We report on experimental and theoretical investigations of light diffraction from opal films of different thickness.
A special attention was paid to the transformation of diffraction patterns upon building up the opal structure from two-dimensional
(2D) film structure towards bulk three-dimensional (3D) structure. In our setup the diffraction patterns are
displayed on a narrow cylindrical screen with a specimen fixed in its center. The diffraction patterns have been studied
visually and recorded in different scattering geometries with the films illuminated with white unpolarized light. With
increasing number of layers, certain regions of 2D diffraction patterns fade out and finally form diffraction spots
characteristic for 3D diffraction. We also found that stacking faults in bulk opals lead to formation of a 2D-like
diffraction pattern, i.e. such structure demonstrate 3D to quasi-2D transition in optical properties.
Photonic crystal and structural properties of synthesized opal films filled with the iron oxides: hematite
magnetite (Fe3O4) were investigated by means of scanning electron microscopy, X-ray diffraction, and Fourier
transforme spectroscopy. Hematite was infiltrated into opal film pores without depositing the oxide onto the outer film
surface by the method of lateral infiltration under capillary forces from a liquid precursor. The synthesis of Fe3O4 was
performed in the opal pores using α→Fe2O3 as a precursor. The evolution of Bragg diffraction line from the (111) planes
of the f.c.c lattice of the opal-α-Fe2O3 film with a various filling degree was studied. The maximum filling degrees both
of opal- α-Fe2O3 and of opal-Fe3O4 films, estimated from the reflection spectra, appeared to be similar and equal to
~ 55% of the pore volume. The reversible chemical transformation of fillers in opal pores α-Fe2O3→Fe3O4→α-Fe2O3
changes only the filler dielectric constant but does not practically produce structural defects that could affect the
photonic crystal properties of the composite.
The opal-GaN-ZnS:Mn composites with various GaN:ZnS ratios were synthesized by chemical bath deposition. These materials are perfect three-dimensional photonic crystals, which produce effective photo- and electroluminescence at room temperature. The emission spectra are considerably modified by the photonic crystal structure to become anisotropic in accordance with the photonic band gap angular dispersion.
Realization of enhancement of second-harmonic generation (SHG) in
three-dimensional (3D) photonic crystals utilizing nonlinear
diffraction is demonstrated. The samples are composed from
close-packed silicon oxide spheres with diameter of 250 to 300 nm in
each sample forming an ordered fcc opal matrix. The opal voids are
filled by noncentrosymmetric gallium nitride and centrosymmetric
silicon with filling factor close to unit. The photonic band gap
(PBG) is obtained for light reflected from the (111) face and
localized in the spectral region from 800 to 950 nm for different
samples. SHG spectra show pronounced peaks as the fundamental
radiation is tuned across the photonic band gap. The intensity
enhancement in SHG is about 100 and the spectral width of the SHG
resonances is approximately 15 nm. The SHG enhancement is attributed
to combination of linear diffraction of the fundamental radiation
from the (111) opal layers and nonlinear diffraction utilizing the
3D periodicity of the quadratic susceptibility of silicon and
gallium nitride nanocrystals in opal voids. The spectral position of
the SHG peak is slightly red-shifted in comparison with the PBG
center and attributed to condition of the group velocity minima.
Transparent polycrystalline diamond films with grain size ranging from a few tens to hundreds of nanometres were prepared on fused silica substrate by Microwave Chemical Plasma Vapour Deposition method (MPCVD). The new technique, called alternating nanodiamonds injection, was applied for substrate pretreatment. It was demonstrated that nanodiamonds injected on fused silica substrate serve as nucleation centres and make possible an increase in nucleation density to 1010 cm-2. The influence of MPCVD parameters such as methane concentration, total pressure and substrate temperature on the crystalline structure and optical properties of diamond films were investigated by using micro-Raman spectroscopy and scanning electron microscopy, transmittance and reflectance measurements in the wavelength range of 400-1000 nm. Under appropriate MPCVD parameters, diamond films with optical transmission ~70% from 650 to 1000 nm and high content of diamond phase were fabricated.
We demonstrate ultrafast shifting of a photonic stop band driven by a photoinduced phase transition in vanadium dioxide (VO2) forming a three-dimensional photonic crystal. An ultrashort 120-fs laser pulse induces a phase transition in VO2 filling the pores of an artificial silica opal, thus changing the effective dielectric constant of the opal. Consequently, the spectral position of the photonic stop band blue-shifts producing large changes in the reflectivity. The observed switching of the photonic crystal is faster that 350 fs. The demonstrated properties of opal-VO2 composite are relevant for potential applications in all-optical switches, optical memories, low-threshold lasers, and optical computers.
Three-dimensional (3D) photonic crystals entirely consisting of GaN have been fabricated for the first time. Detailed investigations of optical Bragg diffraction spectra have shown a high quality of the prepared photonic crystals.
Three-dimensional opal-VO2 photonic crystals were synthesized by the chemical bath deposition technique. The Bragg reflection spectra from the (111) planes of the crystals were measured as a function of the temperature in the range between 15 and 100°C. The thermal hysteresis loop of the reflection peak position due to the phase transition in VO2 filling the opal voids was observed. A theoretical model of the periodic layered medium was proposed to describe quantitatively the reflection spectra of opal-like structures. The values of the dielectric constants of the VO2 below and above the phase transition temperature have been estimated which give the best fit within the model considered.
At low temperatures the autocatalytic process of impact ionization of shallow impurities in high purity semiconductors leads to highly nonlinear current-voltage characteristics. At a critical breakdown voltage, the impact ionization rate of impurities exceeds the capture rate of free carriers at low concentrations, yielding a rapid increase of the current for a practically constant voltage across the sample. At breakdown a strongly ionized channel is formed in the sample constituting a current filament which carries the total current . Any increase of the current is then caused by lateral growth of the current filament.