Jagdeep Shah

Ultrafast luminescence spectroscopy using sum frequency generation

The basic concepts of sum frequency generation are reviewed and its application to the measurement of time-resolved luminescence spectra with subpicosecond time resolution is discussed. The emphasis is on the discussion of those aspects which are important for the optimization of such an upconversion spectrometer. Results of numerical calculations are presented for various nonlinear crystals, and a recently developed upconversion system is reviewed. This system provides excellent time resolution ( >

Detectors and sources for ultrabroadband electro-optic sampling: Experiment and theory

A detailed calculation of the amplitude and phase response of ultrathin ZnTe and GaP electro-optic sensors is presented. We demonstrate that the inclusion of the dispersion of the second-order nonlinearity is essential. Significant structures in experimental data can be explained by the theoretical response function. Correcting for the detector characteristics, we determine the precise shape of electromagnetic transients with a time resolution of 20 fs. In addition, we show that ultrafast transport of photocarriers in semiconductors can act as an efficient source for coherent electromagnetic radiation covering the entire far-to-mid-infrared regime.

Many-body and correlation effects in semiconductors

Solids consist of 1022–1023 particles per cubic centimetre, interacting through infinite-range Coulomb interactions. The linear response of a solid to a weak external perturbation is well described by the concept of non-interacting ‘quasiparticles’ first introduced by Landau. But interactions between quasiparticles can be substantial in dense systems. For example, studies over the past decade have shown that Coulomb correlations between quasiparticles dominate the nonlinear optical response of semiconductors, in marked contrast to the behaviour of atomic systems. These Coulomb correlations and other many-body interactions are important not only for semiconductors, but also for all condensed-matter systems.

Hot electrons and phonons under high intensity photoexcitation of semiconductors

Abstract It has become well established during the last few years that intense photoexcitation of a semiconductor leads to the heating of the carriers and the generation of nonequilibrium phonons. These phenomena which result from the relaxation of photoexcited carriers to the band extrema by interaction with other carriers and by emission of phonons, are reviewed in this paper. At relatively low intensities ( 5 W/cm 2 for GaAs) the photoexcited carrier distribution is Maxwellian with a carrier temperature T e different from the lattice temperature. T e as high as 150K and effective phonon temperatures as high as 3700K have been observed in GaAs. The observed variation of T e with excitation intensity leads to the conclusion that in semiconductors like GaAs the polar optical mode scattering is the dominant energy loss mechanism from the electron gas to the lattice. Similar results are obtained in CdSe and CdS. At higher intensities (>10 5 W/cm 2 for GaAs), the carrier dist0ribution becomes non-Maxwellian for reasons not well understood at present. We will also discuss some recent measurements of variation of T e with excitation wavelength and of the transmission spectra of photoexcited GaAs.

Capture of electrons and holes in quantum wells

The capture of electrons and holes by quantum wells in multiple quantum well samples of InGaAs/InP is investigated using subpicosecond luminescence spectroscopy. For samples with thin barriers, quantum capture or carrier thermalization dominates. For thicker barriers (>500 A), transport of carriers to the well dominates. We show that quantum capture time is <0.3 ps for holes and <1 ps for electrons. No significant dependence on well thickness is observed. Finally, Coulomb interaction between electrons and holes is shown to ‘‘trap’’ the electrons in unbound states in InGaAs before they are captured by the well.

Hot carriers in quasi-2-D polar semiconductors

This paper reviews hot carrier effects in quasi-2-D polar semiconductors (quantum wells and heterostructures), with special emphasis on the GaAs/AlGaAs system. After briefly introducing the basic concepts in hot carrier physics, we discuss theoretical calculations of carrier-phonon interactions and hot carrier energy loss rates to the lattice in quasi-2-D systems. We then discuss how these quantities are affected by degeneracy, plasma effects, and hot phonons. The bulk of the paper is devoted to a discussion of experimental results and their analysis. Three kinds of experiments are discussed: I-V and related transport measurements, direct time-of-flight measurements of velocity-field characteristics, and measurements which use optical spectroscopy to provide direct information about the carrier distribution function in the presence of external perturbations. The optical studies have given valuable new insight into the behavior of hot carrier relaxation processes in quasi-2-D systems from femtosecond to steady-state conditions.

Dynamics of hot carrier cooling in photo-excited GaAs

Abstract Carrier relaxation following excitation with subpicosecond optical pulses, determined from time resolved nearband gap transmission spectrum changes, is described for GaAs. For n

Picosecond dynamics of hot carrier relaxation in highly excited multi-quantum well structures

Abstract Dynamics of hot carrier relaxation, exciton screening and subband level renormalization in GaAsAlGaAs multiple quantum well structures are investigated by time resolved measurements of optical absorption and gain following subpicosecond optical excitation. The cooling rate of the electron hole plasma in these two dimensional structures is approximately the same as for bulk GaAs at comparable photoexcitation density.

Femtosecond intervalley scattering in GaAs

We report the measurement of intervalley scattering rates for optically excited carriers in GaAs. The measurements were performed using optical pulses of 6 fs duration and an energy distribution centered at 2.0 eV. The average rates for Γ→X and Γ→L intervalley scattering were separately estimated by varying the sample temperature.

Optical processes of 2D electron plasma in GaAs-(AlGa)As heterostructures

Abstract We observed the characteristic behavior of a 2D electron plasma in optical emission and absorption across the energy gaps in modulation-doped GaAs-(AlGa)As quantum-well heterostructures. A renormalization of the energy gap with magnitude comparable to the Hartree energy is found in emission spectra. Large breakdown of the parity selection rule of the optical matrix element in quantum-wells is also observed. In absorption there is evidence of final-state electron-hole interactions even at plasma densities n≳ 4 × 1011 cm−2.