The Investigation Of Terahertz Time-domain Spectroscopy Systems Based On Photoconductive Antennae

Terhertz (THz) time-domain spectroscopy(THz-TDS) systems are an important part for THz researches, the THz-TDS based on photonconductive (PC) antennae are received extensive attention due to their significant application values. For a THz spectroscopy, the basic demands are big power,large frequency width,high signal-noise-radio(SNR) and accurate waveform.We use the double-pump-pusle THz emission technique and the double-probe-pulse THz detection technique to investigate the carrier dynamics in the PC antennae, and analyze their influence to the radiation and detection of the THz wave. The details of my work are listed in the following:(1) We calculate the effects of the trapping time of carriers, momentum relaxation time of carriers, the laser pulse width,the bias electric filed and the pump power on the radiated THz filed waveform in time and frequency domain using the one-dimension Drude model. We calculate the effects of structure parameters on the response function for detected antenna. By the experiment method, we investigate the effects of bias voltage and pump power on the radiated THz filed, and the effect of probe pump on the THz waveform and the SNR.(2) We study the effects of crystal thickness and the polarization of the THz and probe pulse on the electric-optical (EO) detection. We put up a THz-TDS system based on PC antenna emission and EO detection, and illustrate the difficulties and focus in the debugging process. We analyze the effect of probe power on the system performance.(3) We investigate the carrier dynamics in the GaAs radiation antenna by the double-pump-pulse THz emission technique. According the equivalent-circuit model,we calculate the time behavior of the radiation and space-charge field screening, and the space-charge field is always larger than the radiation field for our GaAs antenna (the width of the gap is50μm). For the reason,we assume the input reactance of the antenna is inverse proportion to the square of the antenna gap width. The peak radiation filed is stronger than space-charge filed initially, then smaller than the space-charge filed when the gap width increases. Using the model, we investigate the effects of the radiated THz power, the radiation filed and the space-charge field on the antenna gap width and the carrier mobility.(4) We recover the measured THz waveshape by the double-probe-pulse THz detection technique and the equivalent-circle model, and study the radiation and space-charge field screening in detector by the method. We analyze the reason of the distortion for the THz waveform, the limited carrier lifetime and the radiation filed screening determine the distortion of the THz waveshape. This work certify that the radiation and space-charge field screening are existent in detector. The experiment results indicate that the screening fields in detector take the information of the THz field.