V. A. Kosobukin

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.

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.

Optical Characterization of Synthetic Opals

The results of a structural-optical characterization of synthetic opals are presented. Information on the growth-induced features of the opal structure was derived from an analysis of the position and width of the one-dimensional photonic band gap. The structure of the samples was found to vary substantially along the growth axis coinciding with the [111] direction of the fcc lattice. It was shown that the regions corresponding to early stages in the opal structure growth are typically strongly disordered, which manifests itself, in particular, in the crystallites being misoriented relative to the sample growth axis. It was concluded that the regions of synthetic opals most suitable for application as photonic crystals are those corresponding to later growth stages.

Local-field effects in reflectance anisotropy spectra of the (001) surface of gallium arsenide

Characteristic reflectance anisotropy spectra of the naturally oxidized (001) surfaces of GaAs undoped crystals and Ga0.7Al0.3As epitaxial films are measured in the energy range 1.5–5.7 eV. The spectra are interpreted in the framework of the microscopic model proposed for a GaAs(001)/oxide interface and the reflectance anisotropy (difference) theory developed for a multilayer medium with a monolayer of atomic dipoles located near one of the interfaces. The anisotropy of dipole polarizability and the anisotropy of the plane lattice formed by dipoles are taken into account within the unified Green function approach of classical electrodynamics. A good agreement between the measured and calculated reflectance anisotropy spectra of the oxidized GaAs(001) surfaces shows that the local field effects at the semiconductor-oxide interface make the main contribution to these spectra.

Specific features in reflectance and absorbance spectra of one-dimensional resonant photonic crystals

The optical spectra of resonant Bragg and quasi-Bragg quantum-well structures are studied theoretically. The existence of special frequencies in the reflectance and absorbance spectra at which the reflectance or absorbance does not depend on the number of quantum wells in a structure is explained analytically. It is shown that the reflectance and absorbance spectra of structures in which the quantum-well width substantially exceeds the exciton Bohr radius also contain special frequencies and that the smooth spectral component is determined primarily by the interaction of the light wave with the quantum-well ground-state exciton.

Local field effects in magneto-optics of two-dimensional arrays of ferromagnetic nanoparticles

A theory of the polar magneto-optical Kerr effect in a layer of small ferromagnetic particles has been developed in the framework of the Green’s function method of electrodynamics. The manifestation in magneto-optics of the local field effect, which is initiated by the contribution to the effective field of dipole moments induced in particles, has been studied in terms of the model of a square lattice of ferromagnetic ellipsoids. The magneto-optical Kerr effect stimulated under normal incidence of a linearly polarized electromagnetic wave on a layer of particles magnetized perpendicular to the layer plane has been analyzed. The dependence of the Kerr rotation angle in an array of ellipsoidal Co particles embedded in the transparent dielectric CaF2 on the light frequency, the parameters of the ellipsoidal particles, and the lattice period (concentration of the magnet in the layer) has been studied numerically. It has been shown that, within a broad (2.0–4.5 eV) spectral range, the local field effect studied as a function of increasing concentration of Co particles in the layer manifests itself in the reversal of the sign of the Kerr rotation compared to that observed in a single ellipsoid or a solid Co film.

Bragg diffraction of light in high-quality synthetic opals

Abstract The complicated angular and spectral dependencies of scattering of oriented light beams on three-dimensional diffraction grating of synthetic opals were observed experimentally and interpreted theoretically. The results demonstrate the main role of Bragg reflections on several systems of closest-packed (111)-type atomic planes of twinned FCC lattice, built from a-SiO 2 spheres in the condition of one-dimensional disorder in the packing of (111) layers, perpendicular to the growth axis of the sample.

Plasmon-excitonic scattering of light from a nanoparticle located near a quantum well

A solution is presented for the problem of resonant elastic scattering of polarized light from a nanoparticle and a quantum well located near semiconductor surface. Coupling between surface plasmons of the metal particle and quasi-2D excitons of the quantum well is taken into account. The problem is solved by the Green’s functions technique treating the resonant polarization response of particle and quantum well in a self-consistent approximation. The effective polarizability is found for a metal nanoparticle of ellipsoidal shape with account of dynamical effect of “image” charges caused by semiconductor surface and quantum well. Spectra are numerically calculated for a model structure “metal-semiconductor” including a silver nanoparticle and a quantum well AlGaAs/GaAs. Appearance of exciton-plasmon interaction in the resonant scattering of light is interpreted as an enhancement by surface plasmons of the optical response due to quantum-well quasi-2D excitons.

Anisotropic local-field effects of nanoparticles in plasmonic optics and magneto-optics

A theory of anisotropic optical local-field effects caused by resonantly polarizable small particles in multilayer polarizable media is developed. Considered is the model of a rectangular lattice of ellipsoidal nanoparticles with taking account of “image forces” at an interface in a layered medium. The lattice sums for anisotropic dipolar interactions are found using the Green’s function method in the quasi-point dipole approximation, and the effective polarizabilities of particles in a layer located near an interface are calculated self-consistently. The manifestation of an anisotropic local field of nanoparticles in optical radiation and propagation of evanescent waves responsible for optical near-field effects is investigated. Applications of the obtained results in the polar magneto-optical Kerr effect and reflectance anisotropy spectroscopy in propagating the polarized light along the normal to layers are considered. The resonant features in the spectra due to enhancement of the optical effects under excitation of surface (local) plasmons in nanoparticles of a noble metal are studied.

Exciton-polariton absorption in periodic and disordered quantum-well chains

The exciton-polariton transfer and absorption in regular and disordered structures with a finite number of quantum wells are studied theoretically. The transfer matrix method is invoked in the exciton resonance region to calculate the reflectivity, transmissivity, and absorptivity spectra, as well as the integrated absorptivity as a function of the γ/Γ0 ratio of the parameters of nonradiative and radiative damping of quasi-two-dimensional excitons. It is shown that the integrated absorptivity as a function of γ (temperature) follows a universal pattern, more specifically, it increases monotonically from zero at γ = 0 to saturate at γ/Γ0 ≫ 1. Because the exciton-polariton absorption being single mode, the integrated absorptivity in Bragg quantum-well structures is substantially lower than that in short-period structures, in which absorption involves the whole spectral multitude of modes. The intrawell disorder associated with fluctuations in the frequencies of exciton excitation in quantum wells enhances the integrated absorptivity to the level typical of light absorption with no resonance among excitons of different quantum wells. The interwell disorder originating from fluctuations in quantum-well separation likewise leads to an increase in the integrated absorptivity.