%0 Journal Article %J Physical Review Letters %D 1996 %T Undressing a collective intersubband excitation in a quantum well %A Craig, K. %A Galdrikian, B. %A Heyman, J. N. %A Markelz, A. G. %A Williams, J. B. %A Sherwin, M. S. %A Campman, K. %A Hopkins, P. F. %A Gossard, A. C. %X

We have experimentally measured the 1-2 intersubband absorption in a single 40 nm wide modulation-doped Al0.3Ga0.7As/GaAs square quantum well as a function of frequency, intensity, and charge density. The low-intensity depolarization-shifted absorption occurs near 80 cm(-1) (10 meV or 2.4 THz), nearly 30% higher than the intersubband spacing. At higher intensities, the absorption peak shifts to lower frequencies. Our data are in good agreement with a theory proposed by Zaluzny, which attributes the redshift to a reduction in the depolarization shift as the excited subband becomes populated.

%B Physical Review Letters %V 76 %P 2382-2385 %8 Mar 25 %@ 0031-9007 %G eng %M WOS:A1996UB14800044 %R 10.1103/PhysRevLett.76.2382 %0 Journal Article %J Physica D-Nonlinear Phenomena %D 1995 %T NONLINEAR QUANTUM DYNAMICS IN SEMICONDUCTOR QUANTUM-WELLS %A Sherwin, M. S. %A Craig, K. %A Galdrikian, B. %A Heyman, J. %A Markelz, A. %A Campman, K. %A Fafard, S. %A Hopkins, P. F. %A Gossard, A. %X

We discuss recent measurements of the nonlinear response of electrons in wide quantum wells driven by intense electromagnetic radiation at terahertz frequencies. The theme is the interplay of quantum mechanics, strong periodic driving, the electron-electron interaction and dissipation. We discuss harmonic generation from an asymmetric double quantum well in which the effects of dynamic screening are important. Measurements and theory are found to be in good agreement. We also discuss intensity-dependent absorption in a 400 Angstrom square quantum well. A new nonlinear quantum effect occurs, in which the frequency at which electromagnetic radiation is absorbed shifts to the red with increasing intensity. The preliminary experimental results are in agreement with a theory by Zaluzny, in which the source of the nonlinearity is the self-consistent potential in the Hartree approximation for the electron dynamics.

%B Physica D-Nonlinear Phenomena %V 83 %P 229-242 %8 May 15 %@ 0167-2789 %G eng %M WOS:A1995RA41200018 %R 10.1016/0167-2789(94)00266-S %0 Journal Article %J Physical Review B %D 1995 %T RESONANT-ENERGY RELAXATION OF TERAHERTZ-DRIVEN 2-DIMENSIONAL ELECTRON GASES %A Asmar, N. G. %A Markelz, A. G. %A Gwinn, E. G. %A Cerne, J. %A Sherwin, M. S. %A Campman, K. L. %A Hopkins, P. F. %A Gossard, A. C. %B Physical Review B %V 51 %P 18041-18044 %8 Jun 15 %@ 0163-1829 %G eng %M WOS:A1995RF85700093 %R 10.1103/PhysRevB.51.18041 %0 Journal Article %J Solid-State Electronics %D 1994 %T DC TRANSPORT IN INTENSE, INPLANE TERAHERTZ ELECTRIC-FIELDS IN AL(X)GA(1-X)AS HETEROSTRUCTURES AT 300-K %A Asmar, N. G. %A Markelz, A. G. %A Gwinn, E. G. %A Hopkins, P. F. %A Gossard, A. C. %X

We 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.

%B Solid-State Electronics %V 37 %P 693-695 %8 Apr-Jun %@ 0038-1101 %G eng %M WOS:A1994NE79600042 %R 10.1016/0038-1101(94)90278-X %0 Journal Article %J Semiconductor Science and Technology %D 1994 %T ENERGY RELAXATION AT THZ FREQUENCIES IN ALXGA1-XAS HETEROSTRUCTURES %A Asmar, N. G. %A Markelz, A. G. %A Gwinn, E. G. %A Hopkins, P. F. %A Gossard, A. C. %X

We 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.

%B Semiconductor Science and Technology %V 9 %P 828-830 %8 May %@ 0268-1242 %G eng %M WOS:A1994NM75300116 %! Semicond. Sci. Technol. %R 10.1088/0268-1242/9/5S/116 %0 Journal Article %J Semiconductor Science and Technology %D 1994 %T FAR-INFRARED SATURATION SPECTROSCOPY OF A SINGLE SQUARE-WELL %A Craig, K. %A Felix, C. L. %A Heyman, J. N. %A Markelz, A. G. %A Sherwin, M. S. %A Campman, K. L. %A Hopkins, P. F. %A Gossard, A. C. %X

We have performed saturation spectroscopy measurements of the lowest intersubband transition in a single 400 angstrom GaAs/Al0.3Ga0.7As modulation-doped square quantum well. We couple intense tunable far-infrared radiation from the Santa Barbara free electron laser into our sample using an edge-coupling technique and measure absorption as a function of frequency and intensity. Saturation and frequency shifts in the absorption line are clearly observed. We attribute the frequency shifts to reductions in the many-body depolarization shift. From our preliminary measurements, we estimate the intersubband relaxation time to be 600 ps to within a factor of three.

%B Semiconductor Science and Technology %V 9 %P 627-629 %8 May %@ 0268-1242 %G eng %M WOS:A1994NM75300061 %! Semicond. Sci. Technol. %R 10.1088/0268-1242/9/5S/061 %0 Journal Article %J Journal of Luminescence %D 1994 %T PROBING TERAHERTZ DYNAMICS IN SEMICONDUCTOR NANOSTRUCTURES WITH UCSB FREE-ELECTRON LASERS %A Allen, S. J. %A Craig, K. %A Felix, C. L. %A Guimaraes, P. %A Heyman, J. N. %A Kaminski, J. P. %A Keay, B. J. %A Markelz, A. G. %A Ramian, G. %A Scott, J. S. %A Sherwin, M. S. %A Campman, K. L. %A Hopkins, P. F. %A Gossard, A. C. %A Chow, D. %A Lui, M. %A Liu, T. Y. %X

The UCSB free-electron lasers provide kilowatts of continuously tunable radiation from 120 GHz to 4.8 THz. They have the most impact on terahertz science and technology that require a tunable, high power source to explore non-linear dynamics or that sacrifice incident power to recover the linear response of systems with very small cross-section. We describe three experiments that demonstrate the utility of these lasers in experiments on the terahertz dynamics of semiconductor nanostructures: (i) terahertz dynamics of resonant tunneling diodes, (ii) saturation spectroscopy of quantum wells and (iii) photon-assisted tunneling in superlattices.

%B Journal of Luminescence %V 60-1 %P 250-255 %8 Apr %@ 0022-2313 %G eng %M WOS:A1994NR36100065 %R 10.1016/0022-2313(94)90142-2 %0 Conference Proceedings %B SPIE Proceedings %D 1993 %T Far-infrared nonlinear response of electrons in semiconductor nanostructures %A Sherwin, M. S. %A Asmar, N. G. %A Bewley, W. W. %A Craig, K. %A Felix, C. L. %A Galdrikian, B. %A Gwinn, E. G. %A Markelz, A.G. %A Gossard, A. C. %A Hopkins, P. F. %A Sundaram, M. %A Birnir, B. %B SPIE Proceedings %V 1854 %P 36-47 %G eng