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.     

Reflectivity spectra of NaNO<Subscript>2</Subscript>-infiltrated synthetic opal

We analyze the reflectivity spectra of synthetic opal crystals (sphere diameters of 200, 240, and 290 nm) infiltrated with ferroelectric sodium nitrite (NaNO₂) after preannealing in air or argon. The reflectivity spectra of the opal samples show a band corresponding to the photonic band gap. Its position and shape strongly depend on the sphere diameter and annealing conditions. The spectra of the sodium-nitrite-infiltrated opal samples preannealed in air differ markedly from those of the samples preannealed in argon. The experimentally determined band gap position as a function of sphere diameter is compared to calculation results. Our data demonstrate that argon preannealing ensures effective infiltration of molten sodium nitrite into the pores of synthetic opal.     

Reflection spectra of synthetic opal at liquid-nitrogen temperature

Reflection spectra of the (111) surface of synthetic opal crystals with sphere diameters of 210, 250, and 300 nm have been measured at room and liquid-nitrogen temperatures. At cryogenic temperatures, we observed a narrowing and shift of photonic band gaps due to liquid nitrogen infiltration into the opal pores. We have calculated ω(k) dispersion relations and determined the reflectivity of the opal surface. The results have been compared to the measured reflection spectra. We have detected photon conversion from the edges of the band gap to its center.     

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.     

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.     

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.     

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.     

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.     

Normal-incidence reflection spectra of the (111) photonic opal surface in the regions of the first and second photonic band gaps

We have studied the reflection spectra of opal photonic crystals with air-or ethanol-filled pores at different diameters of the silica spheres. An experimental technique has been 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 allowed us to derive a dispersion relation for the refractive index of the infiltrated substance. The calculations were performed using a model for a one-dimensional periodic layered medium with two refractive indices. We obtained ω(k) dispersion curves for electromagnetic waves in a photonic crystal (at normal incidence). The ω(k) dispersion law was used to find a dispersion relation for the reflectance of the photonic crystal.     

Visible reflection spectra of synthetic opal infiltrated with gold nanoparticles

A technique has been developed for infiltrating gold nanoparticles into the pores of synthetic opal using laser ablation and high-temperature annealing, and the reflection spectra of gold-infiltrated opal photonic crystals have been measured. When a halogen lamp was used as a broadband light source, the peak-reflection wavelength of the gold-infiltrated opal was longer than that of the uninfiltrated opal. We calculated the dispersion laws for the two lower photon bands of the synthesized photonic crystals and obtained the corresponding frequency dependences of the refractive index.     

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.     

Effect of photonic bandgap on upconversion emission in YbPO4:Er inverse opal photonic crystals

We obtained upconversion (UC) light-emitting photonic materials (YbPO(4):Er) with an inverse opal structure by the self-assembly technique in combination with a solgel method. The effect of the photonic stopband on the UC luminescence of the (2)H(11/2), (4)S(3/2)→(4)I(15/2), and (4)F(9/2)→(4)I(15/2) transitions of Er(3+) has been observed in the inverse opals of the Er(3+)-doped YbPO(4). Significant suppression of the UC emission was detected if the photonic bandgap overlapped with the Er(3+) ions emission band, while enhancement of the UC emission occurs if the emission band appears at the edge of the bandgap.     

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.     

Phase diagrams of the ternary liquid systems [Ce(NO3)3(TBP)3]-C10H22-[UO2(NO3)2(TBP)2] and [Ce(NO3)3(TBP)3]-C10H22-[Th(NO3)4(TBP)2] and of the quaternary liquid system [Ce(NO3)3(TBP)3]-C10H22-[UO2(NO3)2(TBP)2]-[Th(NO3)4(TBP)2]

The phase diagrams of the ternary liquid systems [Ce(NO₃)₃(TBP)₃]-C₁₀H₂₂-[UO₂(NO₃)₂(TBP)₂] and [Ce(NO₃)₃(TBP)₃]-C₁₀H₂₂-[Th(NO₃)₄(TBP)₂] and of the quaternary liquid system [Ce(NO₃)₃(TBP)₃]-C₁₀H₂₂-[UO₂(NO₃)₂(TBP)₂]-[Th(NO₃)₄(TBP)₂] at T = 298.15 K are constructed. The phase diagrams are characterized by areas of homogeneous solutions and of two-phase liquid systems (systems with phase separation), with one phase (I) enriched in [Ce(NO₃)₃(TBP)₃], [Th(NO₃)₄(TBP)₂], and [UO₂(NO₃)₂(TBP)₂], and the other phase (II), in C₁₀H₂₂. Using the data on the mutual solubility of the components in the systems under consideration and equations of the NRTL model, the parameters of intermolecular interactions and the excess Gibbs energies (G ^ex) were calculated for the binary, ternary, and quaternary systems. Passing from the ternary system [Ce(NO₃)₃(TBP)₃]-C₁₀H₂₂-[Th(NO₃)₄(TBP)₂] to the quaternary system [Ce(NO₃)₃(TBP)₃]-C₁₀H₂₂-[UO₂(NO₃)₂ (TBP)₂]-[Th(NO₃)₄(TBP)₂] does not appreciably affect the distribution of C₁₀H₂₂ between phases I and II, but leads to the redistribution of [Ce(NO₃)₃(TBP)₃] into phase II and of [Th(NO₃)₄(TBP)₂] into phase I.     

Two-gap structure in Yb(Y)Ba2Cu3O7−x single crystals

Tunneling effects in YbBa₂Cu₃O_7?x (YbBCO) and YbBa₂Cu₃O_7?x (YBCO) single crystals have been investigated using an improved technique of break junction preparation. A two-gap structure with BCS-like density of states has been observed. The value of the “large” gap is in the range of 24–28 meV for YBCO (T _c =85–90 K) and 29–30 meV for YbBCO (T _c =87–90 K). A variation of the value of the “small” gap between 0.5 and 11 meV has been detected, even for the same crystal. As a possible explanation, two-band superconductivity, surface properties, or strong gap anisotropy will be discussed.     

Pinning and vortex dynamics in superconducting (K,Ba)BiO 3 single crystals

Pinning and vortex dynamics have been investigated in the 3-dimensional copper free (K, Ba)BiO₃superconductor (T_c 31K) by magnetization and transport measurements up to 30 Tesla. The magnetization curves present a pronounced fishtail effect which persists for time scales down to 10^–4s (pulsed field measurements). We show that it is an intrinsic feature of the critical current which can in part be well described by the collective pinning theory. Furthermore, this system presents evidence for a vortex liquid /glass transition for vanishingly small currents. As the current density is increased, dissipation in the glass state is dominated by creep effects. The temperature and current dependence of the activation energy is discussed.     

Reaction sequences in the systems Ti(OiPr)4 + Ba(CH3COO)2 and TiO2 + Ba(CH3COO)2

The reaction sequences during BaTiO₃ formation from the systems Ti(OⁱPr)₄ + Ba(CH₃COO)₂ and TiO₂ + Ba(CH₃COO)₂ are studied, using a sol-gel method for the former system and solid-state reaction for the latter. The sol-gel system exhibits greater reactivity, forming BaTiO₃ at temperatures as low as 500°C; also, the formation of the intermediate Ba₂TiO₄ occurs to a much lesser extent than in the case of solid-state reaction. An explanation of the structure and behaviour of the gel system is proposed from X-ray diffraction, infrared and thermogravimetric examinations. Under appropriate treatments, both systems yield microstructures with fine and homogeneous grain size; however, the sol-gel method facilitates a complete reaction at temperatures appreciably lower than those needed with the solid-state reaction system.     

三维光子晶体的制备技术研究进展

光子晶体是周期性介电结构．它能象周期性原子结构中的电子禁带一样．产生光子禁带。自从1987年Yablonovitch提出光子晶体的概念以来，有关光子晶体的各种研究非常活跃。本文回顾了三维光子晶体的制备技术研究现状，旨在激发不同学科领域研究人员的想象力和创造力．使他们从一些可能的光子晶体制造途径中有所裨益．并将这种可能性转变为现实。