The effects of the stop band (SB) in colloidal photonic crystals composed of silica spheres containing Eu3+‐ and Tb3+‐doped yttria nanoparticles are analysed. Reflection and transmission spectra indicate movement of the stop band, due to the 111 series of planes, towards shorter wavelengths with increasing angle of observation. The profile of the emission spectra is modified by the presence of the SB depending on the angle of measurement. Such a modification is more effective for a narrow emission band and it is thus more evident in the case of Tb3+ than Eu3+. An angular effect is also observed in the lifetime, which presents two maxima and one minimum. In the case of Tb3+ the maxima are at observation angles of 35 and 50°, and the minimum at 45°. We attribute this behaviour to penetration of the excitation beam at 475 nm modulated by the stop band. The ions excited in this way emit from different depths in the crystal, and therefore their lifetime will be affected differently by the same stop band, depending on the thickness of the crystal that must be crossed. Eu3+ shows a similar but less pronounced effect for two reasons: first, the main stop band (due to the 111 planes) is not effective at the excitation wavelength of 392 nm; second, the broadness of the Eu3+ emission is comparable to the width of the SB, and a decrease in the transition rate at the wavelength of the SB maximum is compensated by an increase at the sides of the SB.相似文献
Advances in zeolites research emerging from interdisciplinary efforts have opened new opportunities beyond conventional applications. Colloids drive much current research owing to their distinct collective behaviors, but so far, using zeolites as a colloidal building block to construct ordered superstructures remains unexplored. Herein we show that self-assembly of colloidal zeolite LTA superball (ZAS) by tilted-angle sedimentation forms macroscopic films with micro-mesoporosity and 3D long-range periodicity featuring a photonic band gap (PBG) that is tunable through the superball geometry and responds reversibly to chemical vapors. Remarkably, self-assembly of ZAS at elevated temperature forms 3D chiral photonic crystals that enable negative circular dichroism, selective reflection of right-handed circularly polarized (CP) light and left-handed CP luminescence based on PBG. We present a novel class of functional colloids and zeolite-based photonic crystals with the ability to manipulate light in several ways. 相似文献
Perovskite nanocrystals (NCs) have revolutionized optoelectronic devices because of their versatile optical properties. However, controlling and extending these functionalities often requires a light‐management strategy involving additional processing steps. Herein, we introduce a simple approach to shape perovskite nanocrystals (NC) into photonic architectures that provide light management by directly shaping the active material. Pre‐patterned polydimethylsiloxane (PDMS) templates are used for the template‐induced self‐assembly of 10 nm CsPbBr3 perovskite NC colloids into large area (1 cm2) 2D photonic crystals with tunable lattice spacing, ranging from 400 nm up to several microns. The photonic crystal arrangement facilitates efficient light coupling to the nanocrystal layer, thereby increasing the electric field intensity within the perovskite film. As a result, CsPbBr3 2D photonic crystals show amplified spontaneous emission (ASE) under lower optical excitation fluences in the near‐IR, in contrast to equivalent flat NC films prepared using the same colloidal ink. This improvement is attributed to the enhanced multi‐photon absorption caused by light trapping in the photonic crystal. 相似文献
Oblique propagation of light through a planar layer of a cholesteric liquid crystal (CLC) is solved by Ambartsumian’s modified layer addition method. Two cases are considered, namely, the case when dielectric boundaries have a minimum influence on light transmission and the case when the CLC layer is in a vacuum. It is shown that in the first case a total internal reflection can happen for the fast eigen-mode at large incidence angles. New important features of light reflection (transmission) spectra, photonic density of states and accumulated energy density in the CLC layer are studied. The light localisation peculiarities in the CLC layer at oblique incidence is investigated too. It is shown that the light localisation for both the long-wavelength edge mode and the short-wavelength edge mode changes differently when the incidence angle changes. The obtained results can be used in the design of low-threshold lasers, in solar cell systems, in chiral photonics, in systems strongly absorbing light at certain wavelengths and when designing systems with absorption suppression created on the base of absorbing media, etc. 相似文献
ABSTRACT Blue phase liquid crystals are soft 3D photonic crystals in which the liquid crystal molecules self-assemble to form a cubic structure with lattice spacing of a few hundred nanometers resulting in selective reflection of colours in the visible spectrum. The corresponding wavelength or the ‘photonic band gap’ can be tuned using various external stimuli such as thermal, electric, magnetic and optical fields. Here, we report efficient tuning of photonic band gap by utilising the combination of electric and optical fields in a blue phase liquid crystalline system. The studies indicate that the chirality of the medium has a direct bearing on the direction of the wavelength shift and the extent of the photonic band gap tunability. More importantly, the synergistic effect of the two fields helps in reversible tuning of the band gap. 相似文献
ABSTRACTWe study the optical properties of a cholesteric liquid crystal doped with a fluorescent dye in the regime of highly distorted helix without full helix unwinding. The distortion was achieved by applying a pulsed AC electric field, perpendicular to the helix axis. If the pulse is in the millisecond range, the helix is deformed but keeps its original pitch even for electric fields higher than the theoretical critical field for helix unwinding. In this field regime, very pronounced high-order photonic band gaps are observed, in agreement with our calculations. We theoretically explore the possibility of obtaining viable laser emission at the second-order photonic band gap, and experimentally find that lasing is not only possible but has a figure of merit similar to that of the usual laser at the main-gap region. Therefore, electric-field-induced high-order photonic band gaps are potentially useful for multiline laser applications. 相似文献
ABSTRACTWe demonstrate nematic and cholesteric liquid crystal (LC) gyroids and show their photonic properties as photonic crystals by using numerical modelling. The LC gyroids are designed as composite optical materials, where we take one labyrinth of passages to be a solid dielectric, whereas the other (complementing) labyrinth of passages is taken to be filled by chiral or achiral nematic LC, with the intermediate gyroid surface imposing homeotropic (perpendicular) surface anchoring. The nematic inside the gyroid matrix is shown to exhibit a variety of possible orientational profiles which are characterised by complex networks of topological defects – from ordered, semi-ordered, to completely disordered. The diversity of possible nematic states is shown to lead to a rich structure of photonic bands, which can be tuned by the LC volume fraction and the cholesteric pitch, including control over full – direct and indirect – band gaps. 相似文献
We described the engineering and characterization of photonic colloidal crystals based on silica spheres with incorporated YVO4:Eu luminescent nanoparticles. These structures exhibit strong angle-dependent luminescent properties. The controlled incorporation of a planar defect in the periodic structures gives rise to the creation of a pass band in the pseudo-gap. In the energy range of this pass band, we observed a strong increase in combination with a sharp width of the emission spectrum, which opens new possibilities for the design of low-threshold and/or single mode photonic crystal lasers. 相似文献
It is shown that narrowband one‐dimensional photonic crystals can be fabricated from polymeric materials using laboratory scale layer‐multiplying coextrusion technology. The tuning of the photonic bandgap is demonstrated with films that selectively filter different regions of the visible electromagnetic spectrum. The layer uniformity of the photonic crystals is evaluated by comparing the measured UV‐vis transmission spectra with model simulations, and is independently confirmed with atomic force microscopy. Assemblies of alternating polystyrene and poly(methyl methacrylate) layers exhibit a narrow reflection band with close to 100% reflection in good agreement with the prediction for uniform layers. The flexibility of the process is demonstrated by the fabrication of elastomeric one‐dimensional photonic crystals. It is anticipated that this technology will enable the rapid and facile realization of new polymeric optical devices.