Woodpile Metallic Photonic Crystals Fabricated by Using Soft Lithography for Tailored Thermal Emission

3D micropatterning of materials can create advanced mechanical, chemical, or electromagnetic functionalities not observed in bulk. This is especially true for 3D periodic structures, called photonic crystals, which significantly modify optical properties of materials for light having wavelengths close to the periodicity of the patterning. Among them, a 3D metallic photonic crystal (MPC), usually in a woodpilelike pattern, has recently attracted attention because it can produce efficient thermal emitters and photovoltaic devices through tailoring of the absorption spectrum. However, its fabrication is still problematic because of challenges in 3D microfabrication at optical scales. In this letter, we report a nonphotolithographic fabrication method using soft lithography and electrodeposition to produce highly layered full-metallic structures with excellent structural fidelity. By adding a homogeneous monolithic backplane to the conventional woodpile structure, the difficulty of alignment in layer-by-layer fabrication is alleviated, while preserving characteristic highly enhanced thermal radiation in a tailorable range of frequencies. Although the tailored enhancement of absorption has been observed from woodpile MPCs fabricated by semiconductor processing, obstacles in multilayer alignment and intricate processing still remain. As an alternative approach, direct laser writing could be considered to create a template for woodpile MPCs. As in all approaches using a template, metalinfiltration is a critical step because of the complex 3D geometry of the template. Electrodeposition has strong potential for complete bottom-up filling rather than vapor-phase deposition, which often results in voids from blocked channels. However, a number of requirements must be satisfied, including: complete wetting of the template, slow deposition to prevent hydrogen generation at the cathode, and good chemical and mechanical stability of the template under electrolytes. Additionally, the surface of the conducting substrate cannot have any insulating residue impeding current flow after forming the template. It is not clear whether other approaches using photolithography, including direct laser writing, are adequate for the fabrication of woodpile metallic structures using electrodeposition. Recently, we reported a soft lithographic technique, called two-polymer microtransfer molding, for the fabrication of layer-by-layer polymer microstructures using nonoptical additive processing. With this technique, a photocurable polyurethane prepolymer (J91, Summers Optical) is filled in linear microchannels of a polydimethylsiloxane-based elastomeric mold (Sylgard 184, Dow Corning) and solidified. The surface of the prefilled polymer is coated with a photocurable polymetacrylate prepolymer (SK9, Summers Optical) as an adhesive layer and the assembly is placed in contact with an indium-tin-oxide (ITO)-coated glass. Then, after curing the adhesive layer and peeling off the elastomeric mold, a layer of microrods are transferred to the ITO-coated glass. Awoodpile polymer template is created after multiple transfer steps. Generally, the layer-bylayer structure requires sophisticated translational alignment to place rods of each layer in the proper position centered between the rods of alternate layers with the same orientation. In this study, we attempted only angular alignment for the parallel arrangement of rods in different layers using a moirefringes-based alignment technique without the translational alignment. We show here that translational alignment is not crucial in the metallic layer-by-layer structure. The multilayer polymer template was backfilled by the electrodeposition of nickel until a homogenous overlayer was formed. The overlayer served as the backplane of the photonic crystal structure and its optical contribution is discussed. The backfilling speed was slower than for a bare ITO surface, which indicates that some electrically insulating residue existed on the surface, although it was not sufficient to prohibit electroplating. The residue may have originated from the elastomer molds used in soft lithography or prepolymers used in forming the polymer templates. Because of weak adhesion of the electrodeposited nickel to ITO, the backfilled structure was easily peeled off manually. The polymer template embedded in the backfilled nickel structure was chemically removed. Figure 1 shows scanning electron microscopy (SEM) images of a free-standing 11 layer nickel structure without the backplane. The size of the fabricated nickel structures was sufficiently wide (4 mm × 4 mm) for a thermal emitter and each layer consisted of more than 1500 rods, 1.1 lm wide and C O M M U N IC A IO N