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

The hyperbolic metamaterials (HMMs) have gained great research efforts, because of their hyperbolic wavevector iso-frequency contour character which leads to great local electric field (EF) enhancements benefiting for boosting optical processes and applications, such as the nonlinear generation, quantum science, biomedical sensing, etc. However, there are mainly three problems of HMMs preventing their practical implementations, which are 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 propose a kind of new HMMs, which is the cross-etched HMMs (CeHMMs). The CeHMMs can be facially obtained by etching periodic cross-shape arrays in the normal HMMs, and exhibit two resonant high-k modes within the spectral range of 700-1100 nm under the linearly, circularly, or elliptically polarized incident lights from the free space. Also, it is calculated that the CeHMMs can provide large surface EF enhancement in a broadband spectral range (over 500 nm). After integrating the single layered WSe2 onto the top surface, the photoluminescence (PL) enhancements of the CeHMMs and their hot spots based on the emission resonance, are calculated to be 9.72 and 62 times, respectively. With the ability of broadband surface EF enhancement, it is believed the CeHMMs can offer considerable potentials for a variety of nanophotonic applications, including nonlinear optics, integrated optics, and quantum photonics.