Effect of nanosphere size on the luminescence of synthetic opal

We have studied the effect of the stop band and coupled photonic modes on the visible luminescence of synthetic opal photonic crystals. The results demonstrate that the position of the photonic stop band in the luminescence spectrum of opal depends on the nanosphere diameter. Optical measurements have been used to determine the refractive index of the photonic crystals and the silica sphere diameter. We have assessed the effect of coupled modes in opal on its intrinsic luminescence spectrum. Coupled modes of visible light in opal can be visualized by applying a matte sapphire plate to the opal surface.     

Reflectivity spectra of gold- and silver-infiltrated opals

We have studied the optical properties of gold- and silver-infiltrated opal photonic crystals composed of close-packed SiO₂ spheres ∼200 nm in diameter. The reflectivity spectra obtained are used to assess the photonic band gap of the crystals in the visible range. We present the characteristics of the emission induced in the opal photonic crystals by monochromatic and broadband light sources and calculated reflectances of the synthetic opal samples.     

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.     

Local surface spectroscopy of magnetic photonic crystals

Visible reflection spectra of opal photonic crystals infiltrated with magnetic materials have been measured in different areas on the (111) surface of the crystals with a spatial resolution of ∼0.1 mm. The observed reflection band corresponds to the photonic band gap of the opal. The peak position of the reflection band depends on the nature of the magnetic material infiltrated into the opal pores and the position of the fiber-optic probe on the opal surface.     

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.     

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.     

Local spectroscopy of band gaps in ferroelectric photonic crystals

We have measured visible to near-UV reflection spectra of opal photonic crystals infiltrated with ferroelectrics: barium titanate, sodium nitrite, potassium iodate, and triglycine sulfate. An experimental procedure has been developed for the infiltration of various ferroelectrics into opal pores through laser ablation and laser implantation. Using a fiber-optic probe, we were able to analyze surface reflection spectra of photonic crystals with a 0.2-mm resolution. A deuterium lamp was used as a broadband UV source, which allowed us to observe both the first and second [111] photonic band gaps in the reflection spectrum of opal crystals.     

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.     

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.     

Ordered Structures from Nanoparticles

In nature many colors arise from ordered structures with periodicities in the submicron range. One prominent example is the gemstone opal. We discuss the origin of the brillant colors of opal and its artificial counterparts: colloidal crystals and three dimensional photonic crystals. In addition we point out possibilities how to prepare colloidal crystals and related materials with inverted structure. Such materials show promising characteristics for innovative future developments in the fields of opitcs.     

Europium superluminescence in optically transparent photonic crystals

Repeatedly filling the octahedral and tetrahedral pores in the quasicrystalline structure of three-dimensional opal matrices with europium-doped silica sols, we obtained two types of nanocomposites, depending on heat treatment conditions: opal photonic crystals filled with mesoporous glass and optically transparent, ordered quantum-dot photonic crystals. According to elemental analysis data, the nanocomposites were identical in europium concentration: within 10–30 ppm. The luminescence spectra of the nanocomposites were analyzed, and their photoluminescence was shown to depend on the morphological and microstructural characteristics of the quasicrystalline materials and the spectral position of the band gap of the ordered photonic crystals. The Eu³⁺ photoluminescence intensity increases considerably when the spectral position of the ⁵ D ₀ → ⁷ F ₂ transition approaches the band gap edge of the transparent photonic crystal. We present analysis of the feasibility of using europium-doped opaline and transparent photonic crystals as gain media for lasing in the red spectral region.     

Reduction of various metal salts in opal matrices with supercritical isopropanol

We have studied the reactions between supercritical isopropanol and salts of various elements (silver, copper, gold, zinc, iron, nickel, cobalt, manganese, europium, ruthenium, antimony, bismuth, and tellurium) infiltrated into opal photonic crystals. According to transmission electron microscopy data, the diameter of the SiO₂ spheres in the opal crystals is 260 nm, and the nanoparticles of the metals and incomplete reduction products range in size from 2 to 80 nm, i.e., are no greater in size than the opal pores.     

Colloidal and transparent opal-matrix nanocomposites filled with europium-doped silica sols

Three-dimensional ordered opal-matrix composites filled with europium-doped silica sols have been produced using methods of colloid chemistry. According to elemental analysis data, the Eu concentration in the nanocomposites was ～30 ppm. A uniform europium distribution over the tetrahedral and octahedral pores of the 3D opal matrix was ensured by repeatedly filling the opal pores with silica sols doped with europium salts or europium oxide. Varying high-temperature annealing conditions, one can control the microstructure of 3D ordered nanocomposites, producing opaline and transparent photonic crystals. The microstructure of opal photonic crystals has the form of an ordered fcc lattice of amorphous silica spheres, whose tetrahedral and octahedral pores are filled with mesoporous europium-doped glass. Partial sintering of the silica spheres and mesoporous glass in transparent photonic crystals results in a periodic arrangement of quantum dots enriched in Eu-doped silica.     

Photonic Bandgaps in Disordered Inverse‐Opal Photonic Crystals

Three‐dimensional photonic crystals with full bandgaps at optical wavelengths can be fabricated with inverse‐opal techniques. We have shown that the bandgap is extremely sensitive to the presence of geometric disorder in the crystals (see Figure). The bandgap closes completely with a disorder strength as small as under two percent of the lattice constant. This fragility persists even at very high refractive index contrasts and is attributed to the creation of a bandgap at high frequency bands (8–9 bands) in inverse‐opal crystals. This should impose severe demand on the quality of lattice uniformity.     

LED-excited emission of opal infiltrated with ferroelectrics and phosphors

We have measured emission spectra of opal photonic crystals infiltrated with ferroelectrics and phosphors. At a given excitation wavelength, the emission spectra of the infiltrated opals differ markedly from the spectrum of plain opal: the emission bands are redshifted, and extra peaks are present. The infiltration effect on the emission spectrum of the opal matrix can be accounted for by the shift of the photonic band gap.     

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.     

Effect of lattice period on the transmission spectrum of synthetic opal

Transmission spectra of synthetic opal photonic crystals have been measured in the wavelength range of their band gap. The results indicate that the minimum-transmission wavelength depends on the lattice constant of opal and, hence, on the position of the photonic band gap. For synthetic opal samples on the order of 1 mm in thickness, the experimentally determined transmittance in the wavelength range of the photonic band gap does not exceed a fraction of a percent.     

LED-excited emission of opal loaded with silver nanoparticles

The emission spectra of opal photonic crystals loaded with silver nanoparticles have been measured under UV excitation. The spectra are found to markedly differ from the spectrum of plain (uninfiltrated) opal: silver nanoparticles give rise to an extra, long-wavelength emission band and change the shape of the spectrum. We have calculated the dispersion laws for the two lower photon branches and the corresponding dependences of the refractive index on frequency for the plain and silver-infiltrated photonic crystals.     

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.