as terahertz spectroscopy have been applied.In a very recent study, J.Petzelt et al.1) used THz spectroscopy with Fourier transform techniques to analyze the dielectric response of BaSrTiO3 alloy; they performed the analysis with particular attention to the soft mode behavior. It has been frequently established that ferroelectric and/or high-permittivity thin films present a smaller dielectric constant (permittivity) than bulk materials.In the present report, microwave investigations of BST films have been undertaken in two directions. The first requires the in-plane design of metallic electrodes to probe the impedance and tuneability of BST films. This coplanar geometry requires high-tech processing of metallic lines with a precise control of the interelectrode channel of micrometer size. Tuneability of the effective impedance of such structures has been effectively achieved but the actual in-depth dielectric parameters are yet to be quantitatively measured. The second will show that the high frequency dielectric parameters closely fit with the low frequency (f<1 MHz) dielectric parameters for a set of BST thin films.To this aim, results from terahertz time domain spectroscopy(100 GHz-2 THz) will be presented. The complex permittivity of the samples are highly sensitive to the film’s microstructure, stoechiometry and roughness. We will show a systematic comparison of the properties of BST films at low and high frequencies and quantitatively characterize BST films2). Our Terahertz spectroscopy data confirms this trend, leading to the encouraging high permittivity of about 300 in the frequency range under investigation.As several research groups, we have taken the composite route decrease the intrinsic losses of ferroelectric materials. However, standard composites obtained on mixing BaTiO3 or BST with dielectric substrates have several drawbacks such as interdiffusion during the sintering. In integrated structures, multilayers of BST alternating with amorphous SiO2 have been designed which resulted in an overall decrease of dielectric losses. These improvements have been obtained at low frequencies and need further confirmation in the mm and submillimeter frequency range.1 J. Petzelt, P. Kuzel, I. Rychetsky, Y, A. Pashkin, and T. Ostapchuk: Ferroelectrics 288 (2003) 169.2 P.Mounaix, M.Tondusson, L.Sarger, D.Michau, V.Reymond and M.Maglione.JJAP Vol 44 N° 7A 2005 p5058
9:00 PM - K3.3
Broadband Terahertz Study of Excitonic Resonances in the High-density Regime in GaAs/AlxGa1-xAs Quantum Wells.
Ben Schmid 1 2 , Rupert Huber 1 2 , Robert Kaindl 1 2 , Daniel Chemla 1 2 Show Abstract
1 Department of Physics, UC Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
The lowest energy excitation modes of a photogenerated electron-hole (e-h) system in a semiconductor are governed by density-dependent Coulomb interactions. At low temperatures and low densities, bound excitons form, whose fundamental excitation is the transition from 1s to 2p levels. Increasing density modifies the pair correlations, changing, for example, binding energies due to screening. Such effects have been intensely discussed because they are believed to underlie the excitonic Mott insulator-metal transition . Near-infrared (NIR) spectroscopy, while frequently employed, is a rather indirect technique monitoring the renormalization of single-particle states masked by intriguing many body effects. Terahertz (THz) spectroscopy, on the other hand, has been demonstrated to directly probe the internal excitonic fine structure, in the dilute limit .In our contribution, we report the first THz study of intraexcitonic transitions at high excitation densities in GaAs/AlxGa1-xAs multiple quantum wells . By applying ultrafast resonant excitation at the NIR 1s interband absorption line, we create pair densities in the range from 2x1010 to 2x1011 cm-2. Time-delayed THz pulses probe the internal 1s-2p transition. A strong redshift, broadening, and ultimately the disappearance of this resonance occur with increasing density. We model the THz response with a dielectric function of a 2D exciton population. With the inclusion of an additional Drude term, the model quantitatively reproduces both real and imaginary parts of the experimentally determined response, allowing us to gauge the densities of bound and unbound pairs. In the dilute limit, only excitons are generated. With increasing density, the share of unbound carriers becomes more significant. Beyond a critical value nc ≈ 2x1011 cm-2, photoexcitation results primarily in a population of unbound e-h pairs, with the excitonic resonance absent.In addition to the THz pulse, we use a NIR probe to study changes of the interband resonance. We observe a slight blueshift, as well as strong broadening and eventually bleaching of the 1s absorption line. By quantitative comparison of the THz response with the NIR transmission changes, we extract details of the density-dependent modification of the exciton fine structure. We believe that the combination of THz and NIR/visible spectroscopy will prove to be a powerful tool in future studies of dense spatially indirect excitons or interactions of strongly confined carriers in quantum dots. L. Kappei, J. Szczytko, F. Morier-Genoud, and B. Deveaud, Phys. Rev. Lett. 94, 147403 (2005). R. A. Kaindl, M. A. Carnahan, D. Hägele, R. Lövenich, and D. S. Chemla, Nature (London) 423, 734 (2003). R. Huber, R. A. Kaindl, B. A. Schmid, and D. S. Chemla, Phys. Rev. B 72, 161314(R) (2005).
9:00 PM - K3.4
Versatile terahertz radiation functionality of photoconductive devices fabricated on multiferroic BiFeO3 thin films.
Kouhei Takahashi 1 , Noriaki Kida 2 , Masayoshi Tonouchi 1 Show Abstract
1 , Institute of Laser Engineering, Osaka University, Suita, Osaka, Japan, 2 , ERATO-SSS, Tsukuba, Ibaraki, Japan
Ferroelectric materials are known to provide a variety of optical functionalities owing to their nonlinear response to light. Terahertz (THz) radiation via optical rectification in ferroelectrics is one such example, which can be observed within satisfaction of a severe phase matching condition under illumination of high intensity laser pulses in the transparent region. Another common approach for THz radiation is a method that utilizes voltage-biased photoconductive switches fabricated on insulating materials. In this photoconductive approach, the material shall not be transparent for it requires carrier excitation via illumination of laser pulses, and hence, THz radiation can be observed from all sorts of insulators if the photon energy of the femtosecond laser exceeds the energy gap of the material. Unfortunately, many of the ferroelectrics do not satisfy this condition since their energy gap generally lies high above the visible region. However, perovskite BiFeO3, which has recently attracted much attention for exhibiting simultaneous ferroelectricity and antiferromagnetism in the same phase, can overcome this impediment. Due to its multiferroic nature, BiFeO3 exhibits a relatively small energy gap as a ferroelectric and allows carrier excitation by utilizing commercially available femtosecond lasers.In the present work, we have developed a new terahertz radiation functionality of photoconductive switches fabricated on BiFeO3 thin films via carrier excitation upon illumination of femtosecond laser pulses. We confirmed that the radiated THz pulse originates from an ultrafast modulation of the spontaneous polarization. As a consequence, THz radiation was observed even under zero-bias electric field. This is in contrast with the ordinary cases observed in conventional semiconductors. Additionally, the phase of the radiated THz pulse at zero-bias electric field depended on the polarity of the initially applied bias electric field, which expresses the memory effect peculiar to ferroelectric materials. This feature provides potential approach to various applications such as nondestructive readout for nonvolatile ferroelectric memory devices and ferroelectric domain imaging microscopy. Detailed THz radiation characteristics of BiFeO3 photoconductive switches along with the demonstrations on the above-mentioned applications will be presented at the meeting.
9:00 PM - K3.5
The Influence of Stability of Charge-ordering to the Terahertz Radiation Characteristics of Pr1-xCaxMnO3 (x = 0.3, 0.5) Thin Films.
Kosuke Terasaka 1 , Kouhei Takahashi 1 , Masayoshi Tonouchi 1 Show Abstract
1 , Institute of Laser Engineering, Osaka University, Suita, Osaka, Japan
9:00 PM - K3.6
Simultaneous Determination of Dielectric and Magnetic Properties by Time-Domain Terahertz Spectroscopy.
Hynek Nemec 1 , Filip Kadlec 1 , Kuzel Petr 1 , Lionel Duvillaret 2 , Jean-Louis Coutaz 3 Show Abstract
1 , Institute of Physics, Czech Academy of Sciences, Prague Czech Republic, 2 , Institut de Microelectronique, Electromagnetisme et Photonique, Grenoble France, 3 , Laboratorie d'Hyperfrequences et de Caracterisation, Universite de Savoie, Le Bourget du Lac France
Since the time-domain THz spectroscopy (TDTS) has been proposed, this technique has known an impressive soaring in many branches of physics, namely in determination of steady-state dielectric properties of materials . Up to now, there are few studies focusing on the characterization of magnetic properties in the THz range . Most of them postulate that the system exhibits only a dielectric or only a magnetic behavior, but not both at the same time, therefore the refractive index and wave impedance of materials are not considered as independent quantities.At the same time, there is a rapidly growing interest in metamaterials and especially in so-called left-handed materials . Despite the discrete nature of these artificial materials on the sub-wavelength scale, they can be considered as continuous media on the scale of the probing wavelength. Consequently, they can be described by effective dielectric and magnetic susceptibilities which exhibit a dispersion within the same spectral range. Indeed, in this case the refractive index and the wave impedance are independent spectral functions, the left-handedness being attributed to the negative sign of the refractive index. Thus, the simultaneous determination of these two complex quantities is essential to demonstrate the left-handedness of a metamaterial.In this contribution we present methods for a simultaneous determination of the refractive index n and wave impedance z of bulk samples taking advantage of the temporal windowing procedure that TDTS offers. Indeed, for sufficiently thick samples internal Fabry-Pérot reflections appear as mutually delayed pulse echoes in the transmission or reflection time-domain wave forms. It is then possible to determine experimentally the transmittance spectrum Tm (or the reflectance spectrum Rm) corresponding to the echo leaving the sample after 2m internal reflections. Two independent complex spectroscopic quantities required for an unambiguous evaluation of n and z can be chosen among Tm's and Rm's. We analyze quantitatively three cases based on particular choices of the measured spectra and perform their comparative test. We show that successful evaluation of the dielectric and magnetic dispersion crucially depends on the accuracy of the wave impedance measurements.References. G. Gruner (ed.), Millimeter and Submillimeter Wave Spectroscopy of Solids, (Springer-Verlag Berlin Heidelberg, 1998). G. Srinivasan, and A. N. Slavin (eds.), High Frequency Processes in Magnetic Materials, pp. 56–97 (World Scientific, Singapore, 1995). S. A. Ramakrishna, Rep. Prog. Phys., 68, 449 (2005).
9:00 PM - K3.7
Ultrafast Photoconductivity in Organic Semiconductors
Oksana Ostroverkhova 1 2 , David Cooke 2 , Svitlana Shcherbyna 2 , Ray Egerton 2 , Frank Hegmann 2 , Rik Tykwinski 3 , John Anthony 4 , Vitaly Podzorov 5 , Michael Gershenson 5 , Oana Jurchescu 6 , Thomas Palstra 6 Show Abstract
1 Physics, Oregon State University, Corvallis, Oregon, United States, 2 Physics, University of Alberta, Edmonton, Alberta, Canada, 3 Chemistry, University of Alberta, Edmonton, Alberta, Canada, 4 Chemistry, University of Kentucky, Lexington, Kentucky, United States, 5 Physics, Rutgers University, Piscataway, New Jersey, United States, 6 Solid State Chemistry Lab, University of Groningen, Groningen Netherlands
Organic semiconductors are of interest due to their potential applications in molecular electronics and photonics. In order to develop organic semiconductor devices, it is important to understand the mechanisms of conductivity in organic materials. However, using thin-film device structures to characterize intrinsic electronic properties of the materials is complicated due to the effects of metal-organic interfaces at contacts, device geometry, and defects on the overall electronic response of the device. Optical pump – terahertz (THz) probe time-resolved spectroscopy allows probing of the transient photoconductivity with sub-ps time resolution and, therefore, represents a sensitive non-contact tool for studying the dynamics of mobile charge carriers before they are trapped at defect sites, i.e. assesses intrinsic electronic properties of materials. We employed this technique to probe transient photoconductivity in pentacene (Pc), functionalized pentacene (FPc), rubrene (Rub), and tetracene (Tc) single crystals and pentacene and functionalized pentacene thin films. In all samples, we observe fast (<400 fs) charge carrier photogeneration and band-like charge transport characterized by an increase in charge carrier mobility as the temperature is lowered from 300 K to 10 K. At room temperature (RT), the product of charge carrier mobility (μ) and photogeneration efficiency (η) calculated from the peak value of the differential transmission of the THz probe yielded μη~0.3-0.35, 0.15-0.2, 0.05-0.06, and 0.03 cm2/(Vs) for Pc, FPc, Rub and Tc crystals, respectively. In thin films at RT, the μη is 0.02-0.04 cm2/(Vs)in Pc films and 0.01-0.06 cm2/(Vs)in FPc films, depending on the structure and morphology of the films. The decay dynamics of the transient photoconductivity provide information about charge carrier recombination and trapping processes. In all single crystal samples at RT, the decay dynamics are characterized by a power-law function (Δt)(-β) with β=0.5-0.7. As the temperature decreases, the decay dynamics remain unchanged in FPc crystals, but become significantly faster in all other crystals at temperatures below 70 K due to charge trapping at shallow structural traps intrinsic for herringbone-type Pc, Rub, and Tc crystals. Out of all thin films studied, the best-performing photoconductive films were polycrystalline FPc films prepared by thermal evaporation at a heated substrate or grown from solution. These films exhibited photoconductivity transients reaching 30-40% of those in FPc single crystals as well as similar power-law decay dynamics as single crystal samples over at least ~100 ps. In Pc films and FPc films with small grain size, smaller photoconductivity transients as well as single or bi-exponential decay dynamics with the fast time constants on the order of 1 ps were observed. We also report on terahertz pulse generation from voltage-biased Pc thin films.
9:00 PM - K3.8
Terahertz Resonances in the Dielectric Response due to Second Order Phonons in a GaSe Crystal.
Baolong Yu 1 , Hakan Altan 1 , Vladimir Kartazayev 1 , Fanang Zeng 1 , Robert Alfano 1 , Krishna Mandal 2 Show Abstract
1 Institute for Ultrafast Spectroscopy and Lasers, Department of Physics, City College of New York, New York, New York, United States, 2 EIC Laboratories, INC., 111 Downey Street, Norwood, Massachusetts, United States
9:00 PM - K3.9
Terahertz Applications and Switching in Amorphous-carbon Quantum Well Structures.
Somnath Bhattacharyya 1 , L Rojas 1 , S Silva 1 Show Abstract
1 , ATI, University of Surrey, Guildford United Kingdom
Oleg Mitrofanov Bell Laboratories, Lucent Technologies
Xi-Cheng Zhang Rensselaer Polytechnic Institute
Richard Averitt Los Alamos National Laboratory
Kazuhiko Hirakawa University of Tokyo
Alessandro Tredicucci NEST-INFM
K4: THz Devices
Friday AM, April 21, 2006
Room 2016 (Moscone West)
9:30 AM - **K4.1
Terahertz Quantum Cascade Lasers and Real-time T-rays Imaging.
Qing Hu 1 Show Abstract
1 EE and Comp. Sci. and Research Lab. of Electronics, MIT, Cambridge, Massachusetts, United States
Based on a robust THz gain medium that rapidly depopulates the lower state using resonant LO-phonons and a novel metal-metal waveguide structure for mode confinement, we have developed THz quantum-cascade lasers with many performance records. These include but not limited to: the highest pulsed operating temperature of 164 K, the highest CW operating temperature of 117 K, the longest wavelength of 161 µm, and the highest output power levels of ~250 mW. Using these lasers and a 320x240 pixel focal-plane array camera, real-time THz imaging is performed at a rate of 20 frames/second. We will discuss more details and our perspective for further improvement of the laser performance based on phonon engineering at the conference.
10:00 AM - K4.2
GaAs-based Quantum Cascade Laser for Dual Frequency Terahertz Emission.
Aaron Andrews 1 2 3 , Gernot Fasching 2 3 1 , Alexander Benz 2 3 1 , Tomas Roch 3 1 , Werner Schrenk 1 3 , Karl Unterrainer 2 3 1 , Gottfried Strasser 3 1 Show Abstract
1 Center for Micro- and Nanostructures, TU Vienna, Vienna, Wien, Austria, 2 Institute for Photonics, TU Vienna, Vienna, Wien, Austria, 3 Institute for Solid State Electronics, TU Vienna, Vienna, Wien, Austria
10:15 AM - **K4.3
Progress on Terahertz Quantum Cascade Lasers
Stefano Barbieri 2 1 , Jesse Alton 1 , Chris Worral 4 , Mark Houghton 1 4 , Owen Marshall 1 4 , Sukhdeep Dhillon 2 3 , Alfredo de Rossi 3 , Michel Calligaro 3 , Carlo Sirtori 2 3 , Harvey Beere 4 , David Ritchie 4 Show Abstract
2 Materiaux et Phenomenes Quantiques, University Denis Diderot, - Paris 7, Paris France, 1 Quantum Devices, Teraview, Cambridge United Kingdom, 4 Semiconductor Physics, Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 3 Optoelectronics, Thales Research and Technology, Orsay United Kingdom
Quantum Cascade Lasers (QCLs) operating in the far-infrared or Terahertz (THz) region of the electromagnetic spectrum have taken important steps forward during the last couple of years . Operation up to 164K in pulsed mode and up to 113K in continuous wave has been recently demonstrated at 3.0 THz. Lasing at frequencies as low as 1.45 THz has been reported under an external magnetic field perpendicular to the hetero-layers, revealing exciting new physics in a regime of strong electronic localisation. The demonstration of distributed feedback THz QCLs is opening the door to sensing applications and to the use of these sources as local oscillators for heterodyne receivers to perform high-resolution spectroscopy in outer space. Finally, the potential of THz QCLs for imaging applications is being assessed by several research groups.The first part of the talk will focus on our latest results on THz QCLs based on superlattice active regions. First, their design and operation will be reviewed based on a device emitting at 2.9 THz. In particular their transport characteristics will be put under spotlight, by stressing the importance of limiting the current flow before the onset of miniband alignment in order to reduce the laser threshold. We will show that addressing this issue is a crucial step for the achievement of lasing at even lower frequencies. The characteristics of the 2.9T Hz device will be compared with those of a recently demonstrated QCL operating at 1.9 THz, the lowest frequency reported to date without the use of an intense external magnetic field. The active region is based on a GaAs/AlGaAs heterstructure with 10% Al concentration, and lasing action takes place between an isolated subband and the upper state of a 14 meV wide superlattice miniband. In pulsed mode, for a 3.16 mm long device we report a threshold current density of 113 A/cm2 at T = 4K, with a maximum measured peak power of 50 mW. The device lases up to 77K. Operation in continuous wave is reported up to 47K, with a maximum power of 20 mW at T = 4K. The second part of the talk will be centred on novel waveguide designs. Initially, a buried double-metal waveguide allowing the realisation of QCLs with ultra-low threshold currents and improved thermal dissipation will be presented. Next a new concept of loss-coupled distributed feedback laser will be demonstrated, based on the tunnelling of the guided surface plasmon mode across a metal grating, of varying thickness and composition, placed on top of the active region. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser”, Nature 417, 156 (2002)
10:45 AM - K4.4
Negative Differential Conductivities in Bulk GaAs in the Terahertz Frequency Range.
YiMing Zhu 1 , Norihiko Sekine 1 , Kaz Hirakawa 1 Show Abstract
1 Institute of Industrial of Science, University of Tokyo, Tokyo Japan
11:00 AM - K4:Devices
11:30 AM - K4.5
THz-Photomixer Based on Vertical Quasi-Ballistic Transport in GaAlAs nipnip superlattices with ErAs- or LT-GaAs-recombination layers.
Gottfried Doehler 1 , F. Renner 1 , S. Preu 1 , M. Eckardt 1 , A. Schwanhausser 1 , S. Malzer 1 , D. Driscoll 2 , M. Hanson 2 , G. Loata 3 , T. Loeffler 3 , A. C. Gossard 2 , H. Roskos 3 , L. Wang 1 Show Abstract
1 Max-Planck Research Group of Optics, Information and Ohotonics, University of Erlangen, Erlangen Germany, 2 Materials Department, University of California, Santa Barbara, California, United States, 3 , Physik. Inst. J.W. Goethe-University, Frankfurt Germany
The standard photoconductive photomixers based on low-temperature-grown- (LT-)GaAs or GaAs with ErAs islands take advantage of the short recombination lifetime of photo-generated carriers in these materials . These short recombination lifetimes (ca. 150 fs) determine the roll-off frequency of about 1 THz and limit the photoconductive gain to values < 10-2. This implies very low conversion efficiency. In pin-photomixers the photogenerated carriers fully contribute to the photocurrent. However, they still suffer from RC- and transittime-roll-offs, even for the most advanced version, the “uni-traveling-carrier” (UTC) mixers . We have recently developed a new concept for THz-photomixers , which overcomes these (intrinsic) drawbacks. In this contribution we present the concept and recent experimental results. Our photomixers consist of a periodic sequence of nano-pin-diodes (forming a “nipnip-hetero-superlattice” with a bandgap grading in the pin diodes). Electron-hole generation in the pin diodes takes place in the region of lowest bandgap close to the p-layers. Thus, the photo-current is mostly due to the (light) electrons. In order to optimize both the THz-photo-current and the THz-photo-voltage, the length of the intrinsic layer is chosen such, that the i-layer thickness corresponds to the maximum distance over which an electron can propagate quasi-ballistically within a time of flight corresponding to about ½ THz-period. By appropriate choice of the number of superlattice periods the capacitance of the mixer can be adjusted independently to avoid RC roll-off. For the realization and optimized performance of the photomixer it is crucial that the electrons and holes generated in the nano pin-diodes and accumulating in the n- and p-layers, respectively, screen the built-in electric field such that the potential drop in the pin-diodes, ΔVe, oscillates around the value at which intervalley scattering sets in. This implies extremely high recombination current densities of about 10 to 100 kA/cm2 at a photo-induced “internal forward bias” of about 1 V in the “recombination-pn-diodes” (whereas the external bias is zero or close to zero!). We achieve these high current densities by introducing (quasi-metallic) ErAs- or LT-GaAs recombination layers into these diodes. Our experimental results confirm that under optimized operating conditions the transport in our mixers, in fact, becomes ballistic. J. E. Bjarnason, et al. APL 85, 3983 (2004) H. Ito, et al.,, Semicond. Sci. Technol. 20, S191–S190 (2005) G.H. Döhler, et al., Semicond. Sci. Technol. 20, S178–S190 (2005)
11:45 AM - K4.6
Quantum Dot Molecules: Time-integrated and Time-resolved Photoluminescence Studies.
William Kerr 1 , Valeria Stoleru 1 , Anup Pancholi 1 Show Abstract
1 , University of Delaware, Newark, Delaware, United States
12:00 PM - K4.7
Er-Doped Silicon Nanolayers For Generation of Terahertz Radiation.
S. Minissale 1 , I. Izeddin 1 , H. Vrielinck 2 1 , Tom Gregorkiewicz 1 Show Abstract
1 , University of Amsterdam, Amsterdam Netherlands, 2 , Ghent University, Ghent Belgium
The THz frequency range remains relatively unexplored due to difficulties in both generation and detection. Optical doping with rare earth (RE) ions has been widely used for development of optical materials and devices. RE doped matrices have also been considered for generation of THz radiation. In that case the idea was to use transitions between individual levels within a single spin-orbit multiplet split by Stark effect of the local crystal field. Er-doped crystalline silicon would offer the unsurpassed advantage of a possibility to build upon and to integrate with the silicon platform. Unfortunately, Si:Er could not be used for THz generation due a large width, and therefore mutual overlap, of emission lines from individual crystal-field split levels of Er. The large linewidth was traced back to multiplicity of Er-related optical centers simultaneously present in Er-doped silicon matrix. Recently, we were able to show that in Si/Si:Er nanolayers grown by sublimation MBE a single type of an Er-related optical center is preferentially formed [1,2]. Measured at a low temperature, the photoluminescence spectrum of this center is characterized by a set of ultra narrow lines with separation of an order of 100 cm-1. That finding opens a prospect for development of a Si:Er–based THz emitter.In the contribution we will review properties of Si:Er nanolayers with respect to generation of THz radiation. We will discuss results of a preliminary experiment in which we have investigated relaxations between Stark-effect-split levels within the ground (4I15/2) and the first excited (4I13/2) state multiplets of Er3+ ion. The experiment conducted at room temperature in Cs2NaYF6 insulating host (where, similarly as in Si/Si:Er nanolayers, Er3+ occupies a well-defined site) revealed that the relaxation process was characterized by a time constant of 2-4 µs. While one cannot be sure that the relaxation process will be similarly slow also in Si nanolayers, this result is certainly encouraging for the prospect of reaching the population inversion. N.Q. Vinh, H. Przybyliñska, Z.F. Krasil’nik, and T. Gregorkiewicz, Phys. Rev. Lett. 90, 066401:1-4 (2003). N.Q. Vinh, H. Przybyliñska, Z.F. Krasil’nik, and T. Gregorkiewicz, Phys. Rev. B 70, 115332:1-11 (2004).
12:15 PM - K4.8
Ultrafast THz Detection Using Self-Assembled ErAs Nanoislands.
John O'Hara 1 , R. Prasankumar 1 , D. Yarotski 1 , J. Zide. 2 , A. Gossard 2 , A. Taylor 1 , R. Averitt 1 Show Abstract
1 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Materials Department, Univeristy of California-Santa Barbara, Santa Barbara, California, United States
We assess the potential of a self-assembled ErAs:GaAs nanoisland structure for THz detection using optical-pump THz-probe spectroscopy and test THz detection based on this material. Optical efficiency, bandwidth, and saturation behavior are compared among ErAs:GaAs, radiation-damaged silicon-on-sapphire (RD-SOS), and low-temperature grown GaAs (LT-GaAs) based photoconductive detectors.A continuing focus of current research in ultrafast optoelectronics is the development of new sources and detectors at terahertz (THz) frequencies. Traditionally, THz detectors based on photoconductive (PC) antennas have utilized low-temperature grown GaAs [1,2] and radiation- damaged silicon-on-sapphire  due to their fast carrier trapping times. However, the development of self-assembled ErAs nanoislands embedded in a GaAs matrix  offers a particularly useful alternative for THz PC devices based on the ability to independently tune photo-excited carrier lifetimes, trap density, and dark resistance .In this work, we use optical-pump THz-probe spectroscopy to compare the optically induced time-dependent THz conductivity upon 800 nm excitation for RD-SOS, LT-GaAs, and ErAs:GaAs superlattice substrates. The ErAs nanoisland sample used in this study was grown by MBE on a semi-insulating (100) GaAs substrate . The sample has 80 superlattice periods, each consisting of 1.2 monolayers of ErAs and 25 nm of GaAs . Optical-pump THz-probe experiments were performed in vacuum using a regeneratively amplified Ti:sapphire laser system operating at 800 nm. Measurements of the optically induced change in the THz-probe electric field permit extraction of the conductivity and carrier capture time for the ErAs:GaAs superlattice, found to be about 839 fs. This is roughly equivalent to that of RD-SOS .For further comparison, we then fabricate three nearly identical THz PC antenna detectors, one made from each substrate and compare their performance in a typical THz time-domain spectroscopy (THz-TDS) system. The detectors were installed and tested in a carefully aligned confocal THz-TDS system similar to that found in ref. . Our results indicate that short THz pulses can be detected with equivalent signal-noise ratio, enhanced bandwidth, and significantly enhanced optical efficiency using ErAs:GaAs substrates. Efficiency gain is observed to decrease with higher optical fluences due to the more rapid saturation behavior in the ErAs:GaAs based detectors. References1) M. C. Nuss and J. Orenstein, in Millimeter and submillimeter wave spectroscopy of solids (Springer, Berlin, 1998).2) M. C. Beard, G. M. Turner, and C. A. Schmuttenmaer, J. Appl. Phys. 90, 5915 (2001).3) N. Katzenellenbogen and D. Grischkowsky, Appl. Phys. Lett. 58, 222 (1991).4) C. Kadow, S. B. Fleischer, J. P. Ibbetson, J. E. Bowers, A. C. Gossard, J. W. Dong and C. J. Palmstrom, Appl. Phys. Lett. 75, 3548 (1999).
12:30 PM - **K4.9
Broadband and Tunable Terahertz Detection Using High-Mobility GaAs-AlGaAs Heterostructures.
Mark Lee 1 , Eric Shaner 1 , Michael Wanke 1 , Albert Grine 1 , John Reno 1 , S. Allen 2 Show Abstract
1 , Sandia National Laboratories, MS 1415, New Mexico, United States, 2 Center for THz Science & Technology, Univ. of California Santa Barbara, Santa Barbara, California, United States
THz radiation can couple to a very high mobility (106 to 107 cm2/V-s at 4 K) electron gas in a III-V semiconductor heterostructure interface or quantum well in two distinct manners that are both useful in direct and heterodyne mixing detection. The first and more conventional response is based simply on electron drift, while the second response is for the radiation to resonantly excite coherent charge density oscillations, or plasmon modes, of the electron gas. We will show examples and contrast the fundamental limitations on frequency response and mixing characteristics resulting from the different physics of the two responses. We have shown that hot-electron bolometer (HEB) mixers made from a very high-mobility two-dimensional electron gas (2DEG) can enter a regime where neither energy nor momentum relaxes in a channel between source and drain electrodes, leading to ballistic electron drift. For a source-drain channel of length L, this minimizes the charge transit time and hence maximizes the mixers' intermediate frequency (IF) bandwidth to f3dB = vF/2πL, where vF, the Fermi velocity, is typically ~ 107 cm/s in a 2DEG. Even for L > 1 micron, IF bandwidths approaching 40 GHz have been observed. The IF spectrum of such an HEB mixer has very low harmonic distortion, as expected for the square-law non-linearity of a bolometer, while the responsivity and conversion gain are generally low, owing to the small temperature coefficient of resistance of the high-mobility 2DEG. In addition, in the ballistic electron drift regime the 2DEG has kinetic inductance that will degrade coupling to the electromagnetic field as local oscillator (LO) frequency increases. We estimate that, for practical antenna/mixer geometries and electron densities, the inductive reactance will set an upper limit on LO frequency coupling of roughly 0.5 THz.By contrast, the plasmon response we have observed is resonant and not limited by kinetic inductance. Using a grating-gated field-effect transistor geometry, resonant plasmon response has been observed from 0.1 to 1 THz in various devices. , In a single device, the resonant frequency of the response can be tuned continuously over a ~ 0.1 THz range by an applied gate voltage bias. Heterodyne experiments show that, unlike an electron-drift device, the IF bandwidth of this detector operated as a mixer is not limited by the plasmon transit time, but more likely by the plasmon lifetime. The measured IF bandwidth of approximately 8 GHz is in rough agreement with the half-width of the observed plasmon resonance peak. The IF spectrum of the plasmon mixer also shows significant harmonic content, indicating that the non-linear mechanism generating the IF is significantly more complicated than a simple square-law. We are also investigating plasmon detector configurations that could significantly improve responsivity and conversion gain and minimize detector noise.
K5: Ultrafast Processes and Material Properties at THz Frequencies
Friday PM, April 21, 2006
Room 2016 (Moscone West)
2:30 PM - **K5.1
Ultrafast THz Probes of Excitonic and Superconducting Correlations in Solids.
Robert Kaindl 1 2 , Marc Carnahan 1 2 , Daniel Haegele 1 2 , Seongshik Oh 3 , James Eckstein 3 , Ben Schmid 1 2 , Rupert Huber 1 2 , Daniel Chemla 1 2 Show Abstract
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Department of Physics, University of California, Berkeley, California, United States, 3 Department of Physics, University of Illinois, Urbana, Illinois, United States
Coulomb interactions between the large number of constituents in a many-particle system can generate correlated states with fundamentally new physical properties. Ultrafast time-resolved spectroscopy is a powerful tool to study key microscopic processes in such systems by following the temporal relaxation kinetics of nonequilibrium quasiparticles. Transient insulating, conducting, or superconducting phases can be identified by following their characteristic THz-frequency elementary excitations. Here, we will discuss experiments that employ ultrashort coherent THz pulses and direct field-resolved detection to probe time-varying correlations of charge carriers in semiconductors and superconductors. In bulk and nanostructured semiconductors, metastable excitonic states and unbound e-h pairs are selectively photoexcited. The ensuing THz dielectric response reveals characteristic transitions between internal exciton levels, much in analogy to intra-atomic transitions but in a completely different frequency range. The difference between the insulating response of an exciton gas (epitomized by its lowest-energy 1s-2p excitation) and the conducting nature of unbound charges provides for new vistas of exciton dynamics. A metal-insulator transformation is observed in time which occurs upon formation of excitons. We can trace the intra-excitonic transition spectrum as a function of temperature, density, excitation energy, and time. Changes in the THz dielectric response can be directly and quantitatively linked to the photoexcited pair densities, allowing a probe of excitonic phase diagrams. In the high-Tc superconductor Bi-2212, the THz-frequency electromagnetic response couples directly to Cooper pairs and quasiparticle excitations. We observe transient changes in the THz conductivity that occur after ultrafast depletion of the superconducting condensate. The temporal decay reveals a bimolecular kinetics of quasiparticle recombination that occurs as Cooper pairs form on a picosecond timescale. The experiments discussed here trace correlated pair states via their transient low-energy response, motivating the use of coherent broadband THz pulses in further studies of microscopic processes and collective excitations in solids.
3:00 PM - K5.2
Generation of Strong Short Terahertz Pulses via Stimulated Raman Adiabatic Passage-assisted Coherent Scattering.
Nikolai Kalugin 1 2 , Yuri Rostovtsev 1 2 , Marlan Scully 1 2 3 Show Abstract
1 Physics, Texas A&M University, College Station, Texas, United States, 2 , Institute for Quantum Studies, TAMU, College Station, Texas, United States, 3 Mech. & Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
3:15 PM - K5.3
THz Quantum Optics in Excitons: Stimulated Emission from Internal Exciton Transitions in Cu2O.
Rupert Huber 1 2 , Ben Schmid 1 2 , Y. Shen 1 2 , Robert Kaindl 1 2 , Daniel Chemla 1 2 Show Abstract
1 Department of Physics, UC Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Excitons are often viewed as the low-energy counterpart of the hydrogen atom: one electron bound to one hole via Coulomb attraction. This analogy motivates the search for atomic-like quantum optical processes in excitons. Ultrashort mid infrared and THz pulses have been shown to induce resonant absorptive Lyman transitions from the 1s bound exciton state to higher bound and unbound levels [1,2]. Theoretical studies have suggested how the underlying elementary quantum processes could be reversed to induce terahertz gain from inverted exciton populations [3,4]. Unlike atoms, excitons are transient quasiparticles embedded in the background of a crystal lattice. Nonradiative relaxation channels and THz absorption into higher bound and continuum states can thus strongly compete with stimulated emission and are poorly understood. Up to now, emission of THz radiation connecting internal exciton levels has not been reported.In our contribution, we present the first direct observation of stimulated emission of electromagnetic radiation from intra-excitonic transitions . Ultrashort visible light pulses photoexcite 3p excitons in the semiconductor Cu2O (T = 6 K) via resonant interband absorption. At a variable femtosecond delay time after the pump pulse, a broadband THz transient probes internal dipole transitions into other exciton states. Measuring the field resolved electromagnetic response of the non-equilibrium system via electro-optic sampling we gain direct access to both the pump induced refractive index and absorption changes. Ultrafast dynamics, line widths, positions, and oscillator strengths of various internal exciton transitions are directly mapped out. Stimulated THz emission from the 3p-2s downward transition manifests itself via a negative change of the absorption coefficient and a characteristic dispersion feature of the refractive index at the resonant photon energy of 6.6 meV. The effect decays within 1.3 ps explained by electron-hole recombination and phonon scattering. We extract a stimulated emission cross section of 10-14 cm2 per 3p exciton, in good qualitative agreement with theoretical estimations . The picture is further corroborated by the independent observation of efficient THz emission at this energy due to 2s-3p quantum beats. Our results demonstrate a new fundamental process of THz quantum optics and highlight differences and analogies between excitons and atomic systems.R. A. Kaindl et al., Nature 423, 734 (2003).M. Kubouchi et al., Phys. Rev. Lett. 94, 016403 (2005).S. Nikitine, J. Phys. Chem. Solids 45, 949 (1984).M. Kira et al., Phys. Rev. Lett. 93, 076402 (2004).R. Huber et al., submitted.
3:30 PM - K5.4
Compact High-resolution THz Spectrometer with kHz Scan Rates.
Albrecht Bartels 1 2 , Arne Thoma 1 , Christof Janke 1 2 , Thomas Dekorsy 1 , Andre Dreyhaupt 3 , Stephan Winnerl 3 , Manfred Helm 3 Show Abstract
1 Department of Physics, University of Konstanz, Konstanz Germany, 2 , Gigaoptics GmbH, Konstanz Germany, 3 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, Dresden Germany
We demonstrate a compact and rapid scanning high-resolution THz spectrometer that is capable of acquiring THz field transients of 1 ns duration without mechanical delay line. The system is based on two Ti:sapphire femtosecond lasers whose 1-GHz repetition rates are linked with a fixed difference ΔfR (Gigajet TWIN and stabilization unit TL-1000, Gigaoptics GmbH, Germany) in order toperform high-speed asynchronous optical sampling (ASOPS). The separate lasers provide two pulse trains whose time-delay is linearly ramped between zero and the inverse laser repetition frequency (i.e. 1 ns) at a rate given by fR. One laser drives a high-efficiency large-area GaAs-based THz emitter. The other laser is used for time-resolved electro-optic detection of the THz-field. The use of femtosecond lasers with 1 GHz repetition rate is essential for high-speed ASOPS in order to obtain rapidscanning in the multi-kHz regime and a high time-resolution (i.e. a high frequency-bandwidth) at the same time.The spectrometer scans THz field transients of 1 ns duration at a rate of 9 kHz with a resolution of 230 fs in the time domain. The data acquisition system allows to display averages of 1024 single scan transients at a rate of 4 Hz on the computer screen, making 'on-line' signal observation and optimization possible. By Fourier transformation of the time-domain data, THz spectra at frequencies up to 3 THz are obtained with a resolution of 1 GHz. As an example, we have measured absorption spectra of water vapor under atmospheric pressure. Absorption lines are observed with >25 dB signal-to-noise ratio within only 25 s of measurement time. 45 dB signal-to-noise ratio is obtained after 2500 s of averaging. The observed linewidth is 11 GHz, limited by saturation and collision broadening.Our compact and rapid scanning high-resolution THz spectrometer has great potential to advance the field of THz-time-domain spectroscopy in general. Due to the absence of mechanical delay lines, it allows to significantly reduce data acquisition times for a wide variety of applications. It opens the way for monitoring of material properties and detection of specific spectroscopic molecular fingerprints with a high-throughput andhas the potential to lead to much accelerated frame acquisition in THz imaging applications.
3:45 PM - K5.5
Highly-Efficient Broadband THz Modulation by Optically Excited Carriers in Si Structure.
I-Chun Chen 1 , Canan Karaalioglu 1 , Martin Brucherseifer 2 , Rainer Martini 1 , Azza Meshal 3 Show Abstract
1 Physics and Engineering Physics, Stevens Institute of Technology, Hoboken, New Jersey, United States, 2 , Georgia Institute of Technology, Atlanta, Georgia, United States, 3 US Army RDECOM, United States Army, Fort Monmouth, New Jersey, United States
With growing interests in the terahertz regime, THz sources, detectors and measurement techniques have been widely discussed [1,2]. And as the scientific community continues to pursue its plan to develop near future THz systems and devices, attention has turned to developing beam controlling optics. Here, an optically controlled modulation scheme of broadband THz radiation using a moderately doped Si-based modulator is presented as one of the key ingredient to a successful THz communication system. Unlike T. Klien-Ostamann et al. , experimental data acquired from a terahertz time-domain spectroscopy setup will focus on high modulation depth (>98%) as well as a uniform broadband response (250GHz to 3.5THz). Special attention was taken to determine the effects of comparably uniform spatial distribution, verified by knife edge method, for the THz spot size as well as the carrier diffusion profile generated by an 800nm CW source. It should be noted that the use of dope silicon, CW-radiation and THz transmission measurements distinguishes this investigation from previously published works [4,5]. For spectral ranges between 50GHz to 3.5 THz, the technically important 3dB attenuation is achieved at only 5mW and 99% power attenuation at 100mW of optical excitation power. This raises the bar in THz attenuation performance and depth. And from fitted the results with Drude behavior, an experimental description for the effect of optically generated carriers (N) on the system dampening rate (Γ) is shown for the first time. Furthermore, possible applications for spatially inhomogeneous charge distribution in THz systems will be introduced and the possibility of high-speed modulation based on frequency dependent modulation measurements will be discussed.Keywords: THz attenuation, Drude theory, modulation, optically excited semiconductors, THz-TDS Bradley Ferguson and Xi-Cheng Zhang, Materials for terahertz science and technology, nature materials, 1, 2002 P. Haring Bolivar, Coherent Terahertz Spectroscopy, Institut fur Halbleitertechnik II, RWTH, Aachen, Germany T. Kleine-Ostmann, K. Pierz, G. Hein, P. Dawson, and M. Kock, Audio signal transmission over THz communication channel using semiconductor modulator, Electronic Letters, 40, 124-126, 2004. K. P. H. Lui and F. A. Hegmann, Ultrafast carrier relaxation in radiation-damaged silicon on sapphire studied by optical-pump-terahertz-probe experiments, Applied Physics Letters, 78, 3478-3480, 2001 L. Fekete and P.Kuzel, Active optical control of the terahertz reflectivity of high-resistivity semiconductors, Optics Letters, 30(15), 1992-1994, 2005
4:00 PM - K5:U-fast
4:30 PM - **K5.6
Dynamical Electric and Magnetic Metamaterial Response at Terahertz Frequencies
Willie Padilla 1 , Antoinette Taylor 1 , Clark Highstrete 2 , Mark Lee 2 , Rick Averitt 1 Show Abstract
1 MST-CINT, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Utilizing terahertz time domain spectroscopy, we have characterized the electromagnetic response of a planar array of split ring resonators (SRRs). The measured frequency dependent magnetic and electric resonances are in excellent agreement with theory and simulation. The SRRs are fabricated upon high resistivity GaAs thus permitting dynamic control of the electric resonance through photoexcitation of free carriers in the substrate. Above a certain critical threshold the excited carrier density is sufficient to short the gap of the SRRs thereby turning off the electric resonance demonstrating the potential of such structures as terahertz switches.
5:00 PM - K5.7
Metal-wire Terahertz Time-domain Spectroscopy.
Markus Walther 1 , Mark Freeman 1 , Frank Hegmann 1 Show Abstract
1 Department of Physics, University of Alberta, Edmonton, Alberta, Canada
5:15 PM - K5.8
Terahertz Transmission Through a Subwavelength Aperture in a Structured Metal Film and its Application to Pulse Shaping.
Amit Agrawal 1 , Hua Cao 1 , Ajay Nahata 1 Show Abstract
1 Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States
Recent developments in the ability to arbitrarily synthesize ultrashort optical waveforms have led to resurgent interest in the fields of high speed communications, quantum control and spectroscopy, among others. While significant effort has been expended in recent years to create shaped femtosecond pulses at optical frequencies using a variety of spectral filtering techniques, these approaches cannot directly be translated to ultrafast pulses at longer wavelengths. With specific regard to the terahertz frequency regime, considerable effort has been put into technology development in this frequency regime, because of the potential applications in sensing,, imaging, and spectroscopy. In order to extend the flexibility of THz radiation to applications, such as coherent control of molecular states and the study of carrier dynamics, it is desirable to have the capability to generate arbitrarily shaped terahertz pulses.The majority of prior work in demonstrating the ability to generate freely propagating shaped THz pulses utilized techniques to alter the properties of bulk electromagnetic waves using nonlinear optical techniques. In this contribution, we demonstrate a novel approach to linear (passive) THz pulse shaping based on altering the properties of surface waves propagating along a metal-dielectric interface. Using terahertz time-domain spectroscopy, we measure the relative coupling and scattering characteristics of surface waves for rectangular cross section grooves on a thick planar metal film. A unique aspect of the measurement technique is that it allows one to discriminate between the contributions of individual grooves to the transmitted terahertz waveform. The structured metal film is used to couple incident terahertz radiation to surface propagating modes and a subwavelength aperture is used to subsequently sample the propagating surface wave. We demonstrate that by varying the location of the grooves and their geometry, it is possible to alter the coupling and scattering characteristics of these surface modes providing the capability to generate complex terahertz pulse profiles. We further show that by surrounding the aperture with grooves on the exit side it is possible to have additional control over the shape of transmitted THz pulse. A simple time-domain model explaining the transmission of THz pulses through these and associated structures is developed and shows excellent agreement with the experimental data. In contrast to already existing techniques, the present approach can be more straightforwardly integrated into potential applications and has the capability for being made dynamically reconfigurable.
5:30 PM - K5.9
High Intense Terahertz Radiation with Superior Signal-to-Noise Ratio from a Large-area Photoconductive Device.
Andre Dreyhaupt 1 , Stephan Winnerl 1 , Thomas Dekorsy 2 , Manfred Helm 1 Show Abstract
1 Institute of Ion-Beam Physics and Materials Research, Forschungszentrum Rossendorf, 01314 Dresden Germany, 2 Fachbereich Physik, Universitaet Konstanz, 78457 Konstanz Germany
We present an approach of photoconductive terahertz (THz) generation providing a broad bandwidth and exceptional electric field amplitude. A large-area interdigitated two-electrode structure is applied to a GaAs substrate to offer high electric fields. Photocarriers excited by a Ti:Sapphire oscillator laser with MHz repetition rate are accelerated there, yielding an intense THz output. An appropriate binary mask covers every second electrode interval and carriers are excited in uniform electric field areas only. Hence contrary carrier acceleration and destructive interference is avoided. Due to the periodic nature of the electrode structure, the size of the excitation spot on the photoconductive THz emitter can be varied. This results in a THz beam of variable divergence. By electro-optic sampling the THz radiation is detected and characteristic properties of the THz source are measured. For an excitation spot diameter of about 300 μm, which corresponds to the central wavelength of the THz pulses, the THz generation is most efficient. THz radiation with an average power of 145 µW is generated with an efficiency of 2 × 10-4 for the conversion from NIR to THz power. Furthermore, the THz radiation has excellent focusing properties. A Gaussian beam profile with a diameter (FWHM) of less than 1.4 of the central wavelength of the THz pulses is found. The THz field amplitude in the center of the focused THz beam is 1.5 kV/cm, which is almost one order of magnitude more of what is achieved with conventional semi-large aperture photoconductive emitters and a similar excitation spot diameter. Exceptionally large signal-to-noise ratios are achieved by modulating the bias voltage in the kHz range and using lock-in technique. We suggest that the THz power can be further improved by a sufficient cooling system, e.g. water cooling. Furthermore the use of LT GaAs instead of semiinsulating GaAs can result in larger THz bandwidth.
5:45 PM - K5.10
Ultrafast THz Conductivity of Nanostructured Silicon
David Cooke 1 , A. MacDonald 1 , A. Hryciw 1 , A. Meldrum 1 , J. Wang 2 , Q. Li 2 , F. Hegmann 1 Show Abstract
1 Physics, University of Alberta, Edmonton, Alberta, Canada, 2 Physics, Chinese University of Hong Kong, Shatin, Hong Kong China
The photoluminescence properties of silicon nanocrystals and the potential integration of their tunable light-emitting properties into current silicon-based electronics have made these materials extremely interesting from both a fundamental and technological point of view. Despite the intense research effort to understand the mechanisms behind their light emission, very few studies exist of the ultrafast carrier dynamics immediately after photoexcitation, which is crucial to unraveling the mystery of light emission in nanocrystalline silicon. We use time-resolved THz spectroscopy (TRTS) to probe the picosecond carrier dynamics of nanocrystalline silicon (nc-Si) thin films and silicon nanocrystals (Si-NCs) embedded in glass, formed by thermal annealing of SiOx with x = 0 and 1, respectively. The decay of the transient photoconductivity for the Si-NCs after 400 nm excitation is dominated by trapping at surface states with a lifetime increasing from < 1 ps to a few 100 ps as the mean crystallite size increases and the surface area to volume ratio decreases. The effect of structural disorder is apparent in measured transient THz conductivity. A bulk-like Drude conductivity is observed for a reference epitaxial silicon-on-sapphire sample, however the nanostructured films showed a shift in Drude weight away from zero frequency which is more dramatic for the Si/SiO2 nanocomposites than for the nc-Si. We find that the complex conductivity can be well described by a classical Drude-Smith model where the carrier localization is described in terms of carrier backscattering at the Si/SiO2 interface. We demonstrate that TRTS is an extremely powerful probe of ultrafast carrier dynamics in nanostructured silicon, revealing both practical information such as trapping of injected carriers at surface states and fundamental issues concerning conduction in disordered media.The authors acknowledge financial support from NSERC, iCORE and CIPI.