Terahertz generation based on the ultrashort laser pulses pumped metal-insulator-semiconductor heterostructure

Thanks to the rapid development of ultrashort laser technology, semiconductor materials, and their processing technology, semiconductor heterojunctions can be designed and fabricated. For example, semiconductor materials and other materials can be stacked layer by layer to form heterojunction elements with special optoelectronic properties. Among them, the ultrashort laser-pumped semiconductor is used to generate photogenerated carriers, so that the carriers can generate ultrafast oscillating currents and broadband terahertz radiation, which has become one of the ways to provide broadband and strong-field terahertz pulse radiation.

Based on the physical model of terahertz radiation generated by ultrashort laser pulse-pumped metal-insulator-semiconductor (MIS) structures, the influence of laser pulses and heterojunction parameters on the terahertz radiation are investigated in detail with numerical calculations. Then the relationships between these parameters and terahertz pulses are obtained, to obtain the ways to optimize the terahertz generation from this structure.

The time-domain waveform (Fig. 4(a)) and frequency spectra (Fig. 4(b)) of the terahertz pulses are obtained numerically, and the influences of the pulse width (Fig. 5), carrier lifetime (Fig. 6) and semiconductor thickness (Fig. 7) on the central frequency and bandwidth of terahertz pluses are obtained and analyzed in detail as well. In addition, the influences of the wavelength and power of ultrashort laser on terahertz pulses were discussed. The physical mechanisms behind are analyzed.

In the physical process of terahertz radiation generated by ultrashort laser pulse-pumped MIS heterojunction elements, the carriers oscillate under the biased electric field and endogenous electric field, which accelerate them in the semiconductor to form a photocurrent, and thereby radiate terahertz pulses. In this paper, the relationship between photogenerated carrier lifetime, ultrashort laser pulse duration, and thickness of semiconductor materials and terahertz pulses is studied through numerical simulation and theoretical analysis. The results show that: 1) The shorter the lifetime of the photogenerated carriers is, the higher the central frequency and broader bandwidth of the terahertz pulses; 2) When the duration of the ultrashort laser increases, the central frequency and bandwidth of the terahertz pulses decrease, and the central frequency changes slowly when the pulse duration is longer than 100 fs; 3) When the thickness of the semiconductor material increases, the central frequency and bandwidth of terahertz will firstly decrease rapidly and then decrease slowly. This work will offer a good reference to the related experiment.