Science Popularization Issue 1: Volume Holographic Grating Materials
Augmented Reality (AR), as a technology based on real-time computer processing and multi-sensor fusion, is regarded as a driving force behind the upgrade and innovation of future tech products. AR devices are expected to become the next-generation computing platform, succeeding smartphones.
In AR devices, the near-eye display optical module is the most critical component, serving as a replacement for traditional screens and enabling human-device interaction. Among various near-eye display solutions, optical modules based on waveguide technology are considered the preferred choice for consumer-grade AR products due to their slim profile and high light transmittance.
Currently, there are three technical approaches to waveguides in the AR field: array waveguides, surface relief grating waveguides, and volume holographic grating waveguides. Among them, volume holographic grating waveguides stand out as the most competitive solution for AR glasses and AR Head-Up Displays (AR-HUD) due to their low cost and suitability for mass production.
luminous efficiency and maximum field of view (FOV) are the two core metrics for evaluating near-eye display waveguides.
For volume holographic grating (VHG) waveguides, luminous efficiency primarily depends on diffraction efficiency and angular bandwidth. Higher diffraction efficiency corresponds to better overall light utilization, while a larger angular bandwidth supports wider FOV display requirements.
For reflective volume holographic gratings, the diffraction efficiency can be calculated using the following formula
where Δn is the refractive index modulation, d is the grating thickness, Λ is the grating period, and θ is half of the two-beam interference angle.
It can be observed that diffraction efficiency is positively correlated with the refractive index modulation (Δn) and grating thickness (d). However, as the grating thickness increases, the angular bandwidth decreases, leading to a significant reduction in FOV.
Therefore, to achieve higher luminous efficiency while maintaining a sufficient FOV, the material must possess as large a refractive index modulation (Δn) as possible.
Among various photosensitive materials used for fabricating volume holographic gratings, photopolymer materials demonstrate the most remarkable refractive index modulation capability. The photochemical reaction in photopolymers follows a chain-growth polymerization mechanism. When exposed to laser interference patterns at specific wavelengths, these materials undergo phase separation that creates periodic refractive index modulation, ultimately forming a three-dimensional volume grating structure.
Mechanism of Chain-Growth Polymerization:
The material undergoes free-radical polymerization upon exposure to form gratings. The entire reaction process can be divided into three stages: chain initiation, chain propagation, and chain termination:
1.Chain Initiation:
The chain initiation step involves two reactions. First, dye molecules or other initiators absorb photon energy and undergo electron transfer, forming free radicals R· with unpaired electrons.
Step 2: These free radicals R· then react with monomer molecules (M), generating monomer radicals (chain initiators) M1·
2.Chain Propagation:
The chain propagation step involves the continuous addition of numerous unreacted monomer molecules (M) to M1·, causing the polymer chain to grow. This process can be represented as:
3.Chain Termination:
The chain termination process occurs through two competing mechanisms: combination termination and disproportionation termination. In holographic materials, both termination reactions typically coexist in a competitive relationship. The overall termination can be represented by the following general expression:
From the aforementioned chain reaction process, it can be observed that the rate of initiator photolysis (kd) generating free radicals and the rate of monomer radical formation (ki) critically influence the overall photochemical reaction. Therefore, in holographic material fabrication, the selection and structural modification of photoinitiators constitute a crucial technical aspect. Nica Optics has established an optimal photoinitiation system through theoretical simulations and extensive experimental validation, providing reliable support for the development of phase-separation materials.
Addition polymerization primarily includes four types: cationic polymerization, anionic polymerization, free-radical polymerization, and metal-catalyzed polymerization, each suitable for different monomers. In cationic polymerization, the initiators typically consist of strong Lewis acids (such as protons, phosphorus pentafluoride, etc.) or fuming sulfuric acid. Under acidic conditions, protons or Lewis acids attack the double bonds or rings of monomers, undergoing addition reactions. While stabilizing themselves, they generate new Lewis acids that continue to attack subsequent monomers. Through repeated reactions, a polymer chain is formed (as shown in the diagram below, where different monomers follow the same reaction mechanism).
Figure: Cationic polymerization is initiated by a proton, forming a carbocation.
Figure: The carbocation continues to attack monomers, resulting in chain growth.
When the polymer chain encounters a Lewis base, the chain terminus forms a coordination bond, terminating the reaction. Additionally, side reactions can also lead to polymerization termination.
Figure: Side reactions may also terminate the chain-growth polymerization.
Figure: The reaction is terminated by a Lewis base.
Principles of Grating Formation:
Photopolymerizable materials can be classified into two types based on the substrate material: photopolymers (PP) and holographic polymer-dispersed liquid crystals (HPDLC).
Before the photochemical reaction occurs, polymerizable monomers and the substrate material (resin/liquid crystal) are uniformly dispersed in the system, forming a homogeneous solution. When exposed to interfering laser beams of appropriate intensity, the monomers undergo chain polymerization in the bright regions under the action of a photoinitiator. As a result, monomer molecules accumulate in the bright zones, forming a solid polymer network that phase-separates from the resin/liquid crystal matrix. This causes the resin/liquid crystal to diffuse toward the dark regions.
Once the free radicals in the system are fully consumed, the reaction terminates, resulting in the formation of a grating structure that achieves refractive index modulation.
Photo-induced phase separation in materials (Image sourced from Naoaki Suzuki's paper at The University of Electro-Communications, Japan)
In phase-separating photopolymer materials, two key parameters govern the phase separation process and final grating morphology: diffusion rate (kd) and polymerization rate (kp).
If kp is too high, the reaction proceeds too rapidly, trapping the substrate material within the polymer network before it can fully migrate. This inhibits high refractive index modulation.
If kp is too low, the resulting polymer network becomes loose and irregular, leading to poor grating diffraction efficiency.
Particularly in HPDLCs, a low kp causes enlarged liquid crystal droplet formation, increasing light scattering and ultimately raising haze.
Thus, only when the kd/kp ratio falls within an optimal range can complete phase separation occur while maintaining a well-defined polymer microstructure, ultimately achieving high refractive index modulation depth.
Photopolymers (PP)
Currently, photopolymer gratings can be broadly categorized into polyurethane-based, polyvinyl acetate (PVAc)-based, and other polymer systems, depending on the film-forming resin composition.
1. Polyurethane Type
A representative example is the Bayfol® HX series by Covestro (Germany). Bayfol® HX is a volume holographic film used in:
Smart glasses display components
Automotive HUD holographic displays
Lens assembly elements
This film features:
Refractive index modulation (Δn) > 0.03 for all three primary colors
16μm thickness, suitable for general display applications
Mass-production capability
Advantages of Polyurethane as a Holographic Binder:
✔ Excellent weather resistance
✔ Low cost due to mature industrial supply chains
✔ Ongoing modifications by research institutions/companies to enhance optical performance
Many Chinese universities and companies are actively developing polyurethane-based holographic polymers.
2. Polyvinyl Acetate (PVAc) Type
PVAc-based photopolymers are represented by products from DuPont (USA).
Advantages:
✔ Low-cost industrial raw material
✔ Tunable glass transition temperature (Tg) via molecular weight selection
✔ Optimizable diffraction efficiency and Δn through post-processing
Limitations:
✖ Moderate water resistance, restricting use in high-humidity/high-temperature environments
Nevertheless, PVAc remains popular in research and industry due to its balanced performance.
3. Other Polymer Systems
Beyond polyurethane and PVAc, researchers are exploring:
Epoxy resins
Poly(methyl methacrylate) (PMMA)
Common Features:
✔ Excellent optical properties (e.g., PMMA as "organic glass" for construction/decoration)
Essential Requirements for All Film-Forming Materials:
① High compatibility with the system (critical for grating formation and haze control)
② Optimal Tg tuning (key for grating performance)
③ Superior weather resistance (stable under high temp/humidity)
Holographic Polymer-Dispersed Liquid Crystals (HPDLC)
HPDLC operates on the same phase-separation-induced Δn principle as photopolymers but enables thinner films and larger FOVs while maintaining high diffraction efficiency.
Challenges in HPDLC:
Liquid crystal (LC) selection requires balancing:
✓ Refractive index
✓ Clearing point
✓ Low-temperature phase stability
✓ Crystallization resistance
Some LCs undergo yellowing under prolonged UV exposure, rendering them unusable despite high birefringence.
Nika Optics’ Solution:
Designed/synthesized multiple LC monomers for volume holography
Developed custom LC mixtures through iterative optimization
Achieved >90% diffraction efficiency and <0.5% haze at 2μm thickness
Nika Optics has pioneered the development of its proprietary material system for volume holographic materials, meticulously considering material selection, optical exposure, and mass production feasibility.
Currently, Nika Optics has achieved breakthroughs in both photopolymer (PP) and holographic polymer-dispersed liquid crystal (HPDLC) materials:
For PP materials:
Achieved >90% diffraction efficiency at 10μm thickness (single green wavelength)
Optimized roll-to-roll (R2R) processing for scalable mass production
For HPDLC materials (a key R&D focus):
Attained >90% diffraction efficiency at an ultra-thin 2μm thickness
Achieved <0.5% haze, positioning its performance among the world’s leading solutions
As one of China’s few companies capable of full-cycle autonomous development—from material synthesis and optical design to industrial-scale manufacturing—Nika Optics is committed to becoming the domestic leader in volume holographic products.
Photographs of Nika Optics' photopolymer holographic film gratings Diffraction performance images of Nika Optics' red-band gratings