Nanoimprinting of Refractive Index: Patterning Subwavelength Effective Media for Flat Optics

Subwavelength effective media offer a powerful tool for tailoring optical properties on the surface of a chip but remain challenging to realize at optical frequencies owing to their demand for nanoscale features. To meet this challenge, a simple two-step method for patterning refractive index on the surface of a chip is introduced. The process is referred to as "nanoimprinting of refractive index" (NIRI) and relies on the direct nanoimprinting and plastic deformation of high-porosity mesoporous silicon thin films. This is shown to enable very wide and patternable tuning of refractive index over a range Delta n approximate to 1 RIU. Investigation of the effective medium response to film compression reveals close agreement to effective medium theory only after factoring in the contribution from the nanoscaled native oxide. NIRI opens a new route for harnessing the subwavelength degree of freedom and offers the prospect of realizing high-performance and low-cost flat optics.