Concept of inverted refractive-index-contrast grating mirror and exemplary fabrication by 3D laser micro-printing.

Highly reflective mirrors are indispensable components in a variety of state-of-the-art photonic devices. Typically used, bulky, multi-layered distributed Bragg (DBR) reflectors are limited to lattice-matched semiconductors or nonconductive dielectrics. Here, we introduce an inverted refractive index-contrast grating (ICG) as compact, single-layer alternative to DBR. In the ICG, a subwavelength one-dimensional grating made of a low-refractive-index material is implemented on a high-refractive-index cladding. Our numerical simulations show that the ICG provides nearly total optical power reflectance for the light incident from the side of the cladding whenever the refractive index of the grating exceeds 1.75, irrespective of the refractive index of the cladding. Additionally, the ICG enables polarization discrimination and phase tuning of the reflected and transmitted light, the property not achievable with the DBR. We experimentally demonstrate a proof-of-concept ICG fabricated according to the proposed design, using the technique of sub-µm 3D laser lithography in which thin stripes of IP-Dip photoresist are micro-printed on a Si cladding. This one-step method avoids laborious and often destructive etching-based procedures for grating structuration, making it possible to implement the grating on any arbitrary cladding material.