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

Terahertz radiation: thin film enhancement A new method for generating terahertz waves exploits the low-permittivity properties of an ultrathin film. Terahertz radiation, falling between the infrared and microwave range, has emerging applications in imaging, communications and materials inspection, and is usually generated through the rectification of optical signals in nonlinear crystals. Yuanmu Yang at Tsinghua University in Beijing and co-workers tried an alternative approach, directing a near-infrared laser onto commercially available films of indium tin oxide just 19 nanometres thick. They observed a large enhancement in the terahertz generation when they tuned the laser to hit the film's 'epsilon-near-zero' state-a point where the permittivity of the thin film becomes vanishingly small. The method is relatively inefficient, but the researchers suggest that it could be enhanced by using other thin films in conjunction with metamaterials. 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.

Automated summary

This study demonstrates the potential of generating broadband terahertz radiation through surface optical rectification on a simple 19 nm-thick indium tin oxide thin film. The enhancement of the generated terahertz signal is observed when the pump laser is tuned around the epsilon-near-zero region of the film. This work opens up new avenues for broadening the bandwidth of terahertz generation in subwavelength thin films made of ENZ materials.