Flexible Polymer Colloidal‐Crystal Lasers with a Light‐Emitting Planar Defect

Interest in photonic crystals (PCs) is increasing from both the fundamental and technological viewpoints of photonics research because of the prospects PCs have for manipulating the flow of light at will. PCs are dielectric materials having spatial structures with a periodic scale comparable to that of electromagnetic waves, leading to the appearance of forbidden regions for photons in the dispersion spectrum; these regions are nowadays known as photonic band-gaps (PBGs). Novel PC systems could cause some intriguing optical phenomena such as the strong localization of photons, sharp bending of the light direction, and suppression or enhancement of light emission. So far, there have been many reports on a wide variety of methodologies with regard to the fabrication of 3D PC structures by using lithographic and selective etching, multiphoton polymerization, and holographic techniques. For optoelectronic applications, it is important to construct 3D PC structures with PBGs in the visible range. The periodicity in the PC structures should be designed on a sub-micrometer scale that is compatible with the wavelength of light. However, the top-down fabrication method cannot be applied to the visible-range PBG structures in a straightforward way. This is because the top-down method might encounter some serious problems that arise from the relative difficulty in preparing visible-range PBGs: the limitation of the optical resolution and the high cost of the method. In this context, colloidal crystals (CCs)—highly ordered 3D architectures of colloidal particles of polymers, silica, and so forth—have received tremendous interest as one of the alternative and facile fabrication techniques of 3D PC structures with visible-range PBGs. The monodispersed microparticles have an intrinsic capability to assemble the face-centered cubic (fcc) lattice structures on the substrate from the suspension media in a spontaneous manner, which is known as bottom-up processing. In other words, we do not require any specific facilities for fabricating the 3D PC structures. When the particle diameter corresponds to several hundred nanometers, the PBG of uniform CC structures can be observed as the Bragg reflection of visible light. Significant progress in this research area has established not only various fabrication techniques for high-quality CCs, but also their versatile application to optical sensing devices with external stimuli. Currently, more emphasis in the CC research field is placed on the design and introduction of specific point, channel, and planar defects within the 3D CC structures for the creation of localized points and pathways for photons in minute optical systems. Specific defect structures at the sub-micrometer scale can be fabricated by using multiphoton-induced polymerization, the Langmuir–Blodgett technique, chemical-vapor deposition, layer-by-layer assembly, and photoresist templates. With regard to the planar defects, recent investigations have revealed the emergence of a localized state of photons within the PBGs, which is known as the defect mode, by introducing non-light-emitting layers between the CC structures. This situation motivated us to demonstrate the introduction of a light-emitting layer as a specific planar defect inside the CC structure for novel photonic devices. Previously, Xia and Fudouzi developed a simple procedure to obtain uniform and large-size CC films of polystyrene (PS) microparticles embedded in poly(dimethylsiloxane) (PDMS) elastomer. The CC films of PS/PDMS possess unique capabilities for not only the desired tuning of the reflection band by adding curable PDMS oligomers, but also the on-demand preservation of the shifted reflection band by subsequent thermal polymerization. Taking advantage of these prominent features, we present herein a new potential of high-quality CC films for low-threshold laser applications by optical excitation. Figure 1 shows the schematic illustration of the designed CC laser (CCL) device structure. This structure is very simple. When a light-emitting polymer layer is introduced between a pair of CC films as a planar defect, highly efficient laser-feedback occurs because of the PBG effect of the CC films. Very recently, Foulger and co-workers demonstrated the lasing action by combining a hydrogel CC film on one side of a substrate with a commercial metallic mirror on the other side. As will be seen below, we successfully demonstrate here the first example of flexible CCL devices created using all-polymer materials. C O M M U N IC A IO N