Optical coherent detection through multi-scattering media by wave-mixing cleaning effect in liquid-crystal OASLM.

Liquid-crystal (LC) optically addressable spatial light modulators (OASLMs) allow control of the phase and amplitude of optical beams. By performing wave mixing in an OASLM, we show that coherent phase detection can be achieved for light beams passing through highly scattering media, such as foam layers with several cm thicknesses. Thanks to the adaptive response of our OASLM, the phase information on the speckle signal is transferred at the output of the OASLM to the plane wave reference beam, allowing the cleaning of optical distortions and the direct measurement of amplitude phase modulations with a small diameter single photodiode.

Doppler Gyroscopes: Frequency vs Phase Estimation.

We consider the fundamental roles of frequency versus phase in parameter estimation, specifically in the Sagnac effect. We describe a novel, ultrasensitive gyroscope based on the extremely steep frequency-dependent gain of a liquid crystal light valve. We provide compelling experimental evidence that the Doppler shift is fundamental in the Sagnac effect giving clarity to a long-debated question.

kHz-speed optically induced phase gratings with liquid crystal light valves in transient dynamic mode.

Liquid crystal light valves (LCLV) are optically addressable spatial light modulators that allow controlling the phase and amplitude properties of optical beams. We show that sub-milliseconds phase and amplitude modulations can be obtained when operating the LCLV in the transient dynamic mode by setting the working point close to the saturation of the response. Thanks to the large birefringence of the liquid crystals, this condition provides enough phase shifts to respond to the needs of several methods for optical measurement, dynamic holography, interferometry, and imaging through phase disturbing media, while providing kilohertz (kHz) speed.

Observation of stochastic resonance in a liquid-crystal light valve with optical feedback induced by colored noise in the driving voltage.

Stochastic resonance is a noise phenomenon that benefits applications such as pattern formation, neural systems, microelectromechanical systems, and image processing. This study experimentally clarifies that the orientation of the liquid crystal molecules was switched between two stable positions when stochastic resonance was induced by colored noises in a liquid crystal light valve with optical feedback. Ornstein-Uhlenbeck and dichotomous noises were used for colored noise, and the noise was applied to the drive voltage of the liquid crystal light valve.

Umbilical defect dynamics in an inhomogeneous nematic liquid crystal layer.

Electrically driven nematic liquid crystals layers are ideal contexts for studying the interactions of local topological defects, umbilical defects. In homogeneous samples the number of defects is expected to decrease inversely proportional to time as a result of defect-pair interaction law, so-called coarsening process. Experimentally, we characterize the coarsening dynamics in samples containing glass beads as spacers and show that the inclusion of such imperfections changes the exponent of the coarsening law.

Nanoscale hyperspectral imaging of tilted cholesteric liquid crystal structures.

Ongoing research on chiral liquid crystals takes advantage of the peculiar behavior of twisted structures subject to curvature. We demonstrate the fine tunability of the characteristics of the bandgap of a cholesteric structure in which the orientation of the helix axis spatially changes. To date, the spectral resolution of the order of 6 nm, herein reached by hyperspectral imaging, has not been solved in tilted helices.

Femtometer displacement resolution with phase-insensitive Doppler sensing.

The measurement of extremely small displacements is of utmost importance for fundamental studies and practical applications. One way to estimate a small displacement is to measure the Doppler shift generated in light reflected off a moving object, converting a displacement measurement into a frequency measurement. Here we show a sensitive device capable of measuring μHz/Hz Doppler frequency shifts corresponding to tens of femtometer displacements for a mirror oscillating at 2 Hz.

Spontaneous light-induced Turing patterns in a dye-doped twisted nematic layer.

Optical pattern formation is usually due either to the combination of diffraction and nonlinearity in a Kerr medium or to the temporal modulation of light in a photosensitive chemical reaction. Here, we show a different mechanism by which light spontaneously induces stripe domains between nematic states in a twisted nematic liquid crystal layer doped with azo-dyes. Thanks to the photoisomerization process of the dopants, light in the absorption band of the dopants creates spontaneous patterns without the need of temporal modulation, diffraction, Kerr or other optical nonlinearity, but based on the different scales for dopant transport processes and nematic order parameter, which identifies a genuine Turing mechanism for this instability.

Shear-enhanced diffraction from thin permanent gratings in dye-doped nematic liquid crystal cells.

Permanent gratings are recorded in planar-aligned dye-doped nematic liquid crystal cells under visible light illumination. By increasing the irradiation intensity and exposure time, several diffraction orders of the recorded gratings are obtained in the Raman-Nath diffraction regime. By applying a dynamic transverse shear on one of the confining plates of the cell, an enhancement of the diffraction efficiency is achieved, which follows the period of the grating.

Dynamical optical response of nematic liquid crystal cells through electrically driven Fréedericksz transition: influence of the nematic layer thickness.

A dynamical optical characterization of planar nematic liquid-crystal cells electrically driven through the Fréedericksz transition is presented. Our method involves applying voltage steps with different starting voltage close to the Fréedericksz threshold. Measurements are performed on cells with various thickness, from a few microns up to 180µm, and highlight the transient molecular disorder occurring close to the Fréedericksz transition.

Tunable angular shearing interferometer based on wedged liquid crystal cells.

The concept of a liquid crystal wedge as a tunable angular shearing interferometer is introduced and demonstrated to combine both high stability and high tunability. Different wedges are fabricated from planar aligned nematic liquid crystal cells with thickness gradients. These wedges are shown to produce stable interferograms from the polarization interference between the ordinary and extraordinary waves propagating in different directions at the output of the cell.

Berry Phase of Light under Bragg Reflection by Chiral Liquid-Crystal Media.

A Berry phase is revealed for circularly polarized light when it is Bragg reflected by a chiral liquid-crystal medium of the same handedness. By using a chiral nematic layer we demonstrate that if the input plane of the layer is rotated with respect to a fixed reference frame, a geometric phase effect occurs for the circularly polarized light reflected by the periodic helical structure of the medium. Theory and numerical simulations are supported by an experimental observation, disclosing novel applications in the field of optical manipulation and fundamental optical phenomena.

Continuously tunable femtosecond delay-line based on liquid crystal cells.

We introduce a new device for group and phase delay steering of femtosecond pulse trains that makes use of cascaded, electrically driven, nematic liquid-crystal cells. Based on this approach we demonstrate a continuously tunable optical delay line. The simple collinear implementation with no moving parts enables to shape the achievable temporal range with sub-femtosecond accuracy.

Adaptive holographic interferometer at 1.55 μm based on optically addressed spatial light modulator.

We report the realization of an adaptive holographic interferometer based on two-beam coupling in an optically addressed liquid crystal spatial light modulator operating at 1.55-μm. The system allows efficient phase demodulation in noisy environment and behaves as an optical high-pass filter, with a cut-off frequency of approximately 10 Hz, thus filtering slow phase disturbances (due to, for example, temperature variations or low frequency fluctuations) and keeping the detection linear without the need of heterodyne or active stabilization.

Self-adaptive vibrometry with CMOS-LCOS digital holography.

A self-adaptive interferometer based on digital holography is here reported for applications involving measurements of very small amplitude vibrations. The two-beam coupling gain is optimized through an electronic feedback, while the dynamic character of the hologram allows reaching a high sensitivity of the interferometric measurements even in unstable environments and with strongly distorted wave-fronts. The frequency bandwidth of the adaptive interferometer and its spatial resolution are determined, respectively, by the maximum frame rate and the pixel size of the camera and of the spatial light modulator used to build the digital holographic setup.

Precision Doppler measurements with steep dispersion.

Controlling the group velocity of light is a valuable resource for quantum and classical optical processing and high performance sensor technologies. In this context, slow-light (SL) and the associated steep dispersion have been proposed to increase the sensitivity of certain types of interferometers. Here, we show that the interaction of two intensity-balanced light beams in a SL medium can be used to detect Doppler shifts with extremely high sensitivity.

Transmissive liquid crystal light-valve for near-infrared applications.

An optical valve is realized by associating a nematic liquid crystal layer with a Cr-doped gallium arsenide as a photoconductive substrate. The light-valve is shown to efficiently operate in transmission at 1.06 μm optical wavelength.

Dynamic ultrasound modulated optical tomography by self-referenced photorefractive holography.

Photorefractive Bi(12)SiO(20) single crystal is used for acousto-optic imaging in thick scattering media in the green part of the spectrum, in an adaptive speckle correlation configuration. Light fields at the output of the scattering sample exhibit typical speckle grains of 1 μm size within the volume of the nonlinear crystal. This heterogeneous illumination induces a complex refractive index structure without applying a reference beam on the crystal, leading to a self-referenced diffraction correlation scheme.

Slow-light birefringence and polarization interferometry.

By performing two-beam coupling experiments in a liquid-crystal light valve, we report a large slow-light birefringence (SLB) phenomenon, with orthogonal polarization states traveling at very different group velocities. We show that SLB can be exploited for realizing a common-path polarization interferometer able to detect phase variations with enhanced sensitivity.

Storage of localized structure matrices in nematic liquid crystals.

We show experimentally that large matrices of localized structures can be stored as elementary pixels in a nematic liquid crystal cell. Based on optical feedback with phase modulated input beam, our system allows us to store, erase, and actualize the localized structures in the matrix.

Fragmentation instability of a liquid drop falling inside a heavier miscible fluid.

We report a different type of drop instability, where the density difference between the drop and the solvent is negative. We show that the drop falls inside the solvent down to a minimum height, then fragmentation takes place and secondary droplets rise up to the surface. We have developed a theoretical model that captures the essentials of the phenomenon and predicts the correct scalings for the rise-up time and the minimum height.

Spontaneous nucleation of localized peaks in a multistable nonlinear system.

In a nonlinear optical experiment we report a unique class of localized structures, which appears as localized peaks of a pattern nucleating over another pattern. We show that this occurs when the system is driven through three pattern branches of solutions, accompanied by the appearance of localized peaks with two different amplitudes. Spontaneous creation and motion of localized peaks are triggered by amplitude and phase fluctuations of the underlying pattern.