Kono, J.; Su, M.Y.; Inoshita, T.; Noda, T.; et al., "Resonant terahertz optical sideband generation from confined magnetoexcitons.", Physical Review Letters, 1 Sept. 1997, vol.79, (no.9):1758-61.
We have probed the internal structure and nonlinear response of magnetoexcitons in GaAs/AlGaAs quantum wells by resonantly driving one- and two-photon internal transitions with intense terahertz electric fields. Strong near-band-gap emission lines, or optical sidebands, appear at frequencies omega /sub NIR/+or-2n omega /sub THz/, where omega /s ub NIR/ is the interband exciton-creation frequency, omega /sub THz/ is the frequency of the driving field, and n is an integer. The intensity of the sidebands exhibits pronounced enhancement when omega /sub THz/ coincides with transitions between magnetically tuned energy levels in the excitons, providing new and accurate information on the internal dynamics of excitons.
Zeuner, S.; Keay, B.J.; Allen, S.J.; Maranowski, K.D.; et al. "THz response of GaAs/AlGaAs superlattices: from classical to quantum dynamics.", Superlattices and Microstructures, 1997, vol.22, (no.2):149-54.
We have investigated the THz response of a series of GaAs/Al/sub x/Ga/sub 1-x/As superlattices with a variety of minibands widths. The samples range from the sequential resonant tunneling limit with a very narrow miniband to superlattices show a transition from classical rectification at frequencies below 600 GHz to quantum response above 1 THz. In the quantum regime, the I-V show distinct peaks due to absorption and stimulated emission of up to seven THz-photons. At high a.c. field strengths (several kV cm/sup -1/) the photon-assisted channels dominate the transport, leading to dynamic localization, absolute negative conductance, and gain just below the Stark splitting of the quantum well ground-states. The transition from classical to quantum behavior takes place at a frequency where h omega ~ Gamma , where Gamma approximately=2 meV is the ground-state level width in a single quantum well.
Unterrainer, K.; Keay, B.J.; Wanke, M.C.; Allen, S.J.; et al., "Strong terahertz-photocurrent resonances in miniband superlattices at the Bloch frequency.", Hot Carriers in Semiconductors, Edited by: Hess, K.; Leburton, J.-P.; Ravaioli,
U. New York, NY, USA: Plenum Press, 1996. p. 135-8.
We have observed resonant changes in the current-voltage characteristics of miniband semiconductor superlattices when the Bloch frequency is resonant with a terahertz field and its harmonics. This corresponds to absorption and gain of THz radiation in the Wannier Stark ladder of the superlattice. The resonant feature consists of a peak in the current which grows with increasing laser intensity accompanied by a decrease of the current at the low bias side. When the intensity is increased further the first peak starts to decrease and a second peak at about twice the voltage of the first peak is observed due to a two-photon resonance. At the highest intensities we observe up to a four-photon resonance.
Keay, B.J.; Allen, S.J., Jr.; Galan, J.; Campman, K.L.; et al., "Photon-assisted electric field domains and multiphoton-assisted tunneling in antenna coupled semiconductor superlattices.", Hot Carriers in Semiconductors, Edited by: Hess, K.; Leburton, J.-P.; Ravaioli, U. New York, NY, USA: Plenum Press, 1996. p. 139-42.
We show that the photon-assisted tunneling (PAT) channels can also support high and low field domains if the terahertz field is strong enough. By using bow tie antennas we have improved the coupling of terahertz radiation into semiconductor superlattices, thereby enabling us to explore PAT induced inhomogeneities as well as multiphoton assisted tunneling and the terahertz electric field dependence of the PAT channels. An extension of the model of Bonilla et al. (Phys. Rev. B vol.50, p.8644, 1994) is put forward that can account in a quantitative way for the detailed I-V characteristics of these systems in strong high frequency fields, including both PAT channels and their effect on the electric field domain distribution.
Wanke, M.C.; Markelz, A.G.; Unterrainer, K.; Allen, S.J.; et al., "Third harmonic generation in a GaAs/AlGaAs superlattice in the Bloch oscillator regime.", Hot Carriers in Semiconductors. Edited by: Hess, K.; Leburton, J.-P.; Ravaioli, U. New York, NY, USA: Plenum Press, 1996. p. 161-3.
A GaAs/AlGaAs superlattice driven at 600 GHz exhibits strong, power dependent third harmonic generation. Conversion efficiency up to 0.1% was observed. The power dependence agrees semi-quantitatively with a simple model describing Bloch oscillations driven by a strong THz electric field.
Cerne, J.; Akiyama, H.; Sherwin, M.S.; Allen, S.J.; et al., "Hot excitons in quantum wells, wires, and dots." Hot Carriers in Semiconductors. Edited by: Hess, K.; Leburton, J.-P.; Ravaioli, U. New York, NY, USA: Plenum Press, 1996. p. 305-8.
Advances in the growth of semiconductor quantum heterostructures have allowed the realization of 2-D, 1-D and 0-D quantum-confined systems. Much can be learned by investigating the equilibrium and non-equilibrium response of quantum-confined carriers to far-infrared (FIR) radiation. Investigations of photoexcited carriers are particularly interesting since neither doping nor contacts are required. We have previously observed that intense FIR radiation from UCSB's free-electron lasers (FEL) heats photoexcited carriers in quantum wells (QWs), thereby quenching excitonic photoluminescence (PL) at low lattice temperatures. In this paper, we show preliminary results on the effects of FIR radiation on photoexcited carriers in quantum wires (QWIs) and dots (QDs).
Moussessian, A.; Wanke, M.C.; Yongjun Li; Jung-Chih Chiao; et al., "A terahertz grid frequency doubler.", 1997 IEEE MTT-S International Microwave Symposium Digest, Edited by: Koepf, G.A. New York, NY, USA, IEEE, 1997, p. 683-6 vol.2.
We present a 144-element terahertz quasi-optical grid frequency doubler. The grid is a planar structure with bow-tie antennas as a unit cell each loaded with a planar Schottky diode. The grid has an output power of 5.5 mW at 1 THz for 3.1- mu s, 500-GHz input pulses with a peak power of 36 W. This is the largest recorded output power for a multiplier at terahertz frequencies.
Reddy, M.; Martin, S.C.; Molnar, A.C.; Muller, R.E.; et al., "Monolithic Schottky-collector resonant tunnel diode oscillator arrays to 650 GHz.", IEEE Electron Device Letters, May 1997, vol.18, (no.5):218-21.
We report monolithic array oscillators incorporating Schottky-collector resonant tunnel diodes (SRTD's). In the SRTD, a 0.1- mu m width Schottky collector contact provides a greatly reduced device series resistance, resulting in an estimated 2.2 THz maximum frequency of oscillation. A 64-element oscillator array oscillated at 650 GHz while a 16-element array produced 28 mu W at 290 GHz.
Harris, J.G.E.; Argaman, N.; Allen, S.J.; Volkov, A.F.; et al., "Absence of Shapiro-like steps in certain mesoscopic S-N-S junctions.", Physical Review Letters, 31 March 1997, vol.78, (no.13):2678-9.
In DC transport through mesoscopic S-N-S junctions, it has been observed that as the temperature is increased the Josephson coupling rapidly disappears, but the phase dependence of the conductance persists to much higher temperatures and decreases only as 1/T. In a recent paper by Volkov and Takayanagi (see ibid., vol.76, p.4026 (1996)), AC dynamical effects in such junctions were discussed, and it was shown that this type of phase dependent conductance should lead to Shapiro step-like features, which would also decrease in size only as 1/T. In this comment Harris et al. point out that such a phase-dependent conductance cannot, by itself, lead to the formation of Shapiro steps. Volkov and Takayanagi reply to the comments.
Zeuner, S.; Keay, B.J.; Allen, S.J.; Maranowski, K.D.; et al., "Transition from classical to quantum dynamics in superlattices in intense THz electrical fields.", Proceedings of the International Conference on Quantum Devices and Circuits. Edited by: Ismail, K.; Bandyopadhyay, S.; Leburton, J.P. Singapore: World Scientific, 1997. p. 124-9.
We have investigated the THz-response of a series of GaAs/Al/sub x/Ga/sub 1-x/As superlattices ranging from the sequential tunneling limit to superlattices with a miniband width of 10 meV. In the sequential resonant tunneling case we observe a transition from classical rectification at frequencies below 600 GHz to quantum response above 1 THz. In the quantum regime, the current-voltage characteristics show distinct peaks due to absorption and stimulated emission of up to three THz-photons. At high THz field strengths (~10 kV/cm) the photon-assisted channels dominate the transport, leading to dynamic localization, absolute negative conductance, and gain just below the Stark splitting of the quantum well ground-states. In the classical regime the irradiated I-Vs show no frequency dependent features. The transition from classical to quantum behavior occurs at h(cross) omega ~ Gamma , where Gamma approximately=2 meV is the ground-state level width in a single quantum well.
Ryu, S.R.; Herold, G.; Kono, J.; Salib, M.; et al., "Free electron laser saturation spectroscopy of neutral donors and negative donor ions confined in GaAs/AlGaAs quantum wells.", Superlattices and Microstructures, 1997, vol.21, (no.2):241-6.
Saturation of the D/sup 0/ 1s-2p/sup +/ transition, the D/sup -/-singlet transition and CR has been studied in donor (Si)-doped GaAs/AlGaAs multiple-quantum-well samples by magneto-transmission and magneto-photoconductivity measurements with the UCSB free electron laser. Effective lifetimes of the D/sup 0/ 1s-2p/sup +/ transition were found to vary systematically with laser frequency, decreasing from 62 ns at 84 cm/sup -1/ to 3 ns at 124 cm/sup -1/. The absorption coefficient of the D/sup -/-singlet transition initially increased by up to 40% and showed complete quenching at higher laser powers.
Muratov, L.S.; Stockman, M.I.; Pandey, L.N.; George, T.F.; et al., "Absorption saturation studies of Landau levels in quasi-two-dimensional systems.", Superlattices and Microstructures, 1997, vol.21, (no.4):501-8.
We report the first far-infrared linear and non-linear (saturation) magneto absorption experiments of coupled subband-Landau level excitations in a series of modulation-doped double-coupled multiple quantum-well (MQW) structures. Non-linear spectroscopy is carried out with a free electron laser. A plot of energy versus magnetic field for 4 laser lines at low intensity and high intensity is compared with linear (Fourier transform) spectroscopy at low and high temperatures. An exact numerical solution of the Schrodinger equation for this coupled system is also obtained and compared with experiments. Results show that the feature observed at high laser intensity and high temperature is due to transitions from the 1st-excited state of the coupled system to the higher excited states. Based on this, a simple 2-level model is developed, and with a layer-by-layer analysis for MQW systems, the electron lifetime of the first-excited state is obtained.
Keay, B.J.; Aversa, C.; Zeuner, S.; Allen, S.J., Jr.; et al., "Virtual states, dynamic localization, absolute negative conductance and stimulated multiphoton emission in semiconductor superlattices.", Semiconductor Science and Technology, Nov. 1996, vol.11, (no.11S):1596-600.
We report the observation of dynamic localization, absolute negative conductance (ANC) and multiphoton stimulated emission in sequential resonant tunnelling semiconductor superlattice bow-tie antenna coupled to intense terahertz electric fields. Perhaps the most remarkable observation is that with increasing terahertz field strength the conductance near zero d.c. bias decreases towards zero and then becomes negative. The results presented here compare favourably with a model in which virtual states, familiar from nonlinear optics, take a role similar to the unperturbed quantum well states.
Zeuner, S.; Allen, S.J.; Maranowski, K.D.; Gossard, A.C., "Photon-assisted tunneling in GaAs/AlGaAs superlattices up to room temperature.", Applied Physics Letters, 28 Oct. 1996, vol.69, (no.18):2689-91.
Photon-assisted transport is shown to be a remarkably robust phenomenon in sequential resonant superlattices with a large energy separation between the ground state and the first excited state of the quantum wells. Photon-assisted tunneling involving up to seven photons per tunneling event, stands out very clearly. The one-photon channel is observed up to 300 K. This implies that these superlattices are a gain medium at THz frequencies even at room temperature and potentially important for the future design of THz sources.
Sundaram, M.; Campman, K.L.; Allen, S.J., Jr.; Gossard, A.C., "Oscillations in the plasma frequency of electrons in a superlattice in a parabolic quantum well.", Semiconductor Devices (Proc. SPIE Vol.2733). Edited by: Lal, K. New Delhi, India: Narosa Publishing House. p. 116-18.
We observe the resonance frequency of electrons in a superlattice in a parabolic quantum well which oscillates periodically with voltage bias applied across the well. The wells were realized as graded Al/sub x/Ga(1-x)As grown by MBE on SI GaAs substrates. The amplitude of this oscillation increases with decreasing electrons; its phase coincides with the depopulation of successive superlattice periods.
Allen, S.J.; Bhattacharya, U.; Campman, K.; Drexler, H.; et al., "Photon assisted transport through semiconductor quantum structures in intense terahertz electric fields. Physica B, Sept. 1996, vol.227, (no.1-4):367-72.
Quantum transport in resonant tunneling diodes, sequential resonant tunneling superlattices and miniband superlattices in the presence of intense terahertz electric fields is marked by new channels opened by the absorption or emission of one or more terahertz photons. In triple barrier resonant tunneling diodes, new transport channels supported by the absorption or stimulated emission of up to three terahertz photons are observed. In sequential resonant tunneling superlattices, dynamic localization is accompanied by absolute negative resistance. Transport in miniband superlattices, controlled by coherent tunneling through several barriers and quantum wells, reveals multi-photon resonances with Bloch oscillation. Photon-assisted transport in these semiconductor quantum structures bears a strong analogy to quasi-particle tunneling and the AC Josephson effect in superconducting junctions and opens the arena of superconducting electronics to semiconductor systems. These experiments are made possible by the UCSB free-electron lasers that deliver kilowatts of tunable radiation from 120 GHz to 4.8 THz.
Keay, B.J.; Allen, S.J., Jr.; Maranowski, K.D.; Gossard, A.C.; et al., "Multiphoton-assisted tunneling, dynamic localization and absolute negative conductance.", Surface Science, 20 July 1996, vol.361-362:176-80.
We report the observation of Absolute Negative Conductance (ANC), multiphoton stimulated emission and dynamical localization in sequential resonant tunneling semiconductor superlattices driven by intense terahertz electric fields. With increasing terahertz field strength the conductance near zero dc bias decreases towards zero and then becomes negative. This is accompanied by new steps and plateaus that are attributed to multiphoton-assisted resonant tunneling between ground states of neighboring quantum wells accompanied by the stimulated emission of a photon.
Drexler, H.; Harris, J.G.E.; Yuh, E.L.; Wong, K.C.; et al., "Superconductivity and the Josephson effect in a periodic array of Nb-InAs-Nb junctions.", Surface Science, 20 July 1996, vol.361-362:306-10.
We have applied high-frequency radiation to a one-dimensional array of superconducting-normal-superconducting junctions, comprised of Nb-2DInAs-Nb, and observed Shapiro steps in the I-V curve which are dominated by a step at V=hv/4e, half the voltage of the usual AC Josephson effect. This result is discussed in view of a coupling between the Nb stripes that differs from the usual j/sub c/ sin Phi form. The zero-bias resistance of the sample is finite and increases exponentially with temperature. The Shapiro steps, however, persist up to the Nb transition temperature. These results imply that the finite resistance of the sample originates from excitations in the superconducting state.
Sadwick, L.P.; Lee, P.P.; Patel, M.; Nikols, M.; et al., "Epitaxial dysprosium phosphide grown by gas-source and solid-source MBE on gallium arsenide substrates.", Journal of Crystal Growth, July 1996, vol.164, (no.1-4):285-90.
We report the first known study of the growth of epitaxial dysprosium phosphide (DyP) grown on gallium arsenide (GaAs). DyP is lattice matched to GaAs, with the room-temperature mismatch being less than 0.01%. We have grown DyP on GaAs by gas-source and by solid-source molecular beam epitaxy using custom-designed group V thermal cracker cells and group III high temperature effusion cells. X-ray diffraction results show the DyP epilayer to be (001) single crystal on GaAs(001) substrate. Electrical and optical measurements performed to date are inconclusive as to whether DyP is a semi-metal or a semiconductor with a small band gap. The undoped films are n-type with measured electron concentrations on the order of 5.10/sup 19/-6.10/sup 20/ cm/sup -3/ with mobilities of 1-10 cm/sup 2//V.s. DyP/GaAs is stable in air with no apparent oxidation taking place, even after months of exposure to ambient untreated air. Material and surface science properties measured for DyP/GaAs include Hall measurements, 2 theta and double-crystal X-ray diffraction spectra and photothermal deflection spectroscopy.
Bogaerts, R.; Herlach, F.; De Keyser, A.; Peeters, F.M.; et al., "Experimental determination of the Fermi surface of thin Sc/sub 1-x/Er/subx/As epitaxial layers in pulsed magnetic fields.", Physical Review B (Condensed Matter), 15 June 1996, vol.53, (no.23):15951-63.
High field magnetotransport measurements on thin Sc/sub 1-x/Er/sub x/As layers buried in GaAs have been performed to obtain a detailed description of the Sc/sub 1-x/Er/sub x/As Fermi surface. The Shubnikov-de Haas (SdH) oscillations observed in the magnetoresistance and in the Hall effect are attributed to electron ellipsoids at the X points of the first Brillouin zone, and two hole spheres at the zone center. The masses of the charge carriers are determined. The assignment of the SdH frequencies is confirmed by the analysis of the relative phases of the rho /sub xx/ and the rho /sub yx/ oscillations. A splitting of the SdH frequencies, due to the exchange interaction between the mobile charge carriers and the 4f Er spins, is observed for the holes as well as for the electrons. Strong evidence is presented for confinement of the charge carriers in the Sc/sub 1-x/Er/sub x/As quantum well. As a consequence of this confinement, the carrier densities decrease with decreasing thickness of the layers and the SdH oscillations show two-dimensional behavior.
Brehmer, D.E.; Kai Zhang; Schwarz, Ch.J.; Chau, S.-P.; et al., "Resonant tunneling through rare earth arsenide, semimetal quantum wells." Solid-State Electronics, June 1996, vol.40, (no.1-8):241-4.
Resonant tunneling is observed in double barrier resonant tunneling diodes (RTDs) with semi-metallic ErAs quantum wells. Magnetic field dependence distinguishes two different resonant channels. From the thickness dependence of the voltage for resonant tunneling the dispersion of the channels is found to be hole-like. The dispersion agrees well with the theory that identifies the two channels with m/sub 1/=+or-1/2 and m/sub 1/=+or-3/2, hole states.