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Rectification effect on THz photoconductive antennas

A rectification effect for terahertz current in an asymmetric photoconductive antenna on a semi-insulating GaAs substrate is reported. The effect is attributed to the Schottky barrier at the interface of the metal contact and the semiconductor substrate. Introduction Photoconductive (PC) antennas [1] are widely used for sampling detection of terahertz (THz) radiation generated by the excitation of emitting devices (Semiconductor surfaces, phase matched electro-optic crystals, PC antennas, etc) with femtosecond laser pulses. By scanning the time-delay between the pump and probe laser pulses for the PC antennas, we can obtain the THz radiation waveform, from which the THz spectrum is obtained by Fourier transformation. This sampling scheme is very successful in detection of pulsed THz radiation and in obtaining the broadband THz spectrum. However, in some applications, such as the THz intensity imaging, spectral information is not required and the integrated intensity (or amplitude) information is just sufficient. In such cases, the time-delay scanning costs too much time and hinders the use of the technique to practical applications. On the other hand, intensity or power detectors, such as the bolometer, are need to be cooled to a cryogenic temperature to obtain an enough sensitivity for the THz radiation. The only THz-intensity detector usable at room temperature is the Schottky barrier diodes. Schottky barrier diodes, however, are not suitable for detection of pulsed THz radiation because it also detect thermal background radiation, whose power is comparable or higher than the averaged power of the pulsed THz radiation generated by femtosecond lasers. This problem is alleviated if the Schottky barrier diodes can be photo-activated by the same laser pulses used for generation of pulsed THz radiation. In other words, if a PC antenna activated by laser pulses can rectify the THz signal, we can detect intensity of the pulsed THz radiation without the time-delay scan. In this paper we report our experimental study on the rectification effect of photoconductive antennas at THz frequencies. Experiment The experiment was carried out by using a photoconductive antenna fabricated on a semi-insulating (SI)-GaAs substrate, whose structure is illustrated in Fig.1. Figure 1. Asymmetric PC antenna structure used in the experiment and excitation beam spot on the antenna. The antenna consists of a coplanar stripline (~8 mm) with a 100-μm separation and a triangular stub (~50 μm)at the center of one of the stripline made with aluminum (the other line was made with an alloy of Ni, Ge, and Au to make an ohmic contact). The width of the striplines and stub was about 10 μm. THz pulsed radiation was generated by another photoconductive antenna fabricated on a low-temperature-grown GaAs (LT-GaAs) and detected by the asymmetric PC antenna, each of which was pumped and probed with a femtosecond laser (λ ~ 790 nm, δt < 100fs, 75 MHz). Pump and probe laser power was 50 mW and 7 mW, respectively. The probe laser was focused on the gap of the asymmetric PC antenna by an objective lens (x 50) to a spot size around 10 μm. Pump laser was modulated at 2 kHz with an mechanical chopper. The photoconductive current signal of the asymmetric PC antenna was amplified and detected with a set of a current amplifier and a lock-in amplifier. Result and Discussion Figure 2 shows the THz time-domain waveforms detected with the asymmetric PC antenna for the emitter biases of +30 V and -30 V. The polarity of the signal for -30-V bias was reversed for comparison with that for +30-V signal. It SI-GaAs Laser Spot