Research And Application Of THz GaAs Photoconductive Antenna With High-gain Quenching Mode

Terahertz time-domain spectroscopy(THz-TDS)is a novel,safe and reliable coherent detection technique.Due to the unique properties of THz wave,THz-TDS has a broad application prospect in the fields of material property detection,sample identification,nondestructive testing and biomedical treatment.Because of its characteristics of simple structure and easy operation,GaAs photoconductive antenna is widely used in THz-TDS.But in the process,we found that the traditional photoconductive antenna usually works in linear mode,because of the space charge shielding effect and radiation field shielding effect,the radiation power of the photoconductive antenna is low.Compared with the linear mode GaAs photoconductive antenna,using the high multiplier effect of GaAs in the strong electric field,each incident photon can produce 103-105 electron hole pairs,which obviously can greatly enhance the power of terahertz wave radiated by the GaAs photoconductive antenna.So far,there is no report of THz radiation produced by GaAs photoconductive antenna with high-gain quenching mode in the world.In this paper,the characteristics of terahertz wave radiated by the high-gain quenching mode of GaAs photoconductive antenna,the terahertz photoconductive antenna array and the terahertz non-destructive detection of biological macromolecules and water containing living cells based on the terahertz time-domain spectral system are studied.The main contents of this paper are as follows:1.The THz radiation characteristics of photoconductive antenna in linear mode are studied.Firstly,based on the Drude-Lorentz model,the radiation principle of GaAs photoconductive antenna is described,the influence of band structure,optical absorption characteristics and electron transfer effect of GaAs semiconductor material on THz wave radiation is analyzed.In addition,the relationship between the electric field intensity of THz radiation and photocurrent is used to calculate the carrier occupancy rate of each energy valley when the intervalley transition occurs under different bias electric field conditions,which provides a theoretical basis for high-gian quenching GaAs photo conductive antenna to obtain high multiplication rate.secondly,the influence of electrode structure and size of GaAs photoconductive antenna is simulated by HFSS three-dimensional electromagnetic simulation software.Finally,based on the Current-Surge Model,the influence of bias voltage,laser pulse width and carrier lifetime on THz wave radiated by GaAs photoconductive antenna issimulated by matlab program,which provides technical support and theoretical basis for GaAs photoconductive antenna working in high-gian quenching mode.2.The THz radiation characteristics of the photoconductive antenna in high-gain quenching working mode are studied.The advantages and disadvantages of GaAs photoconductive antenna in linear mode are verified by experiments.It is innovatively proposed to radiate THz wave by using GaAs photoconductive antenna in high-gain quenching mode,and greatly enhance the power of terahertz wave radiated by GaAs photoconductive antenna by using avalanche multiplication effect of carrier.Firstly,we use the band structure of GaAs and the external trigger light energy and electric field constraints to adjust the degree of photo induced collisional ionization,the duration of the carrier avalanche multiplication effect caused by photoinduced charge domain becomes shorter,so that GaAs photoconductive antenna not only has carrier avalanche multiplication effect,but also can work under the high repetition rate laser pulse trigger condition.Subsequently,the GaAs photoconductive antenna was triggered by a femtosecond laser of nJ magnitude entered into the high-gian quenching mode.Under the bias voltage of 5000 V(83.33kv/cm),the minimum laser energy to enter the high-gian quenching mode was about 0.567 nJ,and the carrier multiplier rate was as high as 998.Thirdly,the frequency repetition characteristics of GaAs photoconductive antenna in the high-gain quenching mode are studied.When the repetition frequency of the triggered laser is 1 kHz,the electrical pulse waveform in the high-gian quenching mode has high stability.Finally,the THz wave radiated by GaAs photoconductive antenna in the high-gain quenching mode is obtained.Compared with GaAs photoconductive antenna working in extreme linear mode,the terahertz electric field intensity increased by 4.19 times when the carrier multiplication rate was 16.92.3.A new type of GaAs photoconductive antenna array is designed.Firstly,it is found that the traditional GaAs photoconductive antenna array can not effectively avoid the influence of the reverse electric field between the elements,whether the antenna array is covered by metal layer or focused by lens,because of the diffusion of the photogenerated carrier in the whole substrate material.Secondly,by adjusting the distance between the array elements to reduce the influence of the reverse electric field,the coherent combination degree of the 2 × 2 GaAs photo conductive array antenna is about 1,and the final radiation power of the 2 × 2 GaAs photoconductive array antenna is about 4 times of that of a single array element.Finally,on the basis of 2 × 2 GaAs photo conductive array antenna,in order to improve the utilization of trigger laser energy,a self focusing microlens GaAs photoconductive array antenna is designed.The simulation results show that the utilization of trigger light energy can be increased to 77.8%by using the self focusing micro lens array.4.Terahertz non-destructive testing of trace living cells and hydrous biomacromolecules.Firstly,based on the strong field THz wave generated by high-gain quenching GaAs photoconductive antenna and LiNbO3 optical rectification,combined with two detection methods of free space electro-optical sampling and single detection based on wavefront tilt technology,a transient THz spectrometer for cell and bio macro molecule applications was constructed.The static measurement method based on GaAs photoconductive antenna radiation-free space electro-optical sampling,the detection results of signal to noise ratio(SNR)>10000:1,spectral width of about 3 THz and spectral resolution of 1 GHz were obtained.Then,the free space THz wave is coupled to the waveguide wave by the horn shaped parallel plate waveguide,and the local field enhancement effect is used to realize the THz non-destructive detection of trace living cells and hydrous biomacromolecules,and the corresponding characteristic absorption peak is obtained.