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Time-dependent spontaneous Raman spectra of optically pumped GaAs have been measured with subpicosecond time resolution. When the optically injected carrier density is less than 10' cm, we temporally resolve the growth of the optically induced nonequilibrium LO-phonon population and deduce an average electron-phonon scattering time of about 165 fs. For higher injected carrier densities, substantial time-dependent changes are observed in the spectra which result from screening of the LO phonons by the free carriers and the relaxation of the free carriers to the band edges.PACS numbers: 78.30.Gt, 63.20.Kr, 72. 10.Di The initial relaxation of optically excited carriers in polar semiconductors is dominated by the emission of small -wave-vector LO phonons via the Frohlich interaction. ' Many recent time-resolved studies of carrier relaxation in GaAs have focused on the temporal dependence of the optical constants near the absorption edge and the hot luminescence from the excited carriers. 2 4 These studies have been unable to resolve in real time the LO-phonon cascade which dominates the energy-relaxation process.In this Letter, we directly detect phonons in the cascade by measuring the time dependence of the LO-phonon population in the presence of a nonequilibrium carrier population.We observe for the first time spontaneous Raman scattering with subpicosecond time resolution from phonons in a semiconductor. These results provide a unique view of the process by which energy is transferred from the electronic to the lattice system in GaAs. They show that for optical excitation energies near 2.0 eV, the excess population of Raman-active phonons with wave vectors near 8&&105 cm ' requires 2 ps to reach its maximum value. In addition, given our subpicosecond time-resolved Raman spectra, we study for the first time the temporal evolution of the free-carrier screening of LO phonons as the optically excited carriers relax over the first several picoseconds from a monoenergetic distribution to a quasiequilibrium distribution at the band edges.Von der Linde, Kuhl, and Klingenburg, using a temporal resolution of 3 ps, were able to show that the lifetime of these optically generated nonequilibrium phonons in GaAs is 7 ps at 77 K. Collins and Yu7 demonstrated that optically created carriers at high densities in quasiequilibrium screen the LO phonon exactly as carriers introduced by doping.But these workers have not directly studied the dynamics of how optically injected nonequilibrium carriers create nonequilibrium phonons, nor have they addressed the problem of the dynamics of screening by nonequilibri um carriers. In order to observe directly the electron -LOphonon dynamics, we have performed pump-probeRaman scattering experiments on GaAs with 600-fs laser pulses. The photoexcited carriers and Raman spectra were excited by compressed9 pulses from a synchronously pumped rhodamine-6G dye laser usually operating at 588 nm. The compressed pulses had an autocorrelation full width at half maximum of 900 fs, average power of about ...

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We present a 4x4 spatially non-blocking Mach-Zehnder based silicon optical switch fabricated using processes fully compatible with standard CMOS. We successfully demonstrate operation in all 9 unique switch states and 12 possible I/O routing configurations, with worst-case cross-talk levels lower than -9 dB, and common spectral bandwidth of 7 nm. High-speed 40 Gbps data transmission experiments verify optical data integrity for all input-output channels.

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