Fig. 1. Characterization of GaAs wafers before and after surface passivation. (a) PL spectra (inset: time-resolved THz transmission for the S-passivated GaAs and the reference bare GaAs measured at a low fluence of 3 mW); (b) schematic of the home-made OPTP setup; (c) response waveform of the bare and S-passivated GaAs based modulators to one pulse of the laser under different power levels; and (d) measured UV/VIS/IR absorption spectra.

Fig. 2. Characterization of THz modulation performance through the S-passivated and bare GaAs samples. (a) Schematic of the S-passivated GaAs based all-optical spatial THz modulator, where an 800 nm pulse laser is adopted as optical excitation, which has a spot diameter of 5 mm to completely encapsulate the incident THz beam (3 mm). Bottom graph illustrates the penetration depth (d) dependence of the effective carrier lifetime (τeff) when the photodoping power is varied. (b) Detected transmitted time domain spectra; and (c) corresponding frequency domain spectra calculated from (b).

Fig. 3. THz modulation performance under 800 nm femtosecond laser with different power: (a) and (c) time-domain spectra; (b) and (d) corresponding frequency-domain spectra for bare and S-passivated GaAs, respectively. (e) THz averaged transmittance over a frequency window from 0.2 THz to 1.2 THz; and (f) calculated MD in dependence on the pumping laser power as measured for bare GaAs and S-passivated GaAs wafers.

Fig. 4. Calculated complex conductivity from (a) bare and (b) S-passivated GaAs under various photodoping powers.

Fig. 5. Calculated carrier densities from bare and S-passivated GaAs as a function of photodoping power.

Fig. 6. Performance of opto-THz modulators based on different semiconductors.