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.

对于诸如无创成像和光谱学之类的应用，在太赫兹光谱范围内发射的宽带光源是非常需要的。常规地，太赫兹脉冲是通过光学整流在具有毫米厚度的块状非线性晶体中产生的，其带宽受相位匹配条件的限制。在这里，我们通过简单的，可商购的19 nm厚的铟锡氧化物（ITO）薄膜通过表面光学整流来演示宽带THz发射。当泵浦激光被调谐到ITO的ε近零（ENZ）区域周围时，由于与ENZ效应相关的泵浦激光场增强，我们显示了产生的THz信号的增强。从ITO薄膜产生的THz信号的带宽可以超过3 THz，不受相位匹配条件的限制。