Holographic Photopolymer Material with High Dynamic Range (Δ) via Thiol-Ene Click Chemistry.

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.

High efficiency Fresnel lens design and fabrication in a two-stage photopolymer.

We show the design and fabrication of high diffraction efficiency, optically recorded gradient-index Fresnel lenses in a two-stage photopolymer. A design analysis reveals that lens f/# is limited by the material refractive index contrast, motivating use of recent high-contrast polymers. The number of pixels required for the optical exposure is typically well beyond available spatial light-modulator resolutions, motivating the use of a photolithographic mask.

Multiple patterning of holographic photopolymers for increased refractive index contrast.

We demonstrate that multiple exposures of a two-component holographic photopolymer can quadruple the refractive index contrast of the material beyond the single-exposure saturation limit. Quantitative phase microscopy of isolated structures written by laser direct-write lithography is used to characterize the process. This technique reveals that multiple exposures are made possible by diffusion of the chemical components consumed during writing into the previously exposed regions.

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.

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 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.

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.

Two-color single-photon photoinitiation and photoinhibition for subdiffraction photolithography.

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.

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.

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.