Jae Won Shim

Flexible, Angle-Independent, Structural Color Reflectors Inspired by Morpho Butterfly Wings

Thin-film color reflectors inspired by Morpho butterflies are fabricated. Using a combination of directional deposition, silica microspheres with a wide size distribution, and a PDMS (polydimethylsiloxane) encasing, a large, flexible reflector is created that actually provides better angle-independent color characteristics than Morpho butterflies and which can even be bent and folded freely without losing its Morpho-mimetic photonic properties.

Integration of Colloidal Photonic Crystals toward Miniaturized Spectrometers

2010 WILEY-VCH Verlag Gm Photonic crystals have emerged as one of the most promising materials for manipulating light because of their photonic bandgaps, which affect photons in a manner similar to the effect of semiconductor energy bandgaps on electrons. These bandgaps arise due to the periodic modulation of the refractive index in space with subwavelength period; photons with energies in these gaps are prohibited in the material. The simplest and most economical approach to fabricating 3D photonic crystals is generally accepted to be via the self-organization of colloidal particles. Monodisperse colloidal particles begin to organize spontaneously into crystal lattices above a certain transition concentration, which depends on the interparticle interactions. Colloidal crystals (CCs) have been used in various photonic and biological applications, including sensors, biological probes, display color pigments, and laser cavities. However, CCs have intrinsic defects, such as vacancies, cracks, and faults, which degrade the optical performance of the crystals. Also, most CCs or derivatives thereof have low physical rigidity and weather resistance. More importantly, precise control of the bandgap is difficult and tailoring the crystals into desired shapes adds complexity to the fabrication procedure. Here we develop a simple and practical platform to overcome the abovementioned drawbacks of CCs. To demonstrate the strategy, we integrate colloidal photonic crystals with 20 different bandgaps in the visible range into a miniaturized photonic device that acts as a spectrometer. In conventional spectrometers, a diffraction grating splits the light source into several beams with different propagation directions according to the wavelength of the light. Thus, to achieve sufficient spatial separation for intensity measurements at a small slit, a long light path (i.e., a large instrument) is required. However, for lab-on-a-chip applications, the spectrometer must be integrated into a sub-centimeter scale device to produce a stand-alone platform. To achieve this, we propose a new paradigm in which the spectrometer is based on an array of photonic crystals with different bandgaps. Because photonic crystals reflect light of different wavelengths selectively depending on their bandgaps, we can generate reflected light spanning the entire wavelength range for analysis at different spatial positions using patterned photonic crystals. Therefore, when the light source impinges on the patterned photonic crystals, we can construct the spectrum using the reflection intensity profile from the constituent photonic crystals. This concept is demonstrated in Scheme 1a. To prepare photonic crystals with the desired bandgap positions, we used non-close-packed CCs of silica particles dispersed in ethoxylated trimethylolpropane triacrylate (ETPTA) photocurable resin. Due to the weakening of the van der Waals attraction caused by index-matching, the repulsive interparticle potential dominates via the disjoining pressure of the solvation layer and weak electrostatic interactions. Therefore, monodisperse silica particles dispersed in ETPTA spontaneously crystallize into non-close-packed face-centered cubic (fcc) structures at volume fractions (f) above 0.1. Previously, a reproducible method was reported for preparing uniform colloidal crystal films of high quality at wafer-scale, based on spin-coating of the silica-in-ETPTA suspension. During spinning, strong viscous shear stresses induce contact between colloidal particles in the axial direction while the interparticle distance remains the same in the radial direction. In the present case, however, the particle volume fraction determines both the lattice constant and the bandgap position, because the nearest neighbor interparticle distance is constant without shearing. We can estimate the bandgap position l using Bragg’s equation for a normal-incident beam impinging on a (111) plane of an fcc structure:

Bioinspired Holographically Featured Superhydrophobic and Supersticky Nanostructured Materials

In this Letter, we present an intriguing method for fabricating polymeric superhydrophobic surfaces by reactive-ion etching of holographically featured three-dimensional structures. Using the proposed strategy, we generated both lotus and gecko surfaces by simply controlling the incident angle of the laser beam during holographic lithography. The adhesion force of the gecko-state superhydrophobic surfaces was the highest yet reported for an artificial superhydrophobic surface. The well-controlled patterns enable an in-depth understanding of superhydrophobic and superadhesive surfaces. In particular, the present observations provide direct evidence of a high adhesive force resulting from surface-localized wetting, which is quite different from previously suggested mechanisms.

High-fidelity optofluidic on-chip sensors using well-defined gold nanowell crystals.

Recent advances in nanofabrication techniques have enabled the creation of various metallic nanostructures in order to engineer the location and properties of electromagnetic hot spots in a controlled manner. However, most previous methods usually require complicated and time-consuming techniques, and the integration of metallic nanostructures into simple, low-cost devices for chemical or biological sensing is still challenging. Here, we report a promising new strategy for the fabrication of large-area gold nanowell arrays with novel geometric features that makes use of the trapping of self-assembled colloidal particles on a polymer surface. Through both systematic experimental and theoretical analysis, we confirm that the strong plasmon resonances of the proposed nanowell structures are associated with localized surface plasmon resonance (LSPR) on the brims of the nanoholes in the top gold films as well as in the bottom gold disks. In addition, we demonstrate a novel optofluidic platform with built-in subwavelength nanowell arrays that exhibits strong plasmon resonances within microfluidic chips. In our optofluidic systems, the plasmon coupling between the brims and the disks of nanowells makes the plasmon resonance more sensitive to surrounding materials. The dependence of the plasmon resonance on the refractive index of the surrounding medium is found to be as high as 570 nm RIU(-1) (refractive index units). These data lead to a figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 4.1.

Microfluidic fabrication of microparticles with structural complexity using photocurable emulsion droplets

Polymeric microparticles with hexagonal surface patterns comprising of colloids or dimples were fabricated using photocurable emulsion droplets. Colloidal silica particles within the interior of the photocurable emulsion droplets formed two-dimensional (2D) crystals at the droplet surface by anchoring on the emulsion interface, and the resulting composite structures were captured by rapid photopolymerization. A microfluidic device composed of two coaxial glass capillaries was used to generate monodisperse microparticles, with the evolution time determining the area of the anchored colloidal silica particles on the microparticle that was exposed to the continuous phase. The exposed region of silica particles could be modified by the introduction of desired functional groups such as dye molecules through simple chemical reaction with a silane coupling agent. This ability to modify the surface should prove useful in many applications such as chemical or biomolecular screening and colloidal barcoding systems.

Photothermolysis of immobilized bacteria on gold nanograil arrays

Photothermolysis technique via array of gold nanograils had been developed by illuminating near infrared laser light onto captured bacteria in metal nanostructure. The strong electromagnetic field enhancement at the sharp edges of the gold nanograils produced local heating that was sufficient to break the thick cell walls of the gram-positive Staphylococcus aureus cells within a short time. Individual cells in the nanograil array can be selectively lysed by adjusting the laser scanning area to the micrometer scale.