Research Of All-optically Controlled Terahertz Wave Amplitude Modulator Based On Gold-doped Silicon

As the terahertz(THz) technology has become a worldwide focus of research, the study of high-speed THz communication devices and systems has prevailed among the domestic and foreign institutions and universities. THz device(absorbers, filters, switches, modulators, etc) is to THz system as sensory organ(eyes, ears, etc) is to human body, which are essential parts. For terahertz modulation, multiple stimuli such as electricity, light, heat can be used, but they all have some flaws. For example, modulation depth is shallow by 2D electron gas(2-DEG) at the semiconductor heterojunction, and modulation speed is slow by thermally induced phase change materials such as vanadium dioxide. Optical modulation of THz wave by semiconductor silicon(Si) gives a large modulation amplitude, broadband characteristics and compatibility with existing semiconductor processes, which is one of the efficient methods to achieve terahertz modulation. However, limited by the relaxation of light excited carriers in intrinsic Si, the maximum modulation rate can only reaches the magnitude of ~kHz.This thesis presents an all-optically THz modulator based on gold-doped silicon. By heavily doping, high-speed modulation is achieved by improving the relaxation rate of light excited carriers in intrinsic Si or doped Si. By investigating the effect of doping temperatures and time on the THz modulation, the optimal processing condition is determined to be 60 min for diffusion at 900 °C. The lifetime test of minority carriers shows that interstitial Au atoms provided effective recombination centers for photo-generated electron-hole pairs in bulk silicon, reducing the life time of the minority carrier from ~μs to 110 ns, namely, two orders of magnitude are achieved. To further enhance the performance of the device, we use micron gold dot arrays to dope, aiming at achieving large modulation rate without losing transmitted amplitude of THz wave. The modulation depth achieved by this modulator was 21%, with a maximum modulation speed of 4.3 MHz at 340 GHz THz wave. In addition, this modulator can work in a wide spectrum of Terahertz waves, and it is also of high modulation speed, polarization insensitivity and non-frequency selectivity. After that, gold-doped Si and metamaterial are combined to achieve frequency-selective modulators. At designated frequency or bandwidth, THz wave is modulated by laser. Finally, the gold-doped Si is integrated with the laser device to construct high-speed optically controlled THz wave modulator. The information of temperature was successfully modulated on a carrier wave of 0.34 THz at100 kHz. Transmission, demodulates and display are realized. This work is to verify the reliability of the modulator, to demonstrate a prototype of THz wave wireless communication system.This thesis presents light-controlled modulation of THz wave by gold-doped silicon, which is low cost, easy to implement, and compatible with large-scale integrated semiconductor technology. It is applicable to dynamic laser coupling with terahertz wave, it is also an important base material to construct other Si-based high-speed THz devices. This technology has great potential in the terahertz communication, detection and imaging, et al.