One photon diffusive photopolymers enable self-developing three dimensional (3D) refractive index patterning of up to cm thick solid volumes for the fabrication of micro-optics. However, one photon absorption in solid, thick materials does not yield complete control of the 3D refractive index distribution due to diffraction and the excessive development time for index features measuring 100’s of microns in diameter or larger. We present a fabrication method and photopolymer formulation that can efficiently create mm3 optical devices with programmable, gradient index of refraction with arbitrary feature size and shape. Index contrast of 0.1 is demonstrated, which is 20 times larger than commercial holographic photopolymers. Devices are fabricated by repetitive micro-fluidic layering of a self-developing photopolymer structured by projection lithography. The process has the unusual property that total fabrication time for a fixed thickness decreases as the number of layers is increased, reducing the fabrication time for high axial resolution micro-optics. We demonstrate the process by fabricating thick waveguide arrays and gradient index lenses.
We investigate holographic optical trapping combined with step-and-repeat maskless projection stereolithography for
fine control of 3D position of living cells within a 3D microstructured hydrogel. C2C12 myoblast cells were chosen as a
demonstration platform because their development into multinucleated myotubes requires linear arrangements of
myoblasts. C2C12 cells are positioned in the monomer solution with multiple optical traps at 1064 nm and then are
encapsulated by photopolymerization of monomer via projection of a 512x512 spatial light modulator (SLM) illuminated
at 405 nm. High 405 nm sensitivity and complete insensitivity to 1064 nm is enabled by a lithium acylphosphinate
(LAP) salt photoinitiator. Use of a polyethylene glycol dimethacrylate (PEGDMA) based monomer is compared to that
of polyethylene glycol (PEG) hydrogels formed by thiol-ene photo-click chemistry for patterning structures with cellular
resolution, and for maintaining cell viability. Cells patterned in thiol-ene with RGD are shown to retain viability up to 4
days after the trapping and encapsulation procedure. Further, cells patterned in thiol-ene with RGD and a degradable
ester link, are shown to fuse, indicating the initial stages of development of multi-nucleated cells.
Traditional photolithography begins with single-photon absorption of patterned light by a photo-initiator to locally
expose a resist. In two-color photo-initiation/inhibition (2PII) lithography, these exposed regions are confined by a
surrounding pattern of inhibitors generated by one-photon absorption of a second color in a photo-inhibitor. Like a
stencil used to confine spray-paint to a thin, sharp line, the inhibitory pattern acts as a remotely programmable,
transient near-field mask to control the size and shape of the modified resist region. The inhibiting species rapidly
recombine in the dark, allowing for fast sequential exposures and thus enabling fabrication of complex two- or threedimensional
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.
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.