Haiwen Wang

High-carrier-density electron dynamics in low-temperature-grown GaAs

Pump-probe differential transmission measurements examine high-carrier-density phenomena in as-grown and annealed GaAs samples grown at temperatures from 210 to 270 °C. We observe trap saturation and Auger recombination, and accurately model the measurements on annealed samples with a simple two level rate equation, allowing us to extract the trapped-electron lifetimes.

Optical phase conjugation and nonlinear optical bandpass filter characteristics in CdSSe microcrystallite-doped glass

Experimental measurements of the backward CW four-wave mixing response in semiconductor-doped glass in a regime important to applications are described. In particular, it is shown that lowering the operating temperature to 125 K yields a chi /sup 3/ approximately=2*10/sup -4/ ESU. Aberration correction through optical phase conjugation is demonstrated with a phase conjugate reflectivity of 0.3% along with a wide-angle field-of-view response. In addition, the nearly-degenerate four-wave mixing response of importance to tunable bandpass optical filters is reported. >

Ultrafast response of As-implanted GaAs photoconductors

The photoconductive response of an optoelectronic switch fabricated from GaAs implanted with arsenic ions is measured to have a duration as short as 0.7 ps and a relaxation time as fast as 0.5 ps. The switching efficiency and relaxation time of the photoswitches using the As-implanted GaAs substrates are determined to be comparable to photoconductive devices employing GaAs grown by low-temperature molecular-beam epitaxy (LT-GaAs). For high dc-bias values, persistent photocurrent tails from transient leakage currents are found to be very prominent in bulk GaAs devices that were implanted with 10/sup 16/ cm/sup -2/ arsenic ions at 200 keV. This behavior has been determined to arise from substrate leakage current underneath the thin implanted layer, which recrystallizes and exhibits, as does LT-GaAs, arsenic-precipitate formation after annealing. In order to reduce this leakage current, multiple ion dosages with various implantation energies have been implemented. An epitaxial GaAs layer has also been implanted with arsenic ions, isolated from its semi-insulating substrate, and bonded onto a fused silica wafer in order to verify that the persistent tail response from the photoconductive switches was not actually due to the implanted region of the GaAs.

Frequency domain four-wave mixing spectroscopy of temperature and optical-intensity-dependent relaxation in CdSSe microcrystallite-doped glass

We describe cw frequency-domain nonlinear laser spectroscopy results obtained in the study of relaxation in glasses doped with microcrystallites of CdSSe. Measurements are made as a function of temperature and optical intensity, using low-intensity cw optical excitation. The results are interpreted based on the assumption that the dynamics of the nonlinear response in this material is controlled by traps.

Demonstration of a 0.54-ps x-ray streak camera

A magnetically focused x-ray streak camera was designed and tested using sub-200 fs soft x-ray pulses generated by high harmonic emission in a gas. The temporal resolution of the camera was demonstrated to be under 0.54 ps for the ultraviolet and 0.88 ps in the soft-x-ray wavelength region. Our streak camera represents the fastest x-ray detector developed to date, and should allow sub-picosecond time resolution experiments to be performed using either synchrotron or laser-plasma-based x-ray sources.

High-resolution nonlinear laser spectroscopy of exciton relaxation in GaAs quantum wells

This paper describes measurements of exciton relaxation in GaAs/AlGaAs quantum well structures based on high resolution nonlinear laser spectroscopy. The nonlinear optical measurements show that low energy excitons can be localized by monolayer disorder of the quantum well interface. We show that these excitons migrate between localization sites by phonon assisted migration, leading to spectral diffusion of the excitons. The frequency domain measurements give a direct measure of the quasi-equilibrium exciton spectral redistribution due to exciton energy relaxation, and the temperature dependence of the measured migration rates confirms recent theoretical predictions. The observed line shapes are interpreted based on solutions we obtain to modified Bloch equations which include the effects of spectral diffusion.

Microfocus x-ray generation with a high repetition rate laser

By focusing 1 mJ, 21 fs pulses to a 1.6 μm spot on solid Cu and Ge targets, x-ray line emission around 10 keV photon energy was produced with a 7 μm source size.

Approaching 10/sup 19/ W/cm/sup 2/ intensity at sub-kHz repetition rate

Summary form only given. The relativistic nonlinear optics is a fundamentally new optical regime, where the product of the laser intensity times the square of the wavelength exceeds /spl sim/10/sup 18/ (W/cm/sup 2/)/spl mu/m/sup 2/. In the last few years, laser-matter interaction at relativistic laser intensity has been studied extensively, which lead to the demonstration of many new phenomena such as relativistic self-focusing, nonlinear Thomson scattering and MeV particle acceleration. These experiments, however, were done with low-repetition-rate/single-shot lasers. In a previous paper, we demonstrated for the first time that relativistic intensity pulses could be generated at 1 kHz repetition rate. In this paper, we showed that 10/sup 19/W/cm/sup 2/ can be produced with a high repetition rate laser, which is much more stable and compact than the low repetition rate lasers.

Generation of Relativistic Intensity Pulses at 300-Hz Repetition Rate

By focusing 21-fs laser pulses down to a 1.6-µm spot with aberration- compensated optics, we demonstrated for the first time that a peak intensity of 4x1018 W/cm2 can be produced at 300-Hz repetition rate.

Measurement of /spl sim/10 fs XUV pulses from high-order harmonic generation

Summary form only given. High harmonic generation is unique as a source of coherent soft-x-ray (XUV) light because of its extremely short pulse duration, high spatial coherence, and compact size. Several theories have predicted that attosecond pulses might be generated using high order harmonic generation excited by ultrashort pulses. However, to date the fastest harmonic pulse duration measurements published were performed using laser pulses longer than 25 fs. In this work, we experimentally demonstrate a cross-correlation measurement which sets an upper limit of /spl sim/10 is on the pulse duration of XUV harmonics.