Showing papers on "Diffraction efficiency published in 2001"

Periodic light coupler gratings in amorphous thin film solar cells

Abstract: Efficient light trapping structures for amorphous hydrogenated silicon (a-Si:H) solar cells have been realized using periodically structured aluminum doped zinc oxide (ZnO:Al) with periods between 390 and 980 nm as a transparent front contact. Atomic force microscopy, optical reflection, and diffraction efficiency measurements were applied to characterize solar cells deposited on such gratings. A simple formula for the threshold wavelength of total internal reflection is derived. Periodic light coupler gratings reduce the reflectance to a value below 10% in the wavelength range of 400–800 nm which is comparable to cells with an optimized statistical texture. Diffraction efficiency measurements and theoretical considerations indicate that a combination of transmission and reflection gratings contribute to the observed reduction of the reflectance.

Diffractive exit-pupil expander for display applications

Abstract: Two-dimensional binary diffraction gratings can be used in wearable display applications as exit-pupil expanders (EPEs) (or numerical-aperture expanders) to increase the size of the display exit pupil. In retinal scanning displays the EPE is placed at an intermediate image plane between the scanners and the display exit pupil. A focused spot scans across the diffractive EPE and produces multiple diffraction orders at the exit pupil. The overall luminance uniformity across the exit pupil as perceived by the viewer is a function of the uniformity of the diffraction-order intensities, focused-spot size, grating period, scanning-beam profile, and the viewer’s eye-pupil size. The design, the diffraction-order uniformity, and the effects of the grating phase angle on the uniformity for binary diffraction gratings are discussed. Also discussed are the display exit-pupil uniformity and the impact of the diffractive EPE on the point-spread function and the modulation transfer function of the display. Both theoretical and experimental results are presented.

Limits of scalar diffraction theory and an iterative angular spectrum algorithm for finite aperture diffractive optical element design.

Abstract: We have designed high-efficiency finite-aperture diffractive optical elements (DOE's) with features on the order of or smaller than the wave-length of the incident illumination. The use of scalar diffraction theory is generally not considered valid for the design of DOE's with such features. However, we have found several cases in which the use of a scalar-based design is, in fact, quite accurate. We also present a modified scalar-based iterative design method that incorporates the angular spectrum approach to design diffractive optical elements that operate in the near-field and have sub-wavelength features. We call this design method the iterative angular spectrum approach (IASA). Upon comparison with a rigorous electromag-netic analysis technique, specifically, the finite difference time-domain method (FDTD), we find that our scalar-based design method is surprisingly valid for DOE's having sub-wavelength features.

Characterization of holographic polymer dispersed liquid crystal transmission gratings

Abstract: We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization suggests that the orientation of a liquid crystal within the ellipsoidal droplets is axial. The distribution of droplet orientations appears to vary for different grating spacings. The switching properties of the gratings are investigated and analyzed with a sinusoidal dielectric tensor modulation model. The analysis also takes into account orientational distribution of average directors and spatial distribution of the droplets. Temperature dependence of diffraction efficiency is tentatively explained by a structural transition within the droplets and the presence of impurities in droplets.

Observation of self-diffraction by gratings in nematic liquid crystals doped with carbon nanotubes.

Abstract: Diffraction gratings were studied in cells of the homogeneously aligned liquid-crystal E7 doped with multiwall carbon nanotubes. These phase gratings were induced by interference modulation of two coherent optical beams, in conjunction with an applied dc field that was perpendicular to the unperturbed director axis. Self-diffraction was observed at all angles of incidence of the writing beams, including normal incidence. A superior nonlinear-index coefficient of 5×10-2 cm2/W was obtained after passage of a 44‐mW/cm2 beam through a film with a grating constant of 18 μm under an external voltage of 15 V. The observed phenomenon depends strongly on the applied dc field, and the memory effect in a nematic film depends strongly on the grating constant.

Photoinduced Birefringence and Surface Relief Gratings in Novel Polyurethanes with Azobenzene Groups in the Main Chain

Abstract: Three novel polyurethanes with V-shaped bisazo groups (PU1 and PU2) or rodlike monoazo groups (PU3) in the main chain were synthesized. On irradiation with a linearly polarized laser beam, birefringence was induced in the three films to the level of 0.035, 0.037, and 0.044 for PU1, PU2, and PU3, respectively. The stable birefringence obtained at Tg − T = 33 °C was 71%, 46%, and 19% of that achieved at room temperature for PU1, PU2, and PU3, respectively. The relative stability of the photoinduced birefringence of PU3 decreased linearly with the increase of temperature, while that of PU1 and PU2 was constant even close to Tg. Surface relief gratings with high diffraction efficiency were inscribed on these main-chain polymers. The photoinduced gratings are only partially erasable by heating above the Tg of the polymers.

Evanescent interferometric lithography.

Abstract: Simulation results are presented to illustrate the main features of what we believe is a new photolithographic technique, evanescent interferometric lithography (EIL). The technique exploits interference between resonantly enhanced, evanescently decaying diffracted orders to create a frequency-doubled intensity pattern in the near field of a metallic diffraction grating. It is shown that the intensity in a grating’s near field can be enhanced significantly compared with conventional interferometric lithography. Contrast in the interference pattern is also increased, owing to a reduction in the zeroth-order transmission near resonance. The pattern’s depth of field reduces as the wavelength is increased beyond cutoff of the first-order diffracted components, and results are presented showing the trade-offs that can be made between depth of field and intensity enhancement. Examples are given for a 270-nm-period grating embedded in material with refractive index n = 1.6 and illuminated with wavelengths near 450 nm. Under these conditions it is predicted that high-intensity, high-contrast patterns with 135-nm period can be formed in photoresists more than 50 nm thick.

Dynamic holography for optical interconnections. II. Routing holograms with predictable location and intensity of each diffraction order

Abstract: An analysis of dynamic phase-only holograms, described by fractional notation and recorded onto a pixelated spatial light modulator (SLM) in a reconfigurable optical beam-steering switch, is presented. The phase quantization and arrangement of the phase states and the SLM pixelation and dead-space effects are decoupled, expressed analytically, and simulated numerically. The phase analysis with a skip–rotate rule reveals the location and intensity of each diffraction order at the digital replay stage. The optical reconstruction of the holograms recorded onto SLM’s with rectangular pixel apertures entails sinc-squared scaling, which further reduces the intensity of each diffraction order. With these two factors taken into account, the highest values of the nonuniform first-order diffraction efficiencies are expected to be 33%, 66%, and 77% for two-, four-, and and eight-level one-dimensional holograms with a 90% linear pixel fill factor. The variation of the first-order diffraction efficiency and the relative replay intensities were verified to within 1 dB by performing the optical reconstruction of binary phase-only holograms recorded onto a ferroelectric liquid crystal on a silicon SLM.

Optical system for diffusing light

Abstract: An improved optical system is provided for diffusing light uniformly over a wide angle, including, a diffractive element for diffracting light received by the system in multiple diffraction orders, and a diffusing element which diffuses the diffracted light. The diffractive element provides diffracted light having an angular distribution of intensities over the diffraction orders which is correlated to the power spectrum of the diffusing element such that the system produces a predetermined intensity distribution of diffused light. The diffraction period of the diffractive element is selected such that the angular separation between the zeroeth and first diffraction orders is approximately one-half the angular extent of the full-width-at-half-maximum of the power spectrum of the diffusing element. The strengths of the diffraction orders are selected such that the combination of diffused light from each diffractive order provides uniformity in the intensity of the diffused light from the system.

Method and system for calibrating a diffractive grating modulator

Abstract: A diffractive grating modulator system, includes: a diffractive grating modulator active to produce a plurality of output orders of diffraction; an illumination source for directing light onto the diffractive grating modulator; an output system arranged to receive one of the orders of diffraction from the diffractive grating modulator; a detector arranged to receive a different one of the orders of diffraction from the diffractive grating and to produce a signal representing the output of the diffractive grating modulator; an electronic driving system responsive to a data stream for operating the diffractive grating modulator; and a feedback system connected to the detector and the electronic driving system and responsive to the signal for calibrating the diffractive grating modulator.

Guided-mode resonant subwavelength gratings: effects of finite beams and finite gratings.

Abstract: The effects of finite beams and finite gratings on the performance of guided-mode resonant subwavelength gratings are characterized by using the rigorous boundary element method. The gratings are strongly modulated, have a finite number of periods, and are illuminated by normally incident Gaussian beams. Quantitative results are presented for silicon-on-sapphire resonant gratings and gallium arsenide-aluminum arsenide resonant gratings.

Holographic optical elements recorded in silver halide sensitized gelatin emulsions. Part 2. Reflection holographic optical elements.

Abstract: Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawback of DCG is its low energetic sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-fine-grain silver halide (AgHal) emulsions. In particular, high spatial-frequency fringes associated with HOEs of the reflection type are difficult to construct when SHSG processing methods are employed. Therefore an optimized processing technique for reflection HOEs recorded in the new AgHal materials is introduced. Diffraction efficiencies over 90% can be obtained repeatably for reflection diffraction gratings. Understanding the importance of a selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs, also including high-quality display holograms of the reflection type in both monochrome and full color.

Controllable diffractive grating array with perpendicular diffraction

Abstract: A linear light valve array including individually addressable diffractive optical elements, the diffractive optical elements having planes of diffraction substantially perpendicular to the longitudinal axis of the array. A method of light modulation, including the steps of: providing a linear light valve array including individually addressable diffractive optical elements, the elements having planes of diffraction substantially perpendicular to the longitudinal axis of the array, illuminating the light valve with a light beam, and selectively activating the optical elements.

Design of a Binary Grating with Subwavelength Features that Acts as a Polarizing Beam Splitter.

Abstract: A binary diffractive optical element, acting as a polarizing beam splitter, is proposed and analyzed. It behaves like a transmissive blazed grating, working on the first or the second diffraction order, depending on the polarization state of the incident radiation. The grating-phase profile required for both polarization states is obtained by means of suitably sized subwavelength groups etched in an isotropic dielectric medium. A rigorous electromagnetic analysis of the grating is presented, and numerical results concerning its performances in terms of diffraction efficiency as well as frequency and angular bandwidths are provided.

Enhanced volume phase grating with high dispersion, high diffraction efficiency and low polarization sensitivity

Abstract: An enhanced volume phase diffraction grating provides high dispersion, uniformly high diffraction efficiency and equal diffraction efficiencies for all polarizations across a wide range of wavelengths. The thickness of the volume phase material and the modulation of its refractive index are jointly established to provide equalization of diffraction efficiencies for all polarizations over a wide range of wavelengths. The equalization occurs where the S and P diffraction efficiencies are both at a maximum.

Fourier gratings as submillimeter beam splitters

Abstract: Over the past years, a number of groups have developed phase gratings as local oscillator beam multiplexers for array receivers in the submillimeter wavelength domain. The most popular type of grating is the Dammann (1977) grating, a simple and versatile binary phase grating. We introduce a new type of grating, the Fourier grating, which, in contrast to the binary structure of Dammann gratings, uses a smooth grating structure. Due to the lack of sharp edges these gratings can be manufactured with high precision as reflection gratings even for two-dimensional (2-D) dispersion. Fourier gratings are designed using a simple optimization procedure that only involves a small number of parameters. Their diffraction efficiency is very high. We have produced a number of gratings and tested them at a frequency of 490 GHz, verifying the theoretical results.

Wavelength tunable diffractive transmission lens for hard x rays

Abstract: We report on the fabrication and testing of linear transmission Fresnel zone plates for hard x rays. The diffractive elements are generated by electron beam lithography and chemical wet etching of 〈110〉 oriented silicon substrates. By tilting the cylindrical lenses with respect to the x-ray beam, the effective path through the phase shifting zones can be varied. This makes it possible to optimize the diffraction efficiency over a wide range of photon energies, and to obtain effective aspect ratios not accessible with untilted optics. The diffraction efficiency of such a lens was measured as a function of the tilt angles for various energies between 8 and 29 keV. Values close to the theoretical limit were obtained for all energies. Because of the coherence preserving properties of diffractive optics, the method opens up opportunities for experiments using coherent hard x rays.

Light diffraction based overlay measurement

Abstract: Optical overlay measurement methods are very effective since they are rapid and non-destructive. Imaging techniques need sophisticated image processing and suffer from the wave-optical resolution drawback. Presently, leading edge devices are offered with 5 through 10 nm measuring accuracy. In this paper a method is proposed that relies on the diffraction of a probing laser beam at a periodic reference pattern. This special pattern is implemented in the circuit layout. After the resist patterning of the second of two consecutive layers, the diffraction at the resulting net grating is measured. If an appropriate grating design is chosen, the misalignment error can be directly extracted from the diffraction efficiency. In order to obtain a strong diffraction signal and thus a sufficient signal-to-noise ratio optimum grating designs have been computed by means of rigorous diffraction modeling. Experimental results supported by rigorous modeling suggest that this technique could have the potential to meet next generation overlay accuracy requirements.

Phase-Type Gratings Formed by Photochemical Phase Transition of Polymer Azobenzene Liquid Crystal. 2. Rapid Switching of Diffraction Beams in Thin Films

Abstract: Dynamic holographic gratings were investigated with thin films of polymer azobenzene liquid crystals by periodic induction of photochemical nematic-to-isotropic phase transition. On irradiation of writing beams (488 nm), multiple diffraction beams of a reading beam (633 nm) were immediately observed. The first-order diffraction efficiency reached to a maximum value within several tens of milliseconds: the rise time was approximately 50 ms for PM6AB2 films and 30 ms for PA6ABCN films. Quite large modulation in refractive index (∼0.08) was obtained in both polymer films. Studies on effects of temperature and light intensity on the grating formation suggested that photochemical reactions of azobenzene moieties followed by photochemical phase transition in bright fringes of the interference pattern would be responsible for the formation of holographic gratings. By turning on and off the writing beams, the diffraction beams could be switched dynamically without significant fatigue.

Photonic crystal diffraction gratings.

Abstract: It is shown from numerical results deduced from a rigorous theory of diffraction that diffraction gratings made with two-dimensional dielectric photonic crystals may present blazing effects. Since these structures are lossless, efficiencies of 100% in the -1st order can be obtained in polarized light. Efficiency curves in Littrow mount are shown.

Deterministic Diffractive Diffusers for Displays

Abstract: A LCD backlighting device that uses a diffractive light extractor has been developed for applications in which pointlike light sources are employed. The novel system eliminates the images of light sources, which appear as bright lines emanating from each source in the conventional diffractive approach. In addition, the system illuminates the LCD uniformly: Modulation of the diffractive structure as a function of position is used to control the output field of this extended planar light source.

Time dependent analysis of the formation of a half-period surface relief grating on amorphous azopolymer films

Abstract: The formation of a surface relief grating (SRG) on amorphous copolymer films containing azobenzene chromophores in side chain positions is conducted using two linearly polarized interfering beams with orthogonal polarization directions (p+s). Time dependent analyses of the diffraction efficiency were carried out by measuring simultaneously the transmitted zero order (I(0)) as well as the first (I(±1)) and second (I(±2)) diffracted orders. Two main contributions to the diffraction efficiency are evidenced: an anisotropic (birefringence) part coming from the normal period grating plus another anisotropic and surface relief part (SRG) due to the half-period grating. The origin of the weak amplitude SRG stems from the interference of particular I(±1) diffracted orders, whose resulting polarization and angular Bragg’s conditions are in accordance with the existence of a double frequency modulation. Using the matricial products of Jones, theoretical expressions of transmitted and diffracted orders are thus deri...

Fast photorefractive response in strongly reduced near-stoichiometric LiNbO(3) crystals.

Abstract: The photorefractive response time of LiNbO3 crystal is of the order of minutes, and such a long response time limits the crystal’s practical applications. We report the photorefractive properties of nominally pure near-stoichiometric LiNbO3 crystal that is strongly reduced in vacuum. A short photorefractive response time of the order of 100 ms is measured at a wavelength of 514.5 nm, with incident light intensity of 1.6 W/cm2, and possible corresponding mechanisms are discussed. To our knowledge this is the first experimental evidence of a subsecond photorefractive response in pure LiNbO3 crystals. The diffraction efficiency of a holographic grating written in this reduced crystal is low but can be enhanced by an externally applied electric field.

Nonlocal dynamic gratings and energy transfer by optical two-beam coupling in a nematic liquid crystal owing to highly sensitive photoelectric reorientation

Abstract: We investigate a new ultrahighly sensitive nonlinear optical response in pure nematic liquid crystals composed of rodlike molecules characterized by conjugated acetylene groups. Under the combined application of low-intensity optical waves and dc electrical fields, a photorefractivelike grating formation has been found in degenerate four-wave mixing experiments. Polarization-dependent pump–probe experiments reveal the underlying orientational effect of a space-charge field. The photorefractive-like origin of the gratings was proved by two beam-coupling experiments in which a refractive-index grating was phase shifted by π/2 with respect to the interference grating. The huge optical nonlinearity found in such materials makes them promising for future applications in all-optical image processing and adaptive optics.

Subwavelength surface-relief gratings fabricated by microcontact printing of self-assembled monolayers.

Abstract: We have designed and tested subwavelength diffractive optical elements consisting of surface-relief gratings made by microcontact printing of self-assembled monolayers. The first device is a beam deflector for 1.55-µm operation consisting of a surface-relief grating made up of eight pillars over one period (9.3 µm) of the grating. The widths of the pillars vary to approximate a linear phase profile within each grating period. The second device is a quarter-wave plate for 632.8-nm operation consisting of a subwavelength surface-relief grating with a 300-nm period and 58% duty cycle.

Design of diffractive phase elements for beam shaping: hybrid approach

Abstract: Hybrid approaches that combine genetic algorithms (GA’s) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive phase elements (DPE’s) for laser beam shaping. These hybrid methods exploit the global nature of the GA’s as well as the local improvement capabilities of the gradient-based local search techniques and will perform a more improved search in comparison with each of the individual approaches. The incorporated local search technique that we used here is the Davidon–Fletcher–Powell method. A cost function that can directly control the performance of the final solutions is also used. By performing the DPE design with different desired diffraction efficiencies, we obtain a set of results that approximately reflect the trade-off between the design objectives, namely, signal-to-noise ratio (SNR) and diffraction efficiency. Reasonable solutions can be chosen on the basis of the knowledge of the problem. Simulation computations are detailed for two rotationally symmetric beam-shaping systems, in which an incident Gaussian profile laser beam is converted into a uniform beam and a zero-order Bessel beam. Numerical results demonstrate that the proposed algorithm is highly efficient and robust. DPE’s that have high diffraction efficiency and excellent SNR can be achieved by using the algorithm that we propose.