High Diffraction Efficiency Research Articles

Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography.

We propose a full-color see-through three-dimensional (3D) display method based on volume holography. This method is based on real object interference, avoiding the device limitation of spatial light modulator (SLM). The volume holography has a slim and compact structure, which realizes 3D display through one single layer of photopolymer. We analyzed the recording mechanism of volume holographic gratings, diffraction characteristics, and influencing factors of refractive index modulation through Kogelnik’s coupled-wave theory and the monomer diffusion model of photopolymer. We built a multiplexing full-color reflective volume holographic recording optical system and conducted simultaneous exposure experiment. Under the illumination of white light, full-color 3D image can be reconstructed. Experimental results show that the average diffraction efficiency is about 53%, and the grating fringe pitch is less than 0.3 μm. The reconstructed image of volume holography has high diffraction efficiency, high resolution, strong stereo perception, and large observing angle, which provides a technical reference for augmented reality.

Vortex beam array generated by a volume compound fork grating in lithium niobite

Here, we report a generation of vortex beam array based on a micro volume compound fork grating fabricated by a femtosecond laser direct writing technique in lithium niobite crystal. The grating is able to convert a Gaussian laser beam into an array of large number of vortex beams with high purity and high diffraction efficiency. The intensity distribution and the topological charges of the vortex beams are investigated, indicating there are multiple pairs of vortex beams with opposite topological charges in the array. Interference synthesizing method is also studied to superpose the opposite order vortex beams to produce integer-order vector-vortex beams, in addition to the generation of the fraction-order vector-vortex beam. The theoretical calculation and the experiment polarization measurement results indicate that the beams produced have spatial anisotropic polarization distribution. The micro structure of the compound fork grating has potential applications in generating vector-vortex beams in integrated optics field.

Modelling of the diffraction efficiency of surface relief gratings with varying spatial frequency, height and shape of the relief

An important goal in the development of diffractive optical elements is to achieve high diffraction efficiency. One class of these elements are the surface relief gratings recorded by polarization holography. For recording media, we used azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly denoted as PAZO. The aim of the present work is to simulate a highly efficient surface relief grating formed in the azopolymer PAZO. The model is based on experimental results already obtained by our group. The tasks of the research are related to the analysis of the diffraction efficiency in the spectral range from 300 nm to 800 nm and for: 1) range of grating spatial frequencies from 1200 ln/mm to 1800 ln/mm, 2) surface relief height varying from 100 nm to 400 nm and 3) different shapes of the formed relief.

Twisting Structures in Liquid Crystal Polarization Gratings and Lenses

Recently, diverse twisting structures have been discovered to be a potential approach to design liquid crystal polarization gratings and lenses (LCPGs and LCPLs) with a high diffraction efficiency, broad bandwidth, wide view, and large diffraction angle. In this review, we divide these twisting structures into two main types, namely, multi-layer twisting structures with phase compensation and twisting structures forming Bragg diffraction. We found that multi-layer twisting structure LCPGs and LCPLs presented a broader bandwidth and a wider view angle by phase compensation. While for transmissive or reflective Bragg LCPGs, a large diffraction angle with high diffraction efficiency could be achieved. Based on the theoretical analysis in the review, potential research directions on novel twisting structures were prospected.

Design and Realization of a Compact Efficient Beam Combiner, Based on Liquid Crystal Pancharatnam–Berry Phase Gratings

We demonstrate a laser beam combiner based on four photo-patterned Pancharatnam–Berry (PB) phase gratings, which is compact and has high diffraction efficiency for incident circularly polarized light. The nematic liquid crystal mixture E7 is used as anisotropic material, and the thickness of the layer is controlled by spacers. The beam combiner can bring two parallel laser beams closer to each other while remaining parallel. This work shows the potential to realize components based on flat optical LC devices.

Compact and highly-efficient broadband surface grating antenna on a silicon platform.

We present a compact silicon-based surface grating antenna design with a high diffraction efficiency of 89% (-0.5 dB) and directionality of 0.94. The antenna is designed with subwavelength-based L-shaped radiating elements in a 300-nm silicon core, maintaining high efficiency with a compact footprint of 7.6 µm × 4.5 µm. The reflectivity remains below -10dB over the S, C and L optical communication bands. A broad 1-dB bandwidth of 230 nm in diffraction efficiency is achieved with a central wavelength of 1550 nm.

5.3: Invited Paper: High Efficiency Phase Modulators

In communication networks, wavelength selective switch (WSS) is one of the key components in reconfigurable optical add‐drop multiplexer (ROADM) [1]. Several technologies have been developed to achieve a high‐performance WSS, such as micro electromechanical systems (MEMS) [2], liquid crystal on silicon (LCOS) [3], arrayed waveguide grating (AWG) [4], and hybrid planar lightwave circuit (PLC)‐MEMS [5]. Among these, LCOS‐based WSS is most attractive for the next‐generation optical networks due to its controllability with individual electrical control on pixel. It shows advantages of flexible bandwidth adjustment, adaptive alignment, and robustness without mechanical movements.A LCOS exhibiting a high diffraction efficiency at a large deflection angle is demonstrated [6]. A liquid crystal (LC) mixture with a large birefringence (Δn = 0.27, at λ = 1550 nm) and a large dielectric constant (Δε = 10.4) is developed to obtain the high‐phase retardation, more than 4π, at 1550 nm within a relatively low operation voltage of 5.5 V on LCOS. When compared with the conventional modulation method of 2π phase retardation, both grating period and phase retardation are doubled in 4π modulo modulation. Using the high‐phase retardation modulation method, the LCOS may achieve high diffraction efficiency, more than 56.6% at 4.5° deflection angle of C‐band wavelength with an optimized phase retardation profile.A polarization‐independent blue phase liquid crystal on silicon (BPLCoS) device with low operation voltage for 2π phase depth is proposed [7]. With optimized reflection structure and two reflection films, the incident light may experience multifold optical path and 2π phase depth can be obtained. For the polarization‐independence, an inclined electric field made by periodical gradient voltage is applied on the blue phase liquid crystal (BPLC) to match the light propagation direction. With the structure, the operation voltage can be lowered to 5.5 V in simulation and 5.9 V in experiment for 2π phase modulation at 1550 nm.The high diffraction efficiency and polarization‐independent phase modulators show the great potential application of beam steering for the optical communication and the holographic display.

Alternative Design of Binary Phase Diffractive Optical Element with Non-π Phase Difference

It was found that binary phase diffractive optical element (DOE) with non-π phase difference had higher diffraction efficiency and adjustable zero-order intensity than a 0-π one. However, existing design methods are all based on the simulated annealing algorithm and thus computationally expensive. In this paper, a simple and efficient method using the iterative Fourier transform algorithm (IFTA) is proposed. In this method, the target pattern is first modified via reducing the zero-order intensity. Then, the IFTA is adopted to design the conventional 0-π DOE. Subsequently, the phase distribution remains unchanged and the phase difference is carefully adjusted to increase the zero-order intensity so that the reconstructed pattern is consistent with the target. To verify this method, several typical DOEs for beam splitting were designed and fabricated, and the result showed that the proposed method is effective.

Rectangular multilayer dielectric gratings with broadband high diffraction efficiency and enhanced laser damage resistance.

Broadband multilayer dielectric gratings (MDGs) with rectangular HfO2 grating profile were realized for the first time using a novel fabrication process that combines laser interference lithography, nanoimprint, atomic layer deposition and reactive ion-beam etching. The laser-induced damage initiating at the grating ridge was mitigated for two reasons. First, the rectangular grating profile exhibits the minimum electric-field intensity (EFI) enhancement inside the grating pillar compared to other trapezoidal profiles. Second, our etching process did not create nano-absorbing defects at the edge of the HfO2 grating where the peak EFI locates, which is unavoidable in traditional fabrication process. The fabricated MDGs showed a high laser induced damage threshold of 0.59J/cm2 for a Ti-sapphire laser with pulse width of 40 fs and an excellent broadband diffraction spectrum with 95% efficiency over 150 nm in TE polarization.

Diamond diffractive lens with a continuous profile for powerful terahertz radiation.

An increase in the radiation power of terahertz (THz) sources requires the development of new optics working with it. The laser-assisted replication technique is proposed to fabricate the diamond cylindrical diffractive lens with a continuous profile for the THz range. The procedure involves the inverted structuring of a silicon substrate by laser ablation for its further replication to the diamond surface utilizing the chemical vapor deposition process. Testing of the fabricated lens performed with a free-electron laser at the wavelength of 141 µm has demonstrated high diffraction efficiency (95±5%) and a good agreement between the measured and expected intensity distribution in the focal plane.

Design and optimization method of a convex blazed grating in the Offner imaging spectrometer

The convex reflective diffraction grating is an essential optical component in Offner systems, which has been widely used in imaging spectrometers. We propose a new design and optimization method for the convex blazed grating in the Offner imaging spectrometer. The method integrates the macro- and microdesign of the optical system, and it can be used to design and optimize the convex blazed grating with high diffraction efficiency. Traditional geometric optics theory and image quality evaluation methods are used to design the macro-optical structure parameters of the Offner system. And then the incident ray information, such as the incident angle and the polarization states are calculated by using the three-dimensional polarization ray-tracing method. To improve the diffraction efficiency, we combine rigorous coupled wave analysis and a particle swarm optimization algorithm to optimize the microstructure parameters of the convex-blazed grating. Further, a convex-blazed grating in a mid-wave infrared Offner imaging spectrometer is designed as an example to illustrate our design method in detail. The design results indicate that the Offner imaging spectrometer has good imaging quality, and the average diffraction efficiency of the -1st diffraction order of the convex-blazed grating in the spectral coverage 3-5 µm is 82.24%. Compared to the traditional design method, the lowest spectral diffraction efficiency is improved from 59.88% to 69.24%, the highest spectral diffraction efficiency is improved from 90.45% to 91.84%, and the standard deviation is reduced from 7.82 to 6.62.

大角带宽高衍射效率体全息光栅的设计和制备

大角带宽高衍射效率体全息光栅的设计和制备

Design of high efficiency achromatic metalens with large operation bandwidth using bilayer architecture

Achromatic metalens composed of arrays of subwavelength nanostructures with spatially varying geometries is attractive for a number of optical applications. However, the limited degree of freedom in the single layer achromatic metasurface design makes it difficult to simultaneously guarantee the sufficient phase dispersion and high diffraction efficiency, which restricts the achromatic bandwidth and efficiency of metalens. Here we propose and demonstrate a high efficiency achromatic metalens with diffraction-limited focusing capability at the wavelength ranging from 1000 nm to 1700 nm. The metalens comprises two stacked nanopillar metasurfaces, by which the required focusing phase and dispersion compensation can be controlled independently. As a result, in addition to the large achromatic bandwidth, the averaged focusing efficiency of the bilayer metalens is higher than 64% at the near-infrared region. Our design opens up the possibility to obtain the required phase dispersion and efficiency simultaneously, which is of great significance to design broadband metasurface-based optical devices.

A new method of implementing simultaneous multiplexing holographic display of wavelength and polarization state with simple structure metasurface

In this paper, we propose a new method to realize both polarization-multiplexing and wavelength-multiplexing using a simple structure, which can realize hologram by the multiplexing of double wavelengths and double polarization in the visible band. Our design can reduce color cross-talk and have a higher diffraction efficiency. We design a transmission metasurface composed of simple rectangular cells. Firstly, we establish the relationship of structural parameters with the transmission phase under various incident conditions of light beams. Then we propose a fitness function that can optimize the structural parameters of the unit cell at each pixel point, which can display different images by 532 nm <i>x</i>-polarization and 633 nm <i>y</i>-polarization incident light beams respectively. Finally, finite difference time domain method is used to analyze the structure, and the holographic result fits the theoretical design very well. This work proposes using single metasurface structure to solve the problems of wavelength cross-talk appearing when using simple structures, and will have great importance in coding and anti-counterfeiting.

Holographic grating fabrication for wide angular bandwidth using polymer thin films

To increase the angular bandwidth of volume holographic grating, we fabricate holographic gratings based on grating multiplexing technique by using thin films of photopolymers and polymer dispersed liquid crystals. Experimental results confirm that the liquid crystal materials increase the refractive index modulation of the grating, enabling high diffraction efficiency with wide angular response compared to pure polymer materials. We observe that the fabricated holographic grating has near 80% of diffraction efficiency and about 18° of angular bandwidth, which can be further improved by modifying the liquid crystal/polymer mixtures and the grating multiplexing technique. The grating can be used to fabricate holographic waveguide structures for emerging applications in the near-eye display systems.

Diffraction Grating Fabricated on Chalcogenide Glass Fiber End Surfaces With Femtosecond Laser Direct Writing

Chalcogenide glass (ChG) optical fibers have excellent transmission performance in the mid-infrared range, and microstructured devices prepared on ChG fibers play an important role in the field of infrared sensing and optical integration. In this study, high-quality chalcogenide optical fibers were prepared, and grating structures were produced on their end surfaces. An easy-to-use FC cable was first prepared and polished to high flatness with the surface roughness of less than 0.08 μm. 1- and 2-D diffraction gratings were then prepared on the end surfaces of the chalcogenide fibers via femtosecond laser line-by-line direct writing technology. The ChG fiber end surface gratings prepared with optimized femtosecond laser parameters exhibited high diffraction efficiency and clear diffraction pattern, indicating that they had excellent optical performance. The performance of the grating was tested by using a laser source with wavelengths covering the visible to the near-infrared regions. This measurement showed that the first-order diffraction efficiency of the grating on the end face could reach close to 50% at 1550 nm.

Optimization method of manufacturing for diamond turning soft-brittle materials' harmonic diffractive optical elements.

Single-point diamond turning is a high-efficiency, low-cost method for manufacturing harmonic diffractive optical elements (HDOEs). Generally, in order to ensure high diffraction efficiency of HDOEs, a half-round tool is used to reduce surface-relief profile errors and a super small feed rate is selected to control surface roughness when hard-brittle materials are turning. However, this method is no longer suitable for soft-brittle materials, which have a strict requirement for the range of the feed rate. It limits the types of materials available for HDOEs. Therefore, according to the range of the feed rate and the cutting depth for soft-brittle materials, an optimized turning model is proposed in this paper. It overcomes the high surface roughness caused by the half-round tool; meanwhile, the advantage of the half-round tool is kept in terms of surface-relief profile errors. On this basis, a mathematical model is proposed to reveal the relationship among diffraction efficiency, period widths, tool radius, and feed rate with different soft-brittle optical materials. As a typical soft-brittle material, barium fluoride (BaF2) was selected to be the material for manufacturing HDOEs. By using the optimized model, the turning experiment of BaF2 HDOEs was completed, and then the BaF2 HDOEs with Ra=2.75nm were obtained. The optimized model is verified to be effective and further provides theory guidance and engineering application in the optical manufactory field of soft-brittle materials for HDOEs. The range of application materials for HDOEs is enriched, and the freedom of advanced optical design is broadened.

Highly sensitive updatable green hologram recording polymer with photoisomerizable azobenzene with highly birefringent acetylene as the side chain

To develop highly sensitive updatable hologram materials, an azobenzene monomer was synthesized herein that supports effective photoisomerization under visible green beam irradiation. Furthermore, an acetylene group was introduced via Sonogashira coupling for higher birefringence. Polymerization was performed on the synthesized azobenzene monomer and on the methyl methacrylate (MMA) monomer, which serves as the skeleton of the film. The structures of all molecules were characterized by nuclear magnetic resonance (NMR) spectroscopy, and UV-Visible absorption spectroscopy was performed to analyze their optical properties. The holographic writing and rewriting capabilities of the newly synthesized azobenzene side-chain polymer were characterized by using a green laser as a writing beam, which showed that a high diffraction efficiency was reliably achievable under low-energy laser irradiation when evaluated with a four-wave-mixing optical scheme. In this study, high birefringent acetylene group containing azobenzene-based polymers were synthesized as materials for rewritable green-laser holograms, and their properties were evaluated. Birefringence change of polymer, which has an absorption wavelength suitable for green laser (532 nm), is 0.018. The diffraction efficiency of polymer was 72% under low-energy irradiation (7 J/cm2). These results show that the hologram recording material made in this study with high birefringent azobenzene are stable, without undergoing decomposition even when repeated recording and rewriting were performed.

Scanning ion beam etching: A method for the fabrication of computer-generated hologram with nanometric accuracy for aspherical testing

The phase type computer generated hologram (CGH) plays an important role in aspherical surface optical testing, especially for low reflectivity material mirror surfaces, owing to its high diffractive efficiency. With the increasing demand for high precision CGHs, traditional methods such as reactive ion etching and focusing ion beam, have difficulty realizing the fabrication of a large aperture and high accuracy phase type CGH. In this study, fabrication of a high precision CGH has been demonstrated via a new method called scanning ion beam etching (SIBE), which is suitable for most common optical materials and the fabrication of high performance CGH. On the basis of the movable radio frequency ion source, we determine an etching depth calculation method to realize high accuracy etching regardless of whether the removal function is a perfect Gaussian distribution. Experiments are conducted to verify the etching depth accuracy. Moreover, the linewidth, sidewall angle, roughness, and etching uniformity results of the CGH are superior to the fabrication of CGH via the conventional method. As a proof of concept, a phase type CGH with an aperture of 80 mm is fabricated, and the wavefront accuracy is 6.2 nm, which can meet the processing requirements of a high precision aspheric mirror. Thus, a SIBE method is developed for high performance CGH, which may aid in the development of high accuracy diffractive optics.

Fully symmetric dispersionless stable transmission-grating Michelson interferometer.

Michelson interferometers have been routinely used in various applications ranging from testing optical components to interferometric time-resolved spectroscopy measurements. Traditionally, plate beamsplitters are employed to redistribute radiation between the two arms of an interferometer. However, such an interferometer is susceptible to relative phase fluctuations between the two arms resulting from vibrations of the beamsplitter. This drawback is circumvented in diffraction-grating-based interferometers, which are especially beneficial in applications where highly stable delays between the replica beams are required. In the vast majority of grating-based interferometers, reflective diffraction gratings are used as beamsplitters. Their diffraction efficiency, however, is strongly wavelength dependent. Therefore transmission-grating interferometers can be advantageous for spectroscopy methods, since they can provide high diffraction efficiency over a wide spectral range. Here, we present and characterize a transmission grating-based Michelson interferometer, which is practically dispersion-free, has intrinsically high symmetry and stability and moderate throughput efficiency, and is promising for a wide range of applications.