Jian-Shen Yu

Observation of terahertz electric pulses generated by nearly filled-gap nonuniform illumination excitation

Terahertz (THz) electric pulses generated by nonuniform illumination excitation in a nearly filled-gap configuration were observed. In this scheme, the excitation beam is focused to a spotsize only slightly smaller than the spacing between the transmission lines and is located symmetrically within the gap. With 100 fs laser pulses excitation on sliding-contact photoconductive switches fabricated on low-temperature-grown GaAs, electric pulse correlation with 190±20 fs full width at half maximum, which corresponds to a 3 dB bandwidth of 1.1 THz, was observed. Moreover, electric pulses with three times larger peak amplitude than those generated with filled-gap illumination from In coplanar striplines were observed. Bias, wavelength, and pump power dependencies were investigated.

Comparison of optical pump-probe characterization of low-temperature-grown GaAs at well-above-bandgap and near-bandedge wavelengths

Annealed low-temperature-grown GaAs was studied by time-resolved photoreflectance measurement at well-above-bandgap photon energies and by photoreflectance as well as transmission measurements at near-bandedge wavelength. At near-bandedge wavelength, the initial changes in reflectivity and transmission were observed to relax at identical relaxation rate, which was attributed to the absence of carrier cooling and the domination of carrier trapping. All the measured photoreflectance traces were found to be well fitted by the previously proposed three-component decomposition procedure. Among the three components, the fast positive peak was attributed to absorption bleaching and its relaxation, that is, the scattering of the photo-carrier out of their initially excited states by carrier cooling and trapping. The decay times of the positive peak, combined with the carrier cooling times extracted from photoreflectance measurement on semi-insulating GaAs, give consistent estimate of carrier trapping time at all wavelengths within the spectral range. Our results verify that well-above-bandgap photoreflectance measurement combined with the three-component fitting procedure can be used to estimate the photo-carrier trapping time which are consistent with that obtained by near-bandedge photoreflectance and transmission techniques.

Intense electric pulses with 187-fs FWHM generated by near-filled-gap nonuniform illumination excitation

We reported the observation of intense ultrashort electric pulses generated by a novel nearly-filled-gap nonuniform illumination (NFGNI) excitation scheme. This NFGNI scheme differs from conventional nonuniform illumination in that the excitation beam is focused to a spotsize only slightly smaller than the spacing between the transmission lines, and is located symmetrically within the gap. With NFGNI of 100 fs laser pulses on sliding-contact photoconductive switches fabricated on low-temperature-grown-GaAs, electric pulse correlation with 187 fs FWHM, which corresponds to a 3 dB bandwidth of 1.1 THz, was observed. To our knowledge, these are the shortest electric pulses ever reported in the literature. Moreover, these electric pulses were found three times stronger than these generated with filled-gap illumination. Bias, wavelength, and pump power dependencies are discussed.