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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 conditi ons, TR-FWM is always completely delayed, peaking roughly at the dephasing time after both beams passed through

Abstract:The behavior of carrier-carrier screemng IS investigated in a GaAs-GaAIAs quantum well structure by measuring the band-to-band polarization dephasing with femtosecond photon echoes. The variation of the electron-hole polarization dephasing time with the carrier concentration reveals the two-dimensional character of the short range screening between the interacting carriers. 1The phase coherence time between an electron and the corresponding hole created during a band-to-band optical transition in a semiconductor decreases with increasing carrier density due to the effects of carrier-carrier scattering. At low carrier densities, ignoring the Coulomb interaction between carriers, the carrier-carrier scattering time is expected to scale inversely with the carrier density. At higher densities, screening due to the Coulomb potential of the charged particles will act to suppress the process of carrier-carrier scattering. Coulomb screening, which was originally studied in metals [1], depends on the electron and hole environment and therefore is a function of the dimensionality of the system under study.We report here measurements that show the influence of carrier-carrier interactions on the dephasing processes which occur in a GaAs-GaAlAs Quantum Well (QW) structure. Due to the confmement of the carriers in the well, they are expected to behave like a two-dimensional (2D) electron gas. Indeed, our measurements performed at different carrier densities indicate a screening behavior that reveals the bi-dimensionality of the system. The 2D screening that we deduce from the experiment is well understood if we assume that a given carrier interacts only with its nearest neighbors, indicating that the range of the screened Coulomb interaction is of the order of the average inter-carrier spacing. We discuss the validity of this nearest-neighbor interaction picture with respect to the experimental conditions in which a nonequilibrium, nondegenerate electron-hole population is created over a large range of energies and wave vectors.Femtosecond optical spectroscopy now allows the dynamics of energy relaxation and dephasing of excited carriers in bulk semiconductors or 2D QW structures to be studied with 10 fs resolution [2]. In bulk GaAs it is known that under high densities of excitation, carriercarrier scattering plays an important role in the thermalization of hot carriers (carriers with high excess energy with respect to the bottom of the conduction or valence band [3]), and it has been shown with both photon echo [4] and time-resolved polarization rotation measurements 2 [5,6] that it is also the dominant mechanism for momentum redistribution. In QW structures the carrier therrnalization has been extensively studied in undoped [7,8] and doped [9,10] structures, where the imponance of collisions between carriers was found to be the main process for the redistribution of energy through inelastic carrier-carrier scattering.Recent work [8,9,10] has also shown that phase space filling due to the Pauli exclusion principle...

We demonstrate a compact waveguide-based high-speed Ge electro-absorption (EA) modulator integrated with a single mode 3 µm silicon-on-isolator (SOI) waveguide. The Ge EA modulator is based on a horizontally-oriented p-i-n structure butt-coupled with a deep-etched silicon waveguide, which transitions adiabatically to a shallow-etched single mode large core SOI waveguide. The demonstrated device has a compact active region of 1.0 × 45 µm(2), a total insertion loss of 2.5-5 dB and an extinction ratio of 4-7.5 dB over a wavelength range of 1610-1640 nm with -4V(pp) bias. The estimated Δα/α value is in the range of 2-3.3. The 3 dB bandwidth measurements show that the device is capable of operating at more than 30 GHz. Clear eye-diagram openings at 12.5 Gbps demonstrates large signal modulation at high transmission rate.

We demonstrate noncontact measurements of the Hall mobility of doped semiconductor wafers with roughly 250 μm spatial resolution, using polarization rotation of focused beams of terahertz (THz) radiation in the presence of a static magnetic field. Quantitative and independent images of both carrier density and mobility of a doped semiconductor wafer have been obtained.