Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D15: Focus Session: THz Devices and Materials II |
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Sponsoring Units: FIAP Chair: Mark Sherwin, University of California-Santa Barbara Room: LACC 405 |
Monday, March 21, 2005 2:30PM - 3:06PM |
D15.00001: Voltage-tunable detectors for Terahertz radiation operating above 100k with ns rise times Invited Speaker: Collective vibrations of proteins, rotations of small molecules, excitations of high-temperature superconductors, and electronic transitions in semiconductor nanostructures occur with characteristic frequencies between 1 and 10 THz [1]. Applications to medicine, communications, security and other fields are emerging. However, mapping the coldest parts of the universe has been the largest driver for developing THz detectors [2]. The result is a family of exquisitely-sensitive detectors requiring sub-4K temperatures. For earthbound THz science and technology, sensitivity remains important but many applications require high speed and operating temperatures. Room-temperature Schottky diodes enable some of these applications [3]. Here we demonstrate a new type of detector in which THz radiation collected by a microscopic antenna excites a collective intersubband oscillation of $\sim $25,000 electrons between two gates in a microscopic four terminal GaAs/AlGaAs transistor. The energy dissipates into other modes of the electron gas, warming it and changing the source-drain resistance. The detector shows amplifier-limited rise times near 1 ns and has detected THz laser radiation at temperatures up to 120K. Theory predicts that rise times should be $^{2}$10 ps, enabling operation as a mixer with $>$10 GHz IF bandwidth [4]. The frequency of the collective oscillation tunes with small gate voltages. The first-generation tunable antenna-coupled intersubband Terahertz (TACIT) detectors tune between 1.5 and 2 THz with voltages $<$2V. Supported by NASA and the NSF. Work performed in collaboration with M. F. Doty, P. Focardi, A. C. Gossard, M. Hanson, W. R. McGrath and M. S. Sherwin. Please address communication to M. S. Sherwin, Physics Dept., UCSB. [1] Sherwin, M. S., Schmuttenmaer, C. and Bucksbaum, P. , editors, ``Opportunities in THz Science,'' \underline {http://www.sc.doe.gov/bes/reports/abstracts.html{\#}THz} [2] de Bernardis, P. et al. A flat Universe from high-resolution maps of the cosmic microwave background radiation. Nature 404, 955-959 (2000). [3] Siegel, P. H. Terahertz technology [Review]. IEEE Transactions on Microwave Theory {\&} Techniques 50, 910-928 (2002). [4] Sherwin, M. S. et al. Tunable antenna-coupled intersubband terahertz (TACIT) mixers: the quantum limit without the quantum liquid., in Proceedings of Far-IR, Sub-mm and mm Detector Technology Workshop (Monterey, CA, 2002). \underline {http://www.sofia.usra.edu/det{\_}workshop/papers/} manuscript{\_}session6.html. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D15.00002: Terahertz Space-Charge Field Oscillations in Multilayer Semiconductor Heterostructures Yuri Glinka, Denis Maryenko, Jurgen Smet We report the first observation of few-cycle terahertz oscillations in cross-beam photocurrent signals from GaAs/LT-GaAs, which are due to the space-charge field oscillations in LT-GaAs layer. The experimental setup for simultaneous measurements of the cross-beam photocurrent and pump-probe reflection change has been used. The frequency of oscillations is tunable with the thickness of the LT-GaAs layer. Two effects contribute to the signals: (1) the photocurrent autocorrelation, and (2) the memory effect. The oscillations arise due to the dynamical reverse of interfacial field between GaAs substrate and LT-GaAs layer resulted from different electron relaxation times in the materials. The dynamical reverse of the field induces the Dynamical Franz-Keldysh effect (DFKE) also appeared in pump-probe reflection change measurements. The effect of the interfacial field induced DFKE on ultrafast carrier dynamics in multilayer heterostructures is discussed. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D15.00003: Superconducting Hot-Electron THz Photon Counter Boris Karasik, Andrei Sergeev We present a concept for the hot-electron transition-edge sensor capable of counting THz photons. The main application for such a sensor is a moderate resolution spectrometer on the Single-Aperture Far-Infrared Observatory with a background-limited NEP $\sim $ 10$^{-20}$ W/Hz$^{1/2}$ expected above 1 THz. Under these conditions, the rate of photon arrival is so low that the photon counting mode will be required. The hot-electron photon counter based on a submicron-size Ti bridge has a very low heat capacity which provides a high energy resolution (170 GHz) at 0.3 K. With the sensor time constant of a few microseconds, the dynamic range would be $\sim $ 30 dB that should be sufficient for most of applications. The sensor couples to radiation via a planar antenna and is read by a SQUID amplifier. A compact array of the antenna-coupled counters can be fabricated on a silicon wafer without membranes. The presentation will describe the concept and the first experimental results. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D15.00004: A field-emission based vacuum device for the generation of THz waves Ming-Chieh Lin, Kuo-Hua Huang, Pu-Shih Lu, Pei-Yi Lin, Ruei-Fu Jao Terahertz waves have been used to characterize the electronic, vibrational and compositional properties of solid, liquid and gas phase materials during the past decade. More and more applications in imaging science and technology call for the well development of THz wave sources. Amplification and generation of a high frequency electromagnetic wave are a common interest of field emission based devices. In the present work, we propose a vacuum electronic device based on field emission mechanism for the generation of THz waves. To verify our thinking and designs, the cold tests and the hot tests have been studied via the simulation tools, SUPERFISH and MAGIC. In the hot tests, two types of electron emission mechanisms are considered. One is the field emission and the other is the explosive emission. The preliminary design of the device is carried out and tested by the numerical simulations. The simulation results show that an electronic efficiency up to 4{\%} can be achieved without employing any magnetic circuits. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D15.00005: Probing Material Dynamics Using Terahertz Time-Domain Spectroscopy Invited Speaker: Terahertz time-domain spectroscopy and related techniques (e.g. THz emission spectroscopy, optical-pump THz-probe spectroscopy) comprise a unique experimental toolset for investigating the dynamics of a host of technologically and scientifically relevant materials. This talk will highlight the ease with which these techniques can be implemented and present recent experimental results including investigations of quasiparticle dynamics in correlated electron materials, terahertz emission from photoexcited ferromagnets, and the generation of far-infrared surface plasmon polaritons at metal dielectric interfaces. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D15.00006: Sub-THz oscillations in liquid water probed with terahertz time-domain spectroscopy. Jason McNary, Harry W.K. Tom Terahertz time domain spectroscopy is used to probe liquid water over a range of temperatures from 0.5 $^{o}$C to 20 $^{o}$C and across frequencies from 20 GHz to 2 THz. The experiments were performed with a THz spectrometer that has been specifically designed and optimized for the generation and collection of sub-THz frequencies and it has a single scan signal-to-noise ratio $<$1{\%} at 150 GHz. The spectrum of water in this range shows a significant deviation from the standard double Debye fit for liquid water that can only be fit by several low frequency Lorentzian oscillators. This is suggestive of collective modes of behavior of water molecules in the hydrogen bond network. The results of this experiment lays the necessary groundwork for being able to examine proteins and biological materials in aqueous solutions. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D15.00007: Broadband THz response of high-density excitons R. Huber, R.A. Kaindl, B.A. Schmid, D.S. Chemla Many-body interactions in electron-hole ($e-h)$ gases determine their nature as a conductive unbound $e-h$ plasma or insulating exciton gas. THz spectroscopy, unlike bandgap luminescence or absorption, probes transitions between internal exciton states. In this way, exciton formation in a dilute, optically-generated $e-h$ gas was recently investigated in GaAs quantum wells (R. Kaindl, \textit{et al}. Nature \textbf{423}, 734, 2003). Here, we report THz studies of a high-density exciton gas. For dilute, insulating excitons, the THz conductivity peaks around 7~meV due to 1s-2p transitions. With increasing density, the peak shows a striking red-shift and broadening, and finally evolves into a Drude shape. Quantitative analysis reveals a broadening that gradually exceeds the level spacing and is larger than expected for 1s excitons. This agrees with enhanced scattering from p-like states. In contrast to optical studies, which are limited due to counteracting bandgap renormalization and reduced $e-h$ attraction, THz radiation provides a direct way to measure the impact of phase-space filling and screening on exciton levels at high densities. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D15.00008: Nonlinear absorption of terahertz radiation due to light impact ionization. Sergey Ganichev, Wilhelm Prettl A nonlinear increase in free carrier absorption in InSb bulk samples with rising intensity of terahertz radiation has been observed. It is shown that the effect is caused by a generation of electron-hole pairs at intense excitation with photon energies several factors of ten smaller than the energy gap. The experimental results give strong evidence that the increase of carrier density is caused by light impact ionization in the radiation field. In contrast to well known impact ionization in static or low frequency fields, when the frequency of the electric field is less than the reciprocal relaxation time of free carriers and ionization is due to the acceleration of electrons in one half period, at terahertz frequencies the opposite limit is achieved and carriers acquire high energies entirely because of collisions. Nonlinear transmission and photoconductive response has been observed in n- and p-type InSb with carrier density in the range of 10$^{13}$ cm$^{-3 }$to 10$^{16}$ cm$^{-3}$ applying 40 ns pulses at wavelengths 90.5 $\mu $m, 152 $\mu $m and 250 $\mu $m with a maximum radiation intensity of 3 MW/cm$^{2}$. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D15.00009: THz-emission mechanisms in impurity compensated GaSb Ricardo Ascazubi, Carl Shneider, Ingrid Wilke, Robinson Pino, Partha Dutta Narrow band gap semiconductors are strong sources of femtosecond optically excited THz radiation. They are also attractive materials for compact and lightweight time-domain THz spectroscopy and imaging systems powered by femtosecond fiber lasers with emission wavelengths at 1.55$\mu $m. Here, we report optically excited THz emission from high purity, Tellurium doped GaSb, a typical narrow band gap semiconductor. In contrast to previous work, we investigate the influence of the majority and minority carrier concentrations on the strength of the THz emission. Strong enhancement of THz emission in GaSb is observed as a result of compensation of native acceptors by Te donors. Surface field acceleration and photo-Dember effect are identified as THz emission mechanisms and modeled in dependence of the majority and minority carrier type and concentrations. THz emission from p-type GaSb is dominated by the photo-Dember effect whereas THz emission from n-type GaSb is dominated by surface field acceleration. The doping conditions, under which THz emission is maximized, are identified for both mechanisms. These results suggest a re-examination of the relative THz emission strengths of different III-semiconductors as reported previously. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D15.00010: Nonlinear Optical Response of Polar Semiconductors in the Terahertz Range Eric Roman, Jonathan Yates, Marek Veithen, David Vanderbilt, Ivo Souza We compute the recently measured \footnote{ T. Dekorsy, V. A. Yakovlev, W. Seidel, M. Helm, and F. Keilmann, Phys. Rev. Lett. {\bfseries 90}, 055508 (2003)} infrared dispersion of the second-order nonlinear susceptibility $\chi^{(2)}$ in zincblende semiconductors from first principles. At infrared frequencies, but above the elastic resonance of the medium, the total $\chi^{(2)}$ depends not only on the purely electronic response $\chi^{(2)}_{\infty}$, but also on three additional parameters: $C_1$, $C_2$, and $C_3$. They relate to the TO Raman tensor, second-order dipole moment, and lattice anharmonicity, respectively. We apply small, finite electric fields and finite ionic displacements along [111], and extract the desired parameters from the forces induced on the atoms and the change in macroscopic polarization. By analyzing the resulting displacements of the Wannier-function centers, we make contact with bond-polarizability models and assess the influence of the cation $d$-electrons. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D15.00011: THz GaAs photonic crystals Nathan Jukam, Mark S. Sherwin THz GaAs photonic crystals were fabricated by reactive ion etching. The $\langle $100$\rangle $ semi-insulating GaAs had an n-doped epi-layer that was used to generate THz radiation from coherent plasma oscillations initiated by femtosecond laser excitation. The emitted THz radiation was detected by electro-optic detection with a ZnTe crystal. When the laser beam was brought to a line focus at normal incidence, emitted THz radiation was observed whose spectrum consisted of sharp peaks. These peaks are attributed to guided slab modes being diffracted at the $\Gamma $ points of the photonic crystals. [Preview Abstract] |
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D15.00012: An optical parametric terahertz beam generator for remote sensing applications Dong Ho Wu Terahertz remote-sensing applications require high sensitivity detectors and high-power terahertz sources, since terahertz signals can be quickly attenuated by water molecules present in the target, as well as in the environment between the source and the target. Of the many terahertz source technologies available, the one based on the optical parametric generation technique seems to be the most promising as it is portable and can produce a relatively high-power terahertz beam. We have developed a terahertz source based on an optical parametric technique. We have used a Nd:YAG Q-switched laser as the pump source, and a LiNbO$_{3}$ crystal as the optical parametric medium. With a LiNbO$_{3}$ crystal our generator could produce a terahertz beam over the frequency range of 100 GHz through 3 THz. The output power was highly dependent on the detailed materials property. For terahertz detection we used a Si-bolometer or an electro-optic (EO) detector, which was specifically developed to detect CW terahertz signals. In addition to the EO sensor, we are presently developing a new detector based on a quantum-dot structure, whose noise equivalent power (NEP) is expected to be about 10$^{-21}$ W/(Hz)$^{1/2}$. This is a few orders of magnitude better than the sensitivity of our bolometer (10$^{-13}$ W/(Hz)$^{1/2})$ at 4.2 K, or our EO detector (10$^{-12}$ W/(Hz)$^{1/2}$ ) at room temperature. [Preview Abstract] |
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