Optical characterization of natural and synthetic opals by Bragg reflection spectroscopy

The Bragg reflection of light from natural and synthetic opals was studied experimentally, and the samples were characterized by atomic-force microscopy. The reflection spectra were theoretically calculated within the model of a planar, periodically layered medium. A comparison of the experimental and calculated data made it possible to determine the parameters of the crystal structure of synthetic opals (lattice constants and sintering coefficients of a-SiO₂ particles). It was concluded that the pores in the structure of natural opals are filled by a material with a refractive index close to that of a-SiO₂.     

Bragg diffraction of light in synthetic opals

Three-dimensional light diffraction from the crystal structure, formed by closely packed a-SiO₂ spheres of submicron size, of samples of synthetic opals was visualized. The diffraction pattern of a monochromatic light beam was established to consist of a series of strong maxima whose number and angular position depend on the wavelength and mutual orientation of the incident beam and the crystallographic planes of the sample. The diffraction patterns were studied under oblique incidence on the (111) growth surface of the sample and with light propagated in the (111) plane in various directions perpendicular to the sample growth axis. The spectral and angular relations of diffracted intensity were studied in considerable detail in both scattering geometries. The experimental data are interpreted in terms of a model according to which the major contribution to the observed patterns is due to Bragg diffraction of light from (111)-type closely packed layers of the face-centered cubic opal lattice. The model takes into account the disorder in the alternation of the (111) layers along the sample growth axis; this disorder gives rise, in particular, to twinning of the fcc opal lattice.     

High Miller-index photonic bands in synthetic opals

We present a detailed study of high Miller-index (h k l) photonic bands in a-SiO₂ synthetic opals. Polarized light transmission spectra of opals were studied in a wide wavelength range for all high symmetry directions on the Brillouin zone (BZ) in the face-centered cubic (fcc) lattice. It is shown that under the conditions of low dielectric contrast the dispersion of high Miller-index photonic bands is described well by the calculated dependences of Bragg wavelengths diffracted from the (h k l) crystallographic planes of the fcc lattice.     

In and Si distribution in synthetic opals

An X-ray photoelectric absorption by samples of synthetic opals (SO) loaded by In and Si has yielded filler distribution profiles over thickness. The SO+In sample exhibited a uniform filling of SO voids throughout the sample thickness (on average, 16.9% of each large void). The SO+Si sample, besides the near-surface region where large voids are completely filled, revealed a region with variable Si concentration in the SO, where the Si content decreases linearly down to a depth where pure SO is found. Fiz. Tverd. Tela (St. Petersburg) 40, 1373–1375 (July 1998)     

Sound velocity in single crystals of synthetic opals

The [111] longitudinal sound velocity (v _L) in a single-crystal synthetic opal has been measured at a frequency of 10 MHz in the temperature range 4.2–300 K. At 300 K, v _L=2.1×10⁵ cm/s. The quantity dv _L/v _{300 K}(T) (where v _T,K?v_{300 K}) in the ranges 4.2–200 and 200–300 K behaves in the way typical of amorphous and crystalline solids, respectively.     

Structural modification of synthetic opals during thermal treatment

The density and porosity of synthetic opals with spheres 315 and 1000 nm in diameter were measured in relation to the annealing temperature. At annealing temperatures of up to 500°C, the seeming density and porosity remain almost unchanged. Then, at temperatures of up to 950°C, the density increases gradually and, accordingly, the porosity decreases due to the collapse of nanopores caused by the sphere substructure. As the annealing temperature increases further, the opal density increases sharply up to 2.22 g/cm³ (which corresponds to the density of amorphous silica) and the open microporosity due to the voids between spheres disappears. Differential thermal and thermogravimetric analyses showed that SiO₂ powders with particles with average size of 315 and 1000 nm can have, respectively, two-and three-level systems of micro-and nanopores.     

On dip broadening in transmission spectra of synthetic opals

This paper reports on a comprehensive immersion spectroscopy study of the spectral position and profile of the dip corresponding to the (111) photonic band in the transmission spectra of synthetic opals. The variation in the opal-filler permittivity contrast in the region of the immersion-mediated disappearance of the (111) band has revealed its anomalous abrupt behavior. The width of the (111) photonic band at the L point of the Brillouin zone of the fcc lattice has been calculated as a function of the opal-filler permittivity contrast in the plane wave approximation. The experimentally observed (111) dip width is found to be substantially larger than that obtained in the calculations, which can be due to deviations in the size and dielectric parameters of the a-SiO₂ particles making up synthetic opals.     

Specific features in the thermal conductivity of synthetic opals

Data on the thermal conductivity ϰ _ph of the cluster lattice of synthetic opals are analyzed. All opals are divided into two groups according to the temperature dependence of their ϰ _ph. These are opals-1, whose thermal conductivity behaves like ϰ _ph(T) of quasi-crystalline materials, and opals-2, with a ϰ _ph(T) dependence typical of classical amorphous solids. Possible reasons for this difference are considered. An attempt is made to explain the complex temperature hysteresis in ϰ _ph(T) observed earlier in opals-2. Fiz. Tverd. Tela (St. Petersburg) 39, 392–398 (February 1997)     

Spectroscopy of the photonic stop band in synthetic opals

The photonic band gap of opals has been studied experimentally from their optical transmission spectra as a function of the incident beam orientation in the opal crystal lattice. The measurements were carried out for all high-symmetry points on the surface of the Brillouin zone of an fcc lattice. The experimental dependence of the energy position of the photonic band gap on the light wave vector direction is well described by the set of theoretical relations developed for the stop bands originating from the Bragg diffraction of light on {111}-type planes of the twinned fcc lattice of synthetic opals.     

Low-temperature heat capacity and heat conductivity of single-crystal synthetic opals

The heat capacity at constant pressure (in the range 3–50 K) and the lattice heat conductivity (from 5 to 75 K) of a single-crystal synthetic opal are measured. It is shown that the heat capacity of the opal behaves at these temperatures in a way similar to porous amorphous materials. The data on the heat conductivity suggest that single-crystal opals can be related to a class of semicrystalline (partially crystallized amorphous) materials. However, because of specific features of their crystal structure, the opals form a nonstandard type of semicrystalline material which we termed semiamorphous.     

Acoustic properties of globular photonic crystals based on synthetic opals

Acoustic properties of globular photonic crystals based on synthetic opals and composed of closely packed SiO₂ globules about 200 nm in diameter are theoretically investigated. Dispersion characteristics of the investigated samples are numerically simulated, and the group velocity of acoustic waves and the effective mass of acoustic phonons are found. It is shown that phononic bandgaps in these photonic crystals are within the gigahertz frequency range. The effective mass of the acoustic phonons corresponding to the edges of the bandgaps is found, and a possibility that bound states of acoustic phonon pairs, biphonons, manifest themselves in the light scattering spectra is discussed.     

Selective control of light beams in diffraction experiments on synthetic opals

This paper reports on an experimental and theoretical investigations of the possibility of selectively controlling the intensity of {hkl} diffraction reflections in synthetic opals. The calculations of the photonic stop-band width for low-contrast multi-component photonic crystals argue for a selective pattern of the vanishing of various {hkl} stop bands (and, accordingly, of the various {hkl} diffraction reflections) under variation of the dielectric permittivity of one of the components. Optical diffraction patterns have been studied visually and recorded in various scattering geometries with bulk opal samples illuminated by white unpolarized light. The experiments have made use of immersion spectroscopy, a method reducing essentially to successive measurements of diffraction patterns under variation of the dielectric permittivity of the liquid filler of the opal matrix. The {111} and {220} Bragg reflections have been observed to weaken noticeably at the filler permittivities matching very well with the values derived earlier from an analysis of transmission spectra.     

Secondary radiation of synthetic opals loaded by the sodium nitrite ferroelectric

This paper reports on a comparison of transmission, reflection, and secondary radiation spectra of unloaded synthetic opals and samples filled with the sodium nitrite ferroelectric (NaNO₂). The radiation is provided by semiconductor light-emitting diodes operating in the ultraviolet and visible spectral regions. Selective excitation of slow electromagnetic waves in a photonic crystal, an effect observed when the exciting radiation frequency approaches the stop-band edge, is studied.     

Determining the porosity of synthetic opals and porous silicon by x-ray methods

A simple method is proposed for determining the porosity of fractal materials by measuring the absorption of x rays as they pass through these samples. The method makes it possible to measure the integrated porosity of objects rapidly, does not require special preparation of the objects, is not contaminating, and does not destroy their structure. The possibilities of this method are demonstrated for the examples of synthetic opals and porous silicon. Fiz. Tverd. Tela (St. Petersburg) 39, 956–958 (March 1996)     

Multiple Bragg diffraction in low-contrast photonic crystals based on synthetic opals

The phenomenon of multiple Bragg diffraction in low-contrast photonic crystals based on synthetic opals has been studied both experimentally and theoretically. The transmission and reflection spectra of opal films near the K point of the Brillouin zone of the face-centered cubic lattice in s-polarization exhibit the effect of anticrossing of dispersion curves corresponding to the (111) and ([`1]11)(\bar 111) photonic stop bands. The effect of quasi-Brewster suppression of stop bands is clearly pronounced in p-polarization. The experimental data are analyzed using the calculation of the band structure of opal with the inclusion of the polarization of incident light.     

Optically excited secondary emission spectra of photonic crystals based on synthetic opals

The photonic dispersion, the group-velocity dispersion, the effective mass, refractive index, and the spectral distribution of the density of photonic states near the edge of the photonic stop band are numerically calculated in the one-dimensional model for photonic crystals based on synthetic opals. The fluorescence spectra of rhodamine 6G and 2,5-bis(2-benzoxazolyl)hydroquinone molecules infiltrated into a synthetic opal are measured. For both substances, it is observed that the spontaneous emission intensity in the range of the photonic stop band is appreciably suppressed. A blue shift of the fluorescence spectrum of rhodamine 6G molecules is revealed. Secondary emission of synthetic opals infiltrated with colloidal silver is observed in the Stokes range under excitation of opals by radiation at λ = 400 nm. The spectrum of the secondary emission is located in the range 450–590 nm, which contains the stop band and intervals near its edges.     

Structural parameters of synthetic opals: Statistical analysis of electron microscopy images

The paper reports on a comprehensive study of the statistical characteristics of an ensemble of a-SiO₂ particles in synthetic opals. The results of processing of the electron microscopy images obtained in the investigation of the (111) growth layer are presented. To calculate the statistical parameters, an algorithm is developed which permits determination of the diameters of a-SiO₂ particles and coordinates of their centers. The algorithm is based on the pattern recognition procedure of objects with radially symmetric boundaries by means of a transformation of the original image similar to the Hough transformation. Our image processing study yields structural parameters of the (111) growth layer, more specifically, the average particle diameter, ～316 nm, the distribution profile half-width, ～7%, and the average distance between the centers of neighboring particles, ～315 nm. The results of the opal image processing are compared with data of optical experiments.     

Optical analysis method for fast plasma characterization of high-speed miniaturized synthetic jet

In this paper, a new optical analysis method for plasma characterization is proposed. Plasma characteristics are obtained directly by measuring the plasma luminous color, rather than the complex spectral diagnosis method, which is difficult to obtain at high speed. By using the light transmittance curve of the human cornea, the RGB coordinates are calculated from the measured plasma spectrum data. Plasma characteristics are diagnosed using the Boltzmann plot method and the Stark broadening metho...     

圆柱形蛋白石和反蛋白石结构光子晶体的制备及其光学性质

研究了在圆柱曲面基底上自组装空心和实心的圆柱形蛋白石和反蛋白石结构光子晶体的方法. 用垂直沉积法在不同曲率半径的毛细管内自组装了空心圆柱形聚苯乙烯(PS)蛋白石结构光子晶体薄膜和二氧化硅 (SiO₂) 反蛋白石结构薄膜; 用类重力沉积法制得实心圆柱形蛋白石和反蛋白石结构光子晶体, 并讨论了这一生长方式中的状态变化过程及影响因素. 用扫描电子显微镜对样品内部结构进行了表征, 用光谱仪测试了光子晶体薄膜的反射光谱, 结果表明: 基底曲率半径和微球粒径的大小是影响空心蛋白石和反蛋白石薄膜质量的主要因素; 微球大小是影响实心结构有序性的主要因素. 关键词： 反蛋白石 空心圆柱 溶胶凝胶协同自组装     

Experimental study of the photonic band structure of synthetic opals at a low dielectric contrast

This paper reports on a comprehensive study of optical transmissivity spectra of synthetic opals as a function of the four major parameters of the observed photonic stop bands, namely, light beam orientation relative to the opal fcc lattice, light polarization, opal-filler dielectric permittivity contrast, and sample thickness. The measurements were performed under low opal-filler dielectric contrast conditions for the principal high-symmetry directions of the twinned fcc lattice of the opals. The experimentally determined dependence of the energy positions of photonic stop bands on the direction of the light wave vector is fitted well by the calculated dispersion relation of Bragg wavelengths in diffraction of light from the (hkl) fcc plane system.