Experimental And Theoretical Analysis Of THz Generated By GaAs Photoconductive Switches

Photo-Conductive Semiconductor Switch (PCSS) is a hybrid device which comprises photoconductivesemiconductors (such as GaAs, InP, GaAs) and ultra-fast lasers. At the beginning of 1990s, researchersdiscovered that the PCSS triggered by ultra-short optical pulse, could not only generate electrical pulses, butalso radiate electromagnetic wave with ultra-wide band, and even terahertz (THz) frequency. Thisobservation indicates a new tool for studying terahertz wave. As THz spectrum has a lot of physics andchemistry information about the material, people find a wide range of applications in object imagery,environmental monitoring, medical diagnosis, radio astronomy, wide-band mobile communication, satellitecommunication, military radar, and national defense safety inspection etc. By now, THz technology has beena hot research area, and many countries are competing to develop their own THz technology.In this dissertation, how to generate high power THz electromagnetic wave by photoconductiveswitches based dipole antennas is studied, and the major work is summarized as below:An experiment was carried out to radiate terahertz electrical wave: Based on photoconductive dipoleantenna, biased GaAs PCSS is used to generate THz wave, and the experimental results are comparedbetween insulated GaAs dipole antenna with Si₃N₄ layer and bare GaAs PCSS with identical electrodegeometry (3mm, 6mm). The results indicate the THz electric field generated from a Si₃N₄ film-coatedantenna is obviously stronger, than that from a normal slice antenna at the same optical energy.The physical mechanisms of the THz wave in the time domain were interpreted: A two-dimensionMonte Carlo method is used to simulate the dynamics of optical-generated carders in a biased electric field,in which the GaAs PCSS is used as a radiating antenna triggered by a femtosecond laser, pulse. Thesimulation results supports the broaden characteristic of THz wave from GaAs antenna comparison with theemitter optical pulse in experiment. The physical mechanism of the broaden characteristic of THz wave is analyzed. The dipole characteristic of THz waveforms emitted from low-temperature grown GaAs andsemi-insulating GaAs photoconductive antennas aresimulated by numerical method and Monte Carlo methodThe different physical mechanism of the dipole characteristic of THz waveforms emitted fromlow-temperature grown GaAs and semi-insulating GaAs photoconductive antennas are illustrated. ForSI-GaAs photoconductive antenna, Simulated the electrical field distribution of time-resolved space and theinfluence of screening of the bias field by the space charge on terahertz emission at different experimentalconditions. Based on the time-resolved space electrical field distribution of photo-generated carriers,identified the major reasons for the limited THz power from GaAs photoconductor trigged by high opticalenergy pulses and biased by low electrical field. The effect on THz power radiation from photoconductiveantenna triggered by different ways (all gap and partial gap illumination) at high optical energy is presented.A new formula to compute the power of THz wave radiated by photoconductive antenna wasproposed: Based on Larmor's formula, a new formula to compute the power of THz wave radiated by GaAsphotoconductive antenna is derived. With this formula, the THz wave power for different experimental casesis calculated in this dissertation. The THz wave powers are compared for the large aperture photoconductiveantenna and small aperture photoconductive antenna working at same experimental conditions. The physicalmechanism of THz wave powers generated by two gaps photoconductive antennas are analyzed as well.A new viewpoint of explaining photoconductive switch nonlinear mode was put forward: Based onequivalent circuit of Auston PCSS, simulated the transmission characteristic, dielectric relaxation properties,energy storage specialty and operating mode during the process of the dc biased PCSS as THz radiationantenna illuminated by fs laser pulse radiating THz electromagnetic wave. The simulation results wellsupport the experimental GaAs photoconductive switch working modes, namely the PCSS operate atnonlinear mode only if the bias field and trigger optical energy are greater than threshold conditions (electricfield threshold and optical pulse energy threshold) for PCSS. From the perspectives of the carrier scatteringmechanisms and the space charge high field formation process, the physical essence of the presence of thethreshold of bias voltage and triggering optical energy when PCSS working at nonlinear mode: electric fieldneed be higher than Gunn electric field. The reason is that a photo-generated hot electron must gain enoughenergy from bias electric field to scatter from the low energy valley to high energy valleys and to formnegative resistance effect. The optical pulse energy needs enough number of carriers which can scatter fromthe low energy valley to high energy valleys and create local high field regions. Only on these conditions, theelectric field can be changed quickly, has a longer relaxation time, and the PCSS operating at nonlinear mode.The simulation indicates that there is possibility to use nonlinear photoconductive switch to radiate terahertzwave, and establish a theoretical ground for generating terahertz wave using nonlinear photoconductiveswitch in experiments.