Giant broadband refraction in the visible in a nanodisordered ferroelectric perovskite

We report broadband refraction and diffraction experiments in a ferroelectric perovskite, a solid-solution K_0.997Ta_0.64Nb_0.36:Li_0.003 (KTN:Li), that demonstrate an index of refraction of n > 26 across the entire visible spectrum [1]. 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 imaging [2]. They also open alternative avenues for the management of light to improve the performance of photovoltaic cells. Recent advances have demonstrated the feasibility of a giant refractive index in metamaterials at microwave and terahertz frequencies [2, 3], but the highest reported broadband index of refraction in the visible is n < 5 [4]. A Giant Refraction (GR) with n≫1 across the visible spectrum implies that light propagates along the normal of the input facet of the sample (Fig.1a) and exits from it without suffering significant chromatic dispersion and diffraction irrespective of beam size, intensity and angle of incidence. The physical origin of this behavior is compatible with the emergence of a periodic 3D lattice of spontaneous polarization (Fig.1b), which has already been observed in other crystals below the the Curie point T_c, known as a Super Crystal (SC)[5]. Basic evidence for GR is reported in Fig.1c-d. In the experiment, white light from a commercial projector is focused using a microscope objective onto the input facet of the sample, tilted appropriately in the horizontal plane. Beam dynamics are detected by collecting scattered light using a top-view high-aperture microscope, while transmitted light is collected and imaged using a high-aperture lens and camera. Figure 1c presents a top-view image showing light undergoing GR: the beam is transmitted orthogonal to the sample facets, irrespective of the actual tilt angle (over a wide range of angles). The beam, which would normally diffract to engulf the entire sample, does not spread, and no chromatic aberrations are seen. An absence of chromaticity in the GR component is evident when compared with light scattered from the sample support (Fig.1d).