Broadband terahertz wave generation from an epsilon-near-zero material

Wenhe Jia,Yuanyuan Huang,Yibo Ni,Xi Feng,Yongchang Lu,Jiaguang Han,Mali Gong,Meng Liu,Futai Hu,Xueqian Zhang,Weili Zhang,Qingwei Wang,Xinlong Xu,Yuanmu Yang

doi:10.1038/s41377-020-00452-y

Abstract

Broadband light sources emitting in the terahertz spectral range are highly desired for applications such as noninvasive imaging and spectroscopy. Conventionally, THz pulses are generated by optical rectification in bulk nonlinear crystals with millimetre thickness, with the bandwidth limited by the phase-matching condition. Here we demonstrate broadband THz emission via surface optical rectification from a simple, commercially available 19 nm-thick indium tin oxide (ITO) thin film. We show an enhancement of the generated THz signal when the pump laser is tuned around the epsilon-near-zero (ENZ) region of ITO due to the pump laser field enhancement associated with the ENZ effect. The bandwidth of the THz signal generated from the ITO film can be over 3 THz, unrestricted by the phase-matching condition. This work offers a new possibility for broadband THz generation in a subwavelength thin film made of an ENZ material, with emerging physics not found in existing nonlinear crystals.

Highlights

Terahertz (THz) radiation spanning from 0.1 to 10 THz falls between the microwave and infrared spectral ranges[1]
THz radiation can be generated through the nonlinear downconversion of optical signals or through the nonlinear upconversion of microwave signals, among which the nonlinear optical rectification method is popular for the generation of broadband THz pulses for spectroscopy-related applications
Due to the deep subwavelength thickness, the bandwidth of the THz emission from the ultrathin indium tin oxide (ITO) film is over 3 THz, free from the phase-matching condition

Summary

Introduction

Terahertz (THz) radiation spanning from 0.1 to 10 THz falls between the microwave and infrared spectral ranges[1]. Despite the recent discoveries of broadband THz generation in air plasma[12,13] and liquids[14], THz emission is more routinely generated by pumping solid-state noncentrosymmetric nonlinear crystals, such as ZnTe15, GaP16, and LiNbO317, with a femtosecond laser typically operating in the near-infrared range. The intensity and bandwidth of the generated THz signal, as well as the pump wavelength, are often limited by the phase-matching condition in the bulk nonlinear crystal. This problem has recently stimulated growing interest in developing ultrathin THz emitters with thicknesses down to even a few atomic layers. Surface THz emission from semiconductors such as InAs, InSb, and GaAs has been investigated[22,23], efficient THz emission has only been shown in the reflection configuration

Methods

Results

Conclusion