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
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
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.