Hyerim Hwang

Controlled Pixelation of Inverse Opaline Structures Towards Reflection‐Mode Displays

Pixelated inverse opals with red, green, and blue colors were prepared by hybridizing convective assembly of colloidal particles and photolithography techniques. The brilliant structural colors, high mechanical stability, and small feature size of the pixels were simultaneously accomplished, thereby providing color reflectors potentially useful for display devices. Moreover, this hybridized method provides a general means to create multi-colored photonic crystals.

Fabrication of highly uniform three-dimensional SERS substrates by control of wettability

We present a simple and reproducible method for the fabrication of three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrates by controlling wettability. A superhydrophilic SU-8 microstructured surface generated using O2 plasma treatment led to the formation of a monolayer of aggregated Ag nanoparticles with nanoscale gaps. The SERS substrate showed a highly sensitive and uniform SERS response.

Fabrication of Robust Optical Fibers by Controlling Film Drainage of Colloids in Capillaries

Optical fibers have enabled the development of fast telecommunication and imaging technologies. Since the development of glass fibers, transmission losses have decreased significantly and optical fibers have become indispensable to everyday life, from telecommunications and networking to illumination tools and medical endoscopes. However, optical fibers based on total internal reflection (TIR) incur inevitable losses at highly bent regions, as occur in compact optical circuits, and at solid cores of fibers owing to absorption and scattering. Researchers have developed photonic crystal fibers that can overcome the shortcomings of TIR-based optical fibers. The band-gap properties of photonic crystal fibers reduce losses and enable new optical applications, such as fiber lasers and nonlinear devices. Photonic crystal fibers are prepared using a stack-and-draw procedure with glass capillaries, or by drawing hollow tubes of concentric multilayers. Herein, we present a new type of photonic crystal fiber using a bottom-up approach involving spontaneous crystallization of colloidal particles. The self-organization of colloidal particles provides a practical approach to creating threedimensional photonic crystals with potential utility in photonic circuits, laser cavities, or biosensors. To fabricate colloidal crystal fibers, Moon et al. and Li et al. coated the outer surfaces of glass fibers with colloidal crystals using evaporation-induced vertical deposition. However, extremely long fabrication times and poor mechanical properties have severely restricted the applications of these fibers, and the effects of the photonic band gap on light guiding have not yet been characterized. To create colloidal photonic crystal fibers in a practical and reproducible fashion, colloids were dispersed in a photocurable resin, resulting in spontaneous crystallization of the colloids with a repulsive potential; this resin is coated on the inner walls of microcapillaries under film-draining protrusion flow. Spontaneous crystallization and fast consolidation of the structures yielded robust photonic crystals with excellent optical performance. The controlled dynamic deposition of film on the microcapillaries permitted manipulation of the thickness and number of layers in the hollow photonic crystal fibers. Using photonic crystal fibers, we demonstrated that a stop band in a colloidal photonic crystal could enhance the efficiency of light guidance through the fibers away from the TIR regime. This strategy relied essentially on film formation over the inner walls of the microcapillaries. To make a hollow fiber with a colloidal structure, a photocurable suspension of silica particles dispersed in ethoxylated trimethylolpropane triacrylate (ETPTA) was injected in the hydrophobic capillary. An aqueous surfactant solution was then forced through the capillary to drain the suspension out, maintaining a thin film of the suspension on the inner wall of the capillary owing to the hydrophobic nature of the surface (Figure 1a); surfactant solutions reduce interfacial energy between the solution and the suspension, enabling a stable formation of the film (Supporting Information, Figure S1). Upon application of UV irradiation, the film polymerized, producing a hollow cylindrical silica–ETPTA composite film on the inner wall of the capillary. During film formation, the small Reynolds number and small radius of the capillary produced insignificant gravitational and inertial forces so that the flow was governed by viscous and surface tension forces. In the limit of a small capillary number (Ca = hV/g), the surface tension forces dominated the viscous forces so that the relative thickness of the film e/(b e) was determined by the capillary number (Figure 1b), where h is viscosity of the resin, V is flow velocity of the aqueous solution, g is the interfacial energy, and e and b are the thickness of the film and the inner radius of the capillary, respectively. The dependence of the thickness on two parameters, V and h, is illustrated in Figure S2 of the Supporting Information. The flow rate was controlled by a syringe pump, and the viscosity was modulated by adjusting the concentration of silica particles in the ETPTA resin. The relative thickness was proportional to Ca, in accordance with Bretherton s equation. Therefore, the relative thickness of the film could be controlled by the flow rate of an aqueous solution in the range 0.05–0.25. This coating method was effective, even for tubes longer than a few tens of centimeters (Supporting Information, Figure S3). A coating formed from a highly concentrated silica– ETPTA suspension on a polyethylene (PE) tube produced structural colors that are due to the silica–ETPTA composite layer, which formed a narrow distribution of interparticle distances without long-range order. Because the silica particles (nsilica = 1.45) dispersed in ETPTA (nETPTA = 1.4689) exhibit negligible van der Waals attractions owing to the small [*] Dr. S.-H. Kim, H. Hwang, Prof. S.-M. Yang National Creative Research Initiative Center for Integrated Optofluidic Systems and Department of Chemical and Biomolecular Engineering, KAIST Daejeon, 305-701 (Korea) E-mail: dmz@kaist.ac.kr smyang@kaist.ac.kr Homepage: http://msfl.kaist.ac.kr

Lithographically-featured photonic microparticles of colloidal assemblies

We have described a new and promising strategy for the fabrication of composite and porous photonic crystal microparticles that combines the self-assembly of colloidal particles with photolithography techniques. We fabricated silica/SU-8 composite microparticles with photonic bandgaps via four steps: (1) deposition of the silica colloidal crystals on the photoresist, (2) embedding of the colloidal crystals in the photoresist, (3) UV exposure through a photomask and subsequent development, and (4) release of the microparticles from the substrate. Embedding was performed above the glass transition temperature (T(g)) of uncrosslinked SU-8. At such temperatures, capillary forces on the silica particles facilitate the migration of colloidal crystals in the SU-8 matrix. Particle migration ceased when the top colloidal crystal layer was trapped at the interface between air and SU-8. In addition, we also prepared porous microparticles with an inverse opaline structure by dissolving the embedded silica particles from the composite structures. The porous microparticles showed enhanced reflectivity at the bandgap position due to the large refractive index contrast. The bandgap position of the microparticles was controlled by the size of the silica particles, which determined the lattice constant. Bilayered composite and porous microparticles with two distinct photonic bandgaps were also prepared by sequential deposition of colloidal crystals composed of two differently sized silica particles.