Synthetic Opals Research Articles

Methods of characterization of synthetic opal films

We developed methods for determination of thickness, number of layers and filling fraction of silica particles for synthetic opals. We show that the filling fraction is considerably less than for ideal close-packed structure, which is important for practical and theoretical applications.

Photonic bandgap examination in an immersed synthetic opal

We inspected photonic bandgap (stop-zone) in a synthetic opal (photonic crystal) immersed with different liquids. Dependences of the bandgap depth and spectral position on the immersed liquid refractive index were measured in the case of normal light incidence to the (111) face. An explanation of the observed spectral displacement of the bandgap is given on the basis of interaction between forward-propagating wave affected by Bragg diffraction and phase-shifted back-scattered wave.

High Resolution Higher Energy X-ray Microscope for Mesoscopic Materials

We developed a novel X-ray microscopy technique to study mesoscopically structured materials, employing compound refractive lenses. The easily seen advantage of lens-based methodology is the possibility to retrieve high resolution diffraction pattern and real-space images in the same experimental setup. Methodologically the proposed approach is similar to the studies of crystals by high resolution transmission electron microscopy. The proposed microscope was applied for studying of mesoscopic materials such as natural and synthetic opals, inverted photonic crystals.

Polymer opals as novel photonic materials

Synthetic opals, based on self-assembled arrays of core – shell (bead – matrix) polymer microparticles, are a promising platform for next-generation bulk-scale photonic structures, coatings, fibres and sensors. This perspective article highlights recent work in this area, ranging from characterization and application, to advances in more fundamental understanding of structural colour effects. These advances include viscoelastically tuned symmetry breaking, the observation of anisotropic optical scattering and the study of polymer opals as an analogue to the intrinsically disordered, low-refractive-index contrast systems associated with opals seen in nature. c � 2013 Society of Chemical Industry

Growth and optical properties of synthetic opal filled with Bi12SiO20 and Bi12GeO20 nanocrystals

We report a technique for producing Bi12SiO20 and Bi12GeO20 nanocrystals in pores of synthetic opal. Bragg reflection and Raman spectroscopies are used to assess the degree of filling of opal pores and diagnose the state of the substance in the pores. The substance in the pores is shown to be in a crystalline state. The measured Raman spectra of the nanocrystals are compared to those of powders and single crystals. The spectra of the photonic crystals obtained contain new lines. We observe intensity redistribution at low frequencies and changes in the frequency of several Raman lines compared to the spectra of powders and single crystals.

Structural changes of synthetic opal by heat treatment

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si–O–Si network comprising a four-membered ring structure of SiO4 tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO4 tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C.

Optical dispersion in synthetic opal matrices filled with europium-oxide-doped silica sols

We have studied typical features in reflection spectra of the (111) surface of opal photonic crystals based on synthetic opals filled with Eu2O3-doped silica sols. The reflection spectrum of the composite of opal and europium-doped silica sols is found to contain a resonance at a wavelength of 617.5 nm. A theory has been developed for describing the optical dispersion of resonant photonic crystals. We have calculated the dispersion curves and reflectivity of undoped and Eu2O3-doped opal crystals and compared the calculated reflection spectra to experimental data. Nanocomposites of opal and europium-doped silica sols can be used in laser cavities.

Vibrational spectra of opal-based photonic crystals

Synthetic silica opals were investigated by infrared and Raman spectroscopies. Vibrational modes associated with molecular groups of opal globules and admixtures were detected. Similarities in Raman and infrared spectra of synthetic opal with reference fused and α-quartz indicate the presence of amorphous phase in opal globules. Also some spectral bands designate on modified optical stretching vibrations at 1000-1200 cm−1 and bigger amount of Si-H defects in photonic crystal.

Novel Photoluminescence-Enhancing Substrates for Image Formation of Biological Objects

The use of photonic crystals, which were fabricated on the basis of synthetic opals, as substrates for the luminescence microscopy of biological objects has been shown. The spatial distributions of the photoluminescence by DNA clusters excited by 365-nm ultra-violet irradiation on opal surfaces and rough gold substrates have been studied. With the use of blood cells as an example, a possibility for the visualization of biological objects in the case where the nanostructure elements of synthetic opals are applied as labels and image amplifiers has been demonstrated.

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 \(\vec E\), \(\vec H\), and \(\vec k\) vectors) and “left-hand” (left-hand system of the \(\vec E\), \(\vec H\), and \(\vec k\) 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.

Secondary Emission From Synthetic Opal Infiltrated by Colloidal Gold and Glycine

A comparison of the secondary emission (photoluminescence) and Bragg reflection spectra of photonic crystals (PC), namely, synthetic opals, opals infiltrated by colloidal gold, glycine, and a complex of colloidal gold with glycine is performed. The infiltration of colloidal gold and a complex of colloidal gold with glycine into the pores of PC causes a short-wavelength shift (about 5–15 nm) of the Bragg reflection and increases the intensity of this band by 1.5–3 times. In photoluminescence, the infiltration of PC by colloidal gold and colloidal gold with glycine suppresses the PC emission band near 375–450 nm and enhances the shoulder of the stop-zone band of PC in the region of 470–510 nm. The shape of the observed PC emission band connected with defects in synthetic opal is determined by the type of infiltrates and the excitation wavelength. Possiblemechanisms of the effects are discussed.

Spontaneous parametric light scattering in spatially inhomogeneous nonlinear media based on photonic crystals

The emission spectra of nonlinear media with periodical modulation of the second-order susceptibility that are based on synthetic opals infiltrated by Ba(NO3)2, LiIO3, and KH2PO4 are studied using radiation with λ = 400 and 407 nm for excitation. The spectral angular distribution of the emission in the range of 420–650 nm is revealed, as well as the dependence of its spectrum on the photonic stop-band parameters and the exciting radiation characteristics. The emission spectrum of samples with a low dielectric contrast has the shape of a symmetric band with a maximum at 525 nm and a half-width of about 100 nm. The observed emission is interpreted as spontaneous parametric light scattering in a spatially inhomogeneous nonlinear medium.

Probing acoustical, optical, and acousto-optical properties of nanostructured materials by picosecond laser ultrasonics

Research results on the characterization of the thin (submicrometers thick) films of the nanostructured materials by the experimental methods of picosecond laser ultrasonics are reviewed. These methods make use of femtosecond lasers to generate and to detect GHz–THz acoustic waves. In this communication, theoretical backgrounds of the fs-laser-based opto-acousto-optic techniques that are used for the evaluation of the material properties are first introduced. Then, the results of the experiments on nanoporous low-k films (for the microelectronics), on nanogranular sol–gel optical coatings (for laser optics), on anodized alumina (for the nanomaterial/nanostructure templates), on synthetic opals, on nanoparticles supra-crystals, and other nanostructured materials are discussed. The emerging opto-acousto-optic technology for the depth-profiling of acoustical, optical, and acousto-optical properties of inhomogeneous transparent films with the nanometers scale spatial resolution is also presented.

Synthesis and properties of nanocrystalline CsI

The structure and properties of fine CsI powders prepared by spray pyrolysis and synthetic opal (SiO2) based composites containing CsI in their pores have been studied by electron microscopy, X-ray diffraction, and luminescence spectrometry. The powders consisted of nanocrystalline particles which increased in size from 35 to 55 nm with increasing pyrolysis temperature. In the opal/CsI composites, the CsI nanocrystallites were about 45–50 nm in size and formed a single three-dimensional network between the SiO2 spheres. In going from bulk crystals to their nanocrystalline analogs, the size effect on CsI luminescence leads to a reduction in luminescence yield, a redshift of the emission bands of the on-center and off-center self-trapped excitons (STEs), and an increase in the contribution of the off-center STEs to the net intrinsic emission yield. The emission decay kinetics of the off-center STEs is insensitive to the crystallite size.

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.

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 \((\bar 111)\) 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.

Two-dimensional light diffraction from thin opal films

This paper reports on experimental and theoretical investigations of light diffraction from thin films of synthetic opal. The diffraction patterns have been studied visually and recorded in different scattering geometries with the films illuminated with white unpolarized light. The diffraction pattern obtained with the film illuminated with a light beam along the [111] axis, which is normal to the film surface, has C 6 symmetry and consists of six strong reflections arranged symmetrically with respect to the incident beam. This pattern becomes substantially more complicated when the film is illuminated by white light at an arbitrary angle to the [111] axis. An experimental study of the spectral response and angular relations of the diffraction patterns has established a fairly full pattern of transformation of diffraction reflections obtained under variation of the angle of light incidence on an opal film. The remarkably good matching of experimental and calculated data provides compelling evidence for light diffraction from thin opal films being two-dimensional.

New Optical Properties of Synthetic Opals Infiltrated by DNA

Optical properties of photonic crystals (synthetic opals) with DNA inserted inside the crystals have been registered and analyzed in the visible and IR ranges. A decrease in the intensity of vibration spectra of DNA infiltrated in opal is observed. Molecules of DNA placed in the cavities of a photonic crystal became invisible for the detection by visible and IR spectroscopies. Spectroscopic features of DNA in a photonic crystal indicate the formation of condensed DNA inside and at the surface of the photonic crystal. The emission of the “DNA – photonic crystal” system has been registered at room temperature. By confocal microscopy, we have visualized DNA at the surface of a photonic crystal. The experimental results are explained by assuming the formation of coupled photonic states of DNA and a photonic crystal.

Propagation of polarized light in opals: Amplitude and phase anisotropy

The interaction of linearly polarized light with photonic crystals based on bulk and thin-film synthetic opals is studied. Experimental transmission spectra and spectra showing the polarization state of light transmitted through opals are discussed. A change in polarization is found for waves experiencing Bragg diffraction from systems of crystallographic planes of the opal lattice. It is shown that the polarization plane of the incident linearly polarized wave at the exit from photonic crystals can be considerably rotated. In addition, incident linearly polarized light can be transformed to elliptically polarized light with the turned major axis of the polarization ellipse. Analysis of polarization states of transmitted light by using the transfer-matrix theory and homogenization theory revealed good agreement between calculated and experimental spectra.

Modification of the refractive-index contrast in polymer opal films

Synthetic opals, based on self-assembled arrays of core–shell (bead/matrix) polymer microspheres, are a promising platform for next-generation photonic structures, coatings and sensors. The refractive index contrast (Δn) between beads and the matrix polymer is essential for the appearance of structural colour in polymer opal films. We report how the index contrast can be modified by engineering the chemical composition of the core–interlayer–shell (CIS) precursor particles. Alternative approaches to emulsion polymerisation, using the fluorinated monomer 2,2,2-trifluoroethyl acrylate and the aromatic monomer benzyl acrylate, yield a much larger range of Δn values than for standard systems made from styrene and ethyl acrylate. Spectroscopic studies reveal striking differences in the transmission properties of thin-films as Δn is varied from 0.045 up to 0.18.