Photorefractive Surface Waves Research Articles

光折变表面波诱导的薄层波导中的导波模式

光折变表面波诱导的薄层波导中的导波模式

Regulation of photorefractive surface apodized and chirped waveguide arrays

The tangent-like and sine-like characteristics in the head and tail regions of photorefractive surface waveguide arrays are elaborately demonstrated, and the regulation of such waveguide arrays on the written beam, background illumination, and external electric field is systematically studied for the first time. For the photorefractive surface apodized waveguide arrays, it is observed that guided mode bands of the fundamental mode gradually split and eventually reach saturation as a result of the influence of the power of photorefractive surface waves on the modulation depth of the refractive index; the period can be effectively adjusted by the wavelength and grazing angle of the written beam. However, there is an interesting chirp near surface for a large grazing angle. For the photorefractive surface nonlinearly chirped waveguide arrays, the amplitude, sign, and initial position of chirp can be conveniently regulated by an external electric field, background illumination, and incident beam, etc., and the modulation depth can also be flexibly adjusted by the influence of external electric fields on the refractive index of substrate.

Photorefractive surface nonlinearly chirped waveguide arrays

We report an alternate type of nonlinear waveguides, photorefractive surface nonlinearly chirped waveguide arrays, which can be directly induced by photorefractive surface waves in virtue of diffusion and drift nonlinearities. The amplitude of such nonlinearly chirped waveguide arrays has an apodized envelope owing to the diffusion nonlinearity. The refractive-index change of the apodized tails converges to a nonzero value which can be handily adjusted by an external electric field. Moreover, the chirp parameters such as amplitude, sign (positive or negative), and initial position can be conveniently adjusted by an external electric field, background illumination, incident beam, etc. Then the guided-wave properties of this type of waveguide arrays are analyzed by using the transfer matrix method. Owing to the flexible tail and the nonlinear chirp, the dispersion curves of the index-guided modes can be tailored by an external electric field and the dispersion curves of ordinary and extraordinary Bragg guided modes couple, intertwine, and anticross with each other. Meanwhile, there is a clear ``competition'' in the coupling hybrid mode near anticrossing.

Apodized waveguide arrays induced by photorefractive nonlinear surface waves.

A new type of nonlinear waveguides, photorefractive surface optical waveguides is suggested, which can be induced by photorefractive surface waves on the boundary of photorefractive crystal. The disturbed refractive index distribution of such waveguides behaves as a periodic lattice with apodized envelope, thus we call them photorefractive surface apodized waveguide arrays. Moreover, the dispersion relation and corresponding modes are analyzed. It is very interesting that the dispersion curves of index-guided modes and Bragg-guided modes couple and intertwine with each other, and anti-crossings instead of crossings between them hence generate some mini-gaps. Moreover there exists a type of extraordinary modes constituted by the splice of index-guided modes and Bragg-guided modes.

Observation of photorefractive surface waves in self-defocusing LiNbO_3:Fe crystal

Photorefractive (PR) surface waves (SWs) in self-defocusing LiNbO(3):Fe are studied theoretically and experimentally. We demonstrate that SWs can also be formed in a self-defocusing nonlinear medium and that the nonlocal nonlinearity (such as the diffusion component of PR nonlinearity in this Letter) is the essential cause. The forming process of PR SWs with a self-deflection course of light beams has been observed. The results indicate the possibility of concentrating light energy in self-defocusing media, taking advantage of SWs.

Effect of external electric field and background illumination on the intensity distribution of optical surface waves in the metal – photorefractive crystal system

The influence of the external electric field and background illumination on the intensity distribution of optical photorefractive surface waves at the metal – photorefractive crystal interface has been numerically simulated. The simulation is performed for a strontium – barium niobate (SBN) crystal using the parameters corresponding to the experimental data. The replacement of a real metal with an ideal one and the choice of the corresponding boundary conditions (depending on the wave power) in the numerical simulation have been substantiated. The calculation results have shown good agreement with the previously published experimental data on the effect of background illumination and a significant discrepancy for the data on the effect of the external electric field. It is found that the effect of the external electric field can be significantly enhanced by reducing the optical power of the photorefractive wave to values close to the threshold ones.

Profiles of optical surface waves formed at the metal — photorefractive crystal interface

Photorefractive surface waves propagating in a stationary regime along the surface of the metal — photorefractive crystal (SBN-75) interface are considered. The transverse structure of theoptical field distribution is calculated in the near- and far-field zones, depending on the angle of incidence of the exciting wave. The calculation results are in good agreement with the published experimental results. It is shown that the photorefractive effect leads to a splitting of the spectrum of a surface plasmon polariton excited at the metal — photorefractive crystal interface.

Optical surface wave in a crystal with diffusion photorefractive nonlinearity

We consider a steady-state nonlinear photorefractive surface wave (PR SW) with TE or TM polarisation when the refractive index of the photorefractive crystal (PRC) depends on the strength of the diffusion crystal electric field emerging upon the wave propagation. We have determined the phase trajectory and transverse structure of the PR SW intensity distribution for different values of the diffusion photorefractive nonlinearity. We have investigated a photorefractive diffraction grating, which arises inthe surface PRC layer during propagation of the nonlinear PR SW.

Polymeric photorefractive surface waves

We theoretically studied photorefractive surface waves supported by polymers with orientationally enhanced photorefractive effect. We find that the orientational enhancement and the dependency of the quantum efficiency of generating mobile holes on the electric field make the diffusion nonlinearity in photorefractive polymers dependent not on only the variation of light intensity but also on external electric field, which is very different from the external electric field independent diffusion nonlinearity in inorganic photorefractive crystals. Consequently the profile, frequency and especially the penetration depth of polymeric photorefractive surface waves strongly depend on external electric field, which makes it more controllable than that in photorefractive inorganic crystals. The stability of polymeric photorefractive surface waves, the effects of birefringence component and electro-optic component of orientationally enhanced photorefractive nonlinearity on the formation of x or y-polarized polymeric photorefractive surface waves are also discussed.

Giant enhancement of surface second-harmonic generation using photorefractive surface waves with diffusion and drift nonlinearities

Giant enhancement of second-harmonic generation (SHG) with 83.4%/W coversion efficiency is obtained, taking advantage of photorefractive surface waves with diffusion and drift nonlinearity. In this method, the self-bending induced by diffusion nonlinearity can be utilized at the surface and can solve the phase-mismatch problem in bulk due to beam self-bending. With drift nonlinearity, an applied external electric filed and background illumination can further constringe surface waves to the surface and consequently enhance SHG, which provides the possibility and flexibility of control.

Photorefractive surface waves in liquid crystal

We predict the formation of electrically and optically controlled photorefractive surfacewaves in liquid crystal, with orientational photorefractive effects. In contrast to the case forphotorefractive surface waves in inorganic crystals, the penetration depth of photorefractivesurface waves in liquid crystal can be controlled by means of the applied external electricfield, while the position of the photorefractive surface waves in the liquid crystal can becontrolled by means of the applied external electric field and the polarization of incidentlight beams.

Surface waves in two-photon photorefractive media

We present the evolution equation of surface waves due to the two-photon photorefractive effect. It is shown theoretically that the surface waves can be supported by two-photon photorefractive media. The spatial frequency of photorefractive surface waves can be changed by an applied external electric field, which indicates a method for transforming the surface waves between high-frequency modes and low-frequency modes.

The Coupling Between Forward Propagating Wave and Backward Propagating Wave for P>hotorefractive Surface Waves

The coupling between forward propagating wave and backward propagating wave of photorefractive surface waves (PR SWs) along the surface of photorefractive crystals (PRC) with periodic corrugations is studied for the first time. The influences of depth and period number of periodical corrugation structures on the coupling between the forward propagating PR SW and backward propagating PR SW are discussed. Especially the influence of nonlinearity on the coupling is studied for the first time. The reflectivity and transmissivity of PR SWs are calculated numerically using coupled-mode theory. The results are significant for the design of integrated optical device utilizing photorefractive surface waves.

TE, TM modes photorefractive surface waves and their coupling

The nonlinear equations of photorefractive surface waves (PR SWs) with diffusion mechanism are established and calculated numerically for both TE mode and TM mode and their coupling for the first time. The characteristics of TE mode and TM mode are analyzed, such as propagation constants, spatial distribution, penetration depths etc. For relative larger propagation constant, most of the surface wave energy is concentrated on the Ax component of TM mode, and the penetration depth of Ay of TE mode is always larger than that of Ax of TM mode because electro-optical coefficient r13 is always smaller than r33 in general PRC. As a result, the energy of the PR SW is better confined near the surface for TM mode than that for TE mode. When both TE and TM modes PR SWs are present, they will be coupled drastically and the Ax component of TM mode and Ay component of TE mode resonate in the volume of PR medium. As a result, the PR SWs cannot remain decay, and the light energy can not be confined near the surface steadily and the modes are destroyed. The results provide the theoretical instructions and are very significant to generate steady and high density PR SW in practice.

Surface waves with photorefractive nonlinearity

The photorefractive surface waves with photorefractive nonlinearity of diffusion, drift, and photovoltaic effect is studied numerically for the first time. We find out that the essential cause of photorefractive surface waves is diffusion mechanism, but not drift and photovoltaic effects. The photovoltaic effect only has strong influence on the profile of photorefractive surface waves; the applied external electric field can also change the profile of the surface waves, and even destroy the mode upon a threshold, which provides a possible method to transform the modes of surface waves between high-frequency modes and low-frequency modes.

Modes of photorefractive surface waves

We consider the modes of surface waves with the diffusion mechanism by the band transport model of the photorefractive effect and the nonlinear equation for the first time. We demonstrate the relationship between the modes of surface waves and the angles of incidence. The conditions for the realization of surface modes at the interface between photorefractive crystal and a metal, a linear dielectric with a lower refractive index, or another photorefractive crystal are discussed.

Photorefractive surface waves at the interface of Sr0.6Ba0.4NbO3 and air with diffusion mechanism

Photorefractive surface waves on the surface of Sr0.6Ba0.4NbO3 with a diffusion mechanism of nonlinearity have been observed. The formation time of the photorefractive surface waves is related to the intensity of the incident beam and the background illumination. The formation time of the photorefractive surface waves is about 40 min with a beam intensity of 3 mW, 25 min with 10 mW, and 30 s with 10 mW when an additional background illumination is applied. The experimental results showed that an appropriate background illumination will shorten the photorefractive response time.

Influence of the external field on photorefractive surface waves

Influence of the external field on photorefractive surface waves is studied theoretically and numerically. We demonstrate that photorefractive surface waves cannot be formed without the diffusion nonlinearity. An externally applied electric field can change the profiles of the photorefractive surface waves, especially the spatial frequency etc., and even destructs photorefractive surface waves upon a threshold value. This provides a possible method to control the formation and the shape of photorefractive surface wave. The influence of dark intensity is also discussed.

Propagation of spatial surface waves along the interface of photorefractive crystals

The propagation of a square beam and a Gaussian beam along the interface of two photorefractive crystals is studied theoretically. Both of the crystals have a diffusion nonlinearity of the gradient type (with a sign opposite that of the diffusion nonlinearity), which is able to balance the self-bending of the beams and hence to produce a photorefractive surface wave. A comparison of this wave with the symmetrical and antisymmetrical surface waves propagating along the photorefractive crystals’ interface is also given.

Near-field optical study of photorefractive surface waves in BaTiO_3

Near-field optical microscopy has been used to study photorefractive surface waves in BaTiO(3). The field distribution of the photorefractive surface wave near the crystal-air interface has been measured and compared with theory. Experimental data indicate that a micrometer-wide transition layer with dielectric and photorefractive properties that are different from the properties of the bulk BaTiO(3) exists near the interface.