In this paper, we present an achievable gradient refractive index in bi-continuous holographic structures that are formed through five-beam interference. We further present a theoretic approach for the realization of gradient index devices by engineering the phases of the interfering beams with a pixelated spatial light modulator. As an example, the design concept of a gradient index Luneburg lens is verified through full-wave electromagnetic simulations. These five beams with desired phases can be generated through programming gray level super-cells in a diffractive spatial light modulator. As a proof-of-concept, gradient index structures are demonstrated using synthesized and gradient phase patterns displayed in the spatial light modulator.

Through the use of a multi-layer phase mask to produce fivebeam interference, three-dimensional photonic crystals can be formed through single exposure to a photoresist. In these holographically formed structures, the interconnectivity is controlled by the relative phase difference among contributing beams. Photonic band gaps are calculated and the simulation shows a maximum bandgap of 18% of the middle gap frequency when the phase difference is optimized. A three-layer phase mask is fabricated by placing a spacer layer between two orthogonally-orientated gratings. The phase difference is controlled by thermal-tuning of the spacer thickness. Photonic crystal templates are holographically fabricated in a photosensitive polymer using the phase mask. ©2011 Optical Society of America

This letter demonstrates holographic fabrication of three-dimensional diamondlike photonic crystal templates in SU8 photoresist using a single diffractive optical element. Five coherent laser beams produced by a two-dimensional phase mask were used to construct face-centered-cubic or tetragonal interference patterns. The superposition of two interference patterns through double exposures yields diamondlike photonic crystal templates in SU8. Photonic bandgap calculation reveals a full bandgap in inverse structures based on the template. Among all fabrication approaches, holographic lithography has been recognized as a primary method to produce 3D structures due to its controllability, scalability, and flexibility. All fourteen Bravais periodic lattices can be fabricated using multiple-beam holographic fabrication. 7,8 In addition, the holographic lithography enables the fabrication of complex 3D photonic structures such as a diamondlike structure with large bandgap. 9 Traditionally, multiple coherent laser beams needed to produce holographic interference patterns were constructed by bulk optical elements such as beamsplitters and mirrors.10 It involves delicate optical alignments, large footprint for optical setup, and stable mechanical platforms. To simplify the construction of multiple coherent beams, both refractive and diffractive optical elements have been recently adapted to construct multiple-beam interference patterns for 3D PhC fabrications.11,12 These approaches replace all beam-forming optical components by a single optical element such as a phase mask. The phase mask approach offers a wafer-scale fabrication approach to produce 3D nanooptical patterns, which can be incorporated into existing photolithography process for photonic device chip fabrication. The flexibility of one-dimensional ͑1D͒ phase mask approaches for PhC fabrications have been demonstrated in the fabrication of woodpile, orthorhombic, and tetragonal structures. 11In this letter we extended the phase-mask approach for holographic fabrication from 1D phase mask to twodimensional ͑2D͒ phase mask. The flexibility of 2D phase mask design enables the construction of multiple-beam interference patterns for the fabrication of complex 3D periodic structure such as diamondlike structures.13 Diamondlike PhC structures possess the largest photonic bandgap among all possible PhCs. However, the complex structures pose a great fabrication challenge. In this letter, we demonstrate this flexibility by fabricating diamondlike PhC structures in SU8 photoresist.The 2D phase masks used in this experiment were fabricated in a thin negative-tone SU8-2035 photoresist ͑Micro-Chem Corp.͒. The photoresist was spin coated on a glass substrate with a thickness of 2 m. After spin coating, the sample was prebaked at 65°C for 5 min and 95°C for 3 min before receiving the laser exposure. The source beam from an Argon Ion laser was collimated and split into two coherent linearly polarized beams with equal intensity using a beam splitter. Two laser beams were recombin...

Abstract:Large-area three-dimensional Penrose-type photonic quasicrystals are fabricated through a holographic lithography method using a lab-made diffractive optical element and a single laser exposure. The diffractive optical element consists of five polymer gratings symmetrically orientated around a central opening. The fabricated Penrose-type photonic quasicrystal shows ten-fold rotational symmetry. The Laue diffraction pattern from the photonic quasi-crystal is observed to be similar to that of the traditional alloy quasi-crystal. A golden ratio of 1.618 is also observed for the radii of diffraction rings, which has not been observed before in artificial photonic quasicrystals.

We report a combined holographic and nanoimprinting lithography technique to produce three-dimensional woodpile photonic crystal templates through only one single exposure. The interference lithography process uses an integratable diffractive optical element for large throughout 3D pattern manufacturing. The diffractive optical element consists of two layers of phase grating separated by an intermediate layer, fabricated by repeated nanoimprinting lithography, followed by an SU8 photoresist bonding technique. Grating periods, relative orientation, diffraction angle, and efficiency, as well as layer to layer phase delay, are well designed during manufacturing. By thermally optimizing the thickness of the intermediate layer, this paper demonstrates the fabrication of interconnected 3D photonic structures with arbitrary symmetry through a single laser exposure. The two-layer phase mask approach enables a CMOS-compatible monolithic integration of 3D photonic structures with other integrated optical elements and waveguides.