Influence of the alignment surface on the characteristics of optically addressed liquid-crystal modulators

Two optically addressed transmission-type liquid-crystal modulators based on a nematic liquid crystal-a-Si: C: H photoconductor structure are studied. The modulators differ in the initial tilt angle of the director and in the barrier height at the interface. It is shown that these parameters influence the dependence of the diffraction efficiency on the feed conditions, recording pulse repetition rate, and spatial frequency of the recorded grating. The spatial resolution of the modulators at a level of half the diffraction efficiency is equal to 75 mm⁻¹. The maximal diffraction efficiency is high, 69%, owing to the asymmetric profile of the grating, which arises at a large initial tilt angle of the liquid crystal director.     

Heat treatment for reduction of surface roughness on holographic gratings

Controlled postdevelopment heat treatment of the photoresist polymer used in the preparation of holographic gratings has been shown to enhance the diffraction efficiency of gratings and reduce the scattering losses. We prove this effect by analyzing the resonant reflection spectra of a waveguide grating and observing the reduction in the arc-shaped light scattering associated with the excitation of waveguide modes.     

Characterization of Index and Surface-Relief Gratings Formed in Methacrylate Photopolymers

We report on an experimental investigation of the dual (index and surface-relief) grating formation in methacrylate photopolymers in the green. It is found that the dynamics of the diffracted signal from high spatial frequency dual gratings recorded in a thin film sample is mainly determined by the evolution of the volume index grating and is strongly dependent on writing intensities. Low spatial frequency dual gratings recorded in a bulk sample are also investigated. Depth profile and magnitude of the index grating are estimated quantitatively from the diffraction efficiency as a function of polish depth from the bulk sample’s surface. It is confirmed that the surface-relief grating is dominant in the light diffraction from the bulk sample.     

Detection of higher nonlinear harmonics of volume photorefractive gratings in reflection geometry

A comprehensive technique has been used to detect diffraction from the second and the third nonlinear spatial harmonic components, along with the fundamental harmonic of volume photorefractive gratings recorded in a channel waveguide in a LiNbO₃ substrate. For the first time to our knowledge, efficient diffraction from the higher spatial harmonic components of a photorefractive grating has been detected in reflection geometry. The dependence of a Bragg wavelength shift on the order of nonlinearity is also reported.     

可见光波段双层浮雕型导模共振滤波器设计与分析

基于傅里叶模式理论分析了双层浮雕型导模共振光栅的共振效应,分别讨论了光栅的槽深、剩余厚度、周期以及填充系数对峰值反射率、带宽、旁带反射率的影响. 数据计算表明,欠刻蚀情形的误差宽容度远远优于过刻蚀情形,两者在光栅槽深相对误差小于15％的范围内,都能保证共振峰的衍射效率高于99.5%,在相同的误差范围内,共振峰线宽的相对误差将分别达到7％和60％,因此厚度误差集中反映在对共振线宽的改变上. 另外,光栅周期和填充系数的变化将明显改变共振峰中心波长和线宽. 关键词： 导模共振 平面波导 窄带滤波     

Profiled hologram diffraction gratings based on As2Se3 inorganic photoresist

We devised a method of producing blaze hologram diffraction gratings by transforming symmetric grooves of the original grating into asymmetric ones using an additional oblique irradiation by monochromatic or polychromatic light and a repeated chemical etching of the additionally irradiated grating. Photosensitive As₂Se₃ film layers are used as a photoconductive material for recording gratings. A numerical computer modeling of the formation of asymmetric grooves is conducted. The calculated shape of the groove profile is in good agreement with experimental data. The manifestation of asymmetry of the profile shape of the produced gratings in angular and spectral relations for their diffraction efficiency is considered. It is shown that by varying the parameters of producing the original gratings and the conditions of their additional treatment it is possible to obtain hologram gratings with the required profile shape and blaze angle. To whom correspondence should be addressed. Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 45, Nauka Ave., Kiev, 252028, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 587–590, July–August, 1999.     

Tunable terahertz radiation from arbitrary profile dielectric grating coated with graphene excited by an electron beam

In this paper, the enhanced terahertz radiation transformed from surface plasmon polaritons, excited by a uniformly moving electron bunch, in a structure consisting of a monolayer graphene supported on a dielectric grating with arbitrary profile is investigated. The results show that the grating profile has significant influence on the dispersion curves and radiation characteristics including radiation frequency and intensity. The dependence of dispersion and radiation characteristics on the grating shape for both the symmetric and asymmetric gratings is studied in detail. Moreover, we find that, for an asymmetric grating with certain profile, there exist two different diffraction types, and one of the two types can provide higher radiation intensity comparing to the other one. These results will definitely facilitate the practical application in developing a room-temperature, tunable, coherent and miniature terahertz radiation source.     

Asymmetric dynamic phase holographic grating in nematic liquid crystal

A new scheme for recording a dynamic phase grating with an asymmetric profile in C60-doped homeotropically aligned nematic liquid crystal(NLC) was presented.An oblique incidence beam was used to record the thin asymmetric dynamic phase holographic grating.The diffraction efficiency we achieved is more than 40%,exceeding the theoretical limit for symmetric profile gratings.Both facts can be explained by assuming that a grating with an asymmetric saw-tooth profile is formed in the NLC.Finally,physical mechanism and mathematical model for characterizing the asymmetric phase holographic grating were presented,based on the photo-refractive-like(PR-like) effect.     

Waveguide gas lasers with frequency-selective cavities

Frequency-selective cavities of waveguide gas lasers are investigated theoretically and experimentally. The problem of finding the field distribution and the radiation loss in a cavity with a diffraction grating is solved theoretically in general form. Frequency-selective properties are determined experimentally for certain particular configurations of waveguide (single-mode, with flat reflectors near the ends, nonselective) and open cavities. The calculations and experimental results agree. It is shown that the selectivity of a waveguide cavity has a nonmonotonic dependence on the length and diameter of the waveguide channel, so that the selectivity can be made several times better. The influence of the capillary wall material and of the location of the reflectors on the frequency properties of the cavity is analyzed. For the case with EH11 mode filtration, a method of accuracy not worse than 1.5% is developed for the analytic calculation of the losses. A cavity with a reflecting interferometer made up of a diffraction selector and a diffraction grating is proposed. It is shown experimentally and theoretically that its use permits separation of any (including sequential) transition from the emission spectrum of a CO₂ laser and of longitudinal cavity modes.Lebedev Physics Institute, Academy of Sciences of the USSR. Translated from Preprint No. 251 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1988.     

Diffraction efficiency and light-scattering power of photothermoplastic holographic gratings

The diffraction efficiency and light-scattering power of holographic gratings formed on the surface of a two-layer photothermoplastic carrier with a pit structure of deformation are studied. The influence of the height and regularity of the pit deformation on the diffraction efficiency and light-scattering power is studied. It is shown that the pronounced resonance character of the dependence of the diffraction efficiency on the spatial frequency in the case of pit deformation is related to the distribution of the deformation depth over spatial frequencies; however, the dominant factor is the mutual arrangement of pits within the interference fringe. It is found that the light-scattering power is determined for the most part by the size and density of the deformation. A new method is proposed for determining the resolution of photothermoplastic carriers with pit deformation, which apparently can be used for photothermoplastic carriers with any irregular structure of deformation. This method is based on the analysis of the dependence of the light-scattering power of holographic gratings on their spatial frequency. It is shown that the resolution depends inversely proportionally on the thickness of the thermoplastic layer of the optical carrier.     

Effect of diffusion nonlinearity and an external electric field on the formation of ion-exchange diffraction gratings in glass

The formation of gratings in glass by the ion exchange technique is analyzed. The ion exchange is modeled, and the effect of diffusion nonlinearity and the external electric field on the diffusant concentration profile and diffraction characteristics of gratings are studied. It is shown that the character of diffusion nonlinearity has a substantial effect on the diffraction efficiency of the gratings produced by thermal ion exchange. It is also shown that the diffraction efficiency can be considerably increased by using an external electric field.     

Autocollimation reflection of light from the surface of corrugated waveguides with leaky modes

The diffraction of light by a corrugated surface of a layered structure is studied. It is shown that the excitation of waveguide modes in a layered structure substantially changes the diffraction efficiency of a corrugated structure. In the autocollimation regime, the diffraction efficiency of a grating with a moderate depth may reach 100%. An increased Q factor of leaky modes was found to be the condition of efficient interaction of light with a grating.     

Analysis and design of grating couplers

Based on an accurate perturbation analysis of the guiding properties of dielectric gratings, simple design criteria are developed for grating couplers which transfer the energy of a beam into or out of an optical waveguide. Gratings having arbitrary groove shapes are considered and explicit formulae are given for the leakage parameters of gratings with symmetric profiles. The results cover TE_v and TM_v modes and they apply to both shallow and deep grating grooves. The variation of the leakage parameter α in rectangular gratings is examined in detail; these rectangular gratings are then used as basic configurations for predicting the characteristics of other grating profiles. Particular attention is given to trapezoidal and triangular profiles and gratings with asymmetric profiles are also discussed.     

Light scanner based on a viscoelastic stretchable grating

We present a new technique for light scanning by use of viscoelastic-based deformable phase diffraction gratings. Mechanical stretching of the grating permits control of its spatial period, and thus the orders of diffraction can be spatially deflected. In the experiments the viscoelastic gratings with triangular and rectangular profiles have been characterized at lambda = 633 nm. It is demonstrated that the reversible elongation can exceed 20% of the initial length. For the triangular profile grating, the diffraction angle of the first order changed from 6.6 degrees to 5.4 degrees while the diffraction efficiency remained almost constant at approximately 17%. Dynamic scanning of a laser beam at frequencies of approximately 1 kHz is demonstrated by use of electromechanically driven viscoelastic gratings.     

Thermomechanical modification of diffraction gratings

The most accurate approaches to fabrication of diffraction gratings are known to be the lithographic and holographic methods. The lithographic methods allow fabrication of arbitrarily chirped gratings whose performance, however, is degraded by stitching errors. The holographic methods are free from stitching errors; however, they are limited in the achievable spatial variations of their grating periods. We suggest a method of diffraction grating modification by nonuniform heating and stretching that is much more flexible than the holographic approach and does not suffer from the problem of stitching error. We demonstrate our approach for quartz phase masks that have a characteristic grating period of 1 microm and a length of several centimeters. Our approach allows the grating periods of the phase masks to vary in a range from a few picometers to a few nanometers and a spatial resolution of a few millimeters. It is shown that the grating period can be modified with a negligible effect on the profile of the gratings.     

Space-charge field of a photorefractive grating in a planar waveguide based on a cubic crystal

We consider the initial stage of formation of photorefractive gratings in planar waveguides based on cubic electrooptic crystals. We have obtained solutions for the components of the space-charge field for arbitrary ratios between parameters characterizing the spatial inhomogeneity of the light interference pattern in the waveguide as well as the diffusion length. The dependence of the component of the space-charge field parallel to the grating vector on its period in the initial section of the recording is also determined by the sum and difference of the transverse propagation constants of the interacting waveguide modes. We show that for waveguide interactions characterized by a small transverse scale of spatial inhomogeneity, the distributions of the space-charge field and the light intensity in the interference pattern are substantially different. These effects must be considered in analysis of the nonlinear interactions occurring in a refractive index grating due to the electrooptical effect. Tomsk University of Control Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, pp. 65–73, June, 1997.     

A theory of excitation of a planar semiconductor optical waveguide using a diffraction grating: Single-scattering approximation

The problem of excitation of a totally reflecting planar optical waveguide using a coupling diffraction grating in the form of a periodic relief of the waveguide-layer thickness is solved within the single-scattering approximation. The polariton mode in the presence of a quantum well near the waveguide is considered. Based on the developed concepts, the following experimental features of the dependence of the intensity of radiation conducted in the waveguide layer on the angle of incidence of the excitation beam on the coupling diffraction grating are interpreted: the dependence on the mode number, the interference effects in the presence of two coupling diffraction gratings, and the influence of the lower substrate boundary on the thermal behavior of the waveguide structure.     

Blazing of smooth gratings

The diffraction efficiency of smooth weak gratings can depend significantly on the profile of the gratings as, in the diffraction process there exists wave coupling. By adjusting the grating profile it is possible to obtain enhancement or suppression of the diffraction efficiency. Possible applications include improvements in grating devices, grating profile reconstruction, and precise control of the slightest periodical disturbances. Some numerical examples are given.     

Recording of dynamic gratings in the nonlinear optical coating of a planar waveguide

The possibility of controlled energy exchange between interfering waveguide modes in a singlemode planar waveguide with a nonlinear optical coating is analyzed. As the coating, a suspension of bacteriorhodopsin D96N was used, which makes it possible to realize two spectrally separated mechanisms of recording and controlling dynamic gratings, i.e., the spatial modulation of the trans-cis excitation rate and the spatial modulation of the cis-trans relaxation rate. The method of phase-modulated beams was used to implement the energy exchange. The dynamic gratings in the coating were recorded by using both radiation with a wavelength within the absorption band of the trans state (630 nm) and radiation with a wavelength within the absorption band of the cis state (440 nm). Efficient control of the energy exchange between the waveguide modes by means of uniform exposure of their interference region to radiation with another wavelength was observed. A completely integral geometrical layout for optically controlled energy exchange was realized. The results obtained are compared with known data on energy exchange between beams in the bulk of a similar nonlinear medium.     

Nonreciprocal diffraction by spatial modulation of absorption and refraction

Linear diffraction gratings that provide strongly asymmetric diffraction without surface modulation are created and studied. The spatial phase shift of the refractive-index grating relative to the absorption grating is the origin of nonreciprocal behavior.