Benjamin A. Kowalski

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.

Quantitative modeling of the reaction/diffusion kinetics of two-chemistry diffusive photopolymers

A general strategy for characterizing the reaction/diffusion kinetics of photopolymer media is proposed, in which key processes are decoupled and independently measured. This strategy enables prediction of a material’s potential refractive index change, solely on the basis of its chemical components. The degree to which a material does not reach this potential reveals the fraction of monomer that has participated in unwanted reactions, reducing spatial resolution and lifetime. This approach is demonstrated for a model material similar to commercial media, achieving quantitative predictions of refractive index response over three orders of exposure dose (~1 to ~103 mJ cm−2) and feature size (0.35 to 500 μm).

Two-color photo-initiation/inhibition lithography

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 structures.

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites

Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) volume elements] via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a dc field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be electrically triggered. Initial experiments indicate that the SWNT-polymer interfaces play a crucial role in enabling the electrostriction reported herein.

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.

Patterning nonisometric origami in nematic elastomer sheets

Nematic elastomers dramatically change their shape in response to diverse stimuli including light and heat. In this paper, we provide a systematic framework for the design of complex three dimensional shapes through the actuation of heterogeneously patterned nematic elastomer sheets. These sheets are composed of nonisometric origami building blocks which, when appropriately linked together, can actuate into a diverse array of three dimensional faceted shapes. We demonstrate both theoretically and experimentally that the nonisometric origami building blocks actuate in the predicted manner, and that the integration of multiple building blocks leads to complex, yet predictable and robust, shapes. We then show that this experimentally realized functionality enables a rich design landscape for actuation using nematic elastomers. We highlight this landscape through examples, which utilize large arrays of these building blocks to realize a desired three dimensional origami shape. In combination, these results amount to an engineering design principle, which provides a template for the programming of arbitrarily complex three dimensional shapes on demand.

Subdiffraction Microholograms in a Single-Photon, Uniformly Inhibited System

Microholograms significantly smaller than the diffraction limit are demonstrated in a photopolymer system with uniformly distributed inhibitor. This subdiffraction performance affords both increased storage density and increased readout signal via suppression of out-of-focus exposure. A model of the micron-scale reaction kinetics of the system is presented, and parameter values are derived experimentally.

Layered liquid crystal elastomer actuators

Liquid crystalline elastomers (LCEs) are soft, anisotropic materials that exhibit large shape transformations when subjected to various stimuli. Here we demonstrate a facile approach to enhance the out-of-plane work capacity of these materials by an order of magnitude, to nearly 20 J/kg. The enhancement in force output is enabled by the development of a room temperature polymerizable composition used both to prepare individual films, organized via directed self-assembly to retain arrays of topological defect profiles, as well as act as an adhesive to combine the LCE layers. The material actuator is shown to displace a load >2500× heavier than its own weight nearly 0.5 mm.Liquid crystalline elastomers (LCE) exhibit shape transformation when subjected to various stimuli, but the achievable thickness of LCE films is limited. Here the authors demonstrate arbitrarily thick LCE films that are continuous in composition and maintain the director orientation, prescribed into the material.

Low-energy, nanoparticle reshaping for large-area, patterned, plasmonic nanocomposites

Compliant, robust films with pixelated, voxelated or gradient distribution of plasmonic properties are enabling for technologies from colorimetric sensors, filters, and gradient index optical elements to art. Spatially multiplexing different plasmonic effects, however, is challenging. To address this challenge, we demonstrate a post-film fabrication process that enhances gold nanorod (AuNR) reshaping with chemistry. Mild annealing or broadband non-coherent light sources provide sufficient heating to drive localized redox processes that lead to an isovolumetric reduction of the surface-to-volume ratio of CTAB stabilized AuNRs in polyvinyl alcohol (PVA). Single crystallinity is retained. The reshaping rate in the presence of these redox processes occurs in excess of 100× faster (seconds) than previous reports that utilize increased surface diffusion as temperatures approach the particle melting point (days). Using the processs dependency on reactant concentration, broadband, multi-exposure optical processing preserves particle alignment, enables multi-color patterning, and produces gradients of the longitudinal plasmon resonance of at least 0.01 eV μm−1 (3 nm μm−1).

Enabling and Localizing Omnidirectional Nonlinear Deformation in Liquid Crystalline Elastomers

Liquid crystalline elastomers (LCEs) are widely recognized for their exceptional promise as actuating materials. Here, the comparatively less celebrated but also compelling nonlinear response of these materials to mechanical load is examined. Prior examinations of planarly aligned LCEs exhibit unidirectional nonlinear deformation to mechanical loads. A methodology is presented to realize surface-templated homeotropic orientation in LCEs and omnidirectional nonlinearity in mechanical deformation. Inkjet printing of the homeotropic alignment surface localizes regions of homeotropic and planar orientation within a monolithic LCE element. The local control of the self-assembly and orientation of the LCE, when subject to rational design, yield functional materials continuous in composition with discontinuous mechanical deformation. The variation in mechanical deformation in the film can enable the realization of nontrivial performance. For example, a patterned LCE is prepared and shown to exhibit a near-zero Poissons ratio. Further, it is demonstrated that the local control of deformation can enable the fabrication of rugged, flexible electronic devices. An additively manufactured device withstands complex mechanical deformations that would normally cause catastrophic failure.