To correctly develop numerical model for any photopolymerization, understanding the initiation chemistry in
both space and time is required. In photopolymer materials both the complex initiator chemistry and diffusion
properties have similar timescales. To fully understand this spacial and temporal evolution we developed a
number of experiments to isolate these effects. We show that for Irgacure 784 in a polymer host that the chemical
reaction and diffusion are strongly coupled. We demonstrate the importance of isolating these processes.
Holographic photopolymers develop permanent index change via the polymerization and subsequent diffusion of
monomer. It is well-known that to achieve high-fidelity recording, the rate of polymerization must be small in
comparison to the rate of diffusion, particularly for strong exposures that consume significant fractions of the available
monomer. When this condition is violated, polymerization is slowed in high-intensity regions by the local depletion of
monomer, resulting in broadening of recorded features. This paper shows that a diffusing inhibitor has analogous
dynamics controlled by the ratio of inhibitor diffusion rate to inhibition rate. When the ratio is small, inhibitor is locally
depleted in bright regions, resulting in localized acceleration of polymerization. This causes recorded index features to
be narrower than the incident optical exposure. Theoretical, numerical and experimental studies are used to illustrate
this fact and show that this narrowing can be used to compensate for the broadening caused by monomer dynamics.
Both effects are emphasized for rapid, strong recordings, suggesting that an inhibitor may be used to increase recording
fidelity in this limit.
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.
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.