Showing papers on "Diffraction efficiency published in 1994"

A photorefractive polymer with high optical gain and diffraction efficiency near 100

Abstract: PHOTOREFRACTIVE materials are of considerable interest for the development of all-optical devices1. The photoref ractive effect appears in materials that exhibit an electric-field-dependent refractive index and that are photosensitive, such that the spatial distribution of photogenerated charge carriers is modified on irradiation with light. The diffraction pattern formed by the interference of two coherent light beams within such a material generates a non-uniform internal electric field that in turn modulates the refractive index. The resulting refractive-index pattern forms a grating that can diffract light and thereby give rise to two-beam coupling, whereby one of the writing beams gains energy at the expense of the other—a property that can be exploited in photonic devices. Although the best photorefractive materials currently available are inorganic crystals such as LiNbO3, there is considerable interest in the development of photorefractive polymers2–8, owing to their structural flexibility, ease of processing and lower cost. We describe here a polymer composite with excellent photorefractive properties. We have achieved a diffraction efficiency approaching 100% and a net two-beam coupling gain of more than 200 cm–1, making these polymeric materials suitable for immediate application in areas such as dynamic holographic storage and optical information processing1.

Electrically switchable volume gratings in polymer‐dispersed liquid crystals

Abstract: We report electrical switching of the diffraction efficiency in volume Bragg gratings written holographically in polymer‐dispersed liquid crystals (PDLCs). Scanning electron microscopy confirms the volume nature of the gratings and shows that they consist of periodic PDLC planes. The diffraction efficiency can be switched from a high value (∼50%) to a value near zero at fields ∼11 V/μm.

Algorithm for the rigorous coupled-wave analysis of grating diffraction

Abstract: Diffraction of light by periodic gratings is analyzed with a characteristic-matrix formalism based on a rigorous coupled-wave approach. This formalism is particularly convenient for modeling the diffraction by nonuniform periodic structures. In order to overcome numerical difficulties that are due to inhomogeneous eigenmodes, we propose a new algorithm that remains stable for gratings of any thickness. We obtain the stability by distinguishing in the computation the growing and the decaying inhomogeneous modes. Numerical examples and comparisons with previous results are given.

Eigenmode method for electromagnetic synthesis of diffractive elements with three-dimensional profiles

Abstract: We extend the rigorous eigenmode theory of binary surface-relief gratings to accommodate three-dimensional-modulation profiles, to formulate a synthesis problem for doubly periodic resonance-domain diffractive elements, and to demonstrate some of the problem’s symmetry properties. Several solutions for multiple beam splitters with ~90% transmission-mode diffraction efficiency are obtained by nonlinear parametric optimization. The polarization sensitivity and the required fabrication accuracy are analyzed for some solutions.

Eigenmode method for electromagnetic synthesis of diffractive elements with three-dimensional profiles

Abstract: We extend the rigorous eigenmode theory of binary surface-relief gratings to accommodate three-dimensional-modulation profiles, to formulate a synthesis problem for doubly periodic resonance-domain diffractive elements, and to demonstrate some of the problem’s symmetry properties. Several solutions for multiple beam splitters with ~90% transmission-mode diffraction efficiency are obtained by nonlinear parametric optimization. The polarization sensitivity and the required fabrication accuracy are analyzed for some solutions.

Limits of scalar diffraction theory for diffractive phase elements

Abstract: The range of validity and the accuracy of scalar diffraction theory for periodic diffractive phase elements (DPE’s) is evaluated by a comparison of diffraction efficiencies predicted from scalar theory to exact results calculated with a rigorous electromagnetic theory. The effects of DPE parameters (depth, feature size, period, index of refraction, angle of incidence, fill factor, and number of binary levels) on the accuracy of scalar diffraction theory is determined. It is found that, in general, the error of scalar theory is significant (∊ > ±5%) when the feature size is less than 14 wavelengths (s < 14λ). The error is minimized when the fill factor approaches 50%, even for small feature sizes (s = 2λ); for elements with an overall fill factor of 50% the larger period of the DPE replaces the smaller feature size as the condition of validity for scalar diffraction theory. For an 8-level DPE of refractive index 1.5 analyzed at normal incidence the error of the scalar analysis is greater than ±5% when the period is less than 20 wavelengths (Λ < 20λ). The accuracy of the scalar treatment degrades as either the index of refraction, the depth, the number of binary levels, or the angle of incidence is increased. The conclusions are, in general, applicable to nonperiodic as well as other periodic (trapezoidal, two-dimensional) structures.

Homogeneous layer models for high-spatial-frequency dielectric surface-relief gratings: conical diffraction and antireflection designs

Abstract: The validity of various homogeneous layer models for high-spatial-frequency rectangular-groove (binary) dielectric surface-relief gratings is examined for both nonconical and conical diffraction. In each model the grating is described by a slab of uniaxial material with its optic axis parallel to the grating vector. The ordinary and principal extraordinary indices of the slab depend on the grating filling factor, the substrate and cover refractive indices, and the ratio of the wavelength to the grating period. These indices can be determined by solving two transcendental equations. Higher-order indices are defined as the exact solution to these equations. Second-order indices (second-order dependence on the wavelengthto- period ratio) and first-order indices (no dependence on the wavelength-to-period ratio) are defined by approximate solutions to these equations. Layer models using higher-order and second-order indices are shown to be accurate for high-spatial-frequency gratings, even at wavelength-to-period ratios near the onset of higher-order propagating diffracted waves. These models are used to design example antireflecting gratings on silicon substrates, including designs for conical incidence. All designs are evaluated and optimized by exact rigorous coupled-wave analysis.

Diffraction efficiency analysis of a parallel-aligned nematic-liquid-crystal spatial light modulator.

Abstract: An optically addressed parallel-aligned nematic-liquid-crystal spatial light modulator is developed for applications in optical information processing and interferometry. Its performance, including diffraction efficiency, is measured, and a theoretical analysis of diffraction efficiency is performed. A comparison between the experimental results and the theoretical analysis shows good agreement. The diffraction efficiency of near-sinusoidal gratings written on the device is of the order of 30%, which is close to theoretical maximum.

Multilayer-coated diffraction gratings: differential method of Chandezon et al. revisited

Abstract: I report significant improvements to the differential method of Chandezon et al. [ J. Opt. Soc. Am.72, 839– 846 ( 1982)]. The R-matrix propagation algorithm is used to remove completely the previously existing limitations on the total coating thickness and on the total number of coated layers. I analyze the symmetry properties of the eigenvalue problem that arises in the differential formalism and use them to speed up the numerical computation. The time needed for computing the eigensolutions of coated gratings in a conical mount is reduced to little more than what is needed for gratings in a classical mount. For gratings with dielectric coatings or gratings with symmetrical profiles the need to invert certain matrices appearing in the formalism is eliminated. Numerical results show that it is possible to make nearly 100% efficient surface-relief reflection gratings in a Littrow mount by the use of dielectric materials only.

Holographically formed liquid‐crystal/polymer device for reflective color display

Abstract: — A new optical device whose reflection intensity can be electrically controlled at specific wavelengths has been demonstrated. This device has a liquid-crystal/polymer multilayer structure formed by holography. The diffraction efficiency for Ar-laser light is about 70%. The reflection intensity can be electrically controlled with a response time of less than 1 ms.

Blazed phase liquid crystal beam steering

Abstract: In this paper we describe a novel liquid crystal device for rapidly steering laser beams for high-power and large-aperture applications. The device consists of an array of optical phase modulators that contain a thin, active, liquid crystal layer sandwiched between two substrates. The unique aspect of the device is that each phase modulator can produce a linear (blazed) phase gradient, rather than the constant phase profile typical of other liquid crystal beam- steering devices. It is designed for use over a wide range of wavelengths and is particularly well suited for the deflection of short-wavelength laser beam. In this paper, we will describe device design, theoretical performance (diffraction efficiency and time response), and present experimental results of a device built to deflect a 1.064-micrometers laser beam.

Dual-grating formation through photorefractivity and photoisomerization in azo-dye-doped polymers

Abstract: Gratings formed by photorefractive and photoisomerization processes in an azo-dye-doped polymer, poly(vinyl carbazole):trinitrofluorenone/Disperse Red I, are investigated by polarization- and field-dependent four-wave mixing experiments. High diffraction efficiencies and long storage times are observed for the photorefractive gratings. Both types of grating are erasable, and we can select them by choosing the polarization of the reading beam. The effect of photoisomerization on photorefractivity is discussed. Our results indicate that azo-dye-doped polymers are promising candidates for reversible optical storage applications.

Characterization of the DuPont photopolymer for three-dimensional holographic storage

Abstract: DuPont's HRF-150 photopolymer film is investigated for use in three-dimensional holographic memories. Measurements of sensitivity, hologram persistence, the lateral spread of the photoinitiated reaction, and the variation of diffraction efficiency with modulation depth, spatial frequency and tilt angle, and intensity are reported. We observed that the diffraction efficiency of the HRF-150 photopolymer for a given exposure decreases with increases in intensity and grating tilt angle. The holograms were nondestructively reconstructed for long periods of time at room temperature. The photoinitiated reaction spread less than 100 µm over a period of 16 h.

High-resolution liquid-crystal phase grating formed by fringing fields from interdigitated electrodes.

Abstract: We report the formation of thin anisotropic phase gratings in a nematic liquid-crystalline film by use of lateral (fringing) electric fields induced by transparent interdigitated electrodes. These gratings yield high diffraction efficiency (>30%) with a strong dependence on the readout beam incidence angle. In addition, the formation of a defect wall is observed that has a significant effect on the diffraction properties of the phase grating.

Grating Light Valves for High-Resolution Displays.

Abstract: : The Grating Light Valve (GLV) is a micromechanical phase grating that can be used for color display applications. Operation is based on electrically controlling the mechanical positions of grating elements to modulate diffraction efficiency. By choosing dimensions of the grating structures carefully, it is possible to produce a digital optical device.

Method and apparatus for profiling surfaces using diffracative optics

Abstract: Optical system for measuring surface topography, comprised of a light source (30), a diffractive optical assembly (70,72,73) comprised of two or more diffraction gratings, holograms or like diffractive optics (120, 130;122, 132;123, 133), electronic detection means and digital signal processing means (10, 110) for determining surface height from interference data. The diffractive optical assembly (70,72,73) divides the incident light into two beams (150,160;152,162;153,163), which subsequently impinge upon the object (20) at the same place on the object surface (70) but at two different angles of incidence. After reflection from the object surface, the beams pass separately back through the diffractive optical assembly (70,72,73), after which they combine once again and form an interference pattern representative of the surface topography. The equivalent wavelength of the resultant interference pattern is much greater than the illumination wavelength.

Etch depth estimation of large-period silicon gratings with multivariate calibration of rigorously simulated diffraction profiles

Abstract: Diffraction from a periodic structure is sensitive to small changes in the shape of that structure. It is possible to exploit this behavior of the scatter data for accurate, precise, rapid, nondestructive, and in situ measurements of grating structures. We present the use of rigorous coupled-wave theory to generate diffraction profiles to train a partial least-squares (PLS) multivariate calibration routine. The resulting PLS calibration model was applied to experimental diffraction data from gratings in etched bulk silicon to predict etch depths. A single-detector scanning scatterometer was used to measure the scatter from 32-μm-pitch structures illuminated with a He–Ne laser beam. The scatterometer measured the diffraction patterns from grating structures at 14 die locations on each of a set of five wafers. The theoretically based PLS estimator was then used to predict etch depths from scatterometry data obtained from the 70 different grating structures. The etch depth predictions were in excellent agreement with those obtained with a scanning force microscope (i.e., 0.9-μm-deep structures were predicted with an average error of 0.007 μm). This is a significant step toward the solution of the parametric inverse grating diffraction problem: that of quantitative prediction of structure dimensions from the measurement of scatter data.

Polychromatic diffractive lens

Abstract: A polychromatic diffractive lens is usable with broadband or multi-spectral illumination to bring a plurality of spectral components of the illumination to a common focus in space. The lens has a Fresnel zone structure of zones and a profile which provides a phase jump delay at each zone boundary to the illumination of at least one of the spectral components which is greater than one period (wave) of that wavelength, and more particularly, a multiple of 2πp, where p is an integer greater than or equal to two. The parameter p and the width of the zones are selected so that the spectral components are directed, and if desired directed so as to be brought to the common focus with high diffraction efficiency in distinct diffractive orders, the lens thereby being a multi-order diffractive (MOD) lens. Applications for MOD lenses are in color (RGB) displays, color laser printers, which printers may utilize one MOD lens to image laser beams in red, blue and green wavelengths (multi-spectral illumination) to the same focal spot for forming color images as the beams scan an image forming element, such as a xerographic receptor.

Fabrication and evaluation of an etched infrared diffraction grating

Abstract: We evaluated the optical performance of an IR echelle grating produced on a silicon wafer with anisotropic etching techniques We measured the diffraction efficiency of a sample with a 55° blaze angle and 25-µm groove spacing We also calculated the efficiency for typical triangular and trapezoidal groove profiles of etched gratings The diffraction efficiency for unpolarized light can be approximately as high as the efficiency of right-angle groove gratings The great potential of the etched silicon grating lies in its ease of fabrication, its excellent surface quality, and the high reproducibility of the production process Compact high-resolution diffraction gratings can be produced by etching the grating pattern into the rear side of a transparent prism When used in internal reflection, this increases the resolving power of the grating by a factor equal to the refractive index of the prism over a front surface grating of the same length

Proximity-compensated blazed transmission grating manufacture with direct-writing, electron-beam lithography

Abstract: Proximity-compensated, as well as uncompensated, blazed transmission gratings with periods of 4, 8, and 16 μm were manufactured with direct-writing, electron-beam lithography in positive resist. The compensated gratings performed better than the uncompensated ones. For the 4-μm compensated grating the measured diffraction efficiency was 67%. It was 35% for the uncompensated grating. The compensation was made by repeated convolutions in the spatial domain with the electron-beam point spread function. We determined this function by retrieving the phase from the measured diffraction pattern of the uncompensated gratings.

Dry photopolymer films for computer-generated infrared radiation focusing elements

Abstract: A new technological approach makes fabrication of relief computer-generated focusing elements for IR radiation by use of a dry photopolymer recording material possible. The formation of a relief structure by self-development takes place in the dark, subsequent to the holographic illumination, without wet processing. Consequently these diffractive elements exhibit low surface scattering. The formation of a surface wave of the monomer along the light–darkness boundary is observed for the first time to our knowledge and confirms the previously proposed thermodynamic model of the mechanism of the hologram formation in photopolymerizable layers. Dye-sensitized polymerization of acrylamide is found to produce nonlinearity of the relief recording. At least partial compensation of this nonlinearity is attained by the introduction of appropriate corrections into the computer-generated amplitude function. A diffraction efficiency of ~ 55% is obtained for CO2 laser radiation (λ = 10.6 μm).

High-performance multilevel blazed x-ray microscopy Fresnel zone plates: Fabricated using x-ray lithography

Abstract: Diffractive lenses are becoming the optical elements of choice for many applications. One type of diffractive lens, the binary zone plate, has already demonstrated high‐resolution performance experimentally. However, in order to increase the diffraction efficiency of these zone plates, a blazed grating profile must be used. This can best be approximated by a staircase grating profile, created by multilevel exposures. Using x‐ray lithograph, we fabricated for the first time circular, linear bi‐ and trilevel zone plates, with gold structures 0.75 μm thick (per level), on silicon nitride substrates. The zone plates were designed for use at a wavelength of 1.54 A, and had a theoretical efficiency of 68.5% for bilevel and 81.5% for trilevel zone plates. Due to the large depth of focus and high resolution inherent to x‐ray lithography, the finished zone plate exhibits very steep sidewall profiles, with linewidth resolution down to 0.25 μm. Such vertical sidewalls are essential for achieving high lens efficiency. Fabrication errors, such as thickness variation in the electroplated gold and misalignment, were considered, and their effect on the optical efficiency of the zone plate was estimated. Alignment errors between levels were minimized, achieving a best result of 25 nm (3σ). In fabricating the zone plates, we employed standard integrated device tools, such as a Leica Cambridge Electron Beam microfabricator (EBMF) 10cs/120 electron‐beam writer for the x‐ray mask fabrication, and a Suss 200 x‐ray stepper for the multilevel exposures. Thus, we have shown that it will be possible to fabricate many lenses, with a variety of optical characteristics, in one wafer.

Holographic double-exposure interferometry in near real time with photorefractive crystals

Abstract: A holographic interferometric technique based on fast sequences of double exposures in photorefractive crystals for the dynamic analysis of reflection-type objects is presented. Methods for optimizing the diffraction efficiency and the decay time of the holographic gratings recorded at short exposure times are discussed. Special attention is paid to the use of external electrical fields. The maximization of fringe contrast in double-exposed interferograms is investigated both theoretically and experimentally. Finally, the automatic evaluation of interferograms is demonstrated by use of a sample sequence of interferograms.

Photopolymer system and its application to a color hologram

Abstract: We present a new type of photopolymer system for recording reflection holograms. The photopolymer system with radical and cationic photopolymerization controlled by the wavelength of light gives us enhanced diffraction efficiencies and a balanced recording sensitivity (20-60 mJ/cm(2)) in the blue to red region of the spectrum. In color hologram recordings, diffraction efficiencies of ˜ 60% are obtained when a photopolymer film is used that is composed of different spectrally sensitive photopolymer layers.

Relaxational self‐enhancement of holographic gratings in amorphous As2S3 films

Abstract: A detailed experimental study of the relaxation of holographic gratings in disordered materials is presented. Relaxation parameters of holographic gratings in nonannealed As2S3 amorphous semiconductor films have been measured as a function of aging time, initial diffraction efficiency, recording light intensity, and grating period. The influence of the readout light intensity and sample thickness and its temperature has also been investigated. Relaxational self‐enhancements of gratings were found up to 18 times with respect to the initial diffraction efficiency, with a saturation value stable over a period of more than 2 years. The relaxational self‐enhancement effect is explained in terms of a phenomenological relaxation model with periodically distributed stress. Under certain conditions, a spatially periodic mechanical stress field resulting from a holographic grating causes anomalous diffusion of unfilled sites. This leads to a density modulation which increases the initial refractive index modulation...

Binary high-frequency-carrier diffractive optical elements : electromagnetic theory

Abstract: Using rigorous electromagnetic diffraction theory, we evaluate the potential performance and the limitations of coding diffractive optical elements in the form of a pulse-frequency-modulated carrier grating. This coding scheme employs the first diffraction order of an ultrahigh-frequency binary carrier grating, with a period below 1.5 wavelengths. We establish that the pulse-frequency-modulation structure can be designed with the standard synthesis techniques based on paraxial scalar diffraction theory. However, we had to optimize the groove depth, the aspect ratio, and the carrier period with rigorous electromagnetic theory to achieve close to 100% efficiency and the desired polarization properties. Our method is compared with another recent coding scheme that utilizes the zeroth order of a subwavelength-period pulse-width-modulated binary carrier grating.

Bragg–Fresnel multilayer gratings: electromagnetic theory

Abstract: We develop an electromagnetic analysis and its numerical implementation for gratings of arbitrary shape etched into a plane multilayer. The multilayer consists of a stack of identical bilayers, each bilayer including two materials (dielectrics and/or metals) with different thicknesses. The analysis is based on the differential theory of gratings, which is generalized to the case of many superimposed gratings by computation of the T matrix of each elementary grating linking the field above the modulated region to the field below it. The product of all the T matrices gives the T matrix of the stack, from which the outgoing wave condition permits derivation of the grating efficiencies. The method, developed for TE polarization, can be used throughout the entire spectrum, but it is particularly useful in the soft-x-ray region. Examples of performances of multilayer gratings in this spectral domain are computed.

Holographic polarization-separation elements.

Abstract: Volume holographic optical elements exhibit a property that we call bidiffringence, in which the diffraction efficiency of the element is strongly dependent on the polarization of the incident beam. Dualelement, volume holographic polarization separators utilizing bidiffringence can provide high extinction ratios, and they can provide greater angular separation of the polarized beams and greater flexibility in the beam-separation parameters than conventional birefringent elements. We designed and constructed holographic polarization separators using dichromated gelatin as the holographic medium. Experimental results are compared with the results predicted by the Kogelnik coupled wave theory.

Development of photopolymer/liquid crystal composite materials for dynamic hologram applications

Abstract: Switchable holographic gratings are desirable for a wide range of applications in diffractive optics. Liquid crystalline materials look attractive for these devices because of their large field- induced birefringence. The combination of electro-optical liquid crystals with photopolymeric holographic materials offers a unique single system approach to the economical fabrication of switchable holograms. We report on the progress in our development of a novel system where holographic gratings are recorded in a single step process and consist of periodic polymer-dispersed liquid crystal planes. Gratings have been recorded with high diffraction efficiency (approaching 100%) and narrow angular selectivity (<1 degree(s) FWHM). The diffraction efficiency can be controlled electrically or thermally

Planar-optic-disk pickup with diffractive micro-optics.

Abstract: An integrated optical-disk pickup with a diffractive planar micro-optic system is proposed. In this device, the beam follows a zigzag optical path inside a glass substrate that is used as a light guide. To fabricate off-axis diffractive optical elements, we have recently developed an electron-beam writing system with a curve-pattern generator. It is demonstrated that a transmission off-axis objective microlens, a reflection twin-focusing beam splitter, and reflection layers were integrated on a glass substrate, and such a diffractive planar micro-optic system exhibited an excellent focusing performance and operated for focus-error signal detection, as designed.