Experimental studies of InAs heterostructures illuminated by far-infrared (FIR) radiation reveal an abrupt increase in the charge density for FIR intensities above a threshold value that rises with increasing frequency. We attribute this charge density rise to interband impact ionization in a regime in which omega tau(m) similar to 1, where tau(m) is the momentum relaxation time, and f=omega/2 pi is the FIR frequency. The dependence of the density rise on the FIR field strength supports this interpretation, and gives threshold fields of 3.7-8.9 kV/cm for the frequency range 0.3-0.66 THz. (C) 1996 American Institute of Physics.

10aenergy10afar-infrared excitation10ainas/alsb quantum-wells10ainplane10amodulation10aPhysics1 aMarkelz, A., G.1 aAsmar, N., G.1 aBrar, B.1 aGwinn, E., G. uhttp://markelz.physics.buffalo.edu/node/26601329nas a2200157 4500008004100000020001400041245007800055210006900133260001100202300001400213490000700227520085100234100002001085700001801105856004801123 1996 eng d a0021-897900aNonlinear response of quantum-confined electrons in nonparabolic subbands0 aNonlinear response of quantumconfined electrons in nonparabolic cAug 15 a2533-25350 v803 aWe show that quantum confinement can dramatically alter the density-dependence of the third-order susceptibility, chi(NP)((3)) that arises from band nonparabolicity. Our results predict an oscillatory dependence of the efficiencies for third-harmonic generation and four-wave mixing on the subband occupation of quantum wells, and for narrow wells with high charge densities predict an enhancement over the bulk susceptibility. We also make a simple estimate of the fields required to saturate this nonparabolicity contribution to chi((3)). We discuss these results in light of recent experiments on third-harmonic generation from narrow-gap quantum wells at frequencies of similar to 1 THz, and show that nonparabolicity may not be the only nonlinearity contributing to the large chi((3)) observed. (C) 1996 American Institute of Physics.

1 aMarkelz, A., G.1 aGwinn, E., G. uhttp://markelz.physics.buffalo.edu/node/26801435nas a2200241 4500008004500000020001400045245009100059210006900150260000800219300001200227490000700239520073900246653001800985653001201003100001801015700001401033700002001047700001801067700002001085700002001105700002001125856004801145 1996 Engldsh a0003-695100aTemperature of quasi-two-dimensional electron gases under steady-state terahertz drive0 aTemperature of quasitwodimensional electron gases under steadyst cFeb a829-8310 v683 aWe use photoluminescence to study the time-average energy distribution of electrons in the presence of strong steady-state drive at terahertz (THz) frequencies, in a modulation-doped 125 Angstrom AlGaAs/GaAs square well that is held at low lattice temperature TL. We find that the energy distribution can be characterized by an effective electron temperature, T-e(>T-L), that agrees well with values estimated from the THz-illuminated, dc conductivity. This agreement indicates that under strong THz drive, LO phonon scattering dominates both energy and momentum relaxation; that the carrier distribution maintains a heated, thermal form; and that phonon drift effects are negligible. (C) 1996 American Institute of Physics.

10ahot-electrons10aPhysics1 aAsmar, N., G.1 aCerne, J.1 aMarkelz, A., G.1 aGwinn, E., G.1 aSherwin, M., S.1 aCampman, K., L.1 aGossard, A., C. uhttp://markelz.physics.buffalo.edu/node/25800654nas a2200217 4500008004100000020001400041245008000055210006900135260001100204300001600215490000700231100001800238700002000256700001800276700001400294700002000308700002000328700002000348700002000368856004800388 1995 eng d a0163-182900aRESONANT-ENERGY RELAXATION OF TERAHERTZ-DRIVEN 2-DIMENSIONAL ELECTRON GASES0 aRESONANTENERGY RELAXATION OF TERAHERTZDRIVEN 2DIMENSIONAL ELECTR cJun 15 a18041-180440 v511 aAsmar, N., G.1 aMarkelz, A., G.1 aGwinn, E., G.1 aCerne, J.1 aSherwin, M., S.1 aCampman, K., L.1 aHopkins, P., F.1 aGossard, A., C. uhttp://markelz.physics.buffalo.edu/node/25901473nas a2200193 4500008004100000020001400041245010700055210006900162260001200231300001200243490000700255520087300262100001801135700002001153700001801173700002001191700002001211856004801231 1994 eng d a0038-110100aDC TRANSPORT IN INTENSE, INPLANE TERAHERTZ ELECTRIC-FIELDS IN AL(X)GA(1-X)AS HETEROSTRUCTURES AT 300-K0 aDC TRANSPORT IN INTENSE INPLANE TERAHERTZ ELECTRICFIELDS IN ALXG cApr-Jun a693-6950 v373 aWe report 300 K studies of the dependence of the in-plane, d.c. conductivity, sigma(d.c.) (E(omega)), of a quasi 2D electron gas on the amplitude E(omega) and frequency of intense, far-infrared fields (omega/2pi = 0.24-3.5 THz). We measure sigma(d.c.) (E(omega) parallel-to E(d.c.)), where E(d.c.) is a small sensing field, and observe a monotonic decrease in sigma(d.c.) with increasing E(omega). Although a simple scaling ansatz collapses the measured sigma(d.c.) (E(omega)) data onto a single curve for frequencies from 0.25-3.45 THz (at low to moderate scaled fields), the decrease in conductivity is substantially more rapid than expected from comparison to similar data taken by Masselink et al. [Solid-St. Electron. 31, 337 (1988)] at 35 GHz. We tentatively attribute this difference to effects of a high-frequency modulation in the electron temperature.

1 aAsmar, N., G.1 aMarkelz, A., G.1 aGwinn, E., G.1 aHopkins, P., F.1 aGossard, A., C. uhttp://markelz.physics.buffalo.edu/node/26001471nas a2200193 4500008004100000020001400041245007200055210006900127260000800196300001200204490000600216520091100222100001801133700002001151700001801171700002001189700002001209856004801229 1994 eng d a0268-124200aENERGY RELAXATION AT THZ FREQUENCIES IN ALXGA1-XAS HETEROSTRUCTURES0 aENERGY RELAXATION AT THZ FREQUENCIES IN ALXGA1XAS HETEROSTRUCTUR cMay a828-8300 v93 aWe report 4.2 K studies of the dependence of the in-plane, DC conductivity of a quasi 2D electron gas on the amplitude E(omega) of applied fields with frequencies from 0.25 THz to 3.5 THz. We analyse the dependence of sigma(DC) on E(omega) assuming that electron-optical phonon scattering dominates energy relaxation, that the absorbed power has a Drude form and that the electron distribution is thermal. This simple analysis is self-consistent: Arrhenius plots of the estimated energy loss rate have a slope near -homega(LO)BAR/k(B) for all frequencies, as expected for energy loss by optical phonon emission. We find that the effective energy relaxation time tau(epsilon) varies with the frequency of the applied field, from tau(epsilon) approximately 4 ps at 0.34 THz to tau(epsilon) approximately 0.3 ps at 3.45 THz. This may indicate a frequency-dependent form for the hot-phonon distribution.

1 aAsmar, N., G.1 aMarkelz, A., G.1 aGwinn, E., G.1 aHopkins, P., F.1 aGossard, A., C. uhttp://markelz.physics.buffalo.edu/node/26100526nas a2200169 4500008004100000245007300041210006900114300001000183490000900193100001800202700001800220700002000238700001900258700001500277700001600292856004800308 1994 eng d00aFar-infrared harmonic generation from semiconductor heterostructures0 aFarinfrared harmonic generation from semiconductor heterostructu a48-550 v18541 aMarkelz, A.G.1 aGwinn, E., G.1 aSherwin, M., S.1 aHeyman, J., N.1 aNguyen, C.1 aKroemer, H. uhttp://markelz.physics.buffalo.edu/node/31900527nas a2200157 4500008004100000245010600041210006900147260001200216300001400228100001800242700001400260700001800274700001300292700001600305856004800321 1994 eng d00aFrequency Dependence of the Third Order Susceptibility of InAs Quantum Wells at Terahertz Frequencies0 aFrequency Dependence of the Third Order Susceptibility of InAs Q c08/1994 a1193-11961 aMarkelz, A.G.1 aCerne, J.1 aGwinn, E., G.1 aBrar, B.1 aKroemer, H. uhttp://markelz.physics.buffalo.edu/node/31801623nas a2200193 4500008004100000020001400041245011200055210006900167260001200236300001400248490000700262520102300269100002001292700001801312700002001330700001501350700001601365856004801381 1994 eng d a0038-110100aGIANT 3RD-ORDER NONLINEAR SUSCEPTIBILITIES FOR INPLANE FAR-INFRARED EXCITATION OF SINGLE INAS QUANTUM-WELLS0 aGIANT 3RDORDER NONLINEAR SUSCEPTIBILITIES FOR INPLANE FARINFRARE cApr-Jun a1243-12450 v373 aThird-order, free-carrier nonlinear susceptibilities, chi(3), have been measured between 19 and 23 cm-1 for three InAs/AlSb quantum wells with sheet densities between 2.5 x 10(12) cm-2 and 8 x 10(12) cm-2. We find that these wells are strongly nonlinear at far-infrared frequencies: odd harmonics ninth order have been observed at high incident intensities, and the peak value of chi(3) reaches approximately 1 esu. This is several orders of magnitude larger than previously reported values for chi(3) in bulk n-GaAs (10(-4) esu)[1] and in polyacetylene (10(-7) esu)[2]. The large magnitude of chi(3) is attributed to the high carrier density in the InAs wells, and to the strong non-parabolicity of the conduction band in InAs. However, the free-carrier chi(3) for bulk InAs predicts a density-dependence different from that observed, and the measured decrease in chi(3) with increasing intensity indicates non-perturbative response. We find that the anisotropy of chi(3) displays the expected 4-fold symmetry.

1 aMarkelz, A., G.1 aGwinn, E., G.1 aSherwin, M., S.1 aNguyen, C.1 aKroemer, H. uhttp://markelz.physics.buffalo.edu/node/26901622nas a2200205 4500008004100000020001400041245011600055210006900171260000800240300001200248490000600260520099500266100002001261700001801281700001801299700002001317700001501337700001601352856004801368 1994 eng d a0268-124200aSUBCUBIC POWER DEPENDENCE OF 3RD-HARMONIC GENERATION FOR INPLANE, FAR-INFRARED EXCITATION OF INAS QUANTUM-WELLS0 aSUBCUBIC POWER DEPENDENCE OF 3RDHARMONIC GENERATION FOR INPLANE cMay a634-6370 v93 aLarge third-order, free-carrier nonlinear susceptibilities, chi(3) (to approximately 0.2 esu), and subcubic dependence of the third-harmonic power on the incident intensity, have been observed between 19 cm-1 and 23 cm-1 for InAs/AlSb quantum wells with electron sheet densities between 2.5 x 10(12) cm-2 and 8 X 10(12) cm-2. We find that the transmission of the fundamental, and the samples' DC conductivity, decrease with increasing incident intensity, indicating a large rise in the scattering rate. Using the intensity-dependent transmission to account for absorption in the sample is not sufficient to recover a cubic power law for the third-harmonic intensity. In addition, given the increased scattering rate indicated by the conductivity data, the bulk free-carrier chi(3) due to non-parabolicity should decrease dramatically with increasing fundamental intensity, contrary to our results. Thus, non-parabolicity alone cannot account for the observed third-harmonic response.

1 aMarkelz, A., G.1 aAsmar, N., G.1 aGwinn, E., G.1 aSherwin, M., S.1 aNguyen, C.1 aKroemer, H. uhttp://markelz.physics.buffalo.edu/node/26700716nas a2200241 4500008004100000245008100041210006900122300001000191490000900201100002000210700001800230700001900248700001400267700001800281700001900299700001800318700001800336700002000354700002000374700001700394700001500411856004800426 1993 eng d00aFar-infrared nonlinear response of electrons in semiconductor nanostructures0 aFarinfrared nonlinear response of electrons in semiconductor nan a36-470 v18541 aSherwin, M., S.1 aAsmar, N., G.1 aBewley, W., W.1 aCraig, K.1 aFelix, C., L.1 aGaldrikian, B.1 aGwinn, E., G.1 aMarkelz, A.G.1 aGossard, A., C.1 aHopkins, P., F.1 aSundaram, M.1 aBirnir, B. uhttp://markelz.physics.buffalo.edu/node/320