Disordered Nonlinear Metalens for Raman Spectral Nanoimaging.

Abstract

Over the past decades, considerable progress has been made toward far-field optical imaging beyond the diffraction limit. However, most working proof-of-concepts are based on either time-consuming scanning of a subdiffraction focal spot over a sample or postrecovery treatment using a priori information on a sought image. To our knowledge, none of these can be regarded as being close to a perfect far-field superlensing system capable of real-time color imaging with subwavelength resolution. In this paper, we suggest a proof-of-concept for far-field nonlinear metalens that is made of a disordered metal-dielectric nanocomposite. Postoxidation of a refractory titanium nitride (TiN) thin film, used as a nonlinear plasmonic material, results in the formation of a titanium oxynitride (TiON) film comprising a mixture of multiple phases of TiOxNy. Due to a double epsilon-near-zero behavior near the percolation threshold, the TiON favors supercoupling of the incident light to surface plasmon resonance within the visible and near-infrared range. Point spread function narrowing is achieved owing to the multiplicative nature of stimulated Raman scattering (SRS) and enhanced third-order optical nonlinearity in TiN and TiO2 particle chains through plasmon resonances and Anderson localization of light, respectively. Combined with a conventional confocal optical microscope, the multimode metalens shows subwavelength resolution of λ/6NA at different visible wavelengths (SRS overtones) using multiwalled carbon nanotubes as a test sample. We are confident that our finding will bring us one step closer to developing a robust and versatile far-field super-resolution color imaging system and, eventually, implementing "eye-on-a-chip" technology.