Volume Phase Holographic Gratings Research Articles

Design and development of a stand-off Raman brassboard (SDU-RRS) for the spectroscopic study of planetary materials

Raman spectroscopy has emerged as a crucial mineral analysis technique in planetary surface exploration missions. Nonetheless, the inherently low Raman scattering efficiency of planetary silicate materials makes it challenging to extract enough Raman information. Theoretical and experimental studies of the remote Raman scattering properties of planetary materials are also urgent requirements for future lunar and planetary explorations. Here, Shandong University Remote Raman Spectrometer (SDU-RRS) was developed to demonstrate the feasibility of lunar remote Raman technology and conduct preliminary research on remote Raman scattering properties. SDU-RRS utilizes a pulsed 532 nm laser, a non-focal Cassegrain telescope, a volume phase holographic grating, an intensified charge-coupled device, and the time-gating technique to detect weak-signal silicate minerals. The spectral resolution obtained with atomic emission lamps was <4.91 cm−1, and the wavelength accuracy was <1cm−1, across the spectral range of 241–2430 cm−1. SDU-RRS can detect natural augite within a feldspar-olivine-augite matrix at a concentration of 20 % at ∼1 m under ambient lighting conditions. A series of experiments were conducted to evaluate the influence of measurement conditions and physical matrix effects on acquired Raman signals, either qualitatively or quantitatively, on geological materials. The study indicates that the transmission of Raman-scattered light conforms to Lambert’s cosine law, and a linear correlation exists between Raman intensity and laser power. The study also evaluated the impact of grain size, surface roughness, porosity, and shadow-hiding effects. Reducing grain size decreases Raman intensity and broadens Raman spectra. These characteristics are essential for achieving definitive mineralogical information from granular materials by remote Raman spectroscopy in lunar and planetary explorations.

Easily variable and scalable terahertz pulse source based on tilted-pulse-front pumped semiconductors

A new type of terahertz source containing only two optical elements - a volume phase holographic grating, and a semiconductor nonlinear slab - is proposed. The setup does not require any microstructuring, has only one diffraction order, and can be scaled to large pump sizes without any principal limitations. Furthermore, it can be easily adapted to different pump wavelengths and THz phase-matching frequencies. The Fresnel loss at the boundary of the materials can be significant at conventional pump polarizations (s-pol), but a single-layer anti-reflection (AR) coating can reduce it. Pumping such a setup with polarization in the dispersion plane (p-pol, TM mode) can reduce the effective nonlinear polarization and consequently the terahertz generation efficiency. However, in the absence of AR coating, this reduction is overcompensated by the reduced Fresnel loss.

Initial measurement of ultrafast charge exchange recombination spectroscopy on the EAST tokamak

Ultrafast charge exchange recombination spectroscopy (UF-CXRS) has been developed on the EAST tokamak (Yingying Li et al 2019 Fusion Eng. Des. 146 522) to measure fast evolutions of ion temperature and toroidal velocity. Here, we report the preliminary diagnostic measurements after relative sensitivity calibration. The measurement results show a much higher temporal resolution compared with conventional CXRS, benefiting from the usage of a prism-coupled, high-dispersion volume-phase holographic transmission grating and a high quantum efficiency, high-gain detector array. Utilizing the UF-CXRS diagnostic, the fast evolutions of the ion temperature and rotation velocity during a set of high-frequency small-amplitude edge-localized modes (ELMs) are obtained on the EAST tokamak, which are then compared with the case of large-amplitude ELMs.

Design of a multichannel polychromator for KTX Thomson scattering diagnostic

A polychromator is designed based on the transmission volume phase holographic grating spectrometer coupled with avalanched photon diodes through fiber bundle and has been successfully implemented on the Keda Torus eXperiment [Plasma Phys. Control. Fusion 56 (2014) 094009] Thomson scattering system. The polychromator is operated with a 1064 nm laser, and the designed spectral range is from about 1000 nm to 1100 nm. The spectral channels have been optimized with a five-channel design to lower the estimated error of the electron temperature and density within 5% and 3% respectively when the temperature is around 100 eV. With a calibration method about the ratio of the channel average responses using bremsstrahlung radiation, we obtain an electron temperature of 7 eV in a typical experiment with low plasma currents.

Optical Design of a Slitless Astronomical Spectrograph with a Composite Holographic Grism

In the present work, we consider an optical design of a slitless spectrograph for an existing 0.5 m-class telescope. This design concept has a number of advantages such as compact size, simplicity, and simultaneous coverage of a large field of view. A challenge with this design is correcting aberrations caused by placing a dispersing element in a converging beam. To overcome this issue, we propose to use a composite grism, which represents a combination of a prism and a volume-phase holographic grating, the latter which is split into zones with independently optimized parameters. We demonstrate two designs of such a grism. In both designs, the spectrograph operates in the range of 450–950 nm in an F/6.8 beam and covers a field of view of 35.6′ × 7.2′. Through advanced modeling, it is shown that a composite grism having four rectangular zones with different thickness and index modulation depth of the hologram and recorded with an auxiliary deformable mirror decreases the astigmatic elongation by a factor of 85, increases the spectral resolving power by 4.4 times, and reaches R1389 while increasing the average diffraction efficiency by a factor of 1.31. If we reduce the number of zones to only two, replace the deformable mirror with two static corrector plates, and fix the hologram thickness, the corresponding performance gains still remain high: the astigmatism is reduced by a factor of 61, the spectral resolving power is up to 1.7 times higher, reaching R1067, and the efficiency is increased by a factor of 1.27. This shows that the proposed design allows the construction of a simple and compact instrument, providing high performance over the entire field of view and spectral range.

Polarization Features of Anisotropic Bragg Light Diffraction on Transmission Volume Phase Holographic Gratings

Polarization Features of Anisotropic Bragg Light Diffraction on Transmission Volume Phase Holographic Gratings

The Circumgalactic Hα Spectrograph (CHαS). I. Design, Engineering, and Early Commissioning

The Circumgalactic Hα Spectrograph (CHαS) is a ground-based optical integral field spectrograph designed to detect ultrafaint extended emission from diffuse ionized gas in the nearby universe. CHαS is particularly well suited for making direct detections of tenuous Hα emission from the circumgalactic medium (CGM) surrounding low-redshift galaxies. It efficiently maps large regions of the CGM in a single exposure, targeting nearby galaxies (d < 35 Mpc) where the CGM is expected to fill the field of view. We are commissioning CHαS as a facility instrument at MDM Observatory. CHαS is deployed in the focal plane of the Hiltner 2.4 m telescope, utilizing nearly all of the telescope’s unvignetted focal plane (10′–15′) to conduct wide-field spectroscopic imaging. The catadioptric design provides excellent wide-field imaging performance. CHαS is a pupil-imaging spectrograph employing a microlens array to divide the field of view into >60,000 spectra. CHαS achieves an angular resolution of [1.3–2.6] arcseconds and a resolving power of R = [10,000–20,000]. Accordingly, the spectrograph can resolve structure on the scale of 1–5 kpc (at 10 Mpc) and measure velocities down to 15–30 km s−1. CHαS intentionally operates over a narrow (30 Å) bandpass; however, it is configured to adjust the central wavelength and target a broad range of optical emission lines individually. A high–diffraction efficiency volume phase holographic grating ensures high throughput across configurations. CHαS maintains a high grasp and moderate spectral resolution, providing an ideal combination for mapping discrete, ultralow–surface brightness emission on the order of a few milli-Rayleigh.

Reactive ion plasma etched surface relief gratings for low/medium/high resolution spectroscopy in astronomy

We report the fabrication of a binary-phase proof-of-concept astronomical diffraction grating embedded in a quartz substrate via reactive ion plasma etching. This grating operates at the first diffraction order within the 450 to 750 nm wavelength band. It features 1400-nm-deep, 188-nm-wide binary grooves at a 566-nm pitch, or 1767 lines/mm groove density, over a 25.4 × 25.4 mm2 area. A high depth-to-width ratio ( ∼ 8 ∶ 1 in this case) is one of the keys to near-theoretical diffraction efficiency being attained by the fabricated grating (94% at center wavelength and 70% at band edges) over a broad bandpass (>200 nm). This performance is also attributed to high-resolution micro-lithographic electron-beam patterning and anisotropic reactive ion etching process fabrication techniques. These types of binary gratings can potentially be high-throughput alternatives to Volume-Phase Holographic Gratings (VPHGs) for general spectroscopic applications. When scaled to appropriate sizes for astronomy, such gratings can serve as main or cross dispersion elements in low-, medium-, and high-resolution spectrographs not only in ground-based telescopes but also in those subject to challenging environmental conditions such as in space observatories.

Enhancement of holographic performance and stability of photopolymer materials by introducing epoxy resin and dyes

The use of a composite photopolymer system together with a photoinitiator have a significant effect on the optical properties of a volume phase holographic grating. This paper reports on the performance of a low refractive index monomer combined with a high refractive index epoxy resin. Performance improvements include the compatibility between the cross-linked network structure of the matrix and the recording medium. The effects of the exposure conditions and the internal composition of the materials on holographic performance were investigated. The results showed that the optimal refractive index modulation depends on the ratio of the epoxy resin and the pre-polymerization temperature. The use of Irgacure 784 as a photoinitiator gave better performance than Rose Bengal (RB) in terms of diffraction efficiency (∼92.5 %), transmittance (∼98 %) and slight shrinkage (0.314 %) was observed. The use of a dual-photoinitiator gave improvements in the radical generation rate and the stability, permitted recordings at an exposure energy of 50 mJ/cm2 and afforded high sensitivity (3.9 × 10-2 cm2/J) as a result of the synergy between the two photoinitiators. In other words, due to the highly reactive Irgacure 784 and the inhibiting polymerization characteristics of RB, enhancement of holographic performance and good stability were realized. It was shown in the photocuring and aging experiments that the photopolymer possessed anti-aging properties. The photopolymer can record multiplexed images at 532 nm and polarization holograms, demonstrating that it has the capability to perform high-density data recording.

Uniformly scalable lithium niobate THz pulse source in transmission geometry.

A novel THz source, based on optical rectification in LiNbO3 using the tilted-pulse-front technique, is proposed and experimentally demonstrated. The pulse-front tilt is introduced by a volume phase holographic grating, efficiently used at perpendicular incidence in transmission, and the THz pulses are produced in a LiNbO3 plane-parallel nonlinear echelon slab, arranged parallel to the grating. As a unique feature, the entire setup has a plane-parallel, transmission-type configuration, which straightforwardly enables distortion-free scaling to large sizes, high pulse energies and high THz field strengths. The possibility of operating the setup at cryogenic temperature for increased THz generation efficiency is also investigated. Calculations predict efficiencies of 95% for diffraction and 2% for THz generation at room temperature with a refractive-index-matching liquid between the grating and the echelon slab.

Universal focal reducer for small telescopes

AbstractThis paper is devoted to the memory of Dr Victor Afanasiev and his immense legacy. The report highlights the capabilities of two new instruments tested at the 1‐m Zeiss‐1000 telescope of SAO RAS: The Stokes Polarimeter (StoP) and the MAGIC focal reducer. Optimized for the study of active galactic nuclei, methodically, these instruments are suitable for a wide range of small telescope tasks. The fields of view of StoP and MAGIC are 6′ and 13′ for direct images, respectively. The StoP device allows one to conduct photometric observations and polarimetric ones with a double Wollaston prism; the spectral mode was added to MAGIC. For a star‐like target up to 14 mag in medium‐band filters with a seeing of 1″ for 20 min of total exposure, the photometry accuracy is better than 0.01 mag and the polarization accuracy is better than 0.6%. The available spectral range obtained with the volume phase holographic grating in MAGIC is 4,000–7,200 AA with a dispersion of 2A/px. StoP and MAGIC received the first light in 2020 and are used in test mode at the Zeiss‐1000. The report discusses the first results obtained by the authors with new instruments, as well as further prospects.

High-efficiency broadband polarization-independent binary fused silica gratings for large astronomical telescopes.

The binary fused silica gratings (BFSGs) with high diffraction efficiency are presented for large astronomical ground-based telescopes. Calculations demonstrate that the BFSGs could obtain high diffraction efficiency in a wider wavelength range and angle of incident (AOI) range compared with volume phase holographic gratings. Several gratings with a size of 60mm×60mm have been fabricated by holographic lithography and reactive ion-beam etching technology. The measured peak diffraction efficiency reaches 94%, and results show that there are 130 nm wavelength bandwidth and 12° AOI bandwidth in which diffraction efficiency is higher than 70%. The stray light causes the diffraction efficiency to decrease by about 0.48%. All measurements have indicated good consistency with the simulation results.

Temperature‐Sensitive Holograms with Switchable Memory

Herein, the dynamics and reversibility of temperature‐induced changes in volume phase holographic gratings (VPHGs) recorded in the low‐toxicity self‐processing photopolymer poly(N‐isopropyl acrylamide) (poly‐NIPA) are reported on. Transmission and reflection gratings are exposed to temperatures ranging 8–60 °C to study the effect of temperature on their properties. First, the role of the photopolymer chemical composition is investigated by comparing layers containing NIPA with those containing acrylamide (AA) and measuring the normalized diffraction efficiency (DE) at 60 °C; higher thermal sensitivity is observed for poly‐NIPA gratings (24% drop) versus low sensitivity for AA gratings (2% increase). The impact of the physical properties of the sensing layers (e.g., thickness/protective top layer) on their thermal response is studied. The reversibility of thermally induced changes in VPHGs is studied, especially the effect of exposure to the elevated temperature of 60 °C on the subsequent low‐temperature response. The results reveal the significance of water desorption/absorption for the observed changes in poly‐NIPA upon exposure to varied temperatures and indicate a thermally controlled memory effect. This effect is observed from DE values in transmission and from the spectral characteristics of the reflection gratings. A full reversibility of the DE and spectral characteristics is achieved at 8 °C.

Optical design of a multi-object fiber-fed spectrograph system for Southern Spectroscopic Survey Telescope

Southern Spectroscopic Survey Telescope (SSST) is a wide-field spectroscopic survey telescope that China plans to build in Chile in the next few years. As an instrument for astronomical spectroscopic survey, the multi-object and fiber-fed spectrograph (MOFFS) is one of the most important scientific instruments for SSST. In this paper, we present a recommended optical design for the MOFFS system based on the Volume Phase Holographic Gratings (VPHG). The whole design philosophy and procedure, including the analytic method to determine the initial structure, optimization procedures of the VPHG and the camera groups, are demonstrated in detail. The numerical results of the final obtained spectrograph show a superior imaging quality and a relatively high transmittance for the whole working waveband and the field of view. The design method proposed in this paper can provide a reference for the design of MOFFS accommodated in spectroscopic survey telescopes.

Cyclic allylic sulfide based photopolymer for holographic recording showing high refractive index modulation

AbstractThe development and characterization of a new holographic photopolymer system showing high refractive index modulation (∆n) is presented. It exploits the ring‐opening polymerization reaction of a cyclic allylic sulfide (CAS) monomer, which is dispersed in a cellulose acetate butyrate (CAB) polymer matrix together with a blue sensitive radical photoinitiator. Volume Phase Holographic Gratings (VPHGs) obtained using these films show a very good fidelity of the transferred pattern, without deviations from the theoretical curves. A high ∆n is obtained due to the addition to this system of a tetrafunctional thiol crosslinker, which maximizes the material potential (formula limit) of the formulation. It also allows for a thermal post process of the gratings, further enhancing the ∆n. The photopolymer properties are evaluated as function of the monomer and thiol concentration to determine the optimal composition, at which a ∆n greater than 0.03 is obtained. This is a considerably high value in the field of photopolymers for holography and it enables the manufacturing of efficient holograms.

Tuning the Δn and scattering in Bayfol® HX based holograms

Self-developing photopolymers are crucial materials for making high efficiency volume holograms. A key feature is the tuning of the refractive index modulation (Δn), but at the same time, it is crucial to minimize the light scattering which reduces the efficiency and increase the unwanted light. Here, we study Bayfol® HX materials in terms of scattering formation and control of the Δn by means the realization of Volume Phase Holographic Gratings (VPHGs), which are intended for the application in spectroscopic instrumentation. We highlight the dependency of the Δn and scattering with the laser light intensity, showing that the increase of the intensity reduces the amount of scattering. In addition, the fine tuning of the Δn with the incoherent light pre- and co-exposure is studied and the full dynamic range can be addressed controlling the scattering. A modified diffusion model is also used to describe and predict the experimental results.

Design and development of volume phase holographic grating based digital holographic interferometer for label-free quantitative cell imaging.

In this paper, a volume phase holographic optical element based digital holographic interferometer is designed and used for quantitative phase imaging of biological cells [white blood cells, red blood cells, platelets, and Staphylococcus aureus (S. aureus) bacteria cells]. The experimental results reveal that sharp images of the S. aureus bacteria cells of the order of ${\sim}{1}\;{\unicode{x00B5}{\rm m}}$∼1µm can be clearly seen. The volume phase holographic grating will remove the stray light from the system reaching toward the grating and will minimize the coherent noise (speckle noise). This will improve the sharpness in the image reconstructed from the recorded digital hologram.

Stacked volume holographic gratings for extending the operational wavelength range in LED and solar applications

A novel stacking procedure is presented for volume phase holographic gratings (VPHGs) recorded in photopolymer material using Corning Willow Glass as a flexible substrate in order to achieve broader angular and spectral selectivity in a diffractive device with high efficiency for solar and LED applications. For the first time to our knowledge, we have shown a device designed for use with a white LED that has the same input and output angles and high efficiency when illuminated by different wavelengths. In this paper, two VPHGs were designed, experimentally recorded, and tested when illuminated at normal incidence. The experimental approach is based on stacking two individual gratings in which the spatial frequency and slant have been tailored to the target wavelength and using real-time on-Bragg monitoring of the gratings in order to control the recorded refractive index modulation, thereby optimizing each grating efficiency for its design wavelength. Lamination of the two gratings together was enabled by using a flexible glass substrate (Corning Willow Glass). Recording conditions were studied in order to minimize the change in diffraction efficiency and peak diffraction angle during lamination and bleaching. The final fabricated stacked device was illuminated by a white light source, and its output was spectrally analyzed. Compared to a single grating, the stacked device demonstrated a twofold increase in angular and wavelength range. The angular and wavelength selectivity curves are in good agreement with the theoretical prediction for this design. This approach could be used to fabricate stacked lenses for white light LED or solar applications.

High-speed line-scan confocal Raman microscope with enhanced diffraction efficiency

Confocal Raman microscopy is commonly employed across different research fields because it allows for label-free sample investigation at high spatial resolution. However, the slow data acquisition speed of Raman microscopy limits its application for imaging large area samples. We developed a line-scan confocal Raman microscope to achieve parallel detection for high-speed data acquisition and proposed a dual mirror scan method to enhance the diffraction efficiency of the spectrometer. We used a Powell lens for the line illumination and demonstrated that it can provide a more uniform intensity distribution than a cylindrical lens. The spectrometer consists of a volume phase holographic grating and two scanning mirrors. Using the proposed dual mirror scan method can provide a high-resolution Raman spectrum at high efficiency over a wide spectral range. The system has a maximum spectral rate of 25 600 spectra s−1 and a theoretical spectral resolution of 1.2 cm−1. With the parallel detection by the line illumination and the enhanced diffraction efficiency, we expect the proposed system can be used in a variety of applications that require a high spectral rate, such as wide-field Raman imaging.

Simulation and partial prototyping of an eight‐junction holographic spectrum‐splitting photovoltaic module

AbstractSpectrum‐splitting photovoltaics incorporate optical elements to separate sunlight into frequency bands, which can be targeted at solar cells with bandgaps optimized for each sub‐band. Here, we present the design of a holographic diffraction grating‐based spectrum‐splitting photovoltaic module integrating eight III‐V compound semiconductor cells as four dual‐junction tandems. Four stacks of simple sinusoidal volume phase holographic diffraction gratings each simultaneously split and concentrate sunlight onto cells with bandgaps spanning the solar spectrum. The high‐efficiency cells get an additional performance boost from concentration incorporated using a single or a compound trough concentrator, providing up to 380X total concentration. Cell bandgap optimization incorporated an experimentally derived bandgap‐dependent external radiative efficiency function. Simulations show 33.2% module conversion efficiency is achievable. One grating stack is experimentally fabricated and characterized.