Xuelian Zhu

Two-dimensional photonic crystals with anisotropic unit cells imprinted from poly(dimethylsiloxane) membranes under elastic deformation

We study structural symmetries of two-dimensional (2D) photonic crystals with anisotropic unit cells, including square- and rectangular-lattices with orientationally modulated elliptic motifs, and a compound structure consisting of circles with sixfold rotational symmetry and elliptical lines with twofold symmetry, which are created through elastic deformation of a single elastomeric membrane with circular pores. We then investigate the photonic bandgap (PBG) properties of the corresponding 2D Si posts and their tolerance to the structural deviation. We find that in the compound structure the overall PBGs are dominated by the sublattice with a higher symmetry, while the total symmetry is determined by the one with a lower symmetry.

Poly(glycidyl methacrylate)s with controlled molecular weights as low-shrinkage resins for 3D multibeam interference lithography

Poly(glycidyl methacrylate) has been shown to be a useful material for fabrication of photonic crystal templates using multibeam interference lithography, since it exhibits lower shrinkage than conventional SU8.

Distortion of 3D SU8 Photonic Structures Fabricated by Four-beam Holographic Lithography with Umbrella Configuration

We present a quantitative study of the distortion from a threeterm diamond-like structure fabricated in SU8 polymer by four-beam holographic lithography. In the study of the refraction effect, theory suggests that the lattice in SU8 should be elongated in the [111] direction but have no distortion in the (111) plane, and each triangular-like hole array in the (111) plane would rotate by ~30 degrees away from that in air. Our experiments agree with the prediction on the periodicity in the (111) plane and the rotation due to refraction effect, however, we find that the film shrinkage during lithographic process has nearly compensated the predicted elongation in the [111] direction. In study of photonic bandgap (PBG) properties of silicon photonic crystals templated by the SU8 structure, we find that the distortion has decreased quality of PBG.

Crack-Free 3D Hybrid Microstructures from Photosensitive Organosilicates as Versatile Photonic Templates

We fabricated diamond-like microstructures from epoxy-functionalized cyclohexyl polyhedral oligomeric silsesquioxanes (POSS) through four-beam interference lithography. The 3D structure was maintained when calcined at a temperature up to 1100 degrees C, and crack-free samples over a large area ( approximately 5 mm in diameter) were obtained when the POSS films were heated at 500 degrees C under an Ar environment or treated with a low intensity oxygen plasma. In the latter, the volume fraction of the 3D porous structures could be fine-tuned by plasma etching time and power. Both Fourier transform infrared (FT-IR) spectroscopy and energy-dispersive X-ray (EDX) spectroscopy analysis suggested that the presence of carbon materials in the films enhanced the crack resistance of 3D POSS structures treated under Ar or oxygen plasma. Since POSS and its derivatives could be easily removed by HF solution at room temperature, we demonstrated high fidelity replication of the 3D porous structures to biocompatible poly(glycidyl methacrylate) (PGMA) and elastomeric poly(dimethylsiloxane) (PDMS). Importantly, the whole fabrication process (template fabrication, infiltration, and removal) was carried out at room temperature. Finally, we illustrated the application of 3D PDMS film as a reversible and repeatable color-changing, flexible photonic crystal.

Capillarity induced instability in responsive hydrogel membranes with periodic hole array

We report capillary force induced instability from drying water swollen poly(2-hydroxyethyl methacrylate) (PHEMA) based hydrogel membranes with micron-sized holes in a square array. When the PHEMA membrane was exposed to deionized-water, the size of the holes became smaller but retained the shape, so-called breathing mode instability. However, during the drying process, the square pore array buckled into a diamond plate pattern. The deformed pattern could be recovered upon re-exposure to water. The instability mechanism was confirmed by comparing the observations from optical and scanning electron microscopy (SEM) images with theoretical prediction. When thermoresponsive poly(N-isopropylacrylamide) was introduced to the PHEMA gel, the poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide) (PHEMA-co-PNIPAAm) membrane underwent pattern transformation only if dried below the lower critical solution temperature of PNIPAAm. Along the pattern transformation, we observed a dramatic change of the optical property of the film, from colourful reflection to transparent window.

Holographic Design and Fabrication of Diamond Symmetry Photonic Crystals Via Dual‐Beam Quadruple Exposure

and direct-write assembly. [ 9 , 10 ] Nevertheless, they are typically laborious, therefore, fabrication is often limited to a few unit cells. In comparison, holographic lithography (HL) [ 11 , 12 ] is an effi cient technique for creating large-area defect-free 3D PCs by recording laser interference patterns in photoresists, where the sample area is determined by the laser beam size and the number of layers is dependent on the resist thickness and transparency to laser. By controlling beam geometry, polarization, phase and intensity, various 3D structures have been demonstrated, including simple-cubic, [ 13 , 14 ] face-centered cubic, [ 11 , 15 , 16 ] woodpile, [ 17 ] and complicated structures. [ 18 ] Design of diamond symmetry PCs by HL [ 19 , 20 ] has been proposed. However, it remains elusive to experimentally demonstrate diamond symmetry PCs due to the complexity in optics required in practice and lattice distortion due to the refraction effect and anisotropic shrinkage of the photoresist fi lm during the lithographic process. [ 14 , 16 , 17 , 21 ]

Complex 2D photonic crystals with analogue local symmetry as 12-fold quasicrystals

We construct fourteen complex periodic two-dimensional (2D) photonic structures with different structural symmetries by arranging the small portions of a 12-fold quasicrystal on square or hexagonal lattices. The corresponding reciprocal lattices confirm that all of them demonstrate the 12-fold-like characteristics due to the analogue short-range arrangements. We then investigate their photonic bandgap properties at different dielectric contrast levels (dielectric rods in air background). Our results suggest that all structures possess analogue transverse magnetic (TM) gaps in both Si and glass photonic crystals due to the similarity of their local geometries. However, the arrangements of the basic elements, total symmetries, and the coupling between the local and the lattice symmetries have greater impact on the glass photonic crystals, which show much larger deviation of gap sizes from different structures. Furthermore, we find that the minimal dielectric contrast to achieve the TM gap in the complex lattices (dielectric-in-air) can be as low as epsilon = 1.44, whereas the inverse structures may open a 2D complete gap in silicon nitride (epsilon = 4.1).

Two-dimensional photonic crystals with anisotropic unit cells imprinted from PDMS membranes under elastic deformation

We study structural symmetries of two-dimensional (2D) photonic crystals with anisotropic unit cells, including square- and rectangular-lattices with orientationally modulated elliptic motifs, a compound structure consisting of circles with 6-fold rotational symmetry and elliptical lines with 2-fold symmetry, and a rectangular lattice of aligned ovals, which are created through elastic deformation of an elastomeric membrane with circular pores. We then investigate the photonic bandgap (PBG) properties of the corresponding 2D Si posts, and their tolerance to the structural deviation. We find that in the compound structure the overall PBGs are dominated by the sublattice with a higher symmetry, while the total symmetry is determined by the one with a lower symmetry.

Fabrication of 3D high index photonic crystals by holographic lithography and their fidelity

In order to create three-dimensional (3D) photonic crystals (PCs) with large photonic bandgap properties (PBG), it is necessary to control the 3D fabrication with desired symmetry, high index contrast, and high structural stability. To rational design the 3D photonic structures fabricated by holographic lithography, we have conducted quantitative analysis to study structural distortion during each processing step and their impact to PBG. Because of the relatively low dielectric contrast between typical polymers and air, the directly patterned polymer structures are usually used as templates for backfilling of high-index materials, followed by removal of the polymer template to realize complete PBGs. Therefore, the fidelity of the final PCs is critically dependent on the thermal and mechanical robustness of the polymer templates, the deposition methods (e.g. dry chemical vapor deposition vs. wet chemistry), and the template removal procedure. Here, we address these challenges using different photoresist systems and deposition methods to create Si and titania 3D PCs.

Fidelity of Holographic Lithography for Fabrication of 3D SU-8 Photonic Structures and How to Minimize Distortion by Optical Design

Theoretical analysis can impart great benefits on the rationale design of 3D photonic structures by revealing the underlying mechanisms of structural distortion during each processing step. In this report, we quantitatively study the distortion of a three-term diamond-like structure fabricated in SU-8 polymer by four-beam interference lithography, which can be attributed to refraction at the air-film interface, and resist film shrinkage during lithographic process. In study of photonic bandgap (PBG) properties of Si photonic crystals templated by the SU-8 structures, we find that the distortion has degraded the quality of PBGs. Furthermore, we theoretically design new optical setups to fabricate three-term diamond-like structure with minimal deformation. Instead of single exposure of four beams, we use triple exposure of two beams, one from the central beam and the other from the side beam each time. A set of new linear polarization vectors is suggested to enhance the contrast between the minimal and maximal intensities of interference pattern.