Digital and Gradient Refractive Index Planar Optics by Nanoimprinting Mesoporous Silicon

We report the realization of digital and gradient index flat‐optics and planar waveguides using the ‘nanoimprinting refractive index’ (NIRI) technique applied to mesoporous silicon. This technique combines the distinct optical and mechanical metamaterial qualities of mesoporous silicon, including its widely tunable effective refractive index and ability to undergo plastic deformation with a near zero Poisson ratio. Nanoimprinting with premastered and reusable stamps containing analog or digital features enables the continuous or discontinuous patterning of refractive index with high contrast Δn ≥ 0.8 and subwavelength resolution. Using NIRI we experimentally demonstrate a wavefront shaping flat microlens array operating in the visible (405–635 nm) and mesoporous silicon and silica waveguides operating near 1310 nm. This study demonstrates the viability of patterning arbitrary refractive index distributions, n(x,y), on the surface of a chip while circumventing the challenges and limitations of top‐down lithographic techniques – thus opening a low‐cost and scalable approach for the realization of advanced planar optical technologies. A method for patterning refractive index distributions, n(x,y), on the surface of a chip is demonstrated based on nanoimprinting mesoporous silicon and applied to realize functional planar optical components with digital and gradient refractive index profiles. Device demonstrations include effective medium waveguides and flat effective medium micro‐lens arrays. This work opens a new route for planar optical design and fabrication.