Color tunability of upconversion emission in YBO3: Yb, Er inverse opal

Upconversion (C) light-emitting photonic band gap materials (YBO₃: Yb, Er) with inverse opal structure were prepared by a self-assembly technique in combination with a sol-gel method. The effect of the photonic stop-band on the upconversion luminescence of Er³⁺ ions has been investigated in the YBO₃: Yb, Er inverse opals. Significant suppression of the green or red UC emission was detected if the photonic band-gap overlaps with the Er³⁺ ions emission band. We successfully achieved the color tuning of the UC optical properties of the inverse opal by controlling the structure of the photonic crystal.     

Infrared to visible upconversion luminescence in Er/Yb co-doped CeO2 inverse opal

Infrared to visible upconversion luminescence has been investigated in Er³⁺/Yb³⁺ co-doped CeO₂ inverse opal. Under the excitation of 980 nm diode lasers, visible emissions centered at 525, 547, 561, 660 and 680 nm are observed, which are assigned to the Er³⁺ transitions of ²H_11/2 → ⁴I_15/2 (525 nm), ⁴S_3/2 → ⁴I_15/2 (547, 561 nm), ⁴F_9/2 → ⁴I_15/2 (660 and 680 nm), respectively. The effect of photonic band gap on the upconversion luminescence intensity was also obtained. Additionally, the upconversion luminescence mechanism was studied. The dependence of Er³⁺ upconversion emission intensity on pump power reveals that it is a two-photon excitation process.     

Annealing effect on reflectivity spectra of opal photonic crystals

We analyze broadband visible reflectivity spectra of opal photonic crystals annealed in an argon atmosphere and air. Scanning electron microscopic examination has shown that the crystals consist of close-packed spheres with a mean diameter in the range 200–290 nm. Incorporation of even small amounts of carbon (?0.5 wt %) increases the density of the opal matrix. The presence of carbon shifts the Bragg reflection peak of the opal and increases its intensity.     

Significant reduction of upconversion emission in CaTiO3: Yb, Er inverse opals

Inverse opal photonic crystals of Yb³⁺, Er³⁺ co-doped CaTiO₃ (CaTiO₃: Yb, Er) were prepared using self-assembled polystyrene templates combined with the infiltration of sol-gel precursor. The influence of the photonic band gap on upconversion emission of Er³⁺ has been investigated in the CaTiO₃: Yb, Er inverse opals. Significant reduction of the upconversion emission was detected if the photonic band-gap overlaps with the Er³⁺ ions emission band.     

Patterning hierarchy in direct and inverse opal crystals

Biological strategies for bottom-up synthesis of inorganic crystalline and amorphous materials within topographic templates have recently become an attractive approach for fabricating complex synthetic structures. Inspired by these strategies, herein the synthesis of multi-layered, hierarchical inverse colloidal crystal films formed directly on topographically patterned substrates via evaporative deposition, or "co-assembly", of polymeric spheres with a silicate sol-gel precursor solution and subsequent removal of the colloidal template, is described. The response of this growing composite colloid-silica system to artificially imposed 3D spatial constraints of various geometries is systematically studied, and compared with that of direct colloidal crystal assembly on the same template. Substrates designed with arrays of rectangular, triangular, and hexagonal prisms and cylinders are shown to control crystallographic domain nucleation and orientation of the direct and inverse opals. With this bottom-up topographical approach, it is demonstrated that the system can be manipulated to either form large patterned single crystals, or crystals with a fine-tuned extent of disorder, and to nucleate distinct colloidal domains of a defined size, location, and orientation in a wide range of length-scales. The resulting ordered, quasi-ordered, and disordered colloidal crystal films show distinct optical properties. Therefore, this method provides a means of controlling bottom-up synthesis of complex, hierarchical direct and inverse opal structures designed for altering optical properties and increased functionality.     

Optical properties of three-dimensional magnetic opal photonic crystals

We have studied the optical properties of opal photonic crystals infiltrated with the M_0.35Zn_0.65Fe₂O₄ (M = Ni, Co) ferrites. The crystals consisted of amorphous SiO₂ nanospheres. The visible reflectivity spectra of the crystals were used to determine parameters of their photonic band gap and their refractive index.     

Emission of opal photonic crystals under pulsed laser excitation

Infiltration of opal with nonlinear optical materials is shown to markedly raise its emission intensity in the visible range under pulsed laser excitation. Evidence is presented for three-photon parametric scattering in both uninfiltrated and infiltrated globular photonic crystals, with excitation of “slow” photons in the visible range. Our results indicate that synthetic opal crystals can be used as photon traps for studying the emission spectra of organic and inorganic materials infiltrated in opal pores.     

Dispersion characteristics of water- and gold-infiltrated opal photonic crystals

This paper presents a theoretical analysis of conditions for the propagation of electromagnetic waves in a wide frequency range (from the infrared to ultraviolet spectral region) in synthetic opals infiltrated with water and gold nanoparticles. A dispersion equation is derived which describes the dispersion law of both “right-hand” (right-hand system of the [(E)vec]vec E, [(H)vec]vec H, and [(k)vec]vec k vectors) and “left-hand” (left-hand system of the [(E)vec]vec E, [(H)vec]vec H, and [(k)vec]vec k vectors) electromagnetic waves in the crystals. We have determined the dispersion characteristics of the refractive index and broadband reflectance of the opals, group velocity dispersion, and effective mass dispersion for phonons and polaritons. Theoretical results are compared to measured reflection spectra.     

Negative refraction in the visible range in water-infiltrated opal photonic crystals

A negative refraction effect has been found in opal photonic crystals in the visible range. We have calculated the dispersion branches of a photonic crystal and determined the position of its photonic band gap. The frequency range has been identified where the refractive index of the opal is negative. An experimental arrangement is proposed for focusing a light beam by passing it through a plane-parallel opal photonic crystal and experimental evidence is presented in favor of negative refraction in the visible range.     

Conversion of short-wavelength electromagnetic radiation in SiO2 opal photonic crystals

Reflection spectra of the (111) growth surface of opal photonic crystals differing in silica sphere diameter have been measured under illumination with narrowband ultraviolet and violet light from a laser and light-emitting diodes and with broadband light from a halogen lamp. We have found narrow strong bands differing in spectral position from the light from the short-wavelength excitation sources. The spectral position of these bands corresponds to that of photonic band gaps and is independent of excitation wavelength. The silica sphere diameter has no effect on the shape of the reflection band, and its position always correlates with that of the band gap of the opal. The present results demonstrate that exposure of a photonic crystal to short-wavelength radiation leads to conversion of the radiation to the spectral range of the band gap. The microscopic mechanism of the conversion process may involve three-photon parametric processes and amplification of the broadband photoluminescence due to structural defects in the silica matrix. Our results open up the possibility of creating new types of optically pumped solid gain media based on opal photonic crystals.     

Raman scattering study of NaNO2-infiltrated opal photonic crystals

We have studied light scattering in synthetic opal crystals infiltrated with ferroelectric sodium nitrite, NaNO₂, and have analyzed simple models for the energy band structure of photonic crystals. Expressions have been derived for the group velocity of photons whose energy is close to the photonic band gap. Our results indicate that the infiltration of photonic crystals with NaNO₂ markedly increases the Raman scattering intensity.     

Influence of dielectrics with light absorption on the photonic bandgap of porous alumina photonic crystals

In this work, the influences of dielectrics with light absorption on the photonic bandgaps (PBGs) of porous alumina photonic crystals (PCs) were studied. Transmittance spectra of porous alumina PCs adsorbing ethanol showed that all the PBGs positions red-shifted; however, the transmittance of the PBG bottom showed different trends when the PBGs were located in different wavelength regions. In the near infrared region, liquid ethanol has strong light absorption, and, with the increase in adsorption, the PBG bottom transmittance of porous alumina PCs first increased and then decreased. However, in the visible light region, liquid ethanol has little light absorption, and thus, with the increase in adsorption, the PBG bottom transmittance of porous alumina PCs increased gradually all the time. Simulated results were consistent with the experimental results. The capillary condensation of organic vapors in the pores of porous alumina accounted for the change in the PBG bottom transmittance. The nonnegligible light absorption of the organic vapors was the cause of the decrease in the transmittance. The results for porous alumina PC adsorbing methanol, acetone, and toluene further confirmed the influences of light absorption on the PBG bottomed transmittance.

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Low-temperature persistent afterglow in opal photonic crystals under pulsed UV excitation

Excitation of synthetic opal with 337.1-nm nitrogen laser pulses gives rise to a persistent afterglow, lasting 15 s at 10 K. The afterglow spectrum correlates with the emission spectrum of opal observed earlier under excitation with UV light-emitting diodes. The effect can be understood in terms of the peaks in the density of photon states near the edges of the photonic band gap in photonic crystals.     

Photoluminescence of terbium nitrate hexahydrate incorporated into pores of opal photonic crystals

We have measured photoluminescence (PL) and broadband reflection spectra of the surface of an opal photonic crystal (OPC) filled with terbium nitrate hexahydrate (TNH). The excitation sources used were a halogen lamp, lasers with emission wavelengths of 266 and 337 nm, and semiconductor light-emitting diodes (369 and 385 nm). It has been shown that resonance excitation of the TNH (Tb(NO₃)₃ · 6H₂O) filled OPC by pulsed laser light at a wavelength of 266 nm gives rise to superluminescence: an increase in the relative intensity of the 545-nm PL band, corresponding to the Tb^{3+ 5} D ₄–⁷ F ₅ transition. The efficiency of pumping the upper laser level of the OPC increases due to the increase in the density of photon states (Purcell effect) near the bandgap edge in the OPC. The lack of clear superluminescence under excitation by the other excitation sources is related to specific features of the absorption of incident light by the rare-earth ion and to the low output power of the cw light sources.     

Emission spectra of silver-infiltrated opal photonic crystals under excitation through optical fibers

The emission spectra of opal photonic crystals loaded with silver nanoparticles have been measured in a 180° geometry under UV and visible excitation. The spectra of silver-infiltrated opal under excitation through optical fibers are found to differ from the spectra of plain (uninfiltrated) opal: the infiltrated silver shifts the emission maximum to longer wavelengths and changes the shape of the spectrum. We have calculated the dispersion laws for two photonic bands and the corresponding frequency dependences of the refractive index for the photonic crystals studied.     

Spontaneous emission and lame shift in photonic crystals

Being motivated by the controversial results based on two dispersion models and Weisskopf–Wigner approximation (WWA), we introduce, for the first time to our knowledge, the position-dependent photon-atom interaction into the Green function method of the evolution operator and develop a universal theoretical approach to study spontaneous emission of atoms in photonic crystals (PCs). A position-sensitive generalized Lorentzian formalism (non-Lorentzian shape) for the decay of an excited atom in PCs is derived, and an exact numerical method for calculating the local coupling strength, proportional to the photonic local density of state (LDOS), is presented. For weak interaction PCs with pseudo gaps, the generalized Lorentzian formalism may be reduced to the famous Lorentzian spectrum. In this case, we introduce a lifetime distribution function for an assembly of atoms and find that it depends strongly on the atomic configuration in space, which clarifies successfully the tremendous discrepancy between different experiments. For the PCs with large full gaps, we find that the atomic position can fundamentally change the decay behavior of an excited atom: in strong interaction positions, the atomic decay is non-classical or exhibits an envelope-damped Rabi oscillation, while in weak interaction positions the WWA is valid. Recently, we also predicted giant Lamb shifts for hydrogen atoms in PCs, and revealed that in inhomogeneous electromagnetic environment, the dominant contribution to the Lamb shift comes from real photon emission, while the contribution from emission and reabsorption of virtual photon is negligible, in vast contrast with the case of free space where the virtual photon processes play a key role.     

Self-assembly route for photonic crystals with a bandgap in the visible region

Three-dimensional photonic crystals, or periodic materials, that do not allow the propagation of photons in all directions with a wavelength in the visible region have not been experimentally fabricated, despite there being several potential structures and the interesting applications and physics that this would lead to. We show using computer simulations that two structures that would enable a bandgap in the visible region, diamond and pyrochlore, can be self-assembled in one crystal structure from a binary colloidal dispersion. In our approach, these two structures are obtained as the large (Mg) and small (Cu) sphere components of the colloidal analogue of the MgCu(2) Laves phase, whose growth can be selected and directed using appropriate wall patterning. The method requires that the particles consist of different materials, so that one of them can be removed selectively after drying (for example, by burning or dissolution). Photonic calculations show that gaps appear at relatively low frequencies indicating that they are robust and open for modest contrast, enabling fabrication from more materials.     

Control over the spectral position of the band gap of opal photonic crystals

We demonstrate that the width and spectral position of the band gap of opal photonic crystals can be controlled by varying the concentration of solution in the opal pores. An experimental technique is proposed which enables identification of both the first and second photonic band gaps in the reflection spectrum of opal. The ability to observe the second band gap allows a dispersion relation to be derived for the refractive index of the infiltrated substance. The calculations are performed using a model for a one-dimensional periodic layered medium with two refractive indices. We obtain an ω(k) dispersion relation and the reflection spectra of a photonic crystal in the [111] direction at different solution concentrations.     

Coated photonic crystals: computation of the Green tensor and analysis of the bandgap

A finite-size two-dimensional photonic crystal composed of dielectric rods with holes centered within each rod is considered. The geometry of the rods, as well as the holes, is of arbitrary shape. A boundary-element method is implemented for computing the Green tensor. The semi-analytical solution is used for validating the numerical results in the case of circular geometry. Different types of configurations and geometry shapes are considered in the computation.     

Modified emission of semiconductor nano-dots in three-dimensional photonic crystals

Mono-dispersed polystyrene spheres were used to grow synthetic opal photonic crystals on glass substrates using controlled evaporation. Commercially available cadmium telluride (CdTe) semiconductor nano-dots with emission in the visible were infiltrated into a slice of opal voids by capillary action. The optical properties of CdTe dot loaded opal were studied by using a white light source and showed a red shift of the opal stop-band due to an increase in the effective refractive index. The emission of the CdTe dots was matched with the edge of the (111) direction stop-band of bare opal. Stop-band confined emission from CdTe dots was observed by pumping with an argon-ion laser. The full width at half maximum of the CdTe emission from an infiltrated section of the opal was significantly reduced due to the stop-band effect of bare opal.