Nonlinear optics in a high-index of refraction material

Nonlinear response in a material increases with its index of refraction as $n^4$. Commonly, $n \sim$ 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 broadband refraction \cite{DiMei2018}. We here perform second-harmonic-generation experiments in KTN:Li with $n>26$. 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 an arbitrarily wide spectral acceptance, more than 100 nm in the near infrared spectrum, an ultra-wide angular acceptance, up to $\pm 40^{\circ}$, with no polarization selectivity, and can be tuned to allow bulk supercontinuum generation. Results pave the way to highly efficient versatile and adaptable nonlinear optical devices with the promise of single-photon-to-single-photon nonlinear optics.Comment: 9 pages, 5 figure

Photonics in the ferroelectric super-crystal phase

Nanodisordered ferroelectric perovskites belong to the family of relaxor ferroelectrics, and have long been attracting considerable attention in view of their unique physical properties. The introduction of compositional disorder on the nanoscale leads to the appearance of a broad temperature and frequency dependent peak in the dielectric susceptibility that manifests thermal, electric field, and strain hysteresis and is associated with anomalous relaxation. The presence of different compounds introduces, for specific composition concentrations, competing structural phases leading to unique polarization properties, such as the anomalous large capacitance and the giant piezoelectric effect. Recently, a new ferroelectric phase of matter, the spontaneous super-crystal phase (SC), has been discovered in bulk solid-solution of nanodisordered ferroelectric perovskite, several degrees below the Curie point. In this phase, domains, instead of locking into a disorganized pattern of clusters, form a 3D regular lattice of spontaneous polarization with micrometer lattice constant across macroscopic samples. This phase mimics standard solid-state structures but on scales that are thousands of times larger. The work presented in this thesis is an experimental investigation, through several photonics techniques, of the SC phase. In order to investigate the properties of the underlying ferroelectric domains, we first analyze the light-polarization dynamics which emerge from the interplay of mesoscopic domain ordering and anisotropy. Results indicate that polarized light propagating through the SC spatially separates in its polarization components, of mutually orthogonal linear polarization states. Furthermore, performing diffraction and refraction experiments, we discover that the SC phase is also accompanied by a broadband giant refraction (GR). Here the effective index of refraction is greater than 26 across the entire visible spectrum, even though no optical resonance is in place. The result is a material with no chromatic aberration and no diffraction. The discovery of GR opens up a wholly new realm of study, allowing us to expand our investigation to the field of nonlinear optics. Enhanced response causes wavelength conversion to occur in the form of bulk Cherenkov radiation with an arbitrarily wide spectral acceptance, more than 100 nm in the near infrared spectrum, an ultra-wide angular acceptance, up to $pm40^ circ$, with no polarization selectivity. From a more fundamental point of view, trying to understand the behavior and physics of complexity-driven GR, in particular the role played by ferroelectric clusters, using a 3D orthographic cross-polarizer projection technique, we provide for the first time, direct imaging of fractal cluster percolation. We also study the effect that the SC, of micrometer-scale, has on the average atomic structure, using several results, obtained through different experimental techniques, from X-ray diffraction, to calorimetry. What we have found, is that the emergence of the SC is accompanied by a large scale and coherent anomalous lattice deformation. Alongside the investigation of the SC phase, we have exploited the strong nonlinear optical response of disordered ferroelectric crystals at the phase transition, which makes these materials suitable to study the physics of nonlinear waves. In our study, we focus principally on the exploration of applications in electro-optic integrated circuits, based on linear and nonlinear waves, and on the analysis of the physical origin of so-called soliton rogue waves

Observation of extreme nonreciprocal wave amplification from single soliton-soliton collisions

We report the observation of strong nonreciprocal soliton amplification mediated by a Raman-scattering-like effect in single isolated collisions. A pump soliton is found to lose the greater part of its energy to a signal soliton, irrespective of the pump-signal relative amplitude. The result is an efficient rectifying mechanism able to accumulate energy into extreme waves. Experiments are carried out through photorefractive soliton two-wave mixing with a gain coefficient of up to 80 cm-1 that emerges in conditions of nonlinear response, leading to the formation of rogue waves

Using Bessel Beams to Induce Optical Waveguides

Optical fabrication of waveguides in a volume is limited by diffraction in the writing beams. We demonstrate the use of nondiffracting waves in the form of Bessel beams to fabricate scalable optical wiring through direct writing in a photosensitive volume. Experiments are performed in paraelectric potassium-lithium-tantalate-niobate (KLTN), where writing occurs through photogenerated space charge while guiding and electro-optic functionality are supported by the quadratic electro-optic effect. The method allows components to be integrated sequentially without interfering with each other during fabrication, an intrinsic superposition property that is used to realize single, double, and multiple waveguides, and 1 × 2, 1 × 3, and 1 × 4 splitters, and electrically controlled optical switching

Anomalous optical properties of KTN:Li ferroelectric supercrystals

Abstract We report a spectroscopic investigation of potassium–lithium–tantalate–niobate (KTN:Li) across its room-temperature ferroelectric phase transition, when the sample manifests a supercrystal phase. Reflection and transmission results indicate an unexpected temperature-dependent enhancement of average index of refraction from 450 nm to 1100 nm, with no appreciable accompanying increase in absorption. Second-harmonic generation and phase-contrast imaging indicate that the enhancement is correlated to ferroelectric domains and highly localized at the supercrystal lattice sites. Implementing a two-component effective medium model, the response of each lattice site is found to be compatible with giant broadband refraction