Time-resolved excite-probe measurements were performed on GaAs multiple quantum wells to determine the spin relaxation time as a function of well width (L) at room temperature, and as a function of temperature in the range of 10 to 150 K. These studies suggest that the room temperature spin relaxation time follows an exponential well width dependence, indicating that at room temperature spin flip is dominated by a combination of the D'Yakonov-Perel and the Bir-Aronov-Pikus mechanisms. The low temperature results suggest that a different mechanism, the exchange interaction, is dominant at low temperatures.
Selective area disordering of multiquantum well structures is used to fabricate an optical switch that consists of an overmoded undisordered section integrated with disordered input and output branching waveguides.
Cr3+:LiSrAlF6, Cr3+:LiSr0.8Ca0.2AlF6, and Cr3+:LiCaAlF6 have been successfully mode-locked to produce a stable train of 90 fs pulses at a repetition frequency of 140 MHz. The ultrashort pulses were initiated by gently rocking the output coupler and the mode-locking was self-sustained for hours after the output coupler had remained stationary. The spectral bandwidth of the pulses was 10 nm.
The gain characteristics of strained layer InGaAs quantum well laser amplifiers grown on GaAs substrates have been characterized using a Ti-sapphire laser in both CW and self-mode- locked configuration. All-optical bistability was achieved in such a device through the nonlinear refractive index that arises as a result of the gain saturation of the diode amplifier at high optical intensities. The Ti-sapphire laser was modified to produce wavelength tunable pulses of the order of 150 fs with a 15 nm spectral bandwidth in order to measure the dynamics of the gain using a time resolved optical pump-probe technique. The gain saturation occurs on the time scale of the pulse width of the laser and the initial recovery is extremely fast. This initial speedy recovery on a time scale of 100 fs is followed by a slower recovery with a time constant of 3 ps followed by a much slower recovery of 500 ps.
We have investigated vertical transport in two different multiple quantum well (MQW) semiconductor optical switching elements; a self electro-optic device and a waveguide directional coupler, using picosecond resolution pump-probe measurements. Cross-well charge transfer mechanisms in semiconductor MQW structures are basic to the operation of a number of optical switching devices currently being researched for applications in signal routing, optical processing, and computing. The cross-well motion of charge carriers in multiple quantum wells, and specifically how rapidly photogenerated carriers can be ejected from quantum well, sets the fundamental limit on the speed of operation of some devices and can be employed to speed the recovery time of others. A self-consistent, time and spatially resolved model of cross-well transport gives good agreement with experimental results. The use of cross-well carrier sweep-out is shown to give significantly improved recovery times for all- optical waveguide directional coupler switches.
We have measured carrier sweep-out times as a function of electric field applied perpendicular to GaAs/AlGaAs multiple quantum wells at different pump power levels in picosecond pump- probe experiments. Resonant tunneling was observed at 5V at low pump pulse energies. At higher excitation levels, the effects of space charge build-up were found to significantly alter the transient nonlinear optical response due to changes in the time constants associated with vertical carrier transport in the screened field.
All-optical switching has been achieved in zero-gap directional couplers containing either a single or multiple quantum wells. The mechanism which is due to free carrier induced refractive nonlinearities at near band-gap resonant frequencies has an instantaneous turn-on time and a recovery time that depends on the speed at which the carriers are dissipated. A recovery time as short as 130 ps has been obtained experimentally by the application of an external dc bias which sweeps out the carriers from the quantum wells.
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