Effective Nonlinear Index Coefficient Research Articles

Radiation pressure and electrostriction induced enhancement for Kerr-like nonlinearities in a nanoscale silicon pedestal waveguide

We report an enhancement of effective nonlinear-index coefficient from the combination of radiation pressure and electrostriction in an air-cladding silicon pedestal waveguide. The effective nonlinear-index coefficient was experimentally measured by characterizing spectral broadening with incident pulses in the pedestal waveguide. Larger spectral broadening was observed in the pedestal waveguide compared to a conventional silicon waveguide. Theoretically, the primary enhancement of the effective nonlinear-index coefficient can be calculated by using Kramers–Kronig relations, which is associated with the change in absorption from stimulated Brillouin scattering in the pedestal waveguide. The calculations were consistent with experimental results. By tailoring the pillar width of the pedestal waveguide, about 27.6% enhancement in the effective nonlinear-index coefficient of the pedestal waveguide was possible at the center wavelength of 1555 nm.

Observation of photorefractive effects in blue-phase liquid crystal containing fullerene-C60.

Photorefractive effects manifested in two beam coupling and side diffractions are observed in fullerene-C60 doped blue-phase liquid crystals (BPLC-C60) upon application of a DC bias field. The mechanism at work here is attributed to BPLC lattice distortion by the combined DC (Edc)+ photorefractive space-charge (Ephoto) fields, in addition to the DC + optical field induced effects reported in previous studies of dye-doped system. The first order diffraction efficiency of ∼2×10-3 and beam coupling gain of over 2% are observed in a 55μm thick sample with input laser beam power of 5mW at an applied DC voltage of 160V. The effective nonlinear index coefficient n2 of BPLC-C60 is measured to be on the order of 10-2 cm2/W, which is slightly lower than their NLC counterparts. Owing to the isotropy of BPLC optical properties, these effects can be observed with more relaxed requirements on the laser polarizations, directions of incidence, and sample orientations.

Anisotropic diffraction pattern formation from a nematic liquid crystals film induced by low-power linearly polarized beam

Anisotropic diffraction patterns were observed on far-field screen induced by a weak (0.16 W/cm2) linearly polarized collimated beam passing through a homeotropic C60-doped nematic liquid crystal (5CB) cell. Based on orientational photorefractive effect, two-wave coupling experiment was carried out to induce liquid crystal molecule re-orientation. The form of space-charge field with the Gaussian distribution induced by the Gaussian beam incident on the sample is investigated. The outline of far-field diffraction pattern is concentric ring, and shows symmetrical gaps in the direction perpendicular to the direction of polarization of light. In addition, diffraction pattern changes when the polarization of the incident light is changed, effective nonlinear index coefficient is n20.3 cm2/W. The results showed that our theoretical analyzsis was consistent with the observed experimental phenomena.

Electrically controlled dynamic holographic gratings in fullerene C60-doped nematic liquid crystals

A new method to demonstrate the dynamic enhancement of the photorefractive effect in C60-doped homeotropically aligned 4-cyano-4′-pentyl-biphenyl (5CB) liquid crystals under the influence of an externally applied dc voltage is presented in the read-out process. We observed clear holographic images due to the transient diffraction efficiency enhancement controlled by the application of a dc voltage. The hidden gratings are due to a light-induced surface-charge modulation and amplified by the Carr–Helfrich effect in the presence of the electric field produced by the dc voltage. A qualitative analysis of the phenomenon, according to which the effective bulk voltage is modulated due to light-induced surface modifications, is proposed. We achieved the highest transient first-order diffraction efficiency of up to 40%. The effective nonlinear index coefficient is as large as 0.2 cm2 W−1.

AC electric field assisted photo-induced high efficiency orientational diffractive grating in nematic liquid crystals

An orientational grating with reduced scattering noise is formed in fullerene-doped and undoped homeotropic nematic liquid crystal cells by simultaneous application of both spatially modulated light and ac electric field. The first-order diffraction efficiency obtained in the fullerene-doped cell reaches ~30 %, approaching the maximum value predicted by theory. Effective nonlinear index coefficient is ~0.9 cm2/W and ~1000 times larger than previous observations in C(60)-doped nematic liquid crystals using a dc field. Grating formation time can be as short as 1 s, which is ~100 times shorter than those found by us using a dc field and previous studies.

Experimental and theoretical study of higher-order nonlinearities in chalcogenide glasses

We investigate experimentally and theoretically the nonlinear properties of two distinct chalcogenide glasses. Nonlinearities are measured through a spatially resolved Mach–Zehnder interferometer. It is shown that the resulting nonlinear index coefficient cannot be correctly described with the usual cubic model. A more convenient theory is developed assuming a medium with two-photon nonlinear absorption and both cubic ( n 2) and quintic ( n 4) nonlinear index variations. The resulting closed-form expression for the effective nonlinear index coefficient allows to extract the cubic and quintic index coefficients.

Nonlinear Schrodinger solitons in media with non-zero second-order nonlinear susceptibility

It is demonstrated theoretically for the first time that, under the strong dispersion condition, media with non-zero second-order nonlinear susceptibility can support the soliton propagation as optical fibres do, but the nonlinear index coefficient n2 in the nonlinear Schrodinger equation should be substituted by the effective nonlinear index coefficient defined here. The results obtained here can be easily extended to discuss the other nonlinear effects connected with the nonlinear index coefficient in the media.

Optical bistability from a thermodynamic phase transition in vanadium dioxide

We demonstrate cw optical bistability at 1.06 μm using the large thermo-optical nonlinearity from the metal-semiconductor phase transition in a thin-film interference filter of vanadium dioxide. Highly stable and reproducible operation has been obtained. By biasing at the transition point, an all-optical external switching between the semiconducting and the metallic states has been demonstrated, with a contrast ratio of 15:1. The effective nonlinear index coefficient is n2 ≈ 1.6 cm2/W with a switching energy density of 135 pJ/μm2 and a speed of 15 ms. The tunability of the material properties of vanadium dioxide provides a means to make devices with a wide range of switching powers and speeds.

SOLITON PROPAGATION IN OPTICAL WAVEGUIDES MADE OF MEDIA WITH NON-CENTRAL SYMMETRY

In this paper, it is demonstrated theoretically that, under the strong dispersion condition, the optical waveguides made of the media with non-central symmetry where the second order nonlinear susceptibility (SONS) is not zero, e.g.the thin-film waveguides and crystalline fibers, can also support these solitons but the nonlinear-index coefficient (NIC) in the nonlinea schro-dinger equation should be substituted by the effective NIC defined here. The results obtained here can be easily extended to discuss the other nonlinear effects connected with the NIC in the media with noncentral symmetry because of the definition of the effective NIC.