Terahertz wave refers to the electromagnetic wave whose frequency is in the range of 0.1?10 THz (1 THz=1012 Hz) and the wavelength range is 30?3000 μm. The terahertz band is located in the transition region between the electronic and optical bands in the entire electromagnetic spectrum, and is also known as the "terahertz gap" because the research on the terahertz band develops more slowly than those on the electronic and optical bands. Because of its special frequency range, terahertz wave has the characteristics of low photon energy, wide band and high penetration. These excellent characteristics make the terahertz wave have huge applications in the fields of nondestructive testing, communication, spectroscopy, and biomedical imaging. High power broadband terahertz radiation source is the important basis of the practical applications of terahertz wave, so it is of great practical significance to improve the power and bandwidth of terahertz radiation source. At present, photoconductor antenna based on the GaAs semiconductor has been a mature terahertz radiation source. Moreover, some materials pumped by ultrashort laser pulses also can generate broadband terahertz radiation from their surface without the biased electric field. The heterojunction of the semiconductor also can generate broadband terahertz radiation from the surface, such as GaAs p-i-n heterojunction structure. In this paper, the mechanism of terahertz generation from the GaAs p-i-n structure pumped by ultrashort laser pulses is studied. The influence of the interference effect in this process on the terahertz yield and its optimization are discussed based on numerical calculations.
Based on the physical model of terahertz radiation generated by the ultrashort laser pumped GaAs p-i-n heterojunction structure, the influence of the interference effect on the terahertz generation is investigated with numerical calculations, and the mechanism of the interference effect is revealed. The influence of the interference effect on terahertz radiation with different thicknesses of i-layer is simulated, and then the correlation between the i-layer thickness and the interference effect is revealed. It is found that this effect is caused by the incoherent oscillations of the plasma which has an uneven distribution of carriers in the i-layer of the GaAs p-i-n structure. The total photocurrent decreases due to the different phase and direction of the photocurrent oscillations in different regions of the i-layer, thus decreasing the intensity of terahertz radiation. By reducing the thickness of i-layer to a certain range, the carrier distribution in the i-layer becomes homogeneous, thus optimizing the influence of interference effect on terahertz generation.
Through numerical calculations, the transient photocurrents generated by the GaAs p-i-n structures with thickness from 0.4 to 4 μm respectively with and without the interference effect are obtained (Fig. 2). The transient photocurrent is a sub-picosecond oscillating current, and its corresponding electromagnetic radiation spectrum is located in the terahertz band. The time-domain waveforms of the terahertz electric field generated from different conditions are calculated, and their frequency spectra are obtained through Fourier transform (Fig. 3). By the numerical experiments, the parameters of terahertz pulse with the thickness of i-layer from 0.4 to 4 μm are given in detail, and an influence factor of the interference effect is defined by the ratio of the terahertz pulse energy loss caused by the interference effect to the terahertz pulse energy without the interference effect (Figs. 4 and 5). By decreasing the thickness of i-layer with the interference effect, the variation trend of the full-width at half-maximum (FWHM) of terahertz pulse is presented (Fig. 6). When the thickness of i-layer decreases in the range of 0.4?4 μm, the influence of interference effect decreases gradually due to the more inhomogeneous distribution of carriers. The FWHM decreases slightly with the decrease of interference effect, and the time-domain peak value and the pulse energy of terahertz pulse increase significantly. Therefore, the influence of interference effect can be optimized by reducing the thickness of i-layer, consequently improving the terahertz intensity.
In summary, this paper reveals the physical mechanism of the interference effect causing the significant reduction of the terahertz radiation of GaAs p-i-n heterojunction structure pumped by ultrashort laser pulses. By comparing the terahertz pulse parameters with and without the interference effect, it is found that the terahertz intensity can be improved by reducing the influence of the interference effect. By comparing the influence of interference effect under different thicknesses of i-layer, the correlation between the thickness of i-layer and the interference effect is revealed. The influence of interference effect can be optimized by reducing the thickness of i-layer, which will increase the terahertz intensity. This work may provide a new way to develop high-power broadband terahertz radiation source with GaAs p-i-n heterojunction structure, and provide a good theoretical reference for the related experiments.
