Constraint-free wavelength conversion supported by giant optical refraction in a 3D perovskite supercrystal

Abstract

Nonlinear response in a material increases with its index of refraction as n4. Commonly, n ~ 1 so that diffraction, dispersion, and chromatic walk-off limit nonlinear scattering. Ferroelectric crystals with a periodic 3D polarization structure overcome some of these constraints through versatile Cherenkov and quasi-phase-matching mechanisms. Three-dimensional self-structuring can also lead to a giant optical refraction. Here, we perform second-harmonic-generation experiments in KTN:Li in conditions of giant broadband refraction. Enhanced response causes wavelength conversion to occur in the form of bulk Cherenkov radiation without diffraction and chromatic walk-off, even in the presence of strong wave-vector mismatch and highly focused beams. The process occurs with a wide spectral acceptance of more than 100 nm in the near infrared spectrum, an ultra-wide angular acceptance of up to ±40∘, with no polarization selectivity, and can be tuned to allow bulk supercontinuum generation. Results pave the way to highly efficient and adaptable nonlinear optical devices with the promise of single-photon-to-single-photon nonlinear optics.

Comprehensive review on Froehlich entropy estimation: a technique to investigate the state of order in condensed matter

“…The physical meaningfulness of FE analysis was pinpointed by its application to disordered ferroelectric single crystals, where this experimental approach was also tested to highlight anisotropic features. The analysis was specifically employed for describing the thermal evolution of long-range phases and state of order in inorganic perovskites for photonics applications [35, 37, 48-51, 66, 115-117], where peculiar dielectric, optical, thermodynamic, and structural effects are still under investigation [49,[118][119][120][121][122][123][124][125][126]. These compounds are known to host 12 nanosized reconfigurable polar regions, the so-called polar nanoregions (PNRs), which are mesoscopic regions with in-herent electric polarization.…”

Observation of an exotic lattice structure in the transparent KTa1−xNbxO3 perovskite supercrystal

“…A wide variety of innovative materials for a range of applications have a perovskite atomic structure. Examples are high-temperature and anomalous superconductors, photovoltaics, and a host of solids with unconventional and giant electromechanical, capacitive, piezoelectric, magnetoresistive, thermal, and optical response [1][2][3][4][5][6][7][8][9][10][11]. While new functional materials with a perovskite structure are still being discovered and engineered, many useful effects still have no general explanatory picture, making the whole subject matter an attractive riddle-ridden arena for condensed matter physics [12][13][14].…”

“…Specifically, illuminating the sample with plane-wave laser beams leads to far-field diffraction patterns typical of three-dimensional photonic crystals with a micrometric lattice constant [15][16][17]. Remarkably, focusing light into a single lattice site causes it to refract and diffract as if it had a broadband giant index of refraction (GR) [8,11]. The effect is thought to be connected with the formation of a ferroelectric supercrystal (SC), a 3D periodic domain mosaic of interwoven polarization vortices, each lattice site being the crossing of six polarization vortex cores (a hypervortex).…”

“…(a) Light is focused onto the input facet into a polarization vortex core, identified using giant refraction (inset). SHG for (b) TM, (c) TE, and (d) a superposition of TM and TE modes maps the signature spatial and polarization structure of the vortex [11].…”

“…The details of the local polar domain structure of the SC are analyzed using SHG experiments in a KTN:Li sample, as reported in Fig. 2 [11]. At room temperature the sample is in its nominal tetragonal phase (T = T C − 35 K) and manifests a 3D SC.…”