Photopolymer

A photopolymer is a light-sensitive material that polymerizes or crosslinks when it is exposed to light, usually ultraviolet or visible light. In vat photopolymerization 3D printing, for example, a liquid light-curable resin of monomers, oligomers and photoinitiators is hardened by light. In augmented reality (AR) and virtual reality (VR) optics, the term usually refers to holographic photopolymers: dry films in which an interference pattern of laser light is recorded as a modulation of the refractive index. The recorded pattern becomes a volume phase grating that works as a lens, mirror or beam deflector, so photopolymers are one of the main recording media for holographic optical elements (HOEs) and for the couplers of holographic waveguide displays.

Holographic photopolymers record holograms without wet chemical development, are made as coated films, and can hold several holograms in one layer. Their refractive index modulation is relatively small, which makes the gratings highly selective in angle and wavelength. Materials and companies linked to photopolymer optics for AR include Covestro's Bayfol HX film, DigiLens, which says it uses its own holographic photopolymers in its waveguides, and Akonia Holographics, which Apple acquired in 2018.

Reviewed 21 October 2026. Checked every cited paper (Crossref metadata and full text or abstract), the Akonia and DigiLens patents, Covestro, DigiLens and Magic Leap pages, and the Road to VR and Register reports against the article's claims. About review dates.

Definition and chemistry

In a general photopolymer formulation, photoinitiators absorb light and release reactive species that start chain formation between monomers and oligomers. Pagac and colleagues, reviewing vat photopolymerization in 2021, describe this chain-forming process as irreversible. Light-triggered polymerization is also the basis of 3D printing resins, imprint resists and holographic films, which differ mainly in how the light is delivered. A 3D printer hardens selected regions of a resin vat layer by layer. A holographic film is exposed to the interference pattern of two or more coherent laser beams, and the resulting chemical changes follow the bright and dark fringes of that pattern.

Holographic recording mechanism

Bruder, Fäcke and Rölle of Covestro explain the mechanism using Bayfol HX film. Polymerization starts in the bright fringes of the interference pattern. During exposure the polymerizable "writing monomers" diffuse through the material until they react with an activated monomer or polymer chain. According to the authors, the refractive index modulation that forms the hologram "is caused by the process of polymerization in the bright regions and net diffusion away from the dark regions". The result is a pure phase hologram through the whole thickness of the layer, and no further processing is needed to form it. A final photo-curing step only bleaches the absorbing dye and uses up the remaining writing chemistry. Xiong and colleagues describe the same effect for photopolymer HOEs: monomers move toward the bright fringes, so the refractive index ends up slightly higher there.

Two-chemistry films

Bruder and colleagues write that liquid recording media tend to shrink, which causes optical aberrations. In simple photopolymer systems, part of the writing chemistry has to be used up by a flood exposure before recording to give the material enough mechanical stiffness, which consumes dynamic range. Bayfol HX instead uses two independent ("orthogonal") chemistries. Matrix precursors are crosslinked thermally during film manufacture into a polymer network that sets the film's physical stability. The second chemistry, the imaging components, consists of photoinitiator systems and polymerizable monomers that stay inside this matrix until exposure. The unexposed film can then be stored, shipped and handled under safe-light conditions.

The review discusses visible-light sensitization with dyes such as Rose Bengal and Safranine O combined with borate co-initiators, with recording examples at 532 nm and 633 nm. It gives photopolymer layer thicknesses from "a few µm up to >50 µm", coated on transparent substrates of about 20 µm to more than 100 µm such as PET, polycarbonate, TAC or amorphous polyamide. After recording, a photo-curing step bleaches the residual dye; the authors describe a bleaching unit that combines a 365 nm UV LED with a visible LED, and warn that UV wavelengths shorter than 365 nm at high doses cause yellowing. Covestro describes the product as "a transparent thermoplastic film coated with a reactive photopolymer formulation", sold in rolls of unexposed material.

History

In 1969 D. H. Close, A. D. Jacobson, J. D. Margerum, R. G. Brault and F. J. McClung of Hughes Research Laboratories in Malibu, California, reported hologram recording in a material based on photo-induced polymerization. Their samples were exposed at 1 to 30 mJ/cm² at a wavelength of 0.694 µm and then given rapid optical, dry processing with ultraviolet radiation. They reported "diffraction efficiencies of up to 45% into the first order". The same year, Herwig Kogelnik published his coupled wave theory for thick hologram gratings in the Bell System Technical Journal. It describes diffraction efficiency and the Bragg condition for the kind of volume gratings that photopolymers record.

Holographic data storage drove much of the later material work. Bruder and colleagues note that "a major motivation for holographic recording material developments has been the field of optical data storage". In 1999 Lisa Dhar and colleagues at Bell Laboratories, Lucent Technologies, described a strategy for making thick, optically flat photopolymer media for digital holographic storage. They reported media with a dynamic range (M/#) as high as 42 in 1 mm thick formats and recorded and recovered 480-kbit digital data pages. Some later work on photopolymers for AR optics grew out of storage research. Colorado-based Akonia Holographics, founded in 2012, bought some of the assets of the failed holographic storage company InPhase, and The Register reported the company's claim that it had developed "the most advanced holographic photopolymer media in the world". A 2020 Hitachi waveguide for head-mounted displays used a photopolymer several hundred micrometers thick that had been developed for holographic data storage.

Covestro's 2017 review called holographic photopolymers "a new technology to create passive diffractive optical elements by a pure laser interference recording" and listed optical combiners for AR glasses and large head-up displays among the uses of volume HOEs.

Properties as a holographic medium

Photopolymer gratings are volume Bragg gratings. They diffract strongly only near the Bragg condition, and light at other angles and wavelengths passes through almost unchanged. Bruder and colleagues write that this high transparency off the Bragg condition makes volume HOEs an enabler for components such as optical combiners for AR glasses. In a 2021 review of AR and VR displays, Xiong, Hsiang, He, Zhan and Wu of the University of Central Florida give the following figures for photopolymer HOEs (PPHOEs). The bandwidth values come from their rigorous coupled-wave analysis simulations of one transmissive and one reflective grating, not from measurements of a product:

Property	Value given by Xiong et al. (2021)
Refractive index modulation (maximum minus minimum index)	Typically 0 to 0.06, adjustable through exposure dosage
Angular bandwidth, transmissive grating (efficiency above 80%, 550 nm)	About 5 degrees
Spectral bandwidth, transmissive grating (7 degree incidence)	About 175 nm
Spectral bandwidth, reflective grating	About 10 nm
Angular bandwidth, reflective grating (varies with wavelength)	From 2 to 20 degrees

The authors state that the "strong selectivity of PPHOE on wavelength and incident angle is directly related to its small δn". Several holograms can be recorded into one film by consecutive exposures, but the total amount of monomer is fixed, so there is usually an efficiency trade-off between the multiplexed holograms. Xiong and colleagues note that their maximum-minus-minimum definition may give twice the value of other definitions. Bruder and colleagues use the amplitude Δn1 of a sinusoidal index modulation; the Rose Bengal and Safranine O test formulations they report reached maximum values of up to 0.0090. They also write that diffraction efficiency can be designed "from sub percent level to close to 100%". Johannes Frank, a Covestro lab head for application development in holographic light guiding, has said that Bayfol HX films allow a diffraction efficiency for light guiding and redirection that "can be tuned to almost 100%".

Shrinkage during recording changes the grating geometry and shifts the Bragg angle. In a 2023 study of a see-through display recorded in Bayfol HX 200 at 532 nm and read at 632.8 nm, Sirvent-Verdú and colleagues at the University of Alicante estimated shrinkage between 0.2 and 4.0 percent depending on the recording geometry, against a manufacturer figure of 1.4 percent for reflection gratings. They found that shrinkage helped guide light in the glass substrate, including in one recording geometry where, in theory, the diffracted light could not be trapped. Akonia's skew mirror patent describes its own recording medium as shrinking by "usually about 0.1% to 0.2%", compared with about 1.0 percent for the 70 µm Bayfol HX TP film used in another embodiment.

Holographic polymer-dispersed liquid crystals

A holographic polymer-dispersed liquid crystal (HPDLC) is, in the words of Xiong and colleagues, a "liquid-crystal-doped photopolymer". In 1994 R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, T. J. Bunning and W. W. Adams reported volume Bragg gratings written holographically in polymer-dispersed liquid crystals whose diffraction efficiency could be switched electrically from about 50 percent to near zero at fields of about 11 V/µm. A DigiLens patent describes how such a switchable Bragg grating is made. A thin film of "photopolymerizable monomers and liquid crystals" is exposed to two coherent laser beams, and photopolymerization separates the mixture into regions dense in liquid crystal droplets and regions of clear polymer, which form the fringe planes. The diffraction efficiency can then be adjusted over a continuous range with an applied voltage.

Xiong and colleagues report that liquid crystal HOEs have much wider bandwidths than photopolymer HOEs, because their maximum index modulation equals the liquid crystal birefringence, which "may vary from 0.04 to 0.5". Researchers at the University of Alicante recorded slanted transmission gratings for a holographic waveguide in an HPDLC based on the monomer dipentaerythritol penta/hexa-acrylate with 28 percent by weight of a nematic liquid crystal. A grating recorded at 532 nm with 1690 lines/mm had an index modulation of 0.0107 and a thickness of about 20 µm, and one grating reached 80 percent diffraction efficiency at 633 nm. A test image coupled into and out of the waveguide showed speckle and a weaker secondary image caused by an unwanted reflection grating.

Applications in AR and VR

Combiners and holographic optical elements

A reflective photopolymer HOE can redirect display light toward the eye while letting most light from the real world pass, which makes it usable as an optical combiner. Covestro says Bayfol HX enables "the mass production of volume holographic combiners for lightweight AR displays". In October 2022 Covestro announced that Bayfol HX film had been used in a Sony research prototype of a transparent 360 degree holographic display, which Covestro showed at the K 2022 plastics trade fair in Düsseldorf. The company named automotive head-up displays and head-mounted displays as other AR uses for its holographic films.

Waveguide couplers

Xiong and colleagues name photopolymer gratings (PPGs) as one of the three main diffractive coupler types for AR waveguides, along with surface relief gratings and liquid crystal gratings. The small angular bandwidth of a high-efficiency transmissive PPG makes it unsuitable as an in-coupler, so both couplers are usually reflective. The gradually increasing out-coupling efficiency needed for uniform brightness can be made by varying the exposure across the film. Because each grating accepts only a narrow range of angles, a reasonable field of view for one color usually requires stacking two or three PPGs.

At the Alicante group, Sirvent-Verdú and colleagues built a see-through display with in- and out-couplers recorded as slanted transmission gratings in Bayfol HX 200 and reported that each grating reached more than 80 percent diffraction efficiency under the best recording conditions.

Multiplexed mirrors and skew mirrors

Multiplexing many gratings in one layer lets a photopolymer imitate a tilted mirror that works over a wide range of colors. Xiong and colleagues note that such a multiplexed PPHOE is sometimes called a skew mirror; more gratings imitate a mirror better, but each grating then has a smaller index modulation. Takeru Utsugi of Hitachi and colleagues at Hitachi-LG Data Storage built a "volume-holographic multiplexed-mirror" waveguide prototype for head-mounted displays and reported 87 percent transparency, a luminance efficiency of 3850 cd/m²/lm and a potential field of view of 35 degrees horizontal by 36 degrees vertical. According to Xiong and colleagues, it used 54 multiplexed photopolymer gratings; each multiplexed hologram had a low efficiency of about 1.5 percent, but the one-dimensional exit pupil expansion system still exceeded 3000 nit/lm.

Akonia Holographics patented skew mirrors in which the reflective axis of a grating structure is "not constrained to coincident with surface normal", unlike a conventional dielectric mirror. In one embodiment, 48 volume holograms were recorded at 405 nm in Akonia's photosensitive polymeric medium AK174-200, which the patent describes as about 200 µm thick, with an M/# of about 18 and a refractive index of about 1.50 at 405 nm. The patent describes a pupil relay with skew couplers on a slab waveguide that "may be useful for head mounted displays". Apple acquired Akonia and confirmed the deal to Reuters in August 2018.

DigiLens

DigiLens states that its "proprietary holographic photopolymers are formulated for high optical efficiency and low haze" and that it uses an inkjet printing process for large-scale manufacturing, with switchable gratings among its waveguide features. In a 2018 press release quoted by Road to VR in its coverage of the company's Series C funding, DigiLens said it had developed "a high-performance photopolymer material and copy process to manufacture precision diffractive optics" by printing rather than etching the nanostructures.

VR displays

Photopolymer HOEs have also been used in research VR optics. Xiong and colleagues describe the thin VR display of Andrew Maimone and Junren Wang as using "a PPHOE-based directional backlight with laser illumination". Only a single-color backlight was demonstrated, but the reviewers note that such a PPHOE could support a full-color laser backlight through multiplexing. Maimone and Wang reported VR display prototypes thinner than 9 mm with fields of view of over 90 degrees horizontally, built from thin, flat films. The same review describes a pupil duplication system by Kim and Park with multiplexed PPHOEs that produced three viewpoints 3 mm apart, with some ghost images and gaps at the transitions.

Manufacturing

Bruder and colleagues describe a contact copy process for mass replication in which Bayfol HX film is laminated to a master volume HOE and a single scanning laser line copies the master's phase hologram into the film, which moves in a step and repeat process. They report a roll-to-roll replication machine with a web width of 30 cm. A demonstration off-axis holographic lens with a 300 mm focal distance reached an estimated real diffraction efficiency of 78 percent including Fresnel losses.

Related light-cured polymers in XR optics

Light-cured polymers are also used to replicate surface relief gratings, a different waveguide technology. In 1996 Jan Haisma and colleagues at Philips Research described "mold lithography", a two-step process that begins with a photopolymerization replication step followed by etching, for patterns of 100 nm or less. Xiong and colleagues note that surface relief gratings for AR waveguides can be replicated by nanoimprint lithography. Magic Leap describes its Jet and Flash Imprint Lithography process for waveguides as pressing a polymer template carrying the waveguide pattern "into liquid photoresist" that is then UV-cured, using a custom high-refractive-index photoresist. Unlike holographic photopolymers, these resists form a surface pattern rather than a refractive index modulation inside the layer.

See also

• Holographic optical element
• Holography
• Waveguide display
• Holographic waveguide
• Diffraction grating
• Optical combiner
• Nanoimprint lithography
• Akonia Holographics
• DigiLens