Showing papers on "Diffraction efficiency published in 2015"

Fabrication of ideal geometric-phase holograms with arbitrary wavefronts

Abstract: Throughout optics and photonics, phase is normally controlled via an optical path difference. Although much less common, an alternative means for phase control exists: a geometric phase (GP) shift occurring when a light wave is transformed through one parameter space, e.g., polarization, in such a way as to create a change in a second parameter, e.g., phase. In thin films and surfaces where only the GP varies spatially—which may be called GP holograms (GPHs)—the phase profile of nearly any (physical or virtual) object can in principle be embodied as an inhomogeneous anisotropy manifesting exceptional diffraction and polarization behavior. Pure GP elements have had poor efficiency and utility up to now, except in isolated cases, due to the lack of fabrication techniques producing elements with an arbitrary spatially varying GP shift at visible and near-infrared wavelengths. Here, we describe two methods to create high-fidelity GPHs, one interferometric and another direct-write, capable of recording the wavefront of nearly any physical or virtual object. We employ photoaligned liquid crystals to record the patterns as an inhomogeneous optical axis profile in thin films with a few μm thickness. We report on eight representative examples, including a GP lens with F/2.3 (at 633 nm) and 99% diffraction efficiency across visible wavelengths, and several GP vortex phase plates with excellent modal purity and remarkably small central defect size (e.g., 0.7 and 7 μm for topological charges of 1 and 8, respectively). We also report on a GP Fourier hologram, a fan-out grid with dozens of far-field spots, and an elaborate phase profile, which showed excellent fidelity and very low leakage wave transmittance and haze. Together, these techniques are the first practical bases for arbitrary GPHs with essentially no loss, high phase gradients (∼rad/μm), novel polarization functionality, and broadband behavior.

Portable waveguide display system with a large field of view by integrating freeform elements and volume holograms

Abstract: A compact waveguide display system integrating freeform elements and volume holograms is presented here for the first time. The use of freeform elements can broaden the field of view, which limits the applications of a holographic waveguide. An optimized system can achieve a diagonal field of view of 45° when the thickness of the waveguide planar is 3mm. Freeform-elements in-coupler and the volume holograms out-coupler were designed in detail in our study, and the influence of grating configurations on diffraction efficiency was analyzed thoroughly. The off-axis aberrations were well compensated by the in-coupler and the diffraction efficiency of the optimized waveguide display system could reach 87.57%. With integrated design, stability and reliability of this monochromatic display system were achieved and the alignment of the system was easily controlled by the record of the volume holograms, which makes mass production possible.

Exit pupil expanding diffractive optical waveguiding device

Abstract: An optical device is disclosed for expanding input light in two dimensions in an augmented reality display. The device comprises a waveguide (12) and three linear diffraction gratings H0, H1, H2. An incident beam from a projector illuminates an input grating H0 with polychromatic light, and the light is coupled into the waveguide (12). The other two gratings H1, H2 are overlaid on top of one another. Light can be diffracted by one grating H1 into a first diffracted order and towards the other grating H2 which can couple the light out of the waveguide (12) towards a viewer. In another arrangement the crossed gratings H1, H2 may be replaced by a photonic crystal (19) having a regular array of pillars (20) which create a number effective diffraction gratings.

Beam shaping of complex amplitude with separate constraints on the output beam.

Abstract: We present a beam shaping technique in controlling the complex amplitude of an optical beam. The constraint on the amplitude of the output beam in the Gerchberg-Saxton algorithm is replaced with constraints both on the amplitude and phase of the output beam in the proposed method. The total areas of the constrained regions and free regions on the complex amplitude of the output beam in the proposed method are maintained. An output beam with arbitrary complex amplitude can be realized with the proposed method. The computing result from the proposed method is a phase-only distribution, which can be fabricated as diffractive optical element for higher diffraction efficiency. Both simulations and experiments are present and the effectiveness of the proposed method is verified.

All-silicon nanorod-based Dammann gratings

Abstract: Established diffractive optical elements (DOEs), such as Dammann gratings, whose phase profile is controlled by etching different depths into a transparent dielectric substrate, suffer from a contradiction between the complexity of fabrication procedures and the performance of such gratings. In this Letter, we combine the concept of geometric phase and phase modulation in depth, and prove by theoretical analysis and numerical simulation that nanorod arrays etched on a silicon substrate have a characteristic of strong polarization conversion between two circularly polarized states and can act as a highly efficient half-wave plate. More importantly, only by changing the orientation angles of each nanorod can the arrays control the phase of a circularly polarized light, cell by cell. With the above principle, we report the realization of nanorod-based Dammann gratings reaching diffraction efficiencies of 50%-52% in the C-band fiber telecommunications window (1530-1565 nm). In this design, uniform 4 x 4 spot arrays with an extending angle of 59 degrees x 59 degrees can be obtained in the far field. Because of these advantages of the single-step fabrication procedure, accurate phase controlling, and strong polarization conversion, nanorod-based Dammann gratings could be utilized for various practical applications in a range of fields. (C) 2015 Optical Society of America

Adjustable diffractive spiral phase plates

Abstract: We report on the fabrication and the experimental demonstration of Moire diffractive spiral phase plates with adjustable helical charge. The proposed optical unit consists of two axially stacked diffractive elements of conjugate structure. The joint transmission function of the compound system corresponds to that of a spiral phase plate where the angle of mutual rotation about the central axis enables continuous adjustment of the helical charge. The diffractive elements are fabricated by gray-scale photolithography with a pixel size of 200 nm and 128 phase step levels in fused silica. We experimentally demonstrate the conversion of a TEM(00) beam into approximated Laguerre-Gauss (LG) beams of variable helical charge, with a correspondingly variable radius of their ring-shaped intensity distribution.

Micro-patterned photo-aligned ferroelectric liquid crystal Fresnel zone lens.

Abstract: In this Letter, we disclose a fast switchable Fresnel zone lens (FZL) by confining the ferroelectric liquid crystals (FLCs) in multiple microscopically defined photo-aligned alignment domains. The photo-alignment (PA) offers good control on the anchoring energy (W) by mean of irradiation doses (ID) and thus excellent alignment for FLCs. Two operational modes of the FLCFZL, i.e., FOCUS/OFF and FOCUS/DEFOCUS, were demonstrated. The proposed diffracting element provides fast response time, high diffraction efficiency (η), with saturated electro-optical (EO) operations up to high frequency (≈2 kHz). Thus, the proposed FLCFZLs with simple fabrication open several opportunities to improve the quality of existing devices and to find new applications.

Large-angle and high-efficiency tunable phase grating using fringe field switching liquid crystal

Abstract: We propose a switchable phase grating using fringe field switching (FFS) cells. The FFS phase grating possesses several attractive features: large diffraction angle, high diffraction efficiency, fast response time, and high contrast ratio. It can diffract >32% light to ± 2nd orders with a large diffraction angle of 12.1°. Meanwhile, its response time remains relatively fast even at −40°C. A simulation model is developed to explain the experimental results and good agreement is obtained. We also demonstrate a blazed phase grating to achieve tunable beam steering between 0th, 1st and 2nd orders.

Diffractive optical elements with asymmetric profiles

Abstract: In an optical display system that includes a waveguide with multiple diffractive optical elements (DOEs), gratings in one or more of the DOEs may have an asymmetric profile in which gratings may be slanted or blazed. Asymmetric gratings in a DOE can provide increased display uniformity in the optical display system by reducing the "banding" resulting from optical interference that is manifested as dark stripes in the display. Banding may be more pronounced when polymeric materials are used in volume production of the DOEs to minimize system weight, but which have less optimal optical properties compared with other materials such as glass. The asymmetric gratings can further enable the optical system to be more tolerant to variations - such as variations in thickness, surface roughness, and grating geometry - that may not be readily controlled during manufacturing particularly since such variations are in the submicron range.

Square lattices of quasi-perfect optical vortices generated by two-dimensional encoding continuous-phase gratings

Abstract: We propose a type of two-dimensional (2D) encoding continuous-phase gratings capable of simultaneously generating a square lattice of multiple quasi-perfect vortices. As an example, a symmetrical and an asymmetrical 5×5 lattice of quasi-perfect vortices are experimentally demonstrated. It is shown that multiple quasi-perfect vortices with different topological charges are generated at different diffraction orders. The ring-width of these vortices is nearly constant, while there is a shift in the average ring-diameter when the carried charges are large enough, or when the ring-diameter is small. Additional axicon phase has been embedded into these 2D encoding gratings for the compensation of such shift in the average ring-diameter, and experimental results show that the shift can be greatly minimized after this compensation.

Improved holographic waveguide display system

Abstract: An improved achromatic holographic waveguide display system with high efficiency is proposed, which is composed of an in-coupled planar hologram and an outcoupled volume hologram. The efficiency is improved, compared with the previous system proposed by Shi et al. (Appl. Opt.51, 4703–4078 [2012]10.1364/AO.51.004703APOPAI1559-128X), and the loss resulting from multiple exposures is reduced by the combination of the planar hologram and the volume hologram. The basic idea and principles are described, and the experiments are performed. The experimental results demonstrate that the improved system can achieve higher diffraction efficiency and reduce the experimental complexity of multiple exposures. It could be a potential method for improving the efficiency and correcting the chromatic dispersion in the waveguide display systems.

Switchable 3D liquid crystal grating generated by periodic photo-alignment on both substrates.

Abstract: A planar liquid crystal (LC) cell is developed in which two photo-alignment layers have been illuminated with respectively a horizontal and a vertical diffraction pattern of interfering left- and right-handed circularly polarized light. In the bulk of the cell, a complex LC configuration is obtained with periodicity in two dimensions. Remarkably, the period of the structure is larger than the period of the interference pattern, indicating that lowering of the symmetry allows a reduction in the elastic energy. The liquid crystal configuration depends on the periodicity of the alignment but also on the thickness of the cell. By applying a voltage over the electrodes, the power going into the different diffracted orders can be tuned. Finite element (FE) simulations based on Q-tensor theory are used to find the 3D equilibrium director distribution, which is used to simulate the near-field transmission profile based on the Jones calculus. A 2D Fourier transform is performed for both the x- and y-component of the transmitted wave to find the diffraction efficiency.

Diffractive optical element using crossed grating for pupil expansion

Abstract: In an optical display system that includes a waveguide (330) with multiple diffractive optical elements (310,340), an in-coupling diffractive optical element (340) couples light into the waveguide (330), an intermediate diffractive optical element provides exit pupil expansion in a first direction, and an out-coupling diffractive optical element (310) provides pupil expansion in a second direction and couples light out of the waveguide. The intermediate diffractive optical element is configured with a crossed grating which is a three-dimensional microstructure that is periodic in two directions. The crossed grating provides multiple optical paths to a given point in the diffractive optical element in which differences in optical path lengths are larger than the coherence length. The crossed grating in the intermediate diffractive optical element may provide increased display uniformity in the optical display system by reducing the "banding" resulting from optical interference that is manifested as dark stripes in the display. The crossed grating can further enable the optical display system to be more tolerant to manufacturing variations.

Properties of DMDs for holographic displays

Abstract: Digital micromirror device’s (DMD) properties as being a display device for holographic displays are investigated. High speed, a large separation between reconstructed image and reconstruction beam, two symmetric diffraction patterns, and low intensity (0,0)th-order beam at a blazed grating condition are the desired properties for the displays. The blazed grating condition of a DMD can reconstruct images with higher diffraction efficiency than the line grating condition. DMD’s high speed enables to present colors and gray levels to the reconstructed image. However, reconstructed images from a gray-level computer-generated hologram (CGH) and its binary form hologram reveal no noticeable difference between them, except the background noise in the image from the CGH.

Diffraction grating-based backlighting having controlled diffractive coupling efficiency

Abstract: Diffraction grating-based backlighting having controlled diffractive coupling efficiency includes a light guide and a plurality of diffraction gratings at a surface of the light guide. The light guide is to guide light and the diffraction gratings are to couple out a portion of the guided light using diffractive coupling and to direct the coupled-out portion away from the light guide surface as a plurality of light beams at a principal angular direction. Diffraction gratings of the plurality include diffractive features having a diffractive feature modulation configured to selectively control a diffractive coupling efficiency of the diffraction gratings as a function of distance along the light guide surface.

Pixel-level fringing-effect model to describe the phase profile and diffraction efficiency of a liquid crystal on silicon device.

Abstract: We propose a fringing-effect model based on the experimentally measured phase response across the phase transition region of a liquid crystal on silicon (LCOS) device. The measured phase profile in the phase transition region is characterized by a scaled error function of the flyback width. The flyback width can be determined by a cubic function of the phase depth between neighboring pixels. This dependence of the flyback width on the phase depth is explained by a linear rotation model of the liquid crystal director. The simulated diffraction efficiency based on the fringing-effect model shows a close agreement with the experimental measurement.

Image magnification in lensless holographic projection using double-sampling Fresnel diffraction.

Abstract: Since the diffraction angle is limited by the spatial resolution of the spatial light modulator (SLM), the size of the optical image of the lensless holographic projection with a SLM is very small. Using a divergent spherical beam to illuminate a SLM is an effective method to physically increase the projection angle; nevertheless, the sampling ranges of the existing Fresnel diffraction algorithms with fast Fourier transform keep unchanged. In this paper, a double-sampling Fresnel diffraction algorithm to enlarge the sampling range is proposed when using a divergent spherical beam to illuminate a SLM, and the magnification of the optical image is realized in lensless holographic projection. The hologram can be easily optimized by the Gerschberg-Saxton algorithm. Simulation and experimental results with enlarged optical image are presented successfully.

High-order multilayer coated blazed gratings for high resolution soft x-ray spectroscopy.

Abstract: A grand challenge in soft x-ray spectroscopy is to drive the resolving power of monochromators and spectrometers from the 104 achieved routinely today to well above 105. This need is driven mainly by the requirements of a new technique that is set to have enormous impact in condensed matter physics, Resonant Inelastic X-ray Scattering (RIXS). Unlike x-ray absorption spectroscopy, RIXS is not limited by an energy resolution dictated by the core-hole lifetime in the excitation process. Using much higher resolving power than used for normal x-ray absorption spectroscopy enables access to the energy scale of soft excitations in matter. These excitations such as magnons and phonons drive the collective phenomena seen in correlated electronic materials such as high temperature superconductors. RIXS opens a new path to study these excitations at a level of detail not formerly possible. However, as the process involves resonant excitation at an energy of around 1 keV, and the energy scale of the excitations one would like to see are at the meV level, to fully utilize the technique requires the development of monochromators and spectrometers with one to two orders of magnitude higher energy resolution than has been conventionally possible. Here we investigate the detailed diffraction characteristics of multilayer blazed gratings. These elements offer potentially revolutionary performance as the dispersive element in ultra-high resolution x-ray spectroscopy. In doing so, we have established a roadmap for the complete optimization of the grating design. Traditionally 1st order gratings are used in the soft x-ray region, but we show that as in the optical domain, one can work in very high spectral orders and thus dramatically improve resolution without significant loss in efficiency.

Optimum generation of annular vortices using phase diffractive optical elements

Abstract: An annular vortex of arbitrary integer topological charge q can be obtained at the Fourier domain of appropriate phase diffractive optical elements. In this context we prove that the diffractive element that generates the vortex with maximum peak intensity has the phase modulation of a propagation-invariant qth order Bessel beam. We discuss additional advantages of this phase element as annular vortex generator.

Machined immersion grating with theoretically predicted diffraction efficiency.

Abstract: An immersion grating composed of a transmissive material with a high refractive index (n>2) is a powerful device for high-resolution spectroscopy in the infrared region. Although the original idea is attributed to Fraunhofer about 200 years ago, an immersion grating with high diffraction efficiency has never been realized due to the difficulty in processing infrared crystals that are mostly brittle. While anisotropic etching is one successful method for fabricating a fine groove pattern on Si crystal, machining is necessary for realizing the ideal groove shape on any kind of infrared crystal. In this paper, we report the realization of the first, to the best of our knowledge, machined immersion grating made of single-crystal CdZnTe with a high diffraction efficiency that is almost identical to that theoretically predicted by rigorous coupled-wave analysis.

Fabrication of a multilevel THz Fresnel lens by femtosecond laser ablation

Abstract: The possibility of fabricating a silicon diffractive fourlevel THz Fresnel lens by laser ablation is studied. For a microrelief to be formed on the sample surface, use is made of a femtosecond Yb : YAG laser with a high pulse repetition rate (f = 200 kHz). Characteristics of the diffractive optical element are investigated in the beam of a 141-mm free-electron laser. The measured diffraction efficiency of the lens is in good agreement with the theoretical estimate.

Electroinduced Diffraction Gratings in Cholesteric Polymer with Phototunable Helix Pitch

Abstract: For the first time the cholesteric mixture containing nematic polymer with small amount of chiral-photochromic dopant is used for electroinduced diffraction gratings production. The gratings are obtained by applying electric field to the planar-aligned cholesteric polymer layer causing its periodical distortion. Material developed permits manipulating supramolecular helical structure by means of UV exposure resulting in helix untwisting. Photo-controlling of helix pitch brings to change parameters of the electroinduced gratings. Due to macromolecular "nature" of the material one can easily stabilize electroinduced gratings by fast sample cooling. All-known cholesteric grating types are realized in the studied polymer material. It is observed that the grating vector can be oriented along or perpendicular to the rubbing direction of the cell. It is shown that the diffraction efficiency is dictated by grating type and the amplitude of the applied electric field and can achieve about 80%. Moreover, the period of gratings can be tuned upon UV light illumination. The possibility of 2D gratings creation is also demonstrated. The described material and approach gives an opportunity to easily fabricate a variety of diffraction gratings with flexibly controllable parameters. Such gratings can be potentially applied in optics, optoelectronics, and photonics as intelligent diffraction elements.

Diffractive interface theory: nonlocal susceptibility approach to the optics of metasurfaces

Abstract: We present a formalism for understanding the electromagnetism of metasurfaces, optically thin composite films with engineered diffraction. The technique, diffractive interface theory (DIT), takes explicit advantage of the small optical thickness of a metasurface, eliminating the need for solving for light propagation inside the film and providing a direct link between the spatial profile of a metasurface and its diffractive properties. Predictions of DIT are compared with full-wave numerical solutions of Maxwell’s equations, demonstrating DIT’s validity and computational advantages for optically thin structures. Applications of the DIT range from understanding of fundamentals of light-matter interaction in metasurfaces to efficient analysis of generalized refraction to metasurface optimization.

Full-color holographic diffuser using time-scheduled iterative exposure.

Abstract: A compact wavelength multiplexing technique is proposed and experimentally investigated to improve the efficiency of a full-color holographic diffuser using photopolymer. The exposure responses of a monochromatic hologram and a three wavelength multiplexed hologram recorded in photopolymer film are presented. The time-scheduled exposure energies at wavelengths of 633, 532, and 473 nm were chosen to optimize the uniform diffraction efficiency of the wavelength multiplexed hologram. These three wavelength iterative sequences of exposures are applied to achieve a specific color balance for a multicolor holographic diffuser. The experimental results confirm that the fabrication method is well suited to the manufacture of holographic diffusers for full-color display applications.

Photorefractive performance of poly(triarylamine)‐Based polymer composites: An approach from the photoconductive properties

Abstract: Photorefractive (PR) and photoconductive properties of methyl-substituted poly(triarylamine) (PTAA) based PR composite is presented. PR composite consisted of PTAA, piperidinodicyanostyrene, (2,4,6-trimethylphenyl)diphenylamine, and [6,6]-phenyl-C61-butyric acid methyl ester. Photocurrent is simultaneously measured when a transient degenerate four wave mixing is recorded. Diffraction efficiency of 16.6%, response time of 5 ms, and sensitivity of 43 cm2 J−1 are measured under an applied field of 45 V μm−1 and 632.8 nm illumination with the intensity of 1.5 W cm−2. Response time of 10.2 ms with diffraction efficiency of 47.0% is obtained under a same field and 532 nm illumination with the intensity of 0.427 W cm−2. Higher diffraction and faster response is due to the large photocurrent in the order of hundreds μA measured. The resultant trap density is in the order of 1014 cm−3. Thus, space–charge field less than 1 V μm−1 is evaluated, which limits the PR response. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 502–508

Holographic humidity response of slanted gratings in moisture-absorbing acrylamide photopolymer.

Abstract: Holographic humidity response is characterized in detail using transmission and reflection geometry in moisture-absorbing acrylamide photopolymer. The diffraction spectrum and its temporal evolution at various relative humidity are measured and analyzed. The quantitative relations between relative humidity and holographic properties of slanted gratings are determined. The responsibility of holographic gratings for various relative humidity is observed by the spectrum response of gratings. The extracted humidity constants reflect the applicability of reflection and transmission gratings at different humidity regions. The humidity reversibility experiment is achieved for confirming repeatability of the sensor. These experiments provide a probability for improving the applicability of a holographic humidity sensor. Finally, the extended diffusion model is derived by introducing the expansion coefficient to describe the dynamic swelling process. This work can accelerate development of the holographic sensor and provide a novel strategy for exploring the swelling mechanism of photopolymer.

300 mm ruling engine producing gratings and echelles under interferometric control in China

Abstract: The designs of 300 mm ruling engine producing gratings and echelles under interferometric control (CIOMP-2) are presented. A new ruling-tool carriage system and its driving mechanism—which are easy to manufacture—are proposed. A new blank carriage system, controlled by a dual-frequency laser interferometer and a piezoelectric actuator, is designed. The CIOMP-2 ruling engine can now rule grating blanks with dimensions up to 300 mm×300 mm, and mainly rules gratings with grating constants between 10 and 2400 lines/mm. The wavelength range of CIOMP-2 gratings is from ultraviolet to mid-infrared wavelengths. Experiments show that the new ruling-tool carriage system works well and stability of motion is improved with the help of a flexure-hinge structure, and the threefold standard deviation values of the blank carriage positioning errors are less than ∼5 nm. For 600 line/mm gratings (diffraction order m=−1), the scatter intensities and ghosts reach 10−5 of the maximum intensity. The scatter intensities and ghost of gratings ruled with CIOMP-2 are low, and no Rowland ghosts are visible. The gratings ruled by CIOMP-2 have high diffraction efficiency and resolving power. CIOMP-2 can also produce varied-line-space, bend-line, and aberration-reducing gratings and echelles.