Self-assembled Photonic Crystals Research Articles

Fano-type resonances in the reflectance spectra from dense colloidal films

Reflectance spectra at normal incidence were recorded from different latex applications during their film formation processes at ambient temperature. Besides oscillations in the reflected spectra due to two-wave interference phenomenon in an effective film, a clear asymmetric Fano-like resonance appeared during long periods of time in some of the latex films studied. We develop an optical model to explain all the observed interference phenomena and elucidate in simple terms the underlying mechanism of the observed Fano resonances. The model reproduces qualitatively very well all the features in the experimental reflectance spectra. We find that the Fano resonances arise due to the interference between the surface reflection and a Bragg reflection from self-assembled photonic crystals within the film.

Enhancement of light collection through flexible polymeric films patterned using self-assembled photonic crystals

Flexible nanopatterned polymeric thin films are fabricated by a simple and cost-effective soft imprint lithography technique. The use of low-cost self-assembled photonic crystals as structured masters is the highlight of this work. Two types of polymeric films, with and without a dye, are patterned. Both have shown very good anti-reflection properties and the dyed films have also shown a polarization-independent enhancement in the absorption for the normal incidence of light. The structures with smaller features have shown better anti-reflection properties. Angle-dependent reflection measurements show an improved reduction in reflection at larger incidence angles. A maximum reduction in reflection of 11% is obtained from the patterned film in comparison to the bare glass substrate when the light is incident at an angle of 60°. This is very useful in photovoltaic devices as wide-angle anti-reflection coating. The reflection calculated using the effective medium approximation supports the experimental results.

Improving brush polymer infrared one-dimensional photonic crystals via linear polymer additives.

Brush block copolymers (BBCPs) enable the rapid fabrication of self-assembled one-dimensional photonic crystals with photonic band gaps that are tunable in the UV-vis-IR, where the peak wavelength of reflection scales with the molecular weight of the BBCPs. Due to the difficulty in synthesizing very large BBCPs, the fidelity of the assembled lamellar nanostructures drastically erodes as the domains become large enough to reflect IR light, severely limiting their performance as optical filters. To overcome this challenge, short linear homopolymers are used to swell the arrays to ∼180% of the initial domain spacing, allowing for photonic band gaps up to ∼1410 nm without significant opacity in the visible, demonstrating improved ordering of the arrays. Additionally, blending BBCPs with random copolymers enables functional groups to be incorporated into the BBCP array without attaching them directly to the BBCPs. The addition of short linear polymers to the BBCP arrays thus offers a facile means of improving the self-assembly and optical properties of these materials, as well as adding a route to achieving films with greater functionality and tailorability, without the need to develop or optimize the processing conditions for each new brush polymer synthesized.

Improving Light Extraction of Organic Light-Emitting Devices by Attaching Nanostructures with Self-Assembled Photonic Crystal Patterns

A single-monolayered hexagonal self-assembled photonic crystal (PC) pattern fabricated onto polyethylene terephthalate (PET) films by using simple nanosphere lithography (NSL) method has been demonstrated in this research work. The patterned nanostructures acted as a scattering medium to extract the trapped photons from substrate mode of optical-electronic device for improving the overall external quantum efficiency of the organic light-emitting diodes (OLEDs). With an optimum latex concentration, the distribution of self-assembled polystyrene (PS) nanosphere patterns on PET films can be easily controlled by adjusting the rotation speed of spin-coater. After attaching the PS nanosphere array brightness enhancement film (BEF) sheet as a photonic crystal pattern onto the device, the luminous intensity of OLEDs in the normal viewing direction is 161% higher than the one without any BEF attachment. The electroluminescent (EL) spectrum of OLEDs with PS patterned BEF attachment also showed minor color offset and superior color stabilization characteristics, and thus it possessed the potential applications in all kinds of display technology and solid-state optical-electronic devices.

Programmable Anisotropic Microparticles for Self-Assembly Applications

AbstractColloids with anisotropic shape and properties can enable the assembly of advanced materials otherwise not attainable by microfabrication. In this study, we present a convenient method using common microfabrication tools to generate a diverse array of non-spherical microparticles with well-defined shapes, sizes, electromagnetic properties for self-assembly applications. Projection photolithography onto SU-8 photoresist enabled the production of large aspect ratio microparticles such as cubes, cuboids, cylinders, hexagonal prisms, and parallelepipeds. We characterized these particles to confirm their anisotropic shape and size monodispersity. Fluorescent stains (e.g., Nile red) were mixed into the photoresist prepolymer to enhance the visualization of particle orientation. Particles designed for passive self-assembly were prepared by conventional photolithographic techniques. Particles designed for active assembly were then decorated with metallic patches in precise locations along the surface (e.g., top, side or multiple sides) using electron beam metal evaporation. This metal deposition process can enable orientational control of particles during their assembly in directed fields. After fabrication, large particles (e.g., 1,000 µm3) were released from the substrate via gentle sheer forces, whereas small particles (e.g., 10 µm3) were released by the dissolution of a sacrificial layer underneath the SU-8. Suspending the particles in water with surfactant (or other suitable solvents) provided amenable conditions for their assembly in static or dynamic systems. These conventional methods have the potential to catalyze new research in the fabrication and assembly of anisotropic patchy particles with controllable properties for the hierarchical development of self-assembled micromirrors, biosensors, and photonic crystals as examples.

Tuneable multicoloured patterns from photonic cross-communication between cholesteric liquid crystal droplets

Cholesteric liquid crystal droplets are self-assembled spherical photonic crystals. Multiple droplets in the same plane communicate optically, giving rise to unique patterns of circularly polarised and strongly coloured spots. We explain the phenomenon and outline routes to application.

Pore size dependence of self-assembled type photonic crystal on dye-sensitized solar cells efficiency utilising Chlorine e6

There are few reports on photoelectric conversion efficiency using naturally-occurring dyes for dye-sensitized solar cells (DSSC). This is because the matching with an excited electronic level of naturally-occurring dye to the conduction band of semiconductor is problematic; the excited electrons are easily relaxed to the steady state with fluorescence or heat emission. We examined the fluorescence inhibition effect of a self-assembled photonic crystal using Chlorine e6 dye. Chlorine e6 is derived from chlorophyll and has a long excited electron lifetime. We prepared TiO2 inverse opals with various particle sizes by liquid phase deposition and described their effect on DSSCs with regard to structural, optical and electrochemical properties. In addition, we explored the implications of fluorescence lifetime measurements relative to the photonic band diagram and the amount of adsorbed dye. Although the main factor affecting the external photoelectric conversion efficiency was the diffusion resistance of the electrolyte and the contact resistance between TiO2 interfaces, the possibility that the dye fluorescence lifetime, i.e. the photonic band structure, can affect the internal quantum efficiency per one dye molecule was also investigated.

Three-dimensional self-assembled photonic crystals with high temperature stability for thermal emission modification

Selective thermal emission in a useful range of energies from a material operating at high temperatures is required for effective solar thermophotovoltaic energy conversion. Three-dimensional metallic photonic crystals can exhibit spectral emissivity that is modified compared with the emissivity of unstructured metals, resulting in an emission spectrum useful for solar thermophotovoltaics. However, retention of the three-dimensional mesostructure at high temperatures remains a significant challenge. Here we utilize self-assembled templates to fabricate high-quality tungsten photonic crystals that demonstrate unprecedented thermal stability up to at least 1,400 °C and modified thermal emission at solar thermophotovoltaic operating temperatures. We also obtain comparable thermal and optical results using a photonic crystal comprising a previously unstudied material, hafnium diboride, suggesting that refractory metallic ceramic materials are viable candidates for photonic crystal-based solar thermophotovoltaic devices and should be more extensively studied.

Inhibition And Enhancement Of Spontaneous Emission Using Photonic Band Gap Structures

We discuss experimental results demonstrating the inhibition and enhancement of spontaneous emission of dye molecules embedded in a nanophotonic structure. This is achieved in our all-solid self-assembled photonic crystals consisting of dyedoped polystyrene spheres. Our samples exhibit well-resolved photonic stop gap with high reflectivity and photonic strength. Laser induced emission experiments reveal an inhibition of ~ 70% of emission intensity in the photonic stop gap wavelength range. Also, we discuss the enhancement of spontaneous emission intensity near the blue side of the stop gap. Our results have implications in photonic devices, such as, low-threshold lasers and efficient lighting devices. Copyright © 2013 VBRI press.

Spectral properties of self-assembled polystyrene nanospheres photonic crystals doped with luminescent dyes

Three-dimensional ordered photonic crystals have been fabricated on solid substrates with a self-assembling method starting from a suspension of dye-doped polymeric nanospheres in water. These photonic crystals showed angle-dependent stop band for light transmission and, correspondingly, Bragg reflection peak due to the photonic crystal lattice. Polystyrene nanoparticles of 306nm and 288nm diameter, respectively in the case of the Rhodamine B and Fluored dye, were used to obtain self-assembled photonic crystals. They show 40% reflectance at 610nm for Rhodamine B and 35% reflectance at 574nm for Fluored doped materials. The size of the spheres calculated by Bragg’s law are in good agreement with those evaluated by scanning electron microscopy (SEM). The appropriate choice of sphere diameters results in an overlap between the photonic stop-band and the dye emission spectrum. The photonic crystals showed angle-dependent suppression of spontaneous emission of the dye in the wavelength range of the photonic stop band and enhancement near the band edge. In reflection geometries, spectral narrowing and directional emission, all indicative of stimulated emission, were observed from the active photonic crystal matrix. The results of laser induced emission studies on the dye doped photonic crystals are presented.

Propagation of vortex beams through self-assembled photonic crystal

This study describes a new and simple approach to produce multiple optical vortices (OVs) using three-dimensional polymeric photonic crystal (PhC). The crystals are fabricated by well-known inward-growing self-assembly technique using polystyrene micro-particles of particle sizes 990 and 1900 nm. This fabrication technique provides the cubic lattice arrangement to the spheres with the ⟨1 1 1⟩ plane parallel to the substrate. When a Gaussian beam from a He–Ne laser at 633 nm is diffracted through the crystal of particle size 990 nm, six bright spots along with a central spot are observed on the screen. Diffractions are observed up to third-order in the case of 1900 nm particle size PhC. Multiple OVs arise when vortex beam with topological charge 1 is diffracted through such a crystal. The diffraction pattern shows similar arrangement of bright spots as for the Gaussian beam except now the diffracted spots contain the property of incident OV, i.e. the central dark core. The diffraction is recorded using a charge coupling device camera.

Self-assembled block copolymer photonic crystal for selective fructose detection

The use of one-dimensional photonic crystals fabricated from a self-assembled lamellar block copolymer as a sensitive and selective fructose sensor is investigated. The polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP) films are functionalized with 2-(bromomethyl)phenylboronic acid. The boronic acid moiety confined within the lamellar morphology can reversibly bind to sugars such as fructose, imparting the photonic properties of the PS-b-P2VP film. The films exhibit a detection limit of 500μM in water and 1mM in phosphate buffered saline. Exposure to a 50mM solution of fructose invokes a highly visible color change from blue to orange. The films are also able to selectively recognize and respond to fructose in competitive studies in the presence of glucose, mannose and sucrose.

Analysis of Lasing in Dye-Doped Photonic Crystals

Recently, we experimentally demonstrated room-temperature lasing of self-assembled opal photonic crystal (PhC) made of rhodamine-B-doped polystyrene colloids. Here, we explain this experimental observations by analyzing the phenomenon of light amplification in dye-activated PhCs via a complex-valued permittivity of the colloids. We show that the lasing is facilitated by the enhanced distributed feedback due to the reduced group velocity in the vicinity of the photonic band edge. This simple approach to the analysis of PhC lasing behavior allows us to calculate the lasing wavelength in close agreement with the experimental value. It also enables the estimation of gain coefficient required for lasing and may prove useful in design of compact PhC-based lasers.

Transmission Spectrum of Two-Dimensional Self-Assembled SiO<sub>2</sub> Photonic Crystals

Controlling the photon propagation in materials by photonic crystals lies at the heart of quantum optics. The motivation of our work is to investigate the direct control of the photon confinement in the self-assembled two-dimensional SiO2 photonic crystals. Here we demonstrate the realization of transmitted light controls in the self-assembled two-dimensional SiO2 PCs. Our experimental results agree well with the theoretical calculations on the photonic band gaps in the spectral region of 400-1100 nm. These results present experimental evidence on the direct control of the photon confinement in the self-assembled two-dimensional SiO2 PCs.

Optical studies of random disorder of colloidal photonic crystals and its evolution in evaporation induced self-assembly

Self-assembled photonic structures have been under theoretical and experimental study for decades, whereas previous theories on optical properties were mainly concerned with perfect structure or some certain limited kinds of disordered photonic crystals (PCs), making them unsuitable for characterizing the real self-assembled PCs. In order to improve our understanding of the mechanism of self-assembly and provide more crucial clues to further grow perfect crystals, we extended previous widely used scalar wave approximation (SWA), making it be able to characterize long-range disorder (β) and short-range disorder (α) in PCs synthetically in a simple and effective way. Excellent agreement with in situ observed reflectance of evaporation induced self-assembled colloidal photonic crystals (CPCs) was obtained, demonstrating that the introduction of the parameters α and β in SWA can successfully characterize the disorder in self-assembled CPCs. Furthermore, extended SWA was further used to study the disorder formation in self-assembly, and it was found that during growing stage both β and α drop down, whereas in drying stage β stays nearly unchanged while α increases significantly. It turned out that the growing stage of self-assembly is a stage when the structure transforms from disordered to ordered one, and growth induced disorder mainly arises in drying stage. The results obtained provide an insight into the growth mechanisms of self-assembly and theoretical basis for characterizing optical properties of disordered PCs.

Self-assembled organic and polymer photonic crystals for laser applications

This article describes an overview of recent developments in fabrication and uses of self-assembled photonic crystals (PCs) of organic and polymer materials, such as chiral liquid crystals (CLCs) and colloidal crystals (CCs), for laser applications. Both CLCs and CCs have intrinsic capabilities to spontaneously assemble 1D-PC and 3D-PC structures, respectively. When a periodic length in the PC structures of CLCs and CCs corresponds to several hundred nanometers of the light wavelength, the photonic band-gaps (PBGs) can be visualized as Bragg reflection colors. When combining fluorescence dyes in the CLCs and CCs, the stimulated laser action at PBG band edge(s) or within the PBG wavelength can be generated by optical excitation. Moreover, the optically excited laser action is controllable by external stimuli due to the self-organization of CLCs and CCs. This review highlights not only the research backgrounds of CLC and CC structures as PCs, but also the experimental results of their versatile soft and tunable laser applications. We believe that a wide variety of CLC and CC structures will have leading roles in the next generation of optoelectronic devices of organic and polymer materials. This review describes the developments of self-assembled photonic crystals (PCs) of chiral liquid crystals (CLCs) and colloidal crystals (CCs) for laser applications. Both CLCs and CCs have intrinsic capabilities to spontaneously assemble 1D-PCs and 3D-PCs, respectively. By combining fluorescence dyes in the structures, the stimulated laser action can be generated by optical excitation. Moreover, the optically excited lasing is controllable by external stimuli due to their self-organization. This review highlights the research backgrounds of CLCs and CCs, as well as the experimental results of soft and tunable laser applications.

Investigation of local structural properties of self-assembled photonic crystals by optical diffraction

An experimental investigation of optical diffraction in three-dimensional self-assembled photonic crystals is herein presented. The crystals are synthesized using polystyrene colloidal particles of four different sphere diameters and diffraction is studied under three different wavelength illuminations. The differences in the relative intensities of diffracted spots indicate the twin face-centered cubic lattice arrangement of the crystals. This is confirmed by monitoring the variation in intensity of diffracted beam with respect to the incident angles. Moreover, the diffraction angle versus the incidence angle plot provides us with an estimate of the colloidal sphere diameter.

Engineering disorder in three-dimensional photonic crystals

We demonstrate the effect of introducing controlled disorder in self-assembled three-dimensional photonic crystals. Disorders are induced through controlling the self-assembling process using an electrolyte of specific concentrations. Structural characterization reveals increase in disorder with increase in concentrations of the electrolyte. Reflectivity and transmittance spectra are measured to probe the photonic stop gap at different levels of controlled disorder. With increase in disorder the stop gap is vanished and that results in a fully random photonic nanostructure where the diffuse scattered intensity reaches up to 100%. The estimated scattering mean free path shows significant reduction for photonic crystals with 100% controlled disorder as compared to those with 0% controlled disorder. Our random photonic nanostructure is unique in which all scatters have the same size and shape. Therefore, we observe the resonant characteristics in the multiple scattering of light.

Photonic-band-edge-induced lasing in self-assembled dye-activated photonic crystals

We report experimental demonstration of photonic band-edge lasing in three-dimensionally ordered self-assembled photonic crystals consisting of rhodamine-B dye doped nanospheres of diameter 295 nm. Our all-solid photonic crystal shows a well-resolved photonic stop gap in the visible region. Laser-induced emission experiments reveal more than 51$%$ inhibition in spontaneous emission intensity of dye molecules within the stop gap and an enhancement near the blue side of the stop gap. With increase in incident pump energy, we achieve photonic-band-edge-induced lasing at room temperature with a lasing threshold of 0.7 mJ. We explain the origin of lasing as due to the enhancement of local density of states near the band edges and the light field distribution within the photonic crystal structure. Our results indicate that such all-solid, dye-activated self-assembled nanostructures are a promising candidate for realizing photonic devices.

Three-Dimensional Structure and Defects in Colloidal Photonic Crystals Revealed by Tomographic Scanning Transmission X-ray Microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.