Search Results (96 total)

Carrier activation dynamics is measured in self-assembled InAs/GaAs quantum dots with a high degree of electronic state symmetry, at room temperature and following resonant excitation in the ground state. Carriers are activated to the first excited state on a 15-ps time scale in the low-excitation regime, and the total activation rate increases quadratically with the fractional dot occupation. Electron-hole interaction is identified as the dominant mechanism of electron scattering within the lowest confined states of a single quantum dot, circumventing the observation of a phonon bottleneck.

The resonance Rayleigh scattering from disordered semiconductor heterostructures is investigated. Within a generalized transfer-matrix approach, the propagation of the optical field and the linear material response in the presence of disorder are solved self-consistently. To investigate the role of radiative coupling, studies for a single quantum well, multiple quantum wells, and a quantum well embedded in a microcavity are performed. It is shown that the dynamics of the resonance Rayleigh scattered signals is not only determined by the underlying disorder potential, but also by the properties of the polaritonic eigenmodes of the system. In certain cases the polaritonic coupling effects even dominate over the disorder.

Resonance Rayleigh scattering by periodic semiconductor multiple quantum-well structures is studied experimentally and theoretically. Polaritonic effects are found to dominate disorder in the secondary emission dynamics. The coexistence of several radiant polaritonic modes with different radiative decay times leads to polarization beating between modes, strongly influences the rise times, and determines the fast decay times of the resonance Rayleigh scattered signals.

A study of nonequilibrium transport of carriers in GaAs and InP at electric fields up to 130 kV/cm and with a temporal resolution of 20 fs is presented. All measurements are carried out at room temperature. The THz radiation originating from the ultrafast current change in a photoexcited semiconductor device is measured by ultrabroadband electro-optic detection. We probe the influences of two important lattice scattering processes on electron acceleration. Distinct differences are seen between GaAs and InP and interpreted in terms of the different band structures and coupling strengths of these important materials. The maximum velocities and carrier displacements achieved under nonequilibrium conditions are measured directly. Peak velocities of 6×10⁷ and 8×10⁷ cm/s are obtained in GaAs and InP, respectively. The distances achieved during the overshoot regime are found to depend strongly on electric field and material. A displacement as large as 120 nm builds up in less than 200 fs at a field of 60 kV/cm in InP. These findings are important for the design of modern high-speed devices. Coherent excitation of the polar crystal lattice is observed and demonstrated to result from the coupling between free carrier displacement and material polarization via the linear dielectric function. Our experiment is sensitive to collective displacements of the lattice ions with an amplitude as small as 10^-16 m.

We develop a self-consistent, microscopic theory of coherent resonant secondary emission from semiconductor microcavities in the normal-mode-coupling regime. Our theory provides a quantitative description of the spectral, temporal, and angular properties of the disorder-induced emission component—resonant Rayleigh scattering—and offers an intuitive physical explanation of emission properties.

We report on the observation of coherent secondary emission from resonantly excited bound and unbound two-exciton states in quantum wells. The emission is intrinsic, in contrast to the well-known Rayleigh scattering from excitons that requires disorder. We measure the spectral amplitude and the phase (and, hence, the femtosecond dynamics) of this coherent emission using ultrafast spectral interferometry, and obtain direct information on the contributions to the nonlinear susceptibility from exciton-exciton correlations beyond the mean-field approximation.

We develop and experimentally verify novel Monte Carlo simulations of ultrafast resonant Rayleigh scattering from quantum well excitons. In contrast to existing theories, these simulations can study the dynamics and spectrum of resonant Rayleigh scattering from a single realization of disorder, and allow direct comparison to experimental data. We find excellent agreement between our experiments and simulations. Our studies demonstrate the high sensitivity of scattering dynamics to a particular realization of disorder, and provide new insights into the nature of spatial correlations of excitons.

The transient current response of bulk GaAs and InP is investigated at ultrahigh electric fields. On ultrashort time scales, the electronic system is far from equilibrium and overshoot velocities as high as 8×10⁷  cm/s are observed. Our studies also lead to a detailed understanding of the ionic response of polar semiconductors. For the first time, carrier motion is determined with a resolution of 20 fs at fields up to 130 kV/cm. The dependence of the ultrafast dynamics on material and electric field provides new insights into the microscopic mechanisms governing nonequilibrium transport.

Using a novel technique, we simultaneously study the femtosecond dynamics of the amplitude and the coherence of resonant secondary emission from GaAs quantum wells. Two phase-locked pulses resonantly excite excitons in the semiconductor and the subsequently emitted radiation is detected by femtosecond up-conversion. In experiments at various delays and phases between the excitation pulses we identify three different contributions to the secondary emission with different coherence properties in time.

The dynamics of one-dimensional (1D) excitons is investigated in a GaAs/AlAs quantum wire array (QWR-A) using picosecond time resolved photoluminescence (PL) measurements. The excitation density is 4×10⁴ cm ^-1, a factor of 20 below the Mott density. The rise and decay time constants of the excitonic PL in the QWR-A are 55 and 450 psec, respectively. The exciton formation time in 1D is determined to be ≈30 psec. Our results suggest that exciton-acoustic-phonon scattering is enhanced in 1D. However, the rates of exciton scattering with carriers and excitons is reduced.

We report the first investigation of resonant secondary emission from quantum well excitons using ultrafast spectral interferometry. Observation of high-contrast spectral fringes demonstrates a significant phase correlation of the spectral components of the Rayleigh scattered light and allows us to determine the temporal dynamics of the coherent emission. The results contradict the present theories for resonant Rayleigh scattering based on ensemble averaging, and show the nonergodic nature of resonance Rayleigh scattering from semiconductor quantum wells.

We investigate the nonlinear dynamics of the composite absorber-cavity system by performing femtosecond time-resolved four-wave mixing experiments on a semiconductor microcavity. The nonlinear response exhibits an unexpected temporal behavior that distinctively differs from that of other multilevel systems. Accompanying model calculations show that the observed behavior is a direct consequence of the mixed electron-photon character of the absorber-cavity system.

Time-resolved photoluminescence (PL) of excitonic molecules in GaAs quantum wells (QW’s) reveals an initial transient characterized by a finite rise and an extremely fast nonexponential decay [H. Wang, J. Shah, and T. C. Damen et al., Solid State Commun. 98, 807 (1996)]. The transient is attributed to coherent quantum evolution towards the molecule ground state of two optically correlated excitons, σ⁺ and σ^-, which undergo the Coulombic attraction. In the present paper, we develop a theory of the transient PL and fit the experimental data. The radiative decay of a quasi-two-dimensional (2D) excitonic molecule is analyzed with the giant oscillator strength model adapted to QW’s and with the bipolariton model. Within the 2D bipolariton model, the main “hidden” channel of the radiative decay of an excitonic molecule is the resonant dissociation into two outgoing interface (surface) polaritons rather than the observable decay into the bulk radiative modes. We conclude that quasi-2D molecules dissociate already within the initial transient of PL, while the following exponential decay of the molecule-mediated PL is due to the escape of secondary interface polaritons into the bulk modes. According to the 2D bipolariton model, the sequence “two incoming photons→two virtual excitons→2D molecule→two outgoing interface polaritons” is a completely coherent process. The fit of the time-resolved molecule-mediated PL provides us with numerical estimates of the exciton-photon coupling in GaAs QW’s.

We measure picosecond time resolved luminescence spectra in GaAs quantum wells using frequency upconversion luminescence spectroscopy. A careful line-shape analysis of the spectra is performed to separate the free exciton and free carrier related luminescence. From the time evolution of the free carrier luminescence, we deduce the characteristic time constant (τ_f) for the bimolecular process of exciton formation by free electron-hole pairs. For an estimated initial carrier density of 4×10¹⁰ cm^-2, τ_f is found to be 50 ps. © 1996 The American Physical Society.

The presence of interactions between carriers in differing states, nondegenerate interactions, is experimentally demonstrated to result in strong, new contributions to the optical coherent response of semiconductors. These interactions are manifest in a new, state-selective, two-pulse four-wave-mixing technique as emission from a state that is excited by only one pulse. The emission arises due to interactions with another state that is excited by both pulses. These interactions are observed between the 1s exciton and continuum states and also between distinct magnetoexciton states. The resulting contributions must be included to properly understand the coherent response.

We present a unified microscopic approach to four-wave mixing (FWM) in semiconductors on an ultrashort time scale. The theory is valid for resonant excitation in the vicinity of the excitonic resonance and at low densities. The most important many-particle effects, i.e., static and dynamical exciton-exciton interaction as well as biexcitonic effects are incorporated. The internal fields resulting from these interaction processes give rise to pronounced many-particle effects in FWM signals. Our results explain the dependence of FWM signals on the polarization geometry, especially if biexcitons contribute. Time-resolved (TR) FWM experiments show that the diffraction of the interaction induced fields dominate the FWM signals completely. This dominance of the interaction induced field at low temperatures is true regardless of density, detuning, or polarization geometry. While spectrally resolved FWM (-FWM) shows biexcitonic or bound excitonic contributions under various experimental conditions, TR-FWM is always completely delayed, peaking roughly at the dephasing time after both beams passed through. © 1996 The American Physical Society.

We make a direct determination of the time constant for free-exciton formation in CdSe at 8 K by measuring the picosecond time-resolved luminescence at a longitudinal-optical-phonon-assisted Stokes sideband of the free exciton. This method allows us to obtain the time evolution of the total exciton population, in contrast to measurements of luminescence due to K→=0→ excitons. The free-exciton formation time constant is found to be 7-10 ps, independent of the exciting photon energy within a few LO-phonon energies above the band-gap energy at the carrier excitation density of ≈6×10¹⁶ cm^-3. The results suggest rapid carrier thermalization and energy relaxation in the continuum before exciton formation, rather than hot exciton formation by the geminate electron-hole pairs followed by a cascade emission of LO phonos by the excitons.

Energy transfer between quantum wells is of fundamental interest and also contributes to the dynamical response of devices based on multiple quantum wells. We report the observation of efficient energy transfer at low temperatures between unequal GaAs quantum wells separated by a thick (10–30 nm) Al_0.3Ga_0.7As barrier. The transfer efficiency is about 30% for transfer from the narrow well to the wide well (Stokes transfer), about 10^-2% for the anti-Stokes transfer, and nearly independent of temperature (2–80 K) and barrier thickness. Tunneling, thermal excitation, and impurity-related transitions cannot explain these observations. We present a calculation for transfer efficiency using Förster-type dipole-dipole interaction between excitons and between excitons and free carriers in quantum wells, and show that this dipole-dipole transfer process can reproduce the observed temperature dependence and the magnitudes of the transfer efficiency. This process has not been considered previously for energy transfer between quantum wells. © 1996 The American Physical Society.

Using transient differential transmission measurements we probe directly the temporal evolution of band-gap renormalization of a photoexcited modulation-doped quantum-well system. The band-gap renormalization is shown to increase while the carriers cool down towards the lattice temperature. The experimental results for the band-gap renormalization are in qualitative agreement with calculations made using the random-phase approximation.

We present a coherent nonlinear optical study of composite exciton-cavity systems in the strong-coupling regime. Quantum oscillations in a transient four-wave-mixing response show the creation of a coherent superposition state between the two normal modes of the composite system and demonstrate in the time domain the coherent energy exchange between the exciton and cavity. The deep oscillation persists when the cavity resonance is far detuned from the exciton resonance. The result shows that dynamics of the nonlinear response can be in a nonperturbative regime while dynamics of the linear response remains in a perturbative regime.

The dynamics of hot carriers has been investigated extensively for III-V compound semiconductors in the past but not as well for II-VI semiconductors. In this paper, we calculate the hot electron and hole energy-loss rates (ELR’s) in CdSe taking into account all relevant phonon emission processes. Interband and intraband scattering in the two uppermost valence bands (A and B) in CdSe are included. We make a detailed comparison of these ELR’s with those for GaAs. We then report our experimental measurements of time- and energy-resolved luminescence in CdSe performed using upconversion luminescence spectroscopy with a time resolution of 2.5 ps. Using these results, we obtain the hot-carrier cooling rates in CdSe in the initial time domain up to 50 ps following photoexcitation at t=0 ps. By varying the excited carrier density (n₀) within the range 10¹⁷–10¹⁸ cm^-3, we find that the hot-carrier cooling behavior has a noticeable dependence on the excited carrier density n₀, the cooling getting slower as n₀ increases. We present a detailed comparison of the experimental cooling results with our calculations of the ELR’s. The measured cooling rates are much smaller (by over two orders of magnitude) than expected in this theory. The slow cooling can be explained within the hot-phonon theory which takes into account the coupled dynamics of hot carriers and nonequilibrium optical phonons. Using this, we deduce that the lattice dynamical lifetime of the optical phonons in CdSe is about 5 ps (at 8 K, n₀=2×10¹⁷ cm^-3). We find that the energy-loss rates (ELR’s) are initially dominated by the Fröhlich interaction of the electrons and holes but later, by transverse optical phonon emission by holes, as the carriers cool. It is shown that the reduction in the ELR’s as a result of the hot-phonon effects is nearly ten times larger in CdSe than in GaAs. We also study the time dynamics of the nonequilibrium optical phonon occupancies in both CdSe and GaAs.

Resonantly excited photoluminescence with femtosecond time resolution reveals the essential role of momentum scattering in emission from excitons in quasi-two-dimensional systems. The gradual rise of the luminescence provides a direct measure of exciton momentum scattering rates. An analysis of quantum beats in the luminescence determines exciton dephasing rates. The results show that the momentum scattering rate is considerably larger than the dephasing rate, indicating a breakdown of the impact approximation commonly assumed in coherent optical studies.

We show that a low-intensity femtosecond pulse is severely distorted while propagating through a relatively thin (≊6300 Å) GaAs multiple-quantum-well sample near the exciton resonance at low temperatures. This pulse distortion depends critically on the dephasing time T₂, the total thickness l, detuning, and inhomogeneous broadening. In thinner, high-quality samples (l<3000 Å, exciton linewidth <1 meV), the distortion is smaller and free-induction-decay-like. An interferometric measurement reveals the existence of well-defined nodes at which the envelope function changes its sign, demonstrating that the source of the pulse distortion is the reradiation of the induced dipoles. While the effects of this pulse distortion on pump-probe or four-wave-mixing experiments are relatively small for samples with l<3000 Å, samples with l>6000 Å are strongly affected.

Band-edge luminescence in photoexcited CdSe is known to show several interesting features due to radiative decay of free and bound excitons and excitonic molecules (biexcitons). In this paper, we report our experiments on the study of the time evolution of some of these processes in CdSe, using up-conversion luminescence spectroscopy with a time resolution of 2.5 ps. We obtain time-dependent luminescence spectra at two e$hyh pair excitation densities (estimated to be n₀=8×10¹⁶ and 8×10¹⁷ cm^-3). The role of hot biexciton dynamics in determining the time evolution of luminescence is evident. At low density (n₀=8×10¹⁶ cm^-3), the luminescence spectrum also reveals the time evolution of the bound exciton emission at long delays (>200 ps). In addition, we obtain time-resolved luminescence of the longitudinal-optical phonon-assisted Stokes sidebands of the free exciton. The exciton and biexciton lifetimes are deduced to be 600 and 10 ps, respectively, on the basis of a theoretical model describing their coupled dynamics.