Yongan Xu

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.

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 ]

Direct fabrication of 3D silica-like microstructures from epoxy-functionalized polyhedral oligomeric silsesquioxane (POSS)

We have fabricated 2D and 3D structured organosilcates from epoxy functionalized polyhedral oligomeric silsesquioxanes (POSS) cage materials using holographic lithography, which can be conveniently converted to silica structures by thermal removal of the organic moieties.

Blast-Induced Color Change in Photonic Crystals Corresponds with Brain Pathology

A high incidence of blast exposure is a 21st century reality in counter-insurgency warfare. However, thresholds for closed-head blast-induced traumatic brain injury (bTBI) remain unknown. Moreover, without objective information about relative blast exposure, warfighters with bTBI may not receive appropriate medical care and may remain in harms way. Accordingly, we have engineered a blast injury dosimeter (BID) using a photonic crystalline material that changes color following blast exposure. The photonic crystals are fabricated using SU-8 via multi-beam interference laser lithography. The final BID is similar in appearance to an array of small colored stickers that may be affixed to uniforms or helmets in multiple locations. Although durable under normal conditions, the photonic crystalline micro- and nano-structure are precisely altered by blast to create a color change. These BIDs were evaluated using a rat model of bTBI, for which blast shockwave exposure was generated via a compressed air-driven shock tube. With prototype BID arrays affixed to the animals, we found that BID color changes corresponded with subtle brain pathologies, including neuronal degeneration and reactive astrocytosis. These subtle changes were most notable in the dentate gyrus of the hippocampus, cerebral cortex, and cerebellum. These data demonstrate the feasibility of using a materials-based, power-free colorimetric BID as the first self-contained blast sensor calibrated to correspond with brain pathology.

Core-shell diamond-like silicon photonic crystals from 3D polymer templates created by holographic lithography

Diamond-like silicon photonic crystals were fabricated by sequential chemical vapor deposition of silica and silicon on polymer templates photopatterned by holographic lithography. The optical properties of the 3D crystals after each processing step were measured and compared to the corresponding bandgap simulation. The core-shell morphology formed during CVD process is approximated using two level surfaces.

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.