Jinhui Wen

Evolution of spin coherence dynamics and g factor with electron excess energy in bulk intrinsic GaAs

Circularly polarized absorption quantum-beat spectroscopy is used to study excess-energy dependence of electron-spin coherence dynamics in intrinsic GaAs. Absorption quantum beats are observed. The quantum-beat oscillatory frequency is used as a high precision measure of electron g factor. Double linear energy dependence of g factor is obtained and disagrees with the prediction of k∙p theory. A theoretical calculation reveals the double linear energy dependence reflects the energy dependence of g factor of electrons measured through light-hole- and heavy-hole-electron transitions, respectively. The phase of the quantum beats provides the key information to distinguish the two transitions experimentally.

Temperature dependence of electron-spin coherence in intrinsic bulk GaAs

Temperature dependence of electron-spin coherence dynamics is investigated for an intrinsic bulk GaAs in the Voigt geometry using the elliptically polarized absorption quantum-beat spectroscopy. Temperature dependences of spin coherence and recombination lifetimes as well as g factor of electrons are reported over a temperature range from 8.1to260K. The temperature dependence of spin coherence lifetime (T2*) agrees well with a reported theoretical calculation and can be fitted well by a relationship T2*∼T−1∕2, which provides an evidence to support electron-spin decoherence dominated by the Bir-Aronov-Pikus mechanism. The temperature dependence of g factor also agrees well with reported results.

Elliptically polarized absorption spectroscopy and observation of spin coherence in intrinsic GaAs

The elliptically polarized absorption spectroscopy is further developed to observe electron spin coherence dynamics in an intrinsic bulk GaAs in Voigt geometry. Absorption saturation quantum beats are observed and explained as spin-precession-dependent state filling. An analytical model is developed quantitatively to describe the quantum beats and used to fit experimental data to retrieve spin coherence lifetime. Comparing to the reported data reveals that the spin coherence dynamics of the real and imaginary of a complex refractive index is much different.

A transmission-grating-modulated pump-probe absorption spectroscopy and demonstration of diffusion dynamics of photoexcited carriers in bulk intrinsic GaAs film

A transmission-grating-modulated time-resolved pump-probe absorption spectroscopy is developed and formularized. The spectroscopy combines normal time-resolved pump-probe absorption spectroscopy with a binary transmission grating, is sensitive to the spatiotemporal evolution of photoinjected carriers, and has extensive applicability in the study of diffusion transport dynamics of photoinjected carriers. This spectroscopy has many advantages over reported optical methods to measure diffusion dynamics, such as simple experimental setup and operation, and high detection sensitivity. The measurement of diffusion dynamics is demonstrated on bulk intrinsic GaAs films. A carrier density dependence of carrier diffusion coefficient is obtained and agrees well with reported results.

The cavity dispersion noncoaxiality effects on broadband few-cycle pulse generation in Ti:sapphire laser

The cavity dispersion noncoaxiality (CDN) effects on broadband few-cycle pulse generation of a Kerr-lens mode-locked Ti:sapphire laser is investigated theoretically and experimentally. It is found that the influence of CDN is comparable with that of self-focusing and self-phase-modulation in the frequency-dependent mode size (FDMS) effects. Spectra extending from 680 nm to 1020 nm with pulse duration shorter than three optical cycles are favorably generated under the minimum CDN in the vicinity of the coaxial point of the sub-cavity.

Sub-10-fs pulse generation directly from a KLM Ti: sapphire laser

By reducing the effects of frequency dependent mode size and intracavity gain saturation of a KLM Ti: sapphire laser, the pulse bandwidth broadening is enhanced significantly. 8.5 fs pulses with the bandwidth of 92 nm at wavelength of 710 nm have been generated directly from this laser and measured with an extracavity group velocity dispersion compensation system.

Ultrafast dephasing of interband transitions in semiconductors

Two basic types of dephasing mechanisms, carrier-carrier and carrier-phonon scattering including hole-hole and hole-phonon scattering are proposed in the theory of ultrafast polarization dephasing of continuum transitions in bulk semiconductors. The contribution of optical phonon scattering to the dephasing rate is the average of the scattering rates for electrons and holes. A weighting factor that reflects the change in the momentum of the particle in a collision is introduced into the usual integral of the carrier-carrier scattering rate to describe the contribution of carrier-carrier scattering to the dephasing rate for the case of static screening interaction. The theoretical calculations are in quantitative agreement with the reported experimental results.

Elliptically polarized absorption quantum beats and temperature dependence of electron-spin coherence lifetime in intrinsic GaAs

An elliptically polarized absorption quantum-beat spectroscopy is developed and used to observe electron-spin coherence relaxation in an intrinsic bulk GaAs in Voigt geometry. The temperature dependence of electron-spin coherence lifetime is first obtained.

Quantum-transfer-efficiency excitation spectroscopy: Application to direct observation of yellow luminescent level of GaN

A quantum-transfer-efficiency excitation spectroscopy (QTEES) is developed. It measures the quantum transfer efficiency from the photoexcited level to the initial level of the luminescence measured. It is self-normalized and based on the relative measurement of photoluminescence intensity. The initial level of the luminescence detected corresponds to the position of the strongest excitation peak in a QTEES spectrum. An experiment of QTEES on an undoped GaN film is carried out, and the initial level of the yellow luminescence is obtained directly from the QTEES spectrum. The initial level measured is very close to the bottom of the conduction band, which provides direct evidence to support shallow donor–deep acceptor recombination of the yellow luminescence in the GaN film. QTEES spectra have the advantages of easy explanation and clear physical meanings. It is also found that the conventional photoluminescence excitation spectra need to be corrected by a factor of the energy of the excitation photon.

Femtosecond-LT-GaAs photoconductive switching

As short as 350 fs of photoconductive switching response time has been measured in a LT-GaAs photoconductive switch with femtosecond photocurrent autocorrelation technique.