Showing papers on "Diffraction efficiency published in 2019"

Pancharatnam–Berry optical elements for head-up and near-eye displays [Invited]

Abstract: Liquid-crystal-based Pancharatnam–Berry phase optical elements (PBOEs), also known as diffractive wave plates (DWPs), geometric phase optics (GPO), or geometric phase holograms (GPHs), are functional planar structures with patterned orientation of anisotropy axis. Several scientifically interesting yet practically useful electro-optical effects, such as focusing, beam splitting, waveguide coupling, and wavelength filtering, have been realized with PBOEs. Because of the high degree of optical tunability, polarization selectivity, nearly 100% diffraction efficiency, and simple fabrication process, PBOEs have found widespread applications in emerging display systems, particularly virtual/augmented/mixed reality displays and head-up displays. In this review, we will describe the basic operation principles, present device fabrication procedures, discuss numerical modeling methods, and address applications of PBOEs in emerging display systems.

Generalized Space-Time-Periodic Diffraction Gratings: Theory and Applications

Abstract: Unlike a conventional diffraction grating, a space-time-periodic (STP) grating produces spatial diffraction orders, each of which is formed by an infinite set of temporal diffraction orders. STP gratings offer enhanced functionalities and exotic characteristics, such as asymmetric diffraction patterns, nonreciprocal and asymmetric transmission and reflection, and an enhanced diffraction efficiency. Here a rigorous, general theory of diffraction by STP gratings is formulated, and a particular practical application in wireless communication is proposed: an inventive multiple-access communication system, featuring full duplex operation and high data-transmission rate.

Stretchable, flexible, rollable, and adherable polarization volume grating film

Abstract: Volume Bragg gratings (VBGs) have many applications, including filters, wavelength multiplexing devices, and see-through displays. As a kind of VBGs, polarization volume gratings (PVGs) based on liquid crystal polymer have the advantages of nearly 100% efficiency, large deflection angle, and high polarization selectivity. However, previous reports regarding PVGs did not address high efficiency, tunable periodicity, and flexibility. Here, we report a stretchable, flexible, and rollable PVG film with high diffraction efficiency. The control of PVG by mechanical stretching is investigated, while the Bragg reflection band shift is evaluated quantitatively. Moreover, we quantified the deflection angle change's behavior, which has promising potential for laser beam steering applications. The mechanical robustness under stretch-release cycles is also scrutinized.

Device simulation of liquid crystal polarization gratings.

Abstract: Liquid crystal polarization gratings manifest several unique features, such as high diffraction efficiency, polarization selectivity, and fast switching time. However, few works address the chiral-doped liquid crystal alignment issue in such gratings. Here, we develop an improved relaxation method to analyze the liquid crystal director distribution in chiral-doped polarization gratings. Our simulation result agrees well with experimental data on a polarization volume grating. The criteria for forming planar or slanted polarization grating are discussed.

Fabrication of Pancharatnam-Berry phase optical elements with highly stable polarization holography.

Abstract: Polarization-dependent diffraction based on Pancharatnam-Berry phase optical elements (PBOEs) offers considerable benefits compared to conventional metasurfaces, such as negligible absorption, nearly 100% diffraction efficiency and an inexpensive fabrication process. Polarization holography is a simple way to fabricate PBOEs, which entails the interference of beams with different polarizations to generate a spatial-varying polarization field. Thus, the quality of recorded PBOEs manifests high sensitivity to the length change and phase shift between polarized beams, usually caused by environmental vibration and air flow. Here, new polarization holography based on modified Sagnac interferometry is developed for fabricating liquid crystal-based PB gratings and lenses, where the pitch of grating and optical power of lens could be easily tuned. This approach offers high tolerance to environmental disturbance during the exposure process. Detailed design parameters are analyzed, and the fabricated PBOEs with high optical quality are also demonstrated.

Optical properties of reflective liquid crystal polarization volume gratings

Abstract: Polarization volume gratings are self-organized liquid crystal helical structures. They exhibit high diffraction efficiency and unique polarization selectivity. In this work, we investigate and compare two different configurations of polarization volume gratings: planar and slanted structures. We present the optical properties of polarization volume gratings with emphasis on their polarizing nature. Further experimental results reveal the existence of the slanted configuration.

Generalized Space-Time Periodic Diffraction Gratings: Theory and Applications

Abstract: This paper studies the theory and applications of the diffraction of electromagnetic waves by space-time periodic (STP) diffraction gratings. We show that, in contrast with conventional spatially periodic grating, a STP diffraction grating produces spatial diffraction orders, each of which is formed by an infinite set of temporal diffraction orders. Such spatiotemporally periodic gratings are endowed with enhanced functionalities and exotic characteristics, such as asymmetric diffraction pattern, nonreciprocal and asymmetric transmission and reflection, and an enhanced diffraction efficiency. The theory of the wave diffraction by STP gratings is formulated through satisfying the conservation of both momentum and energy, and rigorous Floquet mode analysis. Furthermore, the theoretical analysis of the structure is supported by time and frequency domain FDTD numerical simulations for both transmissive and reflective STP diffraction gratings. Additionally, we provide the conditions for Bragg and Raman-Nath diffraction regimes for STP gratings. Finally, as a particular example of a practical application of the STP diffraction gratings to communication systems, we propose an original multiple access communication system featuring full-duplex operation. STP diffraction gratings are expected to find exotic practical applications in communication systems, especially for the realization of enhanced-efficiency or full-duplex beam coders, nonreciprocal beam splitters, nonreciprocal and enhanced-resolution holograms, and illusion cloaks.

Light induced reversible structuring of photosensitive polymer films

Abstract: In this paper we report on photoswitchable polymer surfaces with dynamically and reversibly fluctuating topographies. It is well known that when azobenzene containing polymer films are irradiated with optical interference patterns the film topography changes to form a surface relief grating. In the simplest case, the film shape mimics the intensity distribution and deforms into a wave like, sinusoidal manner with amplitude that may be as large as the film thickness. This process takes place in the glassy state without photo-induced softening. Here we report on an intriguing discovery regarding the formation of reliefs under special illumination conditions. We have developed a novel setup combining the optical part for creating interference patterns, an AFM for in situ acquisition of topography changes and diffraction efficiency signal measurements. In this way we demonstrate that these gratings can be “set in motion” like water waves or dunes in the desert. We achieve this by applying repetitive polarization changes to the incoming interference pattern. Such light responsive surfaces represent the prerequisite for providing practical applications ranging from conveyer or transport systems for adsorbed liquid objects and colloidal particles to generation of adaptive and dynamic optical devices.

Lop-sided Raman–Nath diffraction in PT-antisymmetric atomic lattices

Abstract: Two-dimensional (2D) optical lattices of driven cold atoms can provide a useful platform to construct 2D electromagnetically induced grating (EIG) with parity-time (PT) antisymmetry. This atomic grating is achieved by the spatial modulations of the atomic density and frequency detunings in the four-level double-Λ atomic system. Gain-assisted PT antisymmetry allows us to realize lop-sided Raman–Nath diffraction with high diffraction efficiency at the exception point. It is shown that the nontrivial phenomenon originates from non-Hermitian degeneracy of PT antisymmetry. Our scheme may provide the possibility for active all-optical control and conversion of the spatial beam in optics.

Improving the homogeneity of diffraction based colours by fabricating periodic patterns with gradient spatial period using Direct Laser Interference Patterning

Abstract: This study focuses on the development of a strategy to produce periodic structures with a variable spatial period for increasing the homogeneity of structural colours by means of direct laser interference patterning. Using a four-beam interference configuration, hole-like periodic arrays are produced on stainless steel with a 70 ps pulsed laser source operating at 532 nm laser wavelength. The laser processing parameters are optimised for obtaining patterns with the highest possible diffraction efficiency and thus showing the highest possible colour intensity. A model for calculating the required spatial period to obtain a defined colour under specific conditions of illumination and observation angles is presented. A very good agreement between the captured structural colour spectrum and the real visible spectrum of light was obtained. In addition, a strategy for mixing holographic colours, in particular for obtaining the white colour is developed. Finally, the developed model is successfully integrated into machine software, in order to automatically process images that exhibit required colours at certain viewing conditions. The produced patterns are characterised using confocal microscopy and the efficiency of the first diffraction order was measured by optical spectroscopy.

Topography and longitudinal chromatic aberration characterizations of refractive-diffractive multifocal intraocular lenses.

Abstract: Purpose Most optical systems present chromatic aberration quantified along the optical axis by the longitudinal chromatic aberration (LCA). LCA is controlled by the biomaterial Abbe number combined with diffractive effects, driven by the intraocular lens (IOL) topography. This study experimentally aimed at describing the effect in vitro of LCA in diffractive multifocal IOLs, with the help of dedicated optical benches and topographic characterization. Setting Centre Spatial de Liege, Belgium. Design Optical and topology analysis of various multifocal diffractive IOLs. Methods Seven diffractive multifocal IOLs, available on the market and exhibiting different diffractive profiles, made from various biomaterials, were characterized under different wavelengths. Results Through-focus modulation transfer function (MTF) curves and IOL diffraction efficiency depends on the incident light wavelength. In this study, the topology properties of various multifocal IOLs were investigated and their characteristics were correlated to their optical behavior for various wavelengths. Chromatic properties and their origins were then compared. As expected, diffractive and refractive effects were found to act in opposite ways, and could be partially or completely compensated. Conclusions The LCA of each of the IOLs was evaluated in vitro. In most of the multifocal IOLs studied, some of the foci were found to be refractive, whereas others were diffractive. Although the results were not extrapolated to clinical relevance, it was shown, in some of the cases, that LCA could be fully compensated.

Fast switching ferroelectric liquid crystal Pancharatnam-Berry lens

Abstract: A ferroelectric liquid crystal (FLC) cell with continuously alignment structure is realized by a polarization hologram method for fabricating a Pancharatnam-Berry (PB) lens, which is employed as a concave/convex lens. The PB phase can be maintained by the optical axis in-plane switching; meanwhile, its diffraction efficiency can be tuned in a certain range by electrically controlling azimuthal angle and optical biaxiality of the smectic helical structure realized by deformed helix ferroelectric liquid crystals. The measured diffraction efficiency of the fabricated device is up to 87% and the response time can be 300μs with a low electric voltage. The FLC PB lens can have potential applications in existing optical devices and the realization of FLC with continuous alignment structure can be further used for other LC-based optical devices.

Design of lensless retinal scanning display with diffractive optical element

Abstract: We propose a design of a retinal-scanning-based near-eye display for augmented reality. Our solution is highlighted by a laser scanning projector, a diffractive optical element, and a moist eye with gradient refractive indices. The working principles related to each component are comprehensively studied. Its key performance is summarized as follows. The field of view is 122°, angular resolution is 8.09', diffraction efficiency is 57.6%, transmittance is 80.6%, uniformity is 0.91, luminance is 323 cd/m2, modulation transfer functions are above 0.99999 at 3.71 cycle/degree, contrast ratio is 4878, and distortion is less than 24%.

Dispersion-engineered nanocomposites enable achromatic diffractive optical elements

Abstract: The use of diffractive optical elements (DOEs) in broadband optical systems can often significantly reduce their size or enhance their performance but is mostly prevented by stray light in unwanted diffraction orders. This is because dispersion causes the diffraction efficiency to decrease as a function of the wavelength. Here we introduce nanocomposites as a material platform that allows for the design of dispersion-engineered materials. We show that these materials enable the design of almost dispersion-free, i.e., achromatic, echelette-type gratings with diffraction efficiencies of close to 100% in the entire visible spectral range. Using numerical simulations, we demonstrate that such high efficiencies are maintained across the range of incidence angles and grating periods required for most optical systems. This concept of dispersion-engineered nanocomposites can also be applied to other applications, and nanocomposite-enabled DOEs have the potential to be an enabling technology for a new generation of better and more compact optical systems.

Low-dispersion low-loss dielectric gratings for efficient ultrafast laser pulse compression at high average powers

Abstract: All-dielectric gratings are ideal for high average power laser pulse compressors due to their low loss and high diffraction efficiencies. However, they have not been utilized in petawatt-class lasers with pulse durations below 100 fs due to a variety of challenges. We present a resonance-free design for low-dispersion (1480 lines/mm) diffraction gratings made of dielectric materials resistant to femtosecond laser damage (SiO 2 /HfO 2 ). A 14 cm diameter sample was fabricated resulting in a mean diffraction efficiency of 99.1% at λ = 810 nm with high spatial uniformity using equipment which can produce gratings up to 1 m diagonal. The implementation of these gratings in the compression of 30 fs pulses in an out-of-plane geometry can result in compressor efficiencies approaching 95%. The measured laser absorption is 500× lower than current ultrafast petawatt-class compressor gratings enabling a substantial increase in average power handling capabilities of these laser systems.

Dynamic Diffractive Patterns in Helix-Inverting Cholesteric Liquid Crystals

Abstract: The future of adaptive materials will rely on transduction of molecular motion across increasing length scales, up to the macroscopic and functional level. In this context, liquid crystals have emerged as a promising amplification medium, in view of their long-range order and high sensitivity to external stimuli, and in particular, chiral liquid crystals have demonstrated widely tunable optical properties and invertible handedness. Here, we demonstrate that by applying weak electric fields, regular, periodic and light-tunable patterns can be formed spontaneously in cholesteric liquid crystals. These patterns can be used as light-tunable diffraction gratings for which the period, the diffraction efficiency, and the in-plane orientation of grating vector can be controlled precisely, reversibly, and independently. Such a photoregulation allows generating a variety of one- and two-dimensional complex diffractive patterns in a single material. Our data are also supported by modeling and theoretical calculations. Overall, the fine tunability of cholesteric materials doped with artificial molecular switches makes them attractive for optics and photonics.

Multifocal multi-value phase zone plate for 3D focusing

Abstract: We demonstrate a method for creating a three-dimensional (3D) array of focal spots by combination of a multi-focal diffractive lens and a two-dimensional multi-value phase grating. The multi-focal Fresnel-based lens is created by means of encoding special nonlinearities into the phase structure of a Fresnel zone plate and is represented as a mathematical superposition of this phase function with a refractive lens. The imposed nonlinearity type enables the creation of multiple focal spots with uniform intensity along the optical axis. We demonstrate the example of a 3D multi-value phase grating, which creates five focal planes with a $5 \times 5$5×5 transverse array of focal spots with equal energy distribution in each plane. Experimental results are included to verify the theoretical outcomes, where the phase pattern of a 3D multi-value phase grating is encoded onto a spatial light modulator.

Applicability of the Debye-Waller damping factor for the determination of the line-edge roughness of lamellar gratings

Abstract: Periodic nanostructures are fundamental elements in optical instrumentation as well as basis structures in integrated electronic circuits. Decreasing sizes and increasing complexity of nanostructures have made roughness a limiting parameter to the performance. Grazing-incidence small-angle X-ray scattering is a characterization method that is sensitive to three-dimensional structures and their imperfections. To quantify line-edge roughness, a Debye-Waller factor (DWF), which is derived for binary gratings, is usually used. In this work, we systematically analyze the effect of roughness on the diffracted intensities. Two different limits to the application of the DWF are found depending on whether the roughness is normally distributed or not.

Chirped polarization volume grating with ultra-wide angular bandwidth and high efficiency for see-through near-eye displays.

Abstract: We report a reflective chirped polarization volume grating (CPVG) with a dramatically wider angular bandwidth and significantly higher first-order diffraction efficiency than the holographic volume grating and surface relief grating for large field-of-view (FOV) augmented reality (AR) displays. By introducing gradient pitch structure along the beam propagation direction, the angular bandwidth is extended from 18° to 54° while keeping over 80% diffraction efficiency. We also prepare a two-layer reflective PVG and compare its performance with the chirped structure. Based on the simulation and experimental results, CPVG is a strong contender for large FOV AR displays.

Controlling second-harmonic diffraction by nano-patterning MoS 2 monolayers

Abstract: Monolayers of transition metal dichalcogenides have a strong second-order nonlinear response enabling second-harmonic generation. Here, we control the spatial radiation properties of the generated second harmonic by patterning MoS2 monolayers using focused ion beam milling. We observe diffraction of the second harmonic into the zero and first diffraction orders via an inscribed one-dimensional grating. Additionally, we included a fork-like singularity into the grating to create a vortex beam in the first diffraction order.

Low-index second-order metagratings for large-angle anomalous reflection

Abstract: A large-angle anomalous reflector based on a low-index polylactic acid metagrating is designed around 140 GHz. By breaking the limit of fixed bar-to-bar distance and directing the beam into the second diffraction order through the optimization of the 4π phase supercell, the efficiencies for 70° and 80° reflection under normal excitation both reach 0.82 with a wide bandwidth. In contrast, the efficiency is less than 0.2 in conventional designs. The success of the second-order low-index metagrating for large-angle deflection originates from the appropriate number of Bloch modes with well-engineered mode interactions and couplings. The design can be potentially fabricated by three-dimensional printing, with promising applications in designing flat lenses of high numerical aperture and extending the material system of metadevices.

Diffraction effect and its elimination method for diamond-turned optics

Abstract: In the visible light band, the diffraction effect of a diamond-turned surface will cause the optical performance to heavily deteriorate. Due to the insufficient understanding of diffraction effect, post-treatment, such as polishing technology has to be fulfilled. To reveal the origins of diffraction effect of the diamond-turned surface under visible light, theoretical analyses are carried out with consideration of the influencing factors in diamond turning. Simulation results, coupled with experimental observations, demonstrate that the periodic components of surface roughness are responsible for the diffraction light distribution in the horizontal direction of the receiving screen. However, the aperiodic components of surface roughness, derived from defects in material matrix, result in the diffraction spots on the whole receiving screen. To directly eliminate the diffraction effect in diamond turning, a novel method—with control on tool edge quality, material defects, and processing parameters—is proposed. The measurement results prove the effectiveness of this method, and the diffraction-free surface finish without any post-treatment is successfully acquired.

Controllable electromagnetically induced grating in a cascade-type atomic system

Abstract: A controllable electromagnetically induced grating (EIG) is experimentally realized in a coherent rubidium ensemble with 5S1/2-5P3/2-5D5/2 cascade configuration. In our work, a whole picture describing the relation between the first-order diffraction efficiency and the power of the coupling field is experimentally presented for the first time, which agrees well with the theoretical prediction. More important, by fine tuning the experimental parameters, the first-order diffraction efficiency of as high as 25% can be achieved and a clear three-order diffraction pattern is also observed. Such a controllable periodic structure can provide a powerful tool for studying the control of light dynamics, pave the way for realizing new optical device.

Ultrafast volume holographic recording with exposure reciprocity matching for TI/PMMAs application

Abstract: The range of exposure for which the holographic reciprocity law holds in photopolymers, is mainly dependent on the light exposure intensity and polymerization rate between photo-initiator and monomers. Matching this is the key to improving performance. Characterization of the dependence on diffraction efficiency of the volume transmission gratings on holographic reciprocity matching of TI/PMMAs under different milliseconds with different thickness (1-3mm) has been carried out for the novel high-sensitive TI/PMMA polymers. Diffraction gratings can be recorded in TI/PMMAs under 20ms with the exposure intensity of 115mW/cm2. The physical and chemical mechanism under and after single shot exposure is analyzed which can be divided into three parts, namely, photo-induced polymerization, dark diffusion of photosensitive molecules, and counter-diffusion of photoproducts. Holographic properties of TI/PMMAs of different thickness (1-3mm) under different shingle-shot durations and repetition rates are investigated in detail as well. The diffraction efficiency reaches 67% with the response time of 15.69s. By this way, volume holographic gratings with no reciprocity failure can be recorded under multi-pulse exposure, with high grating strength and rapid sensitivity in TI/PMMAs, which indicates the volume holographic memories have the potential for recording and storing transient information in life and in the military.

Design of dual-band infrared zoom lens with multilayer diffractive optical elements.

Abstract: A mid-wave infrared (MWIR)/long-wave infrared (LWIR) dual-band zoom lens design with multilayer diffractive optical elements (MLDOEs) is presented. The mathematical relationship between the substrate material selection for dual-band MLDOE and polychromatic integral diffraction efficiency (PIDE) is deduced in the oblique incident situation, and further, a method for optimal selection of substrate material is proposed to obtain the high PIDE in an incident angle range. In the optimization process, the optimal substrate material combination is selected based on the proposed method, and the principle of lens material replacement is discussed. After optimization, the 5× hybrid dual-band infrared zoom system is obtained, which consists of seven lenses. The modulation transfer function values in all configurations are larger than 0.5 and 0.3 in MWIR and LWIR, respectively. The distortion values are less than 2% both in MWIR and LWIR for all configurations.

Pancharatnam–Berry optical lenses

Abstract: In this paper, we analyze a few aspects of Pancharatnam–Berry (PB) phase optical lenses. First, we provide theoretical formulas on how the optical efficiency, focal length, and point spread function depend on wavelength and validate them with numerical calculations based on Fresnel diffraction theory. Second, we perform numerical studies on how optical efficiency is affected by discretization of the PB phase. We find that phase discretization significantly lowers the optical efficiency for low f-number PB lenses.

Single-step creation of polarization gratings by scanning wave photopolymerization with unpolarized light [Invited]

Abstract: Liquid crystalline materials with a cycloidal molecular orientation pattern are attractive for fabricating diffractive waveplates, diffracting incident light regardless of its polarization state into left- and/or right-circularly polarized light only in the +1st and/or −1st orders, applicable as next-generation optical devices. However, large-area high-speed processing of such molecular orientation is a challenge, since even state-of-the-art photoalignment methods require a precise spatial modulation of the polarization states of incident light, e.g., polarization holograms. Here, we propose and demonstrate that unpolarized light could easily generate cycloidal molecular orientation patterns over large areas in a single step merely by using our recently developed method of “scanning wave photopolymerization” with a simple optical setup. Importantly, the processing time for fabricating millimeter-scale films was significantly decreased to less than a few minutes. Detailed investigation revealed that the resultant film showed the desired diffraction behavior with a diffraction efficiency of 50%.