Application of broadband terahertz spectroscopy in semiconductor nonlinear dynamics

Semiconductor nonlinearity in the range of terahertz （THz） frequency has been attracting considerable attention due to the recent development of high-power semiconductor-based nanodevices. However, the underlying physics concerning carrier dynamics in the presence of high-field THz transients is still obscure. This paper introduces an ultrafast, time-resolved THz pump/THz probe approach to study semiconductor properties in a nonlinear regime. The cartier dynamics regarding two mechanisms, intervalley scattering and impact ionization, was observed for doped InAs on a sub-picosecond time scale. In addition, polaron modulation driven by intense THz pulses was experimentally and theoretically investigated. The observed polaron dynamics verifies the interaction between energetic electrons and a phonon field. In contrast to previous work which reported optical phonon responses, acoustic phonon modulations were addressed in this study. A further understanding of the intense field interacting with solid materials will accelerate the development of semiconductor devices. This paper can be divided into 4 sections. Section 1 starts with the design and performance of a table-top THz spectrometer, which has the advantages of ultraroad bandwidth （one order higher bandwidth compared to a conventional ZnTe sensor） and high electric field strength （〉 100kV/cm）. Unlike the conventional THz timedomain spectroscopy, the spectrometer integrated a novel THz air-biased-coherent-detection （THz-ABCD） technique and utilized gases as THz emitters and sensors. In comparison with commonly used electro-optic （EO） crystals or photoconductive （PC） dipole antennas, the gases have the benefits of no phonon absorption as existing in EO crystals and no carrier life time limitation as observed in PC dipole antennas. In Section 2, the newly development THz-ABCD spectrometer with a strong THz field strength capability provides a platform for various research topics especially on the nonlinear carrier dynamics of semiconductors. Two mechanisms, electron intervalley scattering and impact ionization of InAs crystals, were observed under the excitation of intense THz field on a sub-picosecond time scale. These two competing mechanisms were demonstrated by changing the impurity doping type of the semiconductors and varying the strength of the THz field. Another investigation of nonlinear carrier dynamics in Section 3 was the observation of coherent polaron oscillation in n-doped semiconductors excited by intense THz pulses. Through modulations of surface reflection with a THz pump/THz probe technique, this work experimentally verifies the interaction between energetic electrons and a phonon field, which has been theoretically predicted by previous publications, and shows that this interaction applies for the acoustic phonon modes. Usually, two transverse acoustic （2TA） phonon responses are inactive in infrared measurement, while they are detectable in second-order Raman spectroscopy. The study ofpolaron dynamics, with nonlinear THz spectroscopy （in the far- infrared range）, provides a unique method to diagnose the overtones of 2TA phonon responses of semiconductors, and therefore incorporates the abilities of both infrared and Raman spectroscopy. Finally, some conclusions were presented in Section 4. In a word, this work presents a new milestone in wave-matter interaction and seeks to benefit the industrial applications in high power, small scale devices.

Compact and high-power broadband terahertz source based on femtosecond photonic crystal fiber amplifier

We present a review of the development of a compact and high-power broadband terahertz （THz） source optically excited by a femtosecond photonic crystal fiber （PCF） amplifier.The large mode area of the PCF and the stretcher-free configuration make the pump source compact and very efficient.Broadband THz pulses of 150 μW extending from 0.1 to 3.5 THz are generated from a 3-mm-thick GaP crystal through optical rectification of 12-W pump pulses with duration of 66 fs and a repetition rate of 52 MHz.A strong saturation effect is observed,which is attributed to pump pulse absorption;a Z-scan measurement shows that three-photon absorption dominates the nonlinear absorption when the crystal is pumped by femtosecond pulses at 1 040 nm.A further scale-up of the THz source power is expected to find important applications in THz nonlinear optics and nonlinear THz spectroscopy.

太赫兹光谱技术在半导体纳米材料中的应用

太赫兹光谱测量技术由于其非接触性、相干性和瞬时性成为研究半导体纳米材料的有利工具。近年来,太赫兹光谱技术在研究半导体纳米晶、量子点等微型结构的光电性能和光电转换特性方面取得了一系列成果。就太赫兹光谱技术在纳米半导体材料中的应用和最新进展进行了较详细的分析和归纳总结。

砷化镓类半导体材料在太赫兹波段的透射特性研究

首先给出了利用光电导天线产生的太赫兹波的时域和频域光谱,进而,基于搭建的太赫兹时域光谱系统,并采用透射光谱法分别测量了砷化镓类型的三种半导体材料在太赫兹波段的透过率。结果表明,基于光电导天线产生的太赫兹波在0~2THz范围内,光谱比较稳定,频率带宽比较宽;砷化镓半导体材料在0~2.0THz范围内的透过率的变化相对较小,具有较高的透过率(>60%),并且明显优于碲化锌以及碲化镉半导体材料在太赫兹波段的透过率。因此,相比于碲化锌以及碲化镉半导体材料而言,砷化镓半导体材料更适用于设计宽频带的太赫兹功能器件。