Amy C. Sullivan

Two-Color Single-Photon Photoinitiation and Photoinhibition for Subdiffraction Photolithography

Subwavelength Patterning Microscopists have recently achieved fluorescence imaging at subwavelength resolution by focusing one beam of light in a halo around another beam, thereby quenching the glow of fluorescent dyes in all but the very center of the illuminated spot. Three studies have now adapted this approach to photolithography (see the Perspective by Perry). Andrew et al. (p. 917, published online 9 April) coated a photo-resist with molecules that, upon absorbing the ultraviolet etching beam, isomerized to a transparent layer but returned to the initially opaque form upon absorption of visible light. Applying an interference pattern with ultraviolet peaks superimposed on visible nodes restricted etching to narrow regions in the center of these nodes, yielding lines of subwavelength width. Scott et al. (p. 913, published online 9 April) used a central beam to activate polymerization initiators, while using a halo-shaped surrounding beam to trigger inhibitors that would halt polymerization. Li et al. (p. 910, published online 9 April) found that use of a different initiator molecule allowed both beams to share the same wavelength (800 nanometers), with a relatively weak quenching beam lagging a highly intense initiating beam slightly in time. Both the latter techniques produced three-dimensional features honed to subwavelength dimensions. Polymerization activated by a beam of light was halted by inhibitors generated by a surrounding halo of a different color. Controlling and reducing the developed region initiated by photoexposure is one of the fundamental goals of optical lithography. Here, we demonstrate a two-color irradiation scheme whereby initiating species are generated by single-photon absorption at one wavelength while inhibiting species are generated by single-photon absorption at a second, independent wavelength. Co-irradiation at the second wavelength thus reduces the polymerization rate, delaying gelation of the material and facilitating enhanced spatial control over the polymerization. Appropriate overlapping of the two beams produces structures with both feature sizes and monomer conversions otherwise unobtainable with use of single- or two-photon absorption photopolymerization. Additionally, the generated inhibiting species rapidly recombine when irradiation with the second wavelength ceases, allowing for fast sequential exposures not limited by memory effects in the material and thus enabling fabrication of complex two- or three-dimensional structures.

Three-dimensional direct-write lithography into photopolymer.

We demonstrate a three-dimensional direct-write lithography system capable of writing deeply buried, localized index structures into diffusion-mediated photopolymer. The system is similar to that used for femtosecond writing in glass, but has a number of advantages including greater flexibility in the writing media and the ability to use low power, inexpensive, continuous-wave lasers. This system writes index structures both parallel and perpendicular to the writing beam in different types of photopolymers, providing control over the feature size and shape. We demonstrate that this system can be used to create single-mode waveguides that are deeply embedded in the photopolymer medium.

Tomographic reconstruction of weak, replicated index structures embedded in a volume

Measurements of weak, embedded index structures are important for material characterization of photopolymers, glass and other optical materials as well as for characterization of fabricated structures such as waveguides. We demonstrate an optical diffraction tomography system capable of measuring deeply-buried, weak, fabricated index structures written in a homogeneous volume. High-fidelity cross sections of these weak index structures are constructed by replicating the structure to be measured to form a diffraction grating. The coherent addition of scattering from each of these objects increases the sensitivity of the imaging system. Measurements are made in the far field, without the use of lenses, eliminating phase aberration errors through thick volumes.

Mode profile imaging and loss measurement for uniform and tapered single-mode 3D waveguides in diffusive photopolymer

We demonstrate single-mode uniform and parabolically tapered three-dimensional waveguides fabricated via direct-write lithography in diffusion-based photopolymers. Modulation of the writing power is shown to compensate Beer-Lambert absorption in the single-photon initiator and to provide precise control of modal tapers. A laminated sample preparation is introduced to enable full 3D characterization of these modal tapers without the need for sample polishing which is difficult for this class of polymer. The accuracy and repeatability of this modal characterization is shown to allow precise measurement of propagation loss from single samples. These testing procedures are used to demonstrate single-mode waveguides with 0.147 dB/cm excess propagation loss and symmetrical tapers up to 1:2.5 using 1.5 microwatts of continuous write power.

Three-dimensional waveguide arrays via projection lithography into a moving photopolymer

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.

Exceeding the diffraction limit with single-photon photopolymerization and photo-induced termination

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.

3D waveguides with fiber couplers and 90 degree bends in holographic photopolymer

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.

High Dynamic Range (Δn) Two-Stage Photopolymers via Enhanced Solubility of a High Refractive Index Acrylate Writing Monomer

Holographic photopolymers capable of high refractive index modulation (Δn) on the order of 10-2 are integral for the fabrication of functional holographic optical elements that are useful in a myriad of optical applications. In particular, to address the deficiency of suitable high refractive index writing monomers for use in two-stage holographic formulations, here we report a novel high refractive index writing monomer, 1,3-bis(phenylthio)-2-propyl acrylate (BPTPA), simultaneously possessing enhanced solubility in a low refractive index (n = 1.47) urethane matrix. When examined in comparison to a widely used high refractive index monomer, 2,4,6-tribromophenyl acrylate, BPTPA exhibited superior solubility in a stage 1 urethane matrix of approximately 50% with a 20% higher refractive index increase per unit amount of the writing monomer for stage 2 polymerizations. Formulations with 60 wt % loading of BPTPA exhibit a peak-to-mean holographic Δn ≈ 0.029 without obvious deficiencies in transparency, color, or scatter. To the best of our knowledge, this value is the highest reported in the peer-reviewed literature for a transmission hologram. The capabilities and versatility of BPTPA-based formulations are demonstrated at varying length scales via demonstrative refractive index gradient structure examples including direct laser write, projection mask lithography of a 1″ diameter Fresnel lens, and ∼100% diffraction efficiency volume transmission holograms with a 1 μm fringe spacing in 11 μm thick samples.

Transport-of-intensity-based phase imaging to quantify the refractive index response of 3D direct-write lithography

Precise direct-write lithography of 3D waveguides or diffractive structures within the volume of a photosensitive material is hindered by the lack of metrology that can yield predictive models for the micron-scale refractive index profile in response to a range of exposure conditions. We apply the transport of intensity equation in conjunction with confocal reflection microscopy to capture the complete spatial frequency spectrum of isolated 10 μm-scale gradient-refractive index structures written by single-photon direct-write laser lithography. The model material, a high-performance two-component photopolymer, is found to be linear, integrating, and described by a single master dose response function. The sharp saturation of this function is used to demonstrate nearly binary, flat-topped waveguide profiles in response to a Gaussian focus.

Localized Recording Approaches and Phase Metrology for Holographic Storage

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.