High Diffraction Efficiency Research Articles

Numerical Study on the Potential Enhancement of Organic Terahertz Sources through Tilted Pulse Front Pumping

Organic crystals offer promising potential for THz generation, but face limitations in wavelength tunability and damage threshold. By applying tilted pulse‐front pumping to organic crystals an additional degree of freedom is introduced into the pumping conditions enabling a wider range of pumping wavelengths without compromising phase matching. Additionally, the lifespan of organic materials can be extended by using longer pumping wavelength and eliminate lower‐order multi‐photon absorption, allowing for higher pumping intensity without significant free‐carrier absorption, thus increasing the damage threshold. Simulations predict significant improvement for four out of six investigated crystals when tilted pulse‐front pumping is applied. By using volume phase holographic grating one can achieve pulse‐front tilt in organic crystals in collinear geometry with high diffraction efficiency. Design parameters are also presented.

Alignment of polarization volume gratings using semi-circular arc patterns via nanofabrication technology

Polarization volume gratings have the potential for future applications such as AR due to their high single-order diffraction efficiency and large angular and wavelength response range. However, the preparation method of photo alignment, which is currently employed, presents challenges to the application of PVGs due to issues of poor uniformity and difficulty in preparing large sizes. To promote the application, a scheme for fabricating the alignment layer of PVGs by nanofabrication technology is proposed. The alignment structure, where the basic unit is a semicircular arc, was designed. We prepared the structure using EBL and spin-coated a layer of chiral liquid crystals (CLC) on top of it. The dispersion effect in white light indicates that the designed alignment structure imparts lateral periodicity to the CLC. The device could efficiently reflect specific right-polarized light and efficiently transmit specific left-polarized light. Therefore, the diffraction performance of the device can be considered to be consistent with the photo-alignment PVGs within the error allowance. This work provides what we believe to be a novel scheme for the alignment of PVGs, creating a new platform for mass production and multifunctional integrated design of PVGs.

Design of polarization-independent multilayer dielectric gratings: a reflection-phase threaded approach

We present a method to design polarization-independent multilayer dielectric gratings. In this method the reflection phases in TE and TM polarizations of the multilayer stack thread surface-relief grating at the top and the multilayer stack at the bottom together, allowing the two parts first to be designed separately and efficiently, and then to be combined to achieve simultaneously high diffraction efficiency and large fabrication tolerance. We find numerically that in general a periodic stack is unable to provide the top-grating-demanded phase difference between TM and TE polarizations; adequate aperiodic layers atop of a periodic stack are needed. The analytic diffraction efficiency formula of a recent work [J. Opt. Soc. Am. A 41, 252 (2024)] is used at various places of the presented optimization algorithm to save computation time. An example grating with rectangular surface-relief profile and another with trapezoidal profile were successfully designed, validating the effectiveness of this design method.

Ultrafast visible fiber laser mode-locked by frequency-shifted feedback of an α-BaTeMo2O9 crystal acousto-optic modulator.

We report the first demonstration, to the best of our knowledge, of visible mode-locked fiber laser using frequency-shifted feedback (FSF) with a visible α-BaTeMo2O9 (α-BTM) crystal acousto-optic modulator (AOM). First, an α-BTM crystal is used as the visible high-quality AOM with a high diffraction efficiency of 85%, a fast rise/fall time of 79/98 ns, and a low insertion loss of 0.2 dB at 635 nm. Then, the 635 nm FSF mode-locked fiber laser is achieved using a Pr3+:doped ZBLAN double-clad fiber as a visible gain medium and the α-BTM AOM as a frequency-shifting element. Self-starting mode-locking at 635 nm directly generates red laser pulses with a pulse duration of 45 ps and a repetition frequency of 31.8 MHz. Furthermore, we investigate the evolution of mode-locking dynamics as the α-BTW-AOM feedback-frequency, which helps further understand the FSF dynamics by directly generating visible ultrashort pulses.

Three-layer polarisation volume grating with enhanced diffraction efficiency and FOV in AR and VR applications

ABSTRACT In the rapidly evolving fields of Augmented Reality (AR) and Virtual Reality (VR), achieving high diffraction efficiency and a wide field of view (FOV) is critical for enhancing visual experience. This paper presents a three-layer Polarisation Volume Grating (TL-PVG) structure designed to address the limitations of single-layer PVGs in AR/VR applications. By stacking multiple PVG layers with different tilt angles, TL-PVG achieves an expanded angular bandwidth and improved diffraction efficiency, enabling brighter and more immersive displays. The proposed TL-PVG structure incorporates polarisation multiplexing to optimise the optical diffraction efficiency by approximately 75%, thereby enhancing image brightness and sharpness. The experimental results demonstrate that TL-PVG significantly outperforms traditional single-layer PVGs, offering a promising solution for waveguide elements in next-generation AR/VR displays. This study provides a comprehensive analysis of the fabrication process, optical properties, and applications of TL-PVGs, highlighting their potential to revolutionise the AR/VR technology.

The effect of photo-thermo-induced crystallization on the properties and microstructure of transmission volume Bragg gratings

Volume Bragg gratings (VBGs) recored on photo-thermo-refractive (PTR) glass are extensively utilized in high power lasers, optical communication, holographic display fields, etc. In the majority of application fields, high refractive index modulation (RIM) value and high transmittance are the required parameters for transmission Volume Bragg gratings (TVBGs) devices. This study indicates that the RIM values and transmittance of TVBGs strongly depend on the size and concentration of sodium fluoride (NaF) crystals within the grating periodic structure. Increasing the UV exposure dose causes a reduction in the size of NaF crystals, leading to a finitely rise in the RIM value of TVBGs. Prolonging the crystallization heat treatment time and elevating the temperature effectively enhance the RIM value of TVBGs, which also significantly enlarge the size of NaF crystals in the grating periodic structure. By fine-tuning the process parameters of photothermal induced crystallization, a RIM value of 20–320 ppm is freely controlled. For 1.5 mm thick TVBGs, the above RIM value range can meet the application requirements of achieving high diffraction efficiency in the visible to near-infrared range. Following UV exposure at a dose of 5 J/cm2 and nucleation at 480 °C for 120 min, crystallization treatments at 550 °C for 30–60 min and at 570 °C for 60–120 min are determined as optimal for fabricating TVBGs with high diffraction efficiency and transmittance in the visible (532 nm) and near-infrared (1053 nm) bands, respectively.

Optimal Design and Analysis of Wide-Band Near-Infrared Hybrid Dielectric Gratings with High Transmission Efficiency.

Since surface relief transmission gratings have very strict requirements on operators and use environment, according to the semiconductor laser external cavity spectral beam combining system, this paper proposes a design scheme for a semiconductor laser array spectral beam combining system based on the grating-external cavity. The finite element approach was used to create a wideband, high-efficiency fill-in multilayer dielectric transmission grating structure for a high-power spectrum beam combining system. The incidence angle, ridge height, duty cycle, and sidewall inclination angle of the transmission grating were tuned and evaluated, and a link between the transmission grating's diffraction efficiency and grating characteristics was discovered. The calculated design of the high-power fused silica transmission grating has a negative first-order peak diffraction efficiency of 99.5% in the 800 nm range. In the spectral region of 765-872 nm, the transmission grating's diffraction effectiveness exceeds 92%. The filled ultra-high diffraction efficiency multilayer dielectric transmission grating design addresses the issue of resistance to high-power lasers under complicated operating settings. It is intended to maintain a high diffraction efficiency even after several cleaning cycles, and it is an ideal component for high-power spectrum beam combining systems.

Temperature effect on acoustic waves reflection condition in anisotropic crystals

The reflection of acoustic waves in anisotropic media has found important applications in acousto-optics (AO). It may be realized both with changing the type of acoustic mode and without it. The latter case is applied for the realization of the quasi-collinear AO diffraction geometry for which the incident light and ultrasound wave group velocity vectors are collinear. The acoustic wave involved into the AO interaction exists due to the reflection from the AO cell input optical face. Quasi-collinear configuration of AO interaction enables achieving exceptionally high spectral resolution and diffraction efficiency, therefore, it is applied for the laser pulse shaping. However, the application of AO tunable filters for such purposes requires high acoustic power values, which induce the temperature gradients in the AO crystal. Temperature variations influence the stiffness constants of the AO crystal, impacting the characteristics of incident and reflected acoustic waves, affecting the acoustic wave reflection condition and consequently, the AO diffraction characteristics. This study presents the theoretical and experimental examination of the temperature influence on the acoustic beam reflection in anisotropic crystals on the example of tellurium dioxide crystal. This paper is supported by the Russian Science Foundation, grant 23-12-00057.

Octaphenyl-POSS nanoparticles dispersed thick polyvinyl alcohol/acrylamide photopolymer with low volumetric shrinkage and high diffraction efficiency.

Relatively high volumetric shrinkage induced by polymerization of photopolymer severely limits its applications in high-density holographic storage. In this paper, what we believe to be a novel organic-inorganic hybrid nanoparticles (NPs) of octaphenyl-polyhedral Oligomeric silsesquioxane (POSS) are employed into polyvinyl alcohol (PVA)/acrylamide photopolymer with 300µm thickness. By optimizing the concentration of octaphenyl-POSS, the volumetric shrinkage is successfully suppressed from 1.11% to 0.41%, and satisfies the requirements of commercial holographic data storage (HDS) material. Moreover, the introduction of POSS NPs does not sacrifice other holographic recording capacities. Contrarily, the diffraction efficiency of photopolymer after dispersing 2‰(w/v) POSS increases from 86% to 90%. The physical mechanism of enhanced capacities is discussed by using photopolymerization and NPs diffusion process. To our knowledge, this is the best results of PVA/acrylamide photopolymer with low volumetric shrinkage, high diffraction efficiency and angular selectivity. The POSS-dispersed photopolymer system is expected to have good application prospects in angular-multiplexing high-density 3-D holographic storage.

Preparation of Panchromatic Silver Halide Emulsion to Fabricate High Efficiency Multiplexed Volume Phase Holograms for Solar Concentration Application

Volume Phase Gratings (VPG) are often recorded using holographic emulsions such as photopolymer, Dichromated Gelatin (DCG), and silver halide. We employed silver halide emulsion to record the VPG in monochromatic red wavelength for solar concentration purposes. According to the Braggs condition, the recorded VPG's diffraction efficiency reaches its peak at the recorded angle and wavelength. For solar applications, we needed high diffraction efficiency over a wide angle and wavelength range. So, we captured multiplexed single VPG in our own panchromatic silver halide emulsion. The procedure of making panchromatic silver halide emulsion is described in depth. This technique is unique in that it achieves peak diffraction efficiency at broad wavelength and wide angle in a single multiplexed VPG. The panchromatic multiplexed VPG produced a high diffraction efficiency in wide-angle response with overall visible wavelengths. This study describes the high diffraction efficiency multiplexed VPG in its own manufactured panchromatic silver halide emulsion for solar concentrating applications.

Polarization-independent and high-efficiency 2D dielectric transmission grating under Littrow incidence

In this paper, a transmission two-dimensional (2D) all-dielectric grating with cuboid arrays is proposed, which has high diffraction efficiency and good polarization independence under Littrow mounting conditions at an incident wavelength of 780 nm. The optimization results indicate that when the incident wavelength is 780 nm, the diffraction efficiency of the (−1, 0) order of transverse electric (TE) and transverse magnetic (TM) polarizations can reach 98.62% and 98.23%, respectively, with the polarization-dependent loss (PDL) of 0.017 dB. To the best of our knowledge, high-efficiency polarization-independent 2D transmission grating with a simpler and more effective structure is proposed for the first time, which demonstrates significant enhancements in bandwidth and manufacturing tolerances while maintaining high diffraction efficiency. The results suggest that the grating has great potential for applications in high-precision displacement measurements such as grating interferometers.

Diffraction efficiency in acousto-optical interaction with short radio pulses

Development of acousto-optic modulators with high diffraction efficiency, which is achieved by using multi-frequency acoustic signals, is of great interest. In this regard, it seems interesting to study the possibility of implementing highly efficient diffraction for wideband signals in the form of radio pulses with short duration. The goal of presented article is to develop a mathematical model based on a quantum description of the interaction between a photon and a phonon, simulate various operating modes of an acousto-optical modulator and compare theoretical results with experimental ones. A comparison between experimental and theoretical results was made, demonstrating the possibility of obtaining high diffraction efficiency (more than 90%) for wideband signals. The proposed method makes it possible to determine the conditions under which maximum diffraction efficiency is observed, and it also seems possible to determine the parameters of the pulse sequence.

High diffraction efficiency complex amplitude modulation technique by using antiwave functions.

Complex amplitude modulation (CAM) is a single-step technique that codes the amplitude and phase of a diffracted optical field into a real function. Loading this function onto a phase-only spatial light modulator enables the reconstruction of 3D images. However, the obtained images have poor brightness because of the low diffraction efficiency. In this work, we show a novel solution, to the best of our knowledge, that overcomes this limitation. This is done by employing antiwave complex exponential functions, which modify the spectrum of the diffracted field. Proper configuration of these functions allows transferring energy across the diffraction spectrum to the informative diffraction order, which significantly improves the brightness of the reconstructed image. Simulations and experiments prove the efficiency of the proposed method.

High-efficiency polarization-selective two-dimensional diffraction gratings based on silver cuboid arrays

A two-dimensional (2D) diffraction grating based on silver cuboid arrays is demonstrated. With the structural optimization by three-dimensional (3D) finite-difference time-domain method, the proposed grating exhibits favorable polarization selectivity. At wavelength 1550 nm, high diffraction efficiency and polarization correlation can be realized in the case of two-port output ((0, ±1) or (±1, 0)) under TE polarization, as well as five-port output ((0, 0), (0, ±1) and (±1, 0)) under TM polarization. Under the two-port output state, the diffraction efficiency is nearly 48% with the extinction ratio of 16.44 dB and insertion loss of 0.20 dB. Under the five-port output status, the efficiency is larger than 19% for each diffraction order, meanwhile, the total efficiency and uniformity are 97% and 0.58%, respectively. The proposed grating with positive implications has good potentials in polarization multiplexing and polarization-correlated grating interferometry.

Highly efficient multifunctional metasurface integrating lens, prism, and wave plate.

The miniaturization of optical systems is crucial for various applications, including compact augmented reality/virtual reality devices, microelectromechanical system sensors, ranging technologies, and microfabricated atomic clocks. However, reliance on bulky discrete optical elements has been a significant obstacle to achieving this miniaturization. This work introduces a highly efficient multifunctional metasurface (MFMS) that seamlessly integrates a lens, prism, and quarter-wave plate (QWP). This innovation allows simultaneous collimation, beam deflection, and polarization conversion within a singular thin element. Specifically, for the prism-QWP bifunctional integration, we achieved a high diffraction efficiency of 72.8% and a degree of circular polarization of -0.955 under exposure to linearly polarized light at a wavelength of 795 nm, proving its potential for ultracompact atomic clock applications. Moreover, the lens-prism-QWP trifunctional integration successfully showed diffraction-limited focusing performance with a numerical aperture of 0.4, which was sufficient to collimate a beam with a divergence angle of 20 ∘, corresponding to the light emitted from a standard vertical-cavity surface-emitting laser.

Fabrication of Quasi-Sinusoidal Surface Relief Optical Transmission Gratings in Pyrex and IOG Glasses by Implantation with Oxygen and Nitrogen Ion Microbeams of the 5-6 MeV Energy Range.

A method for the fabrication of high diffraction efficiency optical transmission gratings with quasi-sinusoidal profile in glasses by microbeams of medium-mass ions of 5-6 MeV energy was devised and demonstrated. Gratings with a 30 μm grating constant have been manufactured and characterized by interference microscopy and microprofilometry. The obtained surface profiles of the gratings were found to be quasi-sinusoidal with up to 265 nm amplitude. Measured highest first-order diffraction efficiencies were around 26% in both Pyrex and IOG glasses. Such gratings could serve as coupling elements in integrated optics and photonic integrated circuits.

On the thermal stability of multilayer optics for use with high X-ray intensities

High-intensity X-ray free electron laser (XFEL) beams require optics made of materials with minimal radiation absorption, high diffraction efficiency, and high radiation hardness. Multilayer Laue lenses (MLLs) are diffraction-based X-ray optics that can focus XFEL beams, as already demonstrated with tungsten carbide/silicon carbide (WC/SiC)-based MLLs. However, high atomic number materials such as tungsten strongly absorb X-rays, resulting in high heat loads. Numerical simulations predict much lower heat loads in MLLs consisting of low atomic number Z materials, although such MLLs have narrower rocking curve widths. In this paper, we first screen various multilayer candidates and then focus on Mo2C/SiC multilayer due to its high diffraction efficiency. According to numerical simulations, the maximum temperature in this multilayer should remain below 300°C if the MLL made out of this multilayer is exposed to an XFEL beam of 17.5 keV photon energy, 1 mJ energy per pulse and 10 kHz pulse repetition rate. To understand the thermal stability of the Mo2C/SiC multilayer, we performed a study on the multilayers of three different periods (1.5, 5, and 12 nm) and different Mo2C to SiC ratios. We monitored their periods, crystallinity, and stress as a function of annealing temperature for two different heating rates. The results presented in this paper indicate that Mo2C/SiC-based MLLs are viable for focusing XFEL beams without being damaged under these conditions.

Soft x-ray high diffraction efficiency and spectral flux laminar-type W/B4C multilayer diffraction grating for 300-1000 eV.

Multilayer diffraction gratings are designed to improve the detection limit and sensitivity of soft x-ray flat-field spectrographs in the region of 300-1000eV, placing emphasis on Fe-L (705eV), Cu-L (930eV), and Zn-L (1012eV) emissions. For this purpose, spectral flux was used as the performance index, which is proportional to the amount of optical flux incident into a detector and correlated with detection sensitivity. A super-mirror-type W/B4C multilayer coating [Koike et al., Rev.Sci. Instrum. 94, 045109 (2023)] was employed to improve diffraction efficiency in a wide energy region. The unique feature of the multilayer structure is that the average refractive index and the period length of W/B4C layer pairs are increased from the bottom to top layers. In addition, the incidence angle was reduced to 86.03° from 88.65° and the nominal groove density was increased to 3200 lines/mm from 2400 lines/mm of our previous design, to improve spectral flux while maintaining dispersion and spectral resolution. A holographic varied-line-spacing spherical grating and a soft x-ray flat-field spectrograph were designed, using the aspherical-wavefront-recording method, assuming the nominal grating constant and incident angle described above. The numerical simulation results showed that the spectrograph employing the newly designed grating with the W/B4C multilayer indicated 3.2-8.2 times higher spectral flux compared with those using the previously designed grating while keeping the same spectral resolution.

Design of Holographic Condenser Lens for Visible Light Lithography

Photolithography with visible light at λ = 405 nm utilizing the concept of nonlinear saturable absorption effect where the size of the spot is diminished to the nanometer scale as proposed by several researchers is an alluring proposition for cost-effective projection lithography. Costly conventional condenser lenses can be replaced by monochromatic aberration-free and cost-effective volume phase holographic lenses for visible light photolithography with enhanced resolution utilizing the concept of off-axis illumination. However, the diffraction efficiency of holographic lenses depends on the wavelength of illuminating light and the angle of illumination. Angular and spectral characteristics of volume phase holograms can be controlled by optimizing processing parameters. The results on the theoretical optimization of three important processing parameters, namely fringe spacing ( ∧ ), thickness of the film ( d ), and refractive index modulation ( Δ n ) in the light of coupled wave theory for recording holographic lenses of high diffraction efficiency at λ = 405 nm are presented. It is pointed out that a properly designed single holographic lens can be used for off-axis illumination of a photomask with a parallel beam of light, whereas a system of two identical holographic lenses cemented together has to be used to realize a condenser lens similar to a converging lens.

Design of a compact wide-band triple grating imaging spectrometer based on freeform surfaces

Wide-band, high spectral resolution, and miniaturized imaging spectrometers have important applications. However, the working band, spectral resolution, and volume of the imaging spectrometers are mutually restricted. To solve this problem, we proposed a method in which multiple working bands share the same optical components except gratings, and the working bands switch with the switching of the working gratings. Based on the freeform surfaces characterized by XY polynomials, we designed a compact wide-band triple grating imaging spectrometer through the design ideas of human-computer interaction and iterative optimization. The imaging spectrometer works in the wavelength band of 200nm∼1600nm, covering ultraviolet (UV), visible, and near-infrared light, and it has a compact optical path volume of 24mm×38mm×80mm. Three gratings with 0.0025 mm, 0.005 mm, and 0.01 mm line spacing are used to realize dispersion with high diffraction efficiency in the bands of 200nm∼400nm, 400nm∼800nm, and 800nm∼1600nm, respectively. The UV detector receives imaging light in the UV band (200nm∼400nm), and the visible near-infrared (VNIR) detector receives imaging light in the visible and near-infrared bands (400nm∼1600nm). The design results show that the spectral resolutions of the three bands are 0.4 nm, 0.8 nm, and 1.6 nm, and the spectral smile and keystone distortion are less than 1.4 µm. The design results have engineering application value.