Asia Shapira

Two-dimensional nonlinear beam shaping

We develop a technique for two-dimensional arbitrary wavefront shaping in quadratic nonlinear crystals by using binary nonlinear computer generated holograms. The method is based on transverse illumination of a binary modulated nonlinear photonic crystal, where the phase matching is partially satisfied through the nonlinear Raman-Nath process. We demonstrate the method experimentally showing a conversion of a fundamental Gaussian beam pump light into three Hermite-Gaussian and three Laguerre-Gaussian beams in the second harmonic. Two-dimensional binary nonlinear computer generated holograms open wide possibilities in the field of nonlinear beam shaping and mode conversion.

Phase-matched nonlinear diffraction

We report on a new (to our knowledge) configuration incorporating both birefringence and quasi-phase-matching, enabling efficient phase-matched nonlinear diffraction in one-dimensional periodically poled nonlinear crystals. We demonstrate the method experimentally, showing an efficient nonlinear diffraction to the first few orders in two types of crystals, MgO doped congruent lithium niobate and congruent lithium niobate, and characterize its efficiency dependence on the fundamental power, the propagation angle, and the crystal temperature. This configuration can increase efficiencies observed in nonlinear diffraction experiments, enables ferroelectric domain characterization by nonlinear microscopy, and can be used to determine the duty cycles of periodically poled nonlinear crystals.

Unified retention model for localized charge trapping nonvolatile memory device

Retention after cycling in an NROM nonvolatile memory cell is investigated. Electrical characterizations combined with two-dimensional simulations were employed. By combining subthreshold current characterizations with gate-induced drain leakage measurements, the retention loss mechanisms are quantified and differentiated. A unified retention model for the NROM technology is proposed, incorporating both lateral charge transport in the nitride layer and hot carrier-induced interface-states formation. The fabrication conditions in the various studies reported in the literature and the resultant impact on interface-states formation are most likely the cause of the formally nonresolved debate regarding the origin of retention loss in NROM technology.

Nonlinear computer-generated holograms

We propose a novel technique for arbitrary wavefront shaping in quadratic nonlinear crystals by introducing the concept of computer-generated holograms (CGHs) into the nonlinear optical regime. We demonstrate the method experimentally showing a conversion of a fundamental Gaussian beam pump light into the first three Hermite-Gaussian beams at the second harmonic in a stoichiometric lithium tantalate nonlinear crystal, and we characterize its efficiency dependence on the fundamental power and the crystal temperature. Nonlinear CGHs open new possibilities in the fields of nonlinear beam shaping, mode conversion, and beam steering.

Spatial quadratic solitons guided by narrow layers of a nonlinear material

We report analytical solutions for spatial solitons supported by layers of a quadratically nonlinear (χ(2)) material embedded into a linear planar waveguide. A full set of symmetric, asymmetric, and antisymmetric modes pinned to a symmetric pair of the nonlinear layers is obtained. The solutions describe a bifurcation of the subcritical type, which accounts for the transition from the symmetric to asymmetric modes. The antisymmetric states (which do not undergo the bifurcation) are completely stable (the stability of the solitons pinned to the embedded layers is tested by means of numerical simulations). Exact solutions are also found for nonlinear layers embedded into a nonlinear waveguide, including the case when the uniform and localized χ(2) nonlinearities have opposite signs (competing nonlinearities). For the layers embedded into the nonlinear medium, stability properties are explained by comparison to the respective cascading limit.

TUNABLE NONLINEAR BEAM SHAPING BY A NON-COLLINEAR INTERACTION

LETER A R IC LE Abstract A method is proposed for nonlinear beam shaping, employing a non-collinear quasi phase-matched interaction in a crystal whose nonlinear coefficient is encoded by a computer generated hologram pattern. In this method the same axis is used for both satisfying the phase-matching requirements and encoding the holographic information, the result is a single shaped beam in the generated frequency. This allows to shape beams in one-dimension using a very simple method to fabricate patterned nonlinear crystals and to shape beams in two-dimensions with high conversion efficiency. The one-dimensional case is experimentally demonstrated by converting a fundamental Gaussian beam into Hermite-Gaussian beams at the second harmonic in a KTiOPO4 crystal. The two-dimensional case is demonstrated by generating HermiteGaussian and Laguerre-Gaussian beams in a stoichiometric lithium tantalate crystal. The suggested scheme enables broad wavelength tuning by simply tilting the crystal.

Interface states formation in a localized charge trapping nonvolatile memory device

Two NROM nonvolatile memory devices that are practically identical but exhibit different postcycling characteristics are investigated. Electrical characterizations combined with two-dimensional drift diffusion simulations were employed to quantify and differentiate the retention loss mechanisms. The comparison demonstrates the coexistence of two retention loss mechanisms, lateral charge transport in the nitride layer, and hot carrier induced interface states formation and anneal. The fabrication conditions in the various studies reported in the literature and the resultant impact on interface states formation are most likely the cause of the formally nonresolved debate regarding the origin of retention loss in NROM technology. The reduction in interface state formation during cycling is a key to achieving high reliability NROM cells and products.

Nonlinear diffraction from high-order Hermite–Gauss beams

We investigate experimentally and theoretically the nonlinearly diffracted second harmonic light from the first-order Hermite-Gauss beam. We investigate the cases of loosely and tightly focused beams in a periodically poled lithium niobate crystal in the temperature range near the birefringent phase matching. Unlike the case of fundamental Gaussian beam, the nonlinear diffracted beam is spatially structured. Its shape depends on the focusing conditions and on the crystal temperature. Furthermore, for the case of tight focusing, the diffracted beam structure depends on the beams position with respect to the domain wall.

Nonlinear beam shaping by non-collinear interactions

Nonlinear diffraction occurs when light encounters a periodicity in a nonlinear coefficient, for example, a periodically altered second order nonlinear coefficient impinged by a pump beam will result in a diffraction pattern in the second harmonic (SH). An interesting case of nonlinear diffraction is when the pump beam enters the crystal with an angle. This results with an asymmetrical diffraction pattern and enlarges the operational bandwidth [1]. In a perfectly periodic structure the result of diffraction for a Gaussian pump beam is a Gaussian beam in the SH. However, if the structure is modified using a computer generated hologram (CGH) encoding technique, the obtained SH beam can have any arbitrary shape. This method enables 1D nonlinear beam shaping by a simple 1D modulation of the nonlinear coefficient. Moreover, it enables fully phase matched, and hence efficient scheme for 2D nonlinear beam shaping.

Two-dimensional nonlinear beam shaping: erratum

In an earlier Letter [Opt. Lett. 37, 2136 (2012)], a conversion efficiency was improperly given. That error is corrected here.