Hyperspectral near-field imaging of light bending in a graded photonic crystal

Artificial materials at optical frequencies have raised a strong interest in the last years, including photonic metamaterials, graded photonic crystals, and simple gradient index structures. The main common objective of these approaches is achieving a tight control of the electromagnetic guided-wave fields to play with light properties and propose versatile optical functions. In this general context, this work is focused on gradual photonic crystals (GPhCs) working in the diffraction regime, i.e. close to the photonic crystal (PhC) bandgap. A two-dimensional chirp of the PhC lattice parameters using a modelling approach based on Hamiltonian Optics (HO) was then proposed in [1] and far-field experimental observations of the light bending effect was shown in [2]. In this work, we directly visualize this fascinating mirage effect in a GPhC at telecommunication wavelengths using a hyperspectral scanning near-field optical microscope (Hyp-SNOM). The Hyp-SNOM allows a spectrally and spatially continuous near-field imaging of the complex electromagnetic waves dispersion inside the GPhCs, and thus giving back insight into the physics underlying the reported phenomena.