Broadband and large surface enhancements of the local electric field enabled by cross-etched hyperbolic metamaterials

Hyperbolic metamaterials (HMMs) have recently attracted significant research attention due to their hyperbolic wavevector iso-frequency contour, which leads to substantial local electric field (EF) enhancements that benefit optical processes, such as the nonlinear generation, quantum science, biomedical sensing, and more. However, three main challenges hinder their practical implementation: the difficulty in exciting their resonant modes using free-space incidence, the weak enhancement of surface EF, and the narrow spectral range of EF enhancements. Herein, we proposed cross-etched HMMs (CeHMMs) as a novel type of HMM, addressing these issues. The CeHMMs can be easily fabricated by etching periodic cross-shaped arrays in conventional HMMs, and exhibited two resonant high-k modes within the spectral range of 700-1100 nm under linearly, circularly, or elliptically polarized incident light from free space. It was also calculated that the CeHMMs can provide a large surface EF enhancement across a broad spectral range (over 500 nm). After integrating a single layer of WSe₂ onto the top surface, the photoluminescence (PL) enhancements of the CeHMMs and their hot spots, based on the emission resonance, were calculated to be 9.72 and 62 times, respectively. With their ability to provide broadband surface EF enhancement, CeHMMs are expected to offer considerable potential for a variety of nanophotonic applications, including nonlinear optics, integrated optics, and quantum photonics.