In a study published in ACS Nano, researchers from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences proposed a broadband ultrafast all-optical switching scheme based on epsilon-near-zero (ENZ) material-enhanced nonlinear absorption. The sub-wavelength corrugated indium tin oxide (ITO) film was used to realize broadband ultrafast all-optical switching in the 1450–1650 nm telecom band. The figure of merit (FOM) of the device is superior to reported all-optical switches based on planar aluminum doped zinc oxide (AZO) films, ITO nanorods, and metal-insulator-metal (MIM) nanocavities.  

Ultrafast all-optical switches are key devices in the fields of optical communication, optical computing and quantum information processing. In recent years, ENZ-enhanced nonlinear refraction has provided an effective solution for ultrafast all-optical switching. However, nonlinear refraction-induced resonant mode shifts are difficult to achieve wide-wavelength operation.  

The ENZ-enhanced nonlinear refraction of planar ENZ films has intrinsic polarization and angle-dependent characteristics, limiting the application scenarios of these all-optical switches. The broadband nonlinear absorption of gapless graphene can solve the narrow-band problem of ultrafast all-optical switches based on nonlinear refraction, providing a new idea for broadband all-optical switches based on ENZ materials. However, unlike graphene, planar ENZ films are difficult to achieve flat perfect absorption over a wide-wavelength range, making their direct use for broadband all-optical switching impossible.  

In this study, researchers proposed a broadband ultrafast all-optical switching scheme based on the enhanced nonlinear absorption of corrugated ITO films. The core of this scheme is to utilize the broadband perfect absorption and field enhancement realized by ENZ and localized surface plasmon resonance (LSPR) modes in corrugated ITO films to enhance broadband nonlinear absorption, and to utilize the unique intra-band ultrafast nonlinear response of ITO materials to realize broadband ultrafast all-optical switching.  

Then, researchers designed and fabricated subwavelength corrugated ITO films based on SiO₂ microsphere masks. By exciting ENZ and LSPR modes, the corrugated ITO film can achieve perfect absorption (A>98%) and large local field enhancement in the range of 1450-1650 nm, which enables the device to achieve a nonlinear saturable absorption coefficient at 1450 nm as high as -1.5 × 10⁵ cm /GW, the corresponding extinction ratio and switching time being 14.32 dB and 350 fs, respectively, at a pump flux of 18.51 mJ/cm². In addition, the good structural symmetry of the corrugated ITO film enables the all-optical switch to exhibit polarization-independent and wide-angle features within the 200 nm absorption band.  

The corrugated ENZ films can overcome the inherent narrow-band, polarization-dependent, and angle-dependent problems of planar ENZ films without increasing the response time, making them a potential ENZ material platform for ultrafast all-optical switching.