콜로이드 입자의 주기적 적층 구조는 독특한 광특성을 보인다. 이는 광결정 물질의 광밴드갭에 의해 나타나는 현상으로, 특정 파장의 빛을 선택적으로 반사하거나, 광결정 내부의 광자의 상태 밀도를 변화시킨다. 이러한 물질의 특성은 콜로이드 광결정을 디스플레이 및 보안 물질을 비롯하여 광소자 개발 등 다양한 분야에 폭넓게 응용될 수 있게 한다. 그러나 현재까지 개발된 결정화 기술은 대부분 그 실용적 사용에 문제가 있었다. 본 연구실에서는 콜로이드 광결정의 실용적 플랫폼을 개발하고, 이를 통해 다양한 광학적 응용을 모색한다. 특히, 기존의 기술로 달성하기 힘든, 초소형 분광소자 및 보안물질 개발, 반사형 디스플레이소자 개발 등을 집중적으로 연구한다.
The dispersion stabilities and rheological properties of colloidal materials have been widely studied during the last two centuries. Recent advances in colloidal synthesis have accelerated the study of colloids—not only for their monodispersity, but also because many properties of the colloidal particles, including density, surface charge, and material affinity, can be controlled by varying the synthetic scheme. Even the design of particles with anisotropic shapes, internal structures, or chemical patterns can be achieved. Based on colloidal particles with controlled properties, the crystallization into various lattices has been studied for two main applications, namely, the attainment of ‘visible’ models for atomic or molecular assemblies and the development of photonic bandgap materials. Monodisperse colloidal particles with high surface charge density dispersed in a polar medium spontaneously form non-close-packed crystals, a process that is induced by the repulsive interparticle potential. Depending on the volume fraction of the colloids and the strength of the repulsion, the particles appear as either face-centered cubic (fcc) or body-centered cubic (bcc) structures in the thermodynamic equilibrium. On the other hand, bidisperse colloidal systems with oppositely charged colloids enable the preparation of various crystal lattices, which have many similarities with atomic or molecular systems, although the valences of atoms are not consistent with those of colloidal systems. In addition to these similarities in regard to the formation of crystals, bandgap properties are also observed in both atomic and colloidal crystals. At the atomic scale, because crystals exhibit a periodic modulation of the potential for the propagation of electrons, they may affect the conductivity of the electrons and sometimes even prevent their propagation at certain energy levels. It is well known that semiconductors have an electronic bandgap between the valence and conduction bands. Analogously, if the periodicity of a colloidal crystal lattice is comparable to the wavelength of light, the lattice will interact with the electromagnetic waves and induce a photonic bandgap. Photons with an energy on the order of this gap cannot propagate through the crystal. In this case, the crystal is a "photoniccrystal"
Photonic pigments with controlled iridescence
Crystalline or glassy materials made of colloidal nanoparticles show distinctive photonic effects; the crystals exhibit sparkling colors with strong iridescence, while the glasses show noniridescent colors. Both colors are the results of constructive interference of the reflected light by the nonadsorbing nanostructures. Such colored materials have potential applications as nonfading colorants in reflective color displays, optical sensors, coatings, and cosmetics. All of these applications require granular format of the nanostructures; however, precise control of the nanostructures from amorphous to crystalline over the submillimeter length scale remains challenging. Here, we present micrometer-level control of photonic nanostructures confined in microcapsules through osmotic-pressure-mediated concentration. We encapsulate aqueous suspensions of colloidal particles using double-emulsion drops with ultrathin layers of photocurable resin. The microcapsules are then isotropically compressed by imposing a positive osmotic pressure difference that forces the water out through the thin resin membrane. We find that the internal nanostructure of our photonic microcapsules can be kinetically controlled from crystalline to amorphous; slow concentration in small pressure gradients yields colloidal crystals with sparkling color patterns, whereas fast concentration in large pressure gradients yields glassy packing with only short-range order, which show uniform color with little iridescence. By polymerizing the thin monomeric shell, we permanently fix these nanostructures. Our findings provide new insights into the design and synthesis of optical materials with controlled structural colors.
1. Shin-Hyun Kim, Jin-Gyu Park, Tae Min Choi, Vinothan N. Manoharan, and David A. Weitz, "Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules," Nature Communications, 5:3068 (2014)
2. Jin-Gyu Park, Shin-Hyun Kim, Sofia Magkiriadou, Tae Min Choi, Young-Seok Kim, Vinothan N. Manoharan, "Full-Spectrum Photonic Pigments with Non-iridescent Structural Colors through Colloidal Assembly ," Angewandte Chemie International Edition , 126, 2943-2947 (2014).
3. Tae Min Choi, Jin-Gyu Park, Young-Seok Kim, Vinothan N. Manoharan, and Shin-Hyun Kim, "Osmotic-Pressure-Mediated Control of Structural Colors of Photonic Capsules," Chemistry of Materials, 27, 1014−1020 (2015).
Display devices based on structural colors
One of the most promising and practical applications of 3D colloidal crystals is color pigments for displays. Reflection colors produced by light interference from regular lattice in colloidal crystals exhibit beautiful metallic luster as we can observe from opals, which can’t be achieved in transmission colors produced by optical filters in conventional liquid crystal display (LCD) devices. If the structural colors are used in display, users can see the clear display even under high intensity of light in daytime. In addition, power consumption can be significantly reduced because the devices use ambient light as a light source without backlight units. Therefore, many researchers have developed colloidal crystal-based color pigments for various types of displays ranging from portable devices to outdoor signboards or billboards of huge size. In order to make structural color displays, we have developed dynamic tuning method of reflection colors by controlling the lattice constant in photonic crystals and rotation-based display device of photonic microparticles of anisotropic optical properties.
1. Shin-Hyun Kim, Seog-Jin Jeon, Woong Chan Jeong, Hyo Sung Park, and Seung-Man Yang, “Optofluidic Synthesis of Electro-responsive Photonic Janus Balls with Isotropic Structural Colors,” Advanced Materials, 20, 4129-4134 (2008)
2. Shin-Hyun Kim, Se-Heon Kim, and Seung-Man Yang, “Patterned Polymeric Domes with 3D and 2D Embedded Colloidal Crystals using Photocurable Emulsion Droplets,” Advanced Materials, 21, 3771-3775 (2009)
3. Shin-Hyun Kim, Jong-Min Lim, Woong Chan Jeong, Dae-Geun Choi and Seung-Man Yang, "Patterned Colloidal Photonic Domes and Balls Derived from Viscous Photocurable Suspensions," Advanced Materials, 20, 3211-3217 (2008)
4. Tae Soup Shim, Shin-Hyun Kim, Jae Young Sim, Jong-Min Lim, and Seung-Man Yang, “Dynamic Modulation of Photonic Bandgaps in Crystalline Colloidal Arrays under Electric Field,” Advanced Materials, 22, 4494-4498 (2010)
Photonic crystal lasers and fibers
When the spontaneous emission spectrum of the dye is overlapped with band edge position of colloidal crystals, directional laser beam was generated under optical pumping. This is caused by stimulated emission at band edge position where photon density of state (DOS) is high and group velocity of light is low. We have studied lasing at bandedge of 3D photonic crystal film which is porous photonic film produced by nonclose-packed silica arrays as a template.
We have developed a new type of photonic crystal fiber using a bottom-up approach involving spontaneous crystallization of colloidal particles. To create colloidal photonic crystal fibers in a practical and reproducible fashion, colloids are crystallized spontaneously with repulsive potential dispersed in a photocurable resin, which is coated on the inner walls of microcapillaries under film-draining protrusion flow. Spontaneous crystallization and fast consolidation of the structures yield robust photonic crystals with excellent optical performance. The controlled dynamic deposition of film on the microcapillaries permits 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 (total internal refraction) regime.
1. Shin-Hyun Kim, Se-Heon Kim, Woong Chan Jeong, and Seung-Man Yang, “Low-Threshold Lasing in 3D Photonic Crystals Derived from Colloidal Assemblies,” Chemistry of Materials, 21, 4993-4999 (2009).
2. Shin-Hyun Kim, Hyerim Hwang, and Seung-Man Yang, "Fabrication of Robust Optical Fibers by Controlling Tsunami-Driven Film Drainage of Colloids in Capillaries," Angewandte Chemie International Edition, 51, 3661-3665 (2012)
New paradigm in Spectrometers
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 lights 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.
We have developed a novel strategy for creating adiabatic changes in the lattice constant of a 3D photonic crystal using colloidal particle diffusion in a photocurable medium. The variations in lattice constant created a color gradient that spanned the entire visible range. The repulsive interparticle potentials accelerated the colloid diffusion by a factor of O(105) relative to dilute hard-sphere colloids, and fast consolidation of the suspension medium captured and solidified the variations in the stop band position as a function of the diffusion distance to produce a chirped 3D photonic crystal (CPhC). By combining the CPhC functionality with a conventional CMOS sensor array, we demonstrated a miniaturized on-chip spectrometer system with a minimum resolution of 0.09 nm in the visible spectral range. Diffusion-driven structural transformations may open new avenues for engineering the optical properties of 3D photonic crystals.
1. Shin-Hyun Kim, Hyo Sung Park, Jae Hoon Choi, Jae Won Shim, and Seung-Man Yang, “Integration of Colloidal Photonic Crystals toward Miniaturized Spectrometers,” Advanced Materials, 22, 946-950 (2010)
2. Shin-Hyun Kim, Woong Chan Jeong, Hyerim Hwang and Seung-Man Yang, “Robust Chirped Photonic Crystals Created by Controlled Colloidal Diffusion,” Angewandte Chemie International Edition, 50, 11649-11653 (2011).