Evidence of double-loop hysteresis in disordered ferroelectric crystal

Double-loop electric-field vs polarization hysteresis is investigated in a depoled compositionally disordered lithium-enriched potassium tantalate niobate crystal. Comparing electro-optic response and dielectric spectroscopy indicates that the anomalous response occurs for those temperatures in which the sample also manifests a temperature hysteresis in the low-frequency dielectric function. An electric-field hysteresis at concurrent temperatures suggests an underlying role of reorienting mesoscopic polar regions that accompany the nonergodic phase. Published under license by AIP Publishing

Programming scale-free optics in disordered ferroelectrics

Using the history-dependence of a dipolar glass hosted in a compositionally-disordered lithium-enriched potassium-tantalate-niobate (KTN:Li) crystal, we demonstrate scale-free optical propagation at tunable temperatures. The operating equilibration temperature is determined by previous crystal spiralling in the temperature/cooling-rate phase-space

Aging solitons in photorefractive dipolar glasses

We study experimentally the aging of optical spatial solitons in a dipolar glass hosted by a nanodisordered sample of photorefractive potassium-sodium-tantalate-niobate (KNTN). As the system ages, the waves erratically explore varying strengths of the nonlinear response, causing them to break up and scatter. We show that this process can still lead to solitons, but in a generalized form for which the changing response is compensated by changing the normalized wave size and intensity so as to maintain fixed the optical waveform

Anti-diffracting beams through the diffusive optical nonlinearity

Anti-diffraction is a theoretically predicted nonlinear optical phenomenon that occurs when a light beam spontaneously focalizes independently of its intensity. We observe anti-diffracting beams supported by the peak-intensity-independent diffusive nonlinearity that are able to shrink below their diffraction-limited size in photorefractive lithium-enriched potassium-tantalate-niobate (KTN:Li)

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

Subwavelength anti-diffracting beams propagating over more than 1,000 Rayleigh lengths

Propagating light beams with widths down to and below the optical wavelength require bulky large-aperture lenses and remain focused only for micrometric distances. Here, we report the observation of light beams that violate this localization/depth- of-focus law by shrinking as they propagate, allowing resolution to be maintained and increased over macroscopic propagation lengths. In nanodisordered ferroelectrics we observe a non-paraxial propagation of a sub-micrometre-sized beam for over 1,000 diffraction lengths, the narrowest visible beam reported to date. This unprecedented effect is caused by the nonlinear response of a dipolar glass, which transforms the leading opticalwave equation into a Klein-Gordon-type equation that describes a massive particle field. Our findings open the way to high-resolution optics over large depths of focus, and a route to merging bulk optics into nanodevices

Measuring state-of-order by dielectric response: A comprehensive review on Fröhlich entropy estimation

The so-called Fröhlich entropy is the entropy variation of a material under the application of an electric field. This quantity can be calculated, under suitable hypotheses, from the measured real part of the static dielectric function, when the imaginary part is negligible. Although Fröhlich entropy is defined for a biased sample, a straightforward physical interpretation correlates it to the state-of-order of the considered physical system in absence of field. When Fröhlich entropy is calculated from experimental results, its trend is able to give several information about the evolution in temperature of the explored compound, especially of its phase transition features. We here provide a comprehensive review of the physical systems (dipolar liquids and nematicons, organic molecular crystals, metallic nanoparticles, inorganic disordered ferroelectrics, etc.) where this approach has been exploited with the aim of evaluating their state of order and its temperature evolution. The variety of compounds where this method has been applied demonstrates that the estimation of the Fröhlich entropy can be considered a trustworthy tool for carrying out study on the state-of-order of different classes of materials. Indeed Fröhlich entropy evaluation can be considered a fruitful and reliable investigation technique which can be exploited alongside more usual experimental approaches

Line-Shaped Illumination: A Promising Configuration for a Flexible Two-Photon Microscopy Setup

An innovative two-photon microscope exploiting a line-shaped illumination has been recently devised and then implemented. Such configuration allows to carry out a real-time detection by means of standard CCD cameras and is able to maintain the same resolution as commonly used point-scanning devices, thus overcoming what is usually regarded as the main limitation of line-scanning microscopes. Here, we provide an overview of the applications in which this device has been tested and has proved to be a flexible and efficient tool, namely imaging of biological samples, in-depth sample reconstruction, two-photon spectra detection, and dye cross-section measurements. These results demonstrate that the considered setup is promising for future developments in many areas of research and applications