Giant broadband refraction in the visible in a ferroelectric perovskite

In principle, materials with a broadband giant index of refraction (n > 10) overcome chromatic aberration and shrink the diffraction limit down to the nanoscale, allowing new opportunities for nanoscopic imaging¹. They also open alternative avenues for the management of light to improve the performance of photovoltaic cells². Recent advances have demonstrated the feasibility of a giant refractive index in metamaterials at microwave and terahertz frequencies^3,4, but the highest reported broadband index of refraction in the visible is n < 5 (ref. ⁵). Here, we report a ferroelectric perovskite with an index of refraction of n > 26 across the entire visible spectrum and demonstrate its behaviour using white-light and laser refraction and diffraction experiments. The sample, a solid-solution K_0.997Ta_0.64Nb_0.36:Li_0.003 (KTN:Li) perovskite^6–12, has a naturally occurring room-temperature phase that propagates visible light along its normal axis without significant diffraction or chromatic dispersion, irrespective of beam size, intensity and angle of incidence.