Constraint-free wavelength conversion supported by giant refraction in a 3D perovskite Super-Crystal

We demonstrate second-harmonic generation (SHG) in a KTN:Li ferroelectric perovskite that manifests giant refraction (n ⪢ 1) [1] . When light is focused on the vortex or anti-vortex cores of an underlying ferroelectric super-crystal (SC) [2] , [3] , enhanced response allows conversion to occur through bulk nonlinear Cherenkov radiation even for highly focused non-phase-matched beams. The result is that wavelength conversion has a wide spectral acceptance of more than 100 nm in the near infrared, an ultra-wide angular acceptance of up to 40°, has no polarization selectivity, and occurs even in the presence of strong transverse wave-vector mismatch (see ± Fig. 1 ). The underlying physical mechanism is associated to the nonlinear response of the SC state, a 3D mesh of ferroelectric polarization that spontaneously forms below the Curie point ( Fig 1a ). In the SC lattice, each site is the core of a 3D vortex or anti-vortex topological defect along which light experiences broadband giant optical refraction and with it, a greatly enhanced nonlinear susceptibility. Results open up new applicative scenarios in the realm of frequency conversion and parametric amplification, the fundamental ingredients for a wide family of applications, including light sources, detection, optical processing, and quantum-state-generation [4] . Reduced constraints on launch angle can considerably reduce alignment requirements in nonlinear-based light sources, while increased tolerances in wavelength and polarization can support multiple simultaneous nonlinear processes with specific impact, for example, in the conversion of infrared images to the visible spectrum. In comparison, conventional wavelength conversion schemes are based on a cumulative wave interaction and constructive interference, a configuration that requires phase-matching and places constraints on polarization, wavelength, and direction of propagation of the interacting waves, apart from requiring highly specific crystal fabrication and poling geometries.