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Physical Review A

Carrier-carrier interaction and ultrashort pulse propagation in a highly excited semiconductor laser amplifier beyond the rate equation limit

Steven C. Hughes

Published Sep 1, 1998• 16 citations
Publisher

Abstract

The influence of carrier correlation processes on the interaction and propagation of femtosecond optical pulses in an inverted semiconductor is theoretically investigated. Microscopically treated Coulomb scattering described from a first-principles theory, including both diagonal and nondiagonal dephasing, is seen to cause considerably different behavior on the dynamics than that modeled by employing a rate-equation approximation. Semiconductor intrinsic optical nonlinearities such as gain saturation and adiabatic following, controlled by microscopic dephasing and relaxation mechanisms, manifest dominantly in ultrafast transient effects that, experimentally, can appear as a typical Kerr-type (instantaneous) nonlinearity. It is shown that one cannot invoke an adiabatic elimination for the polarization equation on femtosecond time scales. The present self-consistent theory successfully explains recent experimental measurements without recourse to parametrized equations. The possible role of non-Markovian relaxation is also discussed.

DOI
10.1103/physreva.58.2567

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The influence of carrier correlation processes on the interaction and propagation of femtosecond optical pulses in an inverted semiconductor is theoretically investigated. Microscopically treated Coulomb scattering described from a first-principles theory, including both diagonal and nondiagonal dephasing, is seen to cause considerably different behavior on the dynamics than that modeled by employing a rate-equation approximation. Semiconductor intrinsic optical nonlinearities such as gain saturation and adiabatic following, controlled by microscopic dephasing and relaxation mechanisms, manifest dominantly in ultrafast transient effects that, experimentally, can appear as a typical Kerr-type (instantaneous) nonlinearity. It is shown that one cannot invoke an adiabatic elimination for the polarization equation on femtosecond time scales. The present self-consistent theory successfully explains recent experimental measurements without recourse to parametrized equations. The possible role of non-Markovian relaxation is also discussed.
Femtosecond-pulse propagation in resonantly excited semiconductors is investigated by numerically solving the semiconductor Maxwell-Bloch equations for plane waves. For excitation at the exciton resonance, it is shown that the pulse absorption exhibits a strongly nonlinear dependence on the input pulse area. Very long propagation distances for strong pulses are observed, but even when all dephasing processes have been neglected, no lossless propagation (self-induced transparency) was found. The influence of the electron-hole many-body effects, nonequilibrium carrier relaxation, and optical dephasing on the pulse-propagation dynamics is studied. The exchange interaction in the electron-hole plasma is shown to support large propagation distances. For excitation of the continuum states, the dependence of the absorption on the intensity of the input pulse is reduced due to the rapid carrier relaxation into quasiequilibrium distributions.
In this work, the ultrafast gain dynamics of a quantum dot (QD)-based semiconductor optical amplifier (SOA) is modeled on the basis of semiconductor Bloch equations that include microscopically calculated nonlinear scattering rates between QD carriers and the surrounding carrier reservoir. This enables us to separately study the dynamics of electrons and holes inside the device as well as the coherent effects related to the fast polarization dynamics. We show that the optical pulse power and the dephasing time of the polarization mainly affect the gain depletion inside the active region, while the electric injection current density and thus the internal carrier dynamics influence the gain recovery. We observe that carrier–carrier scattering is the source of desynchronized behavior of electrons and holes in the recovery dynamics of QD-based SOAs. The amplification of pulse trains in the SOA predicted by our model agrees well with experimental data.
Summary form only given. High-intensity pulse propagation of femtosecond laser pulses in optically thick semiconductor crystals can lead to self-induced transmission when the laser is tuned to a free exciton resonance. This effect means that at intensities on the order of MW/cm/sup 2/, pulses in the subpicosecond range lead to Rabi flopping of the carrier density. This causes the pulses to break up temporally after long-distance coherent propagation. The effect depends strongly on the coherence of the incident light field with the polarization of the exciton transition created in the semiconductor. Despite the expected excitation-induced dephasing, a large amount of coherent nonlinear transmission and a high contrast ratio of the Rabi flops are found at sample temperatures /spl les/10 K. In order to study the dynamics of the coherently driven polarization and the dependence on external dephasing, we introduced phonon-scattering by raising the temperature of our samples. The experiments were conducted with 800 fs pulses of an optical parametric amplifier in epitaxially grown CdSe. Our experiments clearly demonstrate that the coherent coupling of the free carrier polarization to the driving field of a high-intensity subpicosecond pulse with an area larger than /spl pi/ effectively prevents the expected dephasing by carrier-carrier scattering. Opening an additional dephasing channel by introducing phonon-scattering gradually weakens the coupling and suppresses characteristic features of self-induced transmission.
Based on a microscopic theory of a two-band semiconductor light amplifier, we show that plasma heating, cooling, and ultrafast memory effects all act in concert to produce strong distortion of subpicosecond pulses propagating in semiconductor amplifiers. Plasma heating, spectral hole burning, and carrier density depletion are responsible for saturation of the gain seen by a propagating intense femtosecond pulse in the amplifier. Plasma cooling replenishes the carrier population on the trailing edge of the pulse, leading to pulse broadening as a consequence of gain regeneration. The inclusion of memory effects in the description of dephasing processes goes beyond the usual Markov assumption of constant dephasing rates; it significantly affects the dynamical pulse reshaping processes. \textcopyright{} 1996 The American Physical Society.
Summary form only given.Free-exciton resonances in semiconductors cannot be treated as bare two-level systems because Coulomb interaction and correlation/exchange effects between the carriers substantially modify the optical Bloch equations that describe the behavior of a two-level system under the influence of an external light field. The semiconductor Bloch equations (SBEs) give a more realistic description of the dynamic response of the system on ultrafast time scales. Therefore, the coherent transients known from two-level systems in atomic physics have been found to be substantially modified in semiconductors. Whereas self-induced transparency (SIT) on bound excitons, which are known to behave as atomiclike two-level systems, has been found, the question whether SIT could also appear on a free-exciton resonance is still subject of controversial discussions. We addressed this problem in a combined theory-experiment study.
Excitation-induced many-body effects play an important role in semiconductor quantum materials, where ultrafast processes determine the system properties. These effects occur when light excitations dynamically alter light absorption and emission, leading to nonlinear, excitation-induced changes in a semiconductor's optical response. This thesis theoretically investigates ultrafast excitation-induced effects in semiconductor nanostructures. Specifically, using a first-principles analysis based on quantum dynamic cluster expansion, we implement a systematic approach to describe the dynamics of microscopic polarizations and carrier occupations in semiconductor quantum materials precisely, with Coulomb-induced scattering fully included. We explore the dynamic buildup of excitation-induced effects, and develop a quantum memory model to precisely describe it. We also do a perturbative study on the third-order nonlinearities and propose a method based on it to measure the build-up time of excitation-induced effects via ultrafast experiments. Utilizing these theoretical tools, we successfully guided experiments to detect ultrafast processes. We successfully observed the time-delayed buildup of dephasing from the experiments and demonstrated a reversible ultrafast switching that can be switched within femtosecond time scale and is capable for hundreds of switching operations. We also identified nonperturbative ultrafast nonlinearities in the carbon nanotube, with the nonlinear refractive indices orders of magnitude higher than those found from other materials, explained by the strong excitation-induced effects. Our findings are crucial for quantum photonic applications. They offer significant insights into the fundamental quantum processes relevant to ultrafast photonics and open up new possibilities for quantum information and quantum optoelectronics technologies.