Breakthroughs in holographic optical devices frequently rely on advances in high-refractive-index photopolymer materials (HRIPs). While significant progress has been made in the pursuit of HRIPs, additional considerations have prevented broad application of photopolymerization-based materials for fabricating high-performance holographic gratings. To address the deficiency of suitable high-refractive-index monomers for holographic recording, our recent works were conducted from two main aspects, which are (1) monomer synthesis to improve the theoretical refractive index contrast between photopolymer and matrix (or binder), and (2) formulation manipulation for improving segregation degree between photopolymer and matrix (or binder). We have explored several synthetic approaches to obtain high-refractive-index acrylate monomers (nD=1.6) of high miscibility with matrix, multifunctional low-viscosity, high-refractive-index thiol-ene and thiol-yne monomers (nD=1.6). Combining with polyurethane matrix (binder) with a refractive index of 1.48, these monomers exhibit a high theoretical peak-to-valley index contrast of more than 0.12. To fully utilize the high theoretical index contrast, thiol-ene click chemistry in combination with a linear functionalized polymer binder was explored to achieve a high refractive index modulation(peak-to-mean) close to 0.04. Meanwhile, in the thiol-ene formulations, a variety of chemical modification methods, which can be readily translated into other material systems, were proposed, and studied to manipulate the rates of reaction and diffusion processes during holographic recording to optimize the refractive index modulation. The dramatic difference of achievable refractive index modulation in similar thiol-ene formulations with close theoretical index contrast was observed in such study. The difference underscores the importance of customized strategies and systematic formulation manipulation for achieving high-performance holographic photopolymers.
The transition from exploring holographic photopolymer dynamics to designing a holographic display presents several challenges, including the need to create phase-matched holograms over large areas using high-intensity exposure conditions. High-intensity recording conditions result in low haze and highly diffraction efficient holograms, but such exposures are typically limited to a relatively low writing area. Here we demonstrate a method by which a high-intensity writing beam is rastered across a large region of holographic material in a manner which locks the phase of the hologram grating vector across the entirety of the exposed region.
The current surge of interest in holographic photopolymers is motivated by display applications that often call for holograms in reflection geometry. However, the geometry of reflection holograms is uniquely sensitive to problems that arise from non-instantaneous recording, including volume shrinkage and off-target index development during exposure. Here we leverage a high-powered recording laser to compare holograms of varying writing power and exposure time pairings (with a consistent exposure intensity), showing improved hologram quality with shorter (higher-powered) exposures. Shorter, higher-powered reflection hologram exposures result in lower haze and higher diffraction efficiency.
Diffractive optical elements (DOEs) assimilate optical functionality within thin (≤100 μm), lightweight films. With the recent advent of high dynamic range two-stage photopolymers, gradient-index volume DOEs can now achieve diffraction efficiencies competitive with conventional surface-relief DOEs, while also offering the advantages of contact-free, selfprocessing optical recording into a flat film that can be laminated between protective sheets. Here we design and fabricate Fresnel lenses with what we believe to be the highest reported diffraction efficiencies achieved to date using this gradientindex DOE approach. Our analysis demonstrates that these high diffraction efficiencies are crucially enabled by the high index modulation of the photopolymer, Δn < 0.01. Another factor enabling high diffraction efficiency is the pixel count of the recording exposure. Thus, we use a photolithographic chrome mask with 9000 × 9000 pixels of 2.5 μm diameter, significantly exceeding the pixel count available from spatial light modulators. The mask is imaged onto photopolymer films of 50 μm thickness, and Fresnel patterns of up to 23 mm diameter are recorded in one-shot exposures. The resulting lenses range from f/44 – f/79 with diffraction efficiencies up to 83%. The performance of various lens designs is validated by an analysis showing that, for a given Δn, there is a fundamental trade-off between low f/# and high diffraction efficiency. This high performance represents an important step toward practical applications, ranging through solar energy concentrators, customized vision optics, integrated photonics, heads-up displays, and hybrid lenses.
Two-stage holographic photopolymers capable of high refractive index modulation (Δn) on the order of 10–2 enable the fabrication of a myriad of optical elements. While there are commercial products available that meet these requirements, researchers often want the flexibility to customize both the form factor of the samples as well as the mechanical and chemical properties for their specific applications. We present a novel high refractive index acrylate writing monomer in a low refractive index urethane matrix as a model material for customization for optical applications. We discuss the achievable Δn of this custom monomer, 1,3-bis(phenylthio)-2-propyl acrylate (BPTPA) in the urethane matrix as a function of solubility, along with a comparison to a commercially available high refractive index monomer, 2,4,6- tribromophenyl acrylate. Formulations with BPTPA exhibit a peak-to-mean Δn ≈ 0.029 in transmission holograms without any obvious deficiencies in transparency, color, or scatter. This writing monomer and the synthetic processes present a promising platform for the fabrication of high-performance holographic photopolymers for a wide range of research applications.
We present a high performance, removable light trap for the suppression of Fresnel reflections from optical substrates used in holography. When recording holograms with coherent illumination in photosensitive materials, Fresnel reflections lead to undesired interference patterns, producing parasitic gratings. These parasitic gratings consume dynamic range of the material, impact hologram uniformity, and produce stray light on playback. A reflection as small as 4% (glass-air) will produce interference patterns with fringe visibility 0.38. Typical methods for suppressing reflections are often prohibitive in holography, either due to spectral and angular requirements, cost, or potential for contamination. Antireflection-coatings have limited spectral and angular performance, in addition to high cost for one-time use. Relying on polarization suppression of reflections at Brewster’s angle is impractical for the wide range of angles required in most holographic recording applications. Index matching oil with neutral density (ND) filters limit recording orientation and oil often contaminates the samples. We show that by incorporating an absorber into different elastomers, the refractive index of common optical substrates may be matched, and recording light absorbed. Adhesion between the soft elastomer and glass provides sufficient hold while still allowing the layer to be easily removed after use, with no residue or surface damage. Reflection suppression occurs over broad angular and spectral range, with reflected intensity limited only by the index difference between substrate and elastomer. Using carbon black as an absorber, and a poly(dimethylsiloxane) (PDMS) elastomer, we demonstrate a 15 dB reduction of back reflections from a glass slide over a 600 nm bandwidth.
We show the design and fabrication of high diffraction efficiency, optically recorded gradient-index Fresnel lenses in a two-stage photopolymer. A design analysis reveals that lens f/# is limited by the material refractive index contrast, motivating use of recent high contrast polymers. The number of pixels required for the optical exposure is typically well beyond available spatial light modulator resolutions, motivating the use of a photolithographic mask. Thus, we present a photolithographic technique by which a single exposure into a self-developing photopolymer can directly print single custom high efficiency DOEs with freeform phase profiles, in contrast to holographic optical elements that are limited to the interference of two propagating fields. We use a dithered binary chrome mask with 9000 x 9000 pixels of 2.5 μm diameter to write lenses up to 23 mm in diameter. Lenses down to f/44 with 76% diffraction efficiency and f/79 with 83% diffraction efficiency are demonstrated.
Holographic photopolymers capable of high refractive index modulations (Δn) on the order of 10-2 are integral for the fabrication of functional holographic optical elements (HOEs) for use in a range of optical applications. A novel high refractive index liquid writing monomer suitable for two-stage photopolymer systems was designed and synthesized. This monomer exhibits facile synthetic procedure, low viscosity, high refractive index as well as excellent solubility in a low refractive index urethane matrix. The solubility limit, refractive index change and reaction kinetics/conversion were studied against a commercial reference high refractive index monomer, 2,4,6-tribromophenyl acrylate (TBPA). Superior performance in solubility to TBPA is shown with similar reaction kinetics and final conversion as confirmed by realtime Fourier transform infrared spectroscopy (FTIR) and real-time monitoring of diffraction grating growth. We demonstrate the ability to load substantial amounts of these writing monomers enabling a straightforward path to higher achievable Δn values (peak-to-mean Δn ~ 0.03) without sacrificing optical properties (transparency, color or scatter) as validated through recording ~100% efficiency volume transmission holograms in sub-15 μm thick films.
Two-chemistry polymer systems are attractive platforms for a wide range of optical and mechanical applications due to the orthogonal chemistries of the initial thermoset matrix and the subsequent photo-initiated polymerization. This scheme allows the mechanical and optical properties of the materials to be individually addressed. However, the mechanical properties of both the initial matrix and the photopolymer system affect the performance of these materials in many applications from holography to optically-actuated folding. We present a mechanical model along with experimental demonstrations of a two-chemistry holographic photopolymer system. A three-dimensional finite element model is used to simulate the mechanical and chemical responses in time. The model uses standard material measurements to predict both large-scale deformation and more localized stress and strain. To demonstrate the magnitude of mechanical stresses possible in these materials, we show bending of thin strips with UV light activation using an optical absorber to create an intensity gradient in depth. The resulting non-uniform polymerization causes shrinkage and bending toward the light followed by swelling and bending away from the light caused by monomer diffusion. In addition to this large-scale bending, we demonstrate that the model can be used to qualitatively predict surface deformations that can be used for surface relief optical elements. The mechanical model enables understanding of shrinkage and swelling properties of a material system that affect the performance of that system over a wide range of illumination conditions.
Two-beam holographic exposure and subsequent monitoring of the time-dependent first-order Bragg diffraction is a common method for investigating the refractive index response of holographic photopolymers for a range of input writing conditions. The experimental set up is straightforward, and Kogelnik’s well-known coupled wave theory (CWT)[1] can be used to separate measurements of the change in index of refraction (Δn) and the thickness of transmission and reflection holograms. However, CWT assumes that the hologram is written and read out with a plane wave and that the hologram is uniform in both the transverse and depth dimensions, assumptions that are rarely valid in practical holographic testing. The effect of deviations from these assumptions on the measured thickness and Δn become more pronounced for over-modulated exposures. As commercial and research polymers reach refractive index modulations on the order of 10-2, even relatively thin (< 20 μm thick) transmission volume holograms become overmodulated. Peak Δn measurements for material analysis must be carefully evaluated in this regime. We present a study of the effects of the finite Gaussian write and read beams on the CWT analysis of photopolymer materials and discuss what intuition this can give us about the effect other non-uniformities, such as mechanical stresses and significant absorption of the write beam, will have on the analysis of the maximum attainable refractive index in a material system. We use this analysis to study a model high Δn two-stage photopolymer holographic material using both transmission and reflection holograms.
Optically-driven diffusion of high refractive index molecules within a transparent thermoset polymer matrix is a promising platform for hybrid optics that combines a wide range of optical structures from large scale holograms to micron-scale gradient index waveguides in a single integrated optical system. Design of such a system requires characterization of the optical response of the material at a wide range of spatial scales and intensities. While holographic analysis of the photopolymers is appropriate to probe the smaller spatial scales and lower intensity optical response, quantitative phase mapping of isolated structures is needed to probe the response to the higher intensities and lower spatial frequencies used in direct write lithography of waveguides. We apply the transport of intensity equation (TIE) to demonstrate quantitative refractive index measurements of 10 μm-scale localized gradient index structures written into diffusive photopolymer materials using both single- and two-photon polymerization. These quantitative measurements allow us to study the effect of different exposure conditions and material parameters such as writing beam power, exposure time, and wt% loading of the writing monomer on the overall profile of the refractive index structure. We use these measurements to probe the time scales over which diffusion is significant, and take advantage of the diffusion of monomer with a multiple-write scheme that achieves a peak refractive index contrast of 0.025.
We present a general strategy for characterizing the reaction and diffusion kinetics of polymeric holographic recording media by which key processes are decoupled and independently measured. The separate processes are combined into a predictive model that is shown to make accurate quantitative predictions of index response over three orders of exposure dose (~1 to ~103 mJ/cm2) and feature size (0.35 to 500 microns) for a model material similar to commercial media. Several critical performance concepts also emerge from the model, including a prediction of a formulation’s maximum potential index response, insight into why a particular material may not achieve this maximum and the process that limits the recording resolution.
The fabrication of 3D microstructures has been realized by numerous researchers using two-photon polymerization. The
premise of these studies is that the confinement provided by localized, two-photon absorption results in polymerization
only near the focal point of the focused write beam and unwanted polymerization due to superposition of the out-offocus
exposures is significantly reduced, enabling the fabrication of complex structures with features below the
diffraction limit. However, the low cross-section of two-photon absorbers typically requires excitation by pulsed
Ti:Sapphire laser at 800 nm, resulting in polymerized features that are actually larger than those created by one-photon
absorption at half the wavelength. Here we describe a single photon photolithographic technique capable of producing
features not limited by the physics of diffraction by utilizing a resin which is able to be simultaneously photoinitiated
using one wavelength of light and photoinhibited using a second wavelength. Appropriate overlapping of these two
wavelengths produces feature sizes smaller than the diffraction limit and reduces polymerization in the out-of-focus
regions while avoiding the high light intensities demanded by
multi-photon initiation. Additionally, because the
photoinhibiting species are non-propagating radicals which recombine when the irradiation is ceased, memory effects
typical of photochromic initiators are avoided, allowing rapid and arbitrary patterning.
We demonstrate a projection lithography method that induces optical index changes in a flexible polymer cable that is
continuously translated through the image plane. We demonstrate that a static spot pattern generates a grid of
waveguides along the cable length via a continuous extrusion process. Rotations or scaling of the optical spot array can
fabricate image inverters or magnifying face plates in a single process step. The resulting polymer devices have
applications in optical backplanes, endoscopes for medical applications and lightweight imaging systems.
Optical waveguide adiabatic tapers enable low-loss connections between devices with dissimilar mode profiles. Common examples are semiconductor lasers, single-mode optical fibers and planar waveguides. Planar lithographic processes can easily create tapers in the plane but out-of-plane, symmetric tapers are difficult. Three-dimensional direct-write lithography into photopolymer naturally creates radially-symmetric waveguides when the motion is parallel to the optical axis of the writing focus, but absorption in the photopolymer inevitably attenuates the index with depth. We demonstrate that material absorption, translation speed and/or writing power can be used to control this taper, providing an inexpensive mode coupler for integrated optical systems.
Two active areas of research in the field of integrated optics are the coupling of on-chip waveguides to off-chip optical
fibers and the reduction of circuit size which is dominated by the minimum bend radius of waveguides. Traditional
approaches using mask-based lithography involve the complex etching of micro-mechanical on-chip mounts for the fiber
or total-internal-reflection facets for sharp waveguide bends. Holographic photopolymers have several unique properties
that enable a significantly simpler approach to both problems. Chief among these are the ability to be cast with low
stress around embedded components and the ability to create localized 3D index structures. This is demonstrated by
the fabrication of optical waveguides which couple directly to encapsulated fibers after making 90 degree bends off of
encapsulated front-surface mirrors. The results are low loss and significantly simpler than existing approaches.
The number of layers of a micro-holographic disk is limited by wavefront aberration which is strongly dependent on the
photopolymer initiation, termination and inhibition kinetics. 3D metrology is used to validate predicted index profiles.
Studies of development kinetics in volume photopolymers typically use transmission holography to quantify the
index distribution. This method has advantages including simplicity, quantitative index data and natural mapping
onto theories using harmonic expansion of the material response. A particular disadvantage is that the low spatialfrequency
response corresponding to the intensity of the writing beams can never be Bragg matched and thus
remains invisible.
In configurations where the exposure is not primarily sinusoidal, the holographic method is not applicable.
Important examples include bit-oriented data storage, direct-write lithography, and the object beam of page-based
holography. In these cases the exposure intensity is essentially arbitrary and there is a need for metrology tools that
can quantitatively measure the real and imaginary parts of the weak 3D index perturbation. Images produced by
bright-field and phase-contrast microscopes are generally not quantitative and are corrupted by objects out of the
focal plane.
We have developed two methods, a form of optical diffraction tomography and a scanning transmission microscope,
that are specifically designed to measure the 3D index response of holographic materials. Both are optimized to
measure the extremely weak absorption and phase structures typical of photopolymers and have passbands that
match the expected spatial frequencies.
Models of the index response of diffusion photopolymers typically assume that polymerization is proportional to optical intensity. However, common radical initiators self-terminate. This reduces the polymerization rate and has been shown in steady state to result in polymerization that is proportional to the square root of intensity.
We examine the impact of sublinear polymerization rate on the spatial distribution of index in volume photopolymers. In contrast to previous work based on spatial frequency harmonics, we consider a Gaussian focus and examine the index in the spatial domain. This can thus be thought of as the impulse response of the material which, due to the nonlinear response, is not the Fourier transform of the previous studies.
We show that sublinear polymerization rate dramatically impacts the spatial confinement of the index response. A case of particular interest to applications such as shift-multiplexed holography is a Gaussian beam translated orthogonal to its axis. In this geometry, a square-root material response yields an index profile of infinite axial dimension. We verify this prediction experimentally. The axial confinement of cationic (linear) photopolymer is shown to be significantly smaller than a radical (sublinear) photopolymer under the same writing conditions, confirming the prediction.
Traditional planar lightwave circuits fabricated from lithographically-patterned waveguides in glasses, semi-conductors or polymers cannot accommodate the wide range of materials required by typical optical devices. In addition, such waveguides are nearly always defined in the material surface and thus can support only a limited density of interconnects and suffer poor performance at waveguide crossings. Furthermore, the inflexibility of lithographic approaches -- including both waveguides and "silicon-bench" methods -- requires optical sub-components with unreasonable and expensive tolerances. We propose an alternative integrated optics platform based on 3D direct-write lithography into an optically addressable encapsulant. Arbitrary micro-optics are first embedded in a liquid monomer which is then cured into a semi-solid pre-polymer. It is essential that this step take place with minimal shrinkage to avoid stresses. A scanning confocal microscope then nondestructively identifies the component locations and their tolerances. The controller customizes the circuit design to accommodate these tolerances and then scans a 0.3 to 0.6 NA focus within the volume of the holographic polymer to create waveguides, lenses or other passive interconnects with one micron resolution. A final incoherent exposure cures and solidifies the polymer, finishing the process. The resulting hybrid optoelectronic circuits contain 3D routed waveguides interconnecting active and passive micro-optic devices in environmentally robust, hermetically sealed packages. A feature of particular interest is the ability to write waveguides directly off of the tips of embedded fibers, passively interfacing the circuits to fiber. We show that polymers developed for holographic data storage have the properties required for this application.
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