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: