Nonlinear behaviors of LTG-GaAs based MSM TWPDs under telecommunication wavelength excitation

Summary form only given. Recently, record-high power-bandwidth-product performances of low-temperature-grown-GaAs-based metal-semiconductor-metal traveling-wave-photodetector (MSM TWPD) in short (/spl sim/800 nm) and long wavelength (/spl sim/1300 nm) regimes both have been demonstrated due to short carrier trapping time of LTG-GaAs at both wavelengths and its superior microwave guiding structure. The observed nonlinear saturation behaviors of LTG-GaAs MSM TWPD under long and short wavelength excitation are very different. MSM TWPD structures at these two operating wavelengths are the same, except that the device absorption lengths in the long and short wavelength regimes are 70 /spl mu/m and 10 /spl mu/m respectively due to different modal absorption constants. We employed Cr/sup 4+/:forsterite and Ti:sapphire lasers operating at 1230 nm and 800 nm as the light sources for the transient electro-optical (EO) sampling measurements. The dominant absorption processes of LTG-GaAs under long wavelength excitation is the transition from mid-gap defect states to the conduction band. The dominant transport carrier type is electron. Some immobile vacancies also existed in the mid-gap defect states of LTG-GaAs photo-absorption layer after excitation. In order to support charge neutrality, electrons must be re-injected from the negative electrode to neutralize these leftover vacancies. The dominant absorption processes of LTG-GaAs under short wavelength excitation is the transition from valence band to conduction band. Electron and hole are both the dominant transport carrier types. Charge neutralization can be supported by electron-hole recombination. We believe that the distinct nonlinear saturation behaviors observed are thus related to these two distinct carrier transport dynamics.