Study On THz Source Of Nonlinear Photoconductivity

Under the action of bias electric field,femtosecond laser triggers GaAs photoconductive antenna(GaAs PCA)to generate terahertz wave,which has been widely used in terahertz time-domain spectral system.However,up to now,there is no exception in the world that THz wave is generated by linear operation mode of GaAs PCA.There is no report that THz wave is generated by nonlinear mode of GaAs(avalanche multiplication mode).In this paper,we focus on the experimental test and theoretical analysis of the nonlinear mode GaAs PCA.We study the electrical pulse generated by the large aperture GaAs PCA with a gap of 0.891mm between the different electrodes and the small aperture GaAs PCA with a gap of 1 50μm between the same electrodes in the nonlinear quenching mode,as well as the THz time-domain waveform radiated by the GaAs PCA in the non-linear quenching mode.The characteristics of terahertz wave radiated by nonlinear GaAs PCA are analyzed.The main work of this paper is as follows:(1)A femtosecond laser pulse with a pulse width of 100fs and a single pulse energy of 0.567nJ is used to trigger a nonlinear GaAs PCA.At a bias voltage of 5000V,an electric pulse with a pulse width of 3.37ns is obtained.When the GaAs PCA works in the avalanche multiplication quenching mode,the carrier multiplication rate reaches 103.(2)Using a small capacitance(0.2pF,10pF)as the circuit energy storage element,the electric pulse waveform of GaAs PCA with different electrode gap and different electrode structure(same side,different side)under the action of bias electric field is measured.The experimental results are as follows:for GaAs PCA,the ratio of output voltage to bias voltage,voltage conversion efficiency,is equal to a constant for a given GaAs PCA when the incident light pulse,impedance,capacitance and other conditions are constant.The experimental results show that the voltage conversion efficiency of GaAs PCA in linear mode is between 5%and 10%,while that in nonlinear mode is between 20%and 50%.(3)The output characteristics of GaAs PCA in the mode of capacitor energy storage and transmission line energy storage are tested.Under l0pF capacitor energy storage mode,the repetition frequency can not be further increased is due to the existence of parasitic inductance in the circuit.In the energy storage mode of transmission line,GaAs PCA is applied with a bias voltage of 3500V,a femtosecond laser with a repetition frequency of 1kHz,a pulse width of 100fs and a single pulse energy of 1μJ is used as the excitation.The electric pulse with an output amplitude of 901V and a pulse width of 1.48ns is obtained when GaAs PCA works in the nonlinear quenching mode.(4)THz-TDS system was used to measure THz time-domain waveforms radiated by 150μm gap GaAs PCA and 0.892mm gap GaAs PCA.The results show that the peak value of THz wave radiated by GaAs PCA in nonlinear mode is 2-3 times higher than that in linear mode.Combined with the shielding effect and THz radiation mechanism of GaAs PCA,the influence of bias voltage on THz wave radiated by GaAs PCA is analyzed.