This PDF file contains the front matter associated with SPIE Proceedings Volume 12151, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

The year 2022 marks the 75th anniversary of Dennis Gabor’s invention of the holographic method, what he called “microscopy by reconstructed wave-fronts”, as well as the 60th anniversary of the publication in 1962 of two seminal papers in the field of holography: the introduction of the reflection hologram by Yuri Denisyuk, and the description of the holographic process from the point of view of communication theory by Emmett Leith and Juris Upatnieks. Within the framework of these celebrations, a historical review of the origins of holography is presented with special emphasis on the contributions of Gabor, Denisyuk and Leith to the development of holography.

Bayfol^® HX photopolymer films prove themselves as easy-to-process recording materials for volume holographic optical elements (vHOE’s) and are available in customized grade at industrial scale. Their full-color (RGB) recording and replay capabilities are two of their major advantages. Moreover, the adjustable diffraction efficiency, tunable angular and spectral selectivity of vHOE’s recorded into Bayfol^® HX as well as their unmatched optical clarity enables superior invisible “off Bragg” optical functionality. As a film product, the replication of vHOE’s in Bayfol^® HX can be carried out in a highly cost-efficient and purely photonic roll-to-roll (R2R) process. Utilizing thermoplastic substrates, Bayfol® HX was demonstrated to be compatible to state-of-the-art plastic processing techniques like thermoforming, film insert molding and casting, which opened up using a variety of industry-proven integration technologies for vHOE’s. Therefore, Bayfol^® HX makes its way in applications in the field of augmented reality such as Head-up-Displays (HUD) and Head-mountedDisplays (HMD), in free-space combiners, in plastic optical waveguides, and in transparent screens. Also, vHOE’s made from Bayfol^® HX are utilized in highly sophisticated spectrometers in astronomy as well as in narrow band notch filters for eyeglasses against laser strikes. See through applications such as, HMD and HUD, have demanding performance requirements on combiner and imaging technologies such as efficiency, optical function, and clarity. The expanded properties of Bayfol^® HX make it well suited to solve these challenges: In primary display applications such as free space combiners and waveguide based smart glasses applications, as well as integrated applications such as near-infrared imaging as used in eye-tracking and remote sensing. The transparency and performance characteristics of Bayfol^® HX make it perfectly suited for maintaining the requirements on optical performance as demanded in see through applications. We demonstrate practical examples of Bayfol^® HX vHOE’s in see through applications.

Triazene derivatives are studied as potential doping systems in polymeric films for direct laser wiring (DLW) of volume phase hologram. In particular, aryltriazenes can undergo to the group cleavage with the formation of volatile nitrogen and two radicals when irradiated with UV/violet light. Three derivatives, bearing different substituents, are characterized in n-hexane solution with UV-vis spectroscopy and the different photo-reactivity is highlighted. The best candidate, with a nitro group on the phenyl ring, is then added to a polyurethane matrix at 10% wt and the photoreactivity is evaluated together with the phase modulation in thin films deposited on Si wafers. A 5% change in the product of thickness and refractive index is measured by means of spectral reflectance. Simple patterns are transferred to the films using a DLW facility equipped with a 405 nm laser and studied with the phase contrast microscope.

Maximizing phase modulation in photopolymers remains a challenge in order to use these materials to fabricate photonics devices. Different material compositions and irradiation conditions have been studied in order to achieve it. One of the main conclusions has been that with continuous laser exposure better results are achieved. However, our results show that higher phase modulation can be achieved using pulsed laser. The study has been done with crosslinked acrylamide-based photopolymers (AA/PVA), Biophotopol and Holographic Polymer-Dispersed Liquid Crystals (HPDLC) exposed with a pulsed laser (532 nm). Thus, phase modulation increases of 8-15% have been achieved between pulsed laser and continuous laser exposure, with a maximum phase depth of 3π radians in AA/PVA, ~3π/2 in Biophotopol and ~π in H-PDLC. This opens the door to the use of this photopolymer in large-scale manufacturing, such as H-PDLC photopolymers to fabricate tunable lenses using the laser-induced direct transfer (LIFT) technique.

Holographic polymer dispersed liquid crystal devices (H-PDLC) are involved in many applications, e.g. diffraction lenses, optical data storage, and image capture devices. H-PDLC is based on a light-sensitive monomer and liquid crystal (LC) mixture exposed to an interference pattern. The monomer concentration rises in the illuminated area, whereas in the dark zones, the LC is concentrated, setting up LC droplets of fewer nanometers. Accurate knowledge of the elastic behaviour of the LC director distribution and the influence of the boundaries with the polymer matrix can help to optimise the diffraction efficiency or the angular selectivity of these devices, keeping the driving voltage low. Here, a review of the latest analysis for accurately estimating the director distribution in LC-based devices is shown. This analysis is carried on in three steps. Firstly, an accurate model based on creating a considerably high number of droplets surrounding the polymer matrix fringes is performed. In this step, the user can modify the ratio between the areas filled with LC and polymer and the size and aspect ratio of the droplets. This packing step can be very demanding depending on the thickness of the grating and the domain dimensions considered for the analysis (2D or 3D). Secondly, a formulation based on estimating the director distribution is performed to derive the permittivity tensor from the LC director. Thirdly and last, from the information obtained in the previous step, a Finite-Difference Time-Domain simulation is performed to estimate the electromagnetic field distribution inside the domain considered accurately. The diffraction efficiencies and many indirect parameters can be computed from this rigorous analysis.

Based on the Moiré effect, a pairwise ultrathin flat Moiré metalens is designed and fabricated. The diameter of the metalens is 1.6 mm. According to the mutual angles between two metasurfaces, the focal length tuning range of Moiré metalens is ~115 mm with ~40 % transmission efficiency at 532 nm. In addition, the Moiré metalens is implemented to a telecentric design to form the long axial scanning range imaging system with constant magnification. The scanning range of the telecentric imaging system is around 75 μm. The long tuning range with constant magnification is demonstrated by the imaging resolution chart that shows the lateral resolution of the system is around 2 μm. The proposed telecentric imaging system combines with structure illumination-based HiLo imaging principle to obtain the fine optical sectioning fluorescence images with invariant image contrast in the scanning range. The experiment results of the fluorescence beads show the optical sectioning capability of the system is around 7 μm. The ex-vivo fluorescence image results of the mice intestine tissue indicate that the system has the ability to obtain three different depths sectioning images. With the help of the HiLo imaging process, the defocus background noise can be suppressed, and the in focus villi detailed structure can be captured with high signal-to-noise ratio. The proposed varifocal ultrathin size of Moiré metalens has great potentials to replace the conventional bulky varifocal lens for compact system design of optical systems.

Quantitative differential phase contrast (qDPC) microscopy is utilized to observe label-free specimens by asymmetric illumination and intensity measurements. To provide uniform phase contrast images with minimum acquisition time, dual-color linear gradient pupils are applied to generate structured light. It is shown that the proposed pupils in qDPC can outperform half-circle and vortex pupils, and isotropic phase transfer function can be achieved with only 2-axis measurements. With the implantation of digital pupils in our system, the limitation of the time-consuming multi-axis measurement and reconstruction artifacts caused by missing frequencies in a half-circle pupil can be overcome. The required frame of dual-color coded microscopy reduces to two due to the color encoding method used in pupil design so that one-axis information can be obtained within a single shot. The improvement in imaging speed can help in monitoring fast developmental cell processes for various biological applications. Standard micro-lens-array and rat astrocyte cells were used to evaluate the performance of our microscopy system. We demonstrated time-lapse phase imaging of living cells and observed detailed morphology and dynamics changes. Present studies show the potential of the dual-color coded qDPC system for quantitative biomedical imaging for cell research. Due to our label-free approach, the natural contents of the cells remain intact and the dynamic processes through the samples can readily be observed. With our method, the nature of the structural changes in the samples can be evaluated in terms of quantitative phase changes inside acquired images.

We report our latest results on holographic gratings based on nanoparticle-polymer composites (NPC) including nanodiamonds with large refractive index modulation amplitude for cold and very cold neutrons (1 nm < λ < 10 nm). Diamond has the best combined neutron optical properties: high coherent scattering length, low incoherent scattering and low absorption. These unique properties allowed us to create phase gratings with large refractive index modulation, high thermal and mechanical stability, and also exhibiting large area holograms compared to NPC gratings incorporating other types of nanoparticles. We discuss the measured light and neutron diffraction properties of nanodiamond NPC gratings. It is shown that the NPC gratings exhibit extremely large scattering length density modulation amplitudes and as a result high diffraction efficiencies for cold and very-cold neutrons. We also discuss possible applications of nanodiamond NPC gratings in neutron optics.

Multiplexing is a method in which multiple information is stored or combined in a common medium. In photolithographic applications, spatial and temporal multiplexing can be used to design complex structured surfaces as superposition of simple profiles on the surface of a photosensitive materials able to respond to a time-averaged light pattern. Azopolymers are promising material systems in that sense, as they can be directly photo-structured in a reversible way over large scales with high quality, by simply controlling the spatiotemporal distribution of light irradiated on their surface. Not involving the typical chemical development of standard photoresists, the direct light-induced surface topography of azopolymer can be further modified after structuration, resulting a suitable platform to encode multiplexed information in a topographic surface relief pattern. Here, we show a method that allows the fabrication of multiplexed azopolymer surface relief gratings through a computer controlled holographic illumination system. The ability of our setup in accurately and digitally manipulating the evolution of the geometry of a simple sinusoidal intensity pattern is here exploited for the realization of quasicrystalline surface relief gratings, that can be tuned to have both positive and negative topography while preserving the multiplexed grating-vector information encoded in their far-field light diffraction pattern. Our results pave the way toward the realization structured surfaces able to convert multiple information in a single complex topographic profile for application in optics and cryptography.

The photorefractive effect of flexoelectric smectic liquid crystal mixtures was investigated and applied to a laser ultrasonic remote sensing. Smectic liquid crystal mixtures, composed of smectic-C liquid crystals, photoconductive chiral compounds and a sensitizer, are known to exhibit a large photorefractive effect. The principle of the ultrasonic remote sensing is that a laser beam is irradiated on an object and the variation in the reflected light is detected by photorefractive two-beam coupling. This remote sensing method can be used to probe the internal structure of an object or to measure the thickness of a plate object without contact.

The ability to control the alignment of liquid crystals (LCs) is a prerequisite for all LC applications such as LC displays, LC polarization gratings, and even for liquid crystal elastomer actuators. Out of the many alignment techniques available, photoalignment, i.e., spin-coating a thin layer of a photosensitive organic material and exposing it to polarized light, proves to be an attractive method. This method allows the non-contact and complete control over the alignment by just controlling the polarization state of the light exposure. We present a method to in situ align (in-plane) the nematic liquid crystals with a single-step exposure with the photoalignment technique. For this method, we have used the commercially available azo-dye, Brilliant Yellow (BY), which is photoaligned with linearly polarized 450 nm light. An in-house built, digital micromirror device (DMD) based projection setup allows the spatial structuring of the light, thereby enabling the pixel-by-pixel photoalignment of BY, and hence of LCs. This setup makes it possible to project computer-generated patterns on the substrate, thereby aligning the LCs in bulk in any arbitrary pattern. The photoalignment of BY is done in presence of LCs, i.e., after filling the LCs inside the glass cell. The achieved alignment is stable and rewritable. To the authors’ knowledge, this is the first demonstration of in situ, spatially varying photoalignment of liquid crystals inside the glass cell with single-step exposure and with commercially available chemicals.

The integration of photosensitively functionalized materials into integrated microfluidic systems allows controlling small amounts of fluids by light. One of the most promising application is the use of optoelectronic substrates that lead to the optical induction of electric fields that in turn allow manipulating microfluidic droplets. In particular, photovoltaic crystals have attractive capabilities in the spatial control of liquid droplets by means of optically-induced space charge distributions. The distribution of these electric carriers acts as virtual electrode creating strong electric fields, which have been effectively demonstrated as a versatile tool for trapping and moving small droplets. Up to several kV mm-1 electric fields can be achieved by the photovoltaic effect, for instance exploiting iron-doped lithium niobate crystals (Fe:LN). Despite promising results, this crystal has never been directly integrated in lab-on-a-chip system for active manipulation of aqueous droplets which are essential for microfluidics applications. Indeed, the development of light-based manipulation of electric fields enable a large number of operational functions within lab-on-a-chip protocols, for instance to merge on-demand droplet pairs. Herein, we propose the integration of Fe:LN inside a microfluidic droplet device in order to realize light-actuated merging of microfluidic confined aqueous droplets. The working principle is based on electro-coalescence of droplet pairs due to the presence of light-induced electric fields. Droplets are generated within a T-junction integrated circuit, and are further merged on-demand by optically-induced virtual electrodes. The droplets’ behavior is analyzed in this novel light functionalized chip compared to standard materials devices. We show that the dynamics of the droplets follow the same scaling laws and demonstrate unique light-induced merging by virtual electrodes on Fe:LN.

Phase change materials (PCMs) are materials whose resistive and/or optical properties can be tuned via a phase shift triggered by an external excitation. Within this class of materials, Ge₂Sb₂Te₅ (GST) is the most well-known and widely used PCM, and is currently employed in applications including read/write phase-change memories. Recently, it and similar materials have also drawn interest for use in photonics applications, due to the high optical contrast upon the phase change. However, for applications in the visible range (380-780 nm), GST and many related materials (such as GeTe and GeSnSbTe) exhibit large absorbance, restricting their use to layers of a few nm thick and limiting the achievable phase shift. One promising alternative PCM for photonics applications is Sb₂S₃. Despite its previous characterization as a “write once/read many times” material, recent work has shown it well-suited to repeated switching, with an activation energy comparable to that of GST. In addition, it exhibits much lower absorbance in the visible range than GST, with strong optical contrast between its crystalline and amorphous phases; and may be thermally, electrically or optically switched, with a switching time on the nanosecond scale. In this paper we present a comparison of tunable color coating designs which exploit the phase changes of Sb₂S₃ and GST. We show that Sb₂S₃ offers superior color contrast between its phases, can be used in thicknesses of up to several hundred nm while still realising saturated color, and that a large color shift on switching of up to ΔE=122.4 is obtained.

In this work, the kinetics of photoinduced changes in sputtered ternary Ge₂₉Sb₈Se₆₃ chalcogenide thin films with different thicknesses is studied. The optical bandgap energy of virgin thin films is 1.87±0.02 eV and the refractive index at 1 550 nm is 2.55±0.01 as determined by spectroscopic ellipsometry using Cody-Lorentz oscillator model. An annealing treatment caused bleaching of thin films resulting in optical bandgap energy increase to 1.96±0.02 eV accompanied with refractive index decrease down to 2.54±0.01. Subsequently, the photoinduced shift of the absorption edge was determined by the analysis of transmission data obtained by fibre-coupled high-resolution spectrometer. The irradiation of virgin thin films by near-bandgap light coming from continuous-wave diode-pumped solid-state laser leads to a fast photodarkening (PD) followed by slow photobleaching (PB) effect. The PB effect persists in virgin films and the maximum magnitude of this effect was found in film with the thickness of ~ 350 nm. Rise of the optical bandgap energy was ~ 0.04±0.02 eV using optical intensity of 125.0±5.0 mW ∙ cm⁻². On the other hand, in annealed thin films, only PD occurs under the same conditions indicating that the PB component of the photoinduced change disappears when the film is annealed. Maximum decrease in optical bandgap energy due to the PD effect in annealed films was about ~ 0.05±0.02 eV found in film with the thickness of ~ 650 nm. An influence of the thickness and laser optical intensity onto the kinetics of photoinduced changes is discussed.

In this work a simulation software is employed to study how the different shapes of the phosphor material may affect the white light production of the light source. In particular, while usually the phosphors that are being used in a laser driven configuration are flat plates, in this case we examine what happens when curvature is introduced. It is shown that curvature affects the way the produced white light is distributed and it can also increase the conversion of laser light to white light by the phosphor due to its curved shape. Sample phosphors of different curvature and geometry were studied and compared to the flat shaped ones.

We experimentally study the UV and IR radiation impact on the dynamics of YAG:Yb3+ polycrystal microparticle levitated in a quadrupole Paul trap at atmospheric pressure. Micromotions of trapped particle are interpreted into instantaneous kinetic energy with a certain statistical mode. The microparticle kinetic energy statistical mode was determined for various powers of laser radiation. The kinetic energy statistical mode shows a nonlinear pattern with a dip at a power of 1.10 W for 1020 nm laser radiation. In turn, the dependence of the kinetic energy statistical mode on the power of 405 nm laser radiation remains monotonous. The kinetic energy dip of YAG:Yb3+ trapped microparticle under infrared radiation is discussed in terms of both the internal and the translational laser cooling