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.

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.

A high-performance holographic recording medium was developed based on a unique combination of photoinitiated thiol−ene click chemistry and functional, linear polymers used as binders. Allyl reactive sites were incorporated along the backbone of the linear polymer binder to enable facile film casting and to facilitate cross-linking by photopolymerization of the thiol−ene monomers that also serve as the writing monomers in this distinctive approach to holographic materials. The allyl content and the ratio of the linear polymer to the writing monomers were varied to maximize and control the refractive index contrast. A blade-coatingbased film preparation method was developed to form films from the mixture of linear polymer and the thiol−ene monomers. This approach results in a holographic material with a peak to mean index contrast (Δn) that reaches 0.04. The refractive index contrast was stable for at least two weeks. Haze in holograms with a high writing monomer loading was significantly reduced when a higher allyl content was incorporated into the binder, resulting in the lowest haze around 0.2%. Finally, the media exhibit high resolution as demonstrated by the ability to record reflection holograms with 140 nm pitch and diffraction efficiency in excess of 90%.

Photopolymers formed from the family of thiol-X "click" reactions notably form sulfur-containing thioether linkages. While sulfur-containing materials are conventionally expected to result in high refractive index (n D /20 °C > 1.6) materials, this expectation is rarely realized with thiol-X photopolymers, because of the lack of monomers with sufficiently high refractive indices. Here, an efficient, modular synthetic strategy to obtain high refractive index thiol-X monomers is presented. The efficacy of the overall approach is demonstrated using only commercially available starting compounds to yield low viscosity (<500 cP) liquid multifunctional thiol and diallyl ether monomers with high refractive indices (n D /20 °C > 1.64). These synthesized monomers underwent rapid thiol-ene photopolymerizations to high conversions, achieving refractive index values (n D /20 °C) up to 1.669 in uniform crosslinked networks exhibiting typical narrow and well-defined loss tangent (tan δ) peaks. The convenience of the low-viscosity, high optical quality thiol-ene resins was demonstrated in the fabrication of two functional optical components. First, a plano-convex lens was aspherized using an overmolding procedure. Second, a two-dimensional grating was fabricated using a two-stage material and a photomask exposure.

Holographic photopolymers capable of high refractive index modulation (Δn) on the order of 10 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.