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Terahertz generation based on the ultrashort laser pulses pumped metal-insulator-semiconductor heterostructure
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摘要
利用超短激光脉冲泵浦半导体表面或者半导体异质结可以产生太赫兹脉冲辐射。基于超短激光脉冲泵浦金属-绝缘体-半导体异质结的太赫兹辐射产生物理模型，通过数值模拟和理论分析研究了载流子密度和速度在半导体内部的变化规律，分析了超短激光脉宽、载流子寿命以及半导体厚度等参数对太赫兹辐射的影响和物理机制。结果显示，超短激光脉宽的增加会提高太赫兹脉冲的幅值，降低太赫兹脉冲中心频率和半峰全宽；载流子寿命和半导体材料厚度的增加对太赫兹辐射的中心频率和半峰全宽有不同程度的降低作用。通过分析不同参数对产生太赫兹辐射的影响，获得了该作用过程优化宽带太赫兹脉冲产生的途径和参数。本文结果对开展相关实验提供了较好的理论参考。
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关键词:
- 太赫兹 /
- 超短激光 /
- 金属-绝缘体-半导体 /
- 异质结 /
- 脉宽
Abstract
Terahertz pulse radiation can be generated by pumping semiconductor surfaces or semiconductor heterojunctions with ultrashort laser pulses. Based on the model of terahertz generation from metal-insulator-semiconductor heterostructure pumped by ultrashort laser pulses, the changes of carrier density and velocity in semiconductors are studied through numerical simulations and theoretical analysis. The influences and physical mechanisms of ultrashort laser pulse duration, carrier lifetime, and semiconductor thickness on the terahertz generation are analyzed as well. The results show that the increase of the laser pulse duration increases the amplitude of the terahertz pulse but decreases its central frequency and bandwidth. The increases of the carrier lifetime and thickness of the semiconductor have different influences on the central frequency and bandwidth of terahertz pulse. By analyzing the influence of different parameters on the terahertz generation, the pathways and parameters of optimizing the terahertz generation are obtained. The results of this paper provide a good theoretical reference for the related experiments.
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Key words:
- terahertz /
- ultrashort laser /
- metals-insulators-semiconductors /
- heterostructure /
- pulse duration
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Overview
Overview: 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.
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图 1 超短激光脉冲泵浦金属-绝缘体-半导体异质结产生太赫兹辐射示意图
Figure 1. Schematic of the terahertz generation from metal-insulator-semiconductor heterostructure pumped by the ultrashort laser pulses
图 2 不同脉宽的激光脉冲泵浦产生的载流子密度变化趋势
Figure 2. Trends of carrier density generated by the laser pulses with different pulse durations
图 4 脉宽不同的激光脉冲泵浦产生的太赫兹脉冲。(a)时域波形；(b)对应的频谱分布
Figure 4. Terahertz pulses produced by laser pulse with different pulse durations. (a) Time-domain waveforms; (b) Corresponding spectra distribution
图 5 激光脉宽对(a)太赫兹脉冲中心频率和(b)带宽的影响
Figure 5. Influence of the laser pulse duration on the (a) terahertz pulses central frequency and (b) bandwidth
图 6 载流子寿命对(a)太赫兹脉冲中心频率和(b)带宽的影响
Figure 6. Influence of the carrier lifetime on (a) the terahertz pulse central frequency and (b) bandwidth
图 7 异质结中半导体厚度对(a)太赫兹脉冲中心频率和(b)带宽的影响
Figure 7. The influence of semiconductor thickness in heterojunction on (a) the terahertz pulse central frequency and (b) bandwidth
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