A. Miller

Fast quantum-dot saturable absorber for passive mode-locking of solid-State lasers

We demonstrate stable mode-locking in a Yb : KYW laser by using a quantum-dot (QD) saturable absorber. A fast component in the absorption decay in this QD structure has been measured by pump-probe technique to be /spl sim/1 ps.

Simultaneous measurements of electron and hole sweep-out from quantum wells and modeling of photoinduced field screening dynamics

Electron and hole escape times from a GaAs-AlGaAs quantum well in an electric field at room temperature are measured by picosecond optical pump-probe techniques on samples containing a single quantum well in a waveguide. The use of a single well avoids multiple well transport and resonant tunneling effects. Carriers excited in the quantum well by the pump beam result in a transient bleaching signal from excitonic saturation, and, as they leave the well, a transient electroabsorption signal because the movement of charge partially screens the electric field. Both processes are modeled, including important electrical equilibration processes of the sample as a whole. This modeling and the use of two samples with asymmetric barrier heights allows the measurement of the electron and hole emission as a function of applied electric field. Preliminary analysis suggests that the emission mechanism is thermionic rather than by tunneling, but the results are not well explained by conventional thermionic emission models. >

Direct measurement of the group index of photonic crystal waveguides via Fourier transform spectral interferometry

The authors report a direct, single-shot measurement of the group index profile of photonic crystal waveguides, combining spectral interferometry with Fourier transform analysis. This technique’s versatility allows them to resolve subtle changes in dispersion and to quantify the “slow light” effect at the photonic crystal waveguide mode cutoff. For a waveguide 99μm long, they measure a group index up to 85, whereas for lengths of 397 and 695μm, they measure maximum values of 30 and 25, respectively. These results show the relationship between transmission characteristics and the maximum group delay observed in photonic crystals.

Room temperature spin relaxation in GaAs/AlGaAs multiple quantum wells

We have explored the dependence of electron spin relaxation in undoped GaAs/AlGaAs quantum wells on well width (confinement energy) at 300 K. For wide wells, the relaxation rate tends to the intrinsic bulk value due to the D’yakonov–Perel (DP) mechanism with momentum scattering by phonons. In narrower wells, there is a strong dependence of relaxation rate on well width, as expected for the DP mechanism, but also considerable variation between samples from different sources, which we attribute to differences in sample interface morphology.

Generation of 150-fs tunable pulses in Cr:LiSrAlF6.

We report ultrashort-pulse generation in a Cr:LiSrAlF6 solid-state laser. The 15-mm crystal was pumped by 1 W of red light from a cw krypton laser and actively mode locked at an 82-MHz repetition frequency using an acousto-optic modulator. Wavelength tuning was demonstrated over the range 800–880 nm, limited by the bandwidth of the optical components. By including intracavity dispersion compensation, pulse widths of 150 fs were obtained with evidence that self-phase modulation contributes to spectral broadening and pulse shortening.

130 PS RECOVERY OF ALL-OPTICAL SWITCHING IN A GAAS MULTIQUANTUM WELL DIRECTIONAL COUPLER

A significantly improved recovery time of 130 ps has been achieved in an all‐optical zero‐gap directional coupler containing multiple quantum wells. The mechanism for the all‐optical switching is due to free‐carrier induced refractive nonlinearities at near‐band‐gap resonant frequencies. The large reduction in the switch recovery time was obtained by the application of an external dc bias which sweeps out the carriers from the quantum wells. From our experimental results on the laser pulse width limited switch‐up time of 2 ps, we deduce that the contribution of self‐electro‐optic effect nonlinearity is negligible in this case.

Laser-induced acoustic phonon gratings in semiconductor thin films

Laser induced ultrasonic standing waves are observed in molecular beam deposited ZnSe thin films using the transient grating technique. This observation is attributed to a very short trapping time for single photon absorption in the band tail. The period of the standing wave is used to determine the acoustic phonon velocity in the ZnSe thin film which indicates that it is longitudinal acoustic phonons which are excited.

Modeling of cross‐well carrier transport in a multiple quantum well modulator

A self‐consistent time and spatially resolved model is developed to explain the nature of cross‐well transport in multiple quantum well devices. It is this cross‐well transport that sets fundamental limits on the speed of such devices. This model is solved numerically to obtain a fit to experimental excite‐probe data. A good fit to rise times is obtained with resonant tunneling features predicted and observed. It is also predicted that as the transient features in the transmission data relax, many carriers are still bound in the quantum wells.

Self-mode-locked Cr3+:LiCaAlF6 laser.

A cw-pumped Cr3+:LiCaAlF6 laser has been configured to produce chirp-free, self-mode-locked pulses of 170-fs duration. The prism-dispersion-compensated cavity contained no active or passive mode-locking device. Mode locking was accomplished through self-focusing in the LiCaAlF6 crystal with 2.5 W of red light from a cw krypton laser. Rocking of the output coupler initiated the ultrashort-pulse formation.

Time resolved measurements of cross‐well transport in a multiple quantum well p‐i‐n modulator at high photogenerated carrier densities

Carrier sweep‐out rates from GaAs/AlGaAs multiple quantum wells have been measured as a function of excitation level on picosecond time scales at different values of applied electric field. A minimum rate of ∼20 ps was observed under resonant tunneling conditions at low generated carrier densities. 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 constant associated with vertical carrier transport.