Showing papers in "Optics and Spectroscopy in 2009"

Material parameters of metamaterials (a Review)

Abstract: A theory of the homogenization of a certain class of metamaterials is stated. These metamaterials are volume lattices of electric and magnetic dipoles that are resonant with frequencies that are considerably lower than that of the first Bragg resonance of the lattice. It was shown that, for plates of a metamaterial, which are described by bulk material parameters, transition layers play an important role, and the known Drude notion of transition layers is significantly revised. The paper also discusses a more widespread method of determining the material parameters of metamaterials based on the extraction of the refractive index and the characteristic impedance from the scattering matrix of the plate of the metamaterial. The physical meaning of the material parameters obtained in this way is clarified, and the concept of Bloch lattices related to it is discussed. It is shown that the bulk material parameters and the parameters of transition layers can also be extracted from components of the scattering matrix of plates.

Linear-wavenumber spectrometer for high-speed spectral-domain optical coherence tomography

Abstract: An equidistant (in the wavenumber) spectrometer based on a diffraction grating, a compensation prism, and a CCD linear array is developed and implemented for spectral-domain optical coherence tomography. A criterion is introduced for estimating the level of residual nonequidistance. This criterion allows one to determine the threshold compensation level necessary for obtaining the spectrally limited spatial resolution. The method is tested in spectral-domain optical coherent tomography systems at wavelengths of 1270 and 830 nm.

Luminescence properties of phosphors based on Tb 3 Al 5 O 12 (TbAG) terbium-aluminum garnet

Abstract: The processes of excitation energy transfer in phosphors based on single-crystal Tb3Al5O12:Ce (TbAG:Ce) and Tb3Al5O12:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb3+ cations) to Ce3+ and Eu3+ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb3+ ions to Ce3+ and Eu3+ ions and from Ce3+ ions to Eu3+ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb3+ cation sublattice.

Multimodal polarization system for imaging skin cancer

Abstract: An optical system is created that is capable of detecting tumor formations in vivo in real time by means of the spectrally resolved polarization imaging of light elastically scattered by tissue and imaging of fluorescence polarization of exogenous fluorophores. The performance characteristics of the system, such as the resolution, field of view, and power density and stability of the radiation, as well as the calibration G factor, are determined. The functionality of the system is tested under clinical conditions. Spectrally resolved signals of elastic scattering and fluorescence polarization images are detected both from the wound surface in vivo and from the bioptic material. The reliability of the method is proven by comparing the results with the data of histological studies.

Area of ultimately short light pulses

Abstract: The evolution of the area of a pulse of optical radiation propagating in a nonlinear medium is analyzed. The role played by the medium relaxation and the correctness of the formulation of the problem on the propagation of short pulses are discussed.

Extremely short dissipative solitons in an active nonlinear medium with quantum dots

Abstract: A medium consisting of quartz with embedded active (amplifying) or passive (absorbing) impurities, i.e., quantum dots, is proposed for producing extremely short dissipative solitons on the basis of the effect of enhanced self-induced transparency. The calculations show that, in such a medium, the initial standard femtosecond pulses can be transformed into extremely short dissipative solitons with a peak intensity of ∼1011 W/sm2, with a duration corresponding to the inverse frequency of transitions in impurities, and with the coherent spectral supercontinuum covering almost the entire transmission region of quartz.

Coherent noise compensation in Spectral-Domain optical coherence tomography

Abstract: An efficient technique for the separation of and compensation for coherent noise in spectral optical coherence tomography with parallel spectrum detection is proposed and validated The coherent noise is separated during one exposure by modulating the mutual delay of the signal and reference waves by a certain law It is shown that the influence of internal motions in an object on the quality of the coherent noise separation can be reduced by the modulation frequency increasing The technique has been numerically and experimentally validated with the help of an optical coherence tomography (OCT) setup with a radiation source operating at a wavelength of 1277 nm and a width of the recorded spectrum of about 100 nm

Radiative characteristics of nitrogen upon excitation by volume discharge initiated by runaway electron beam

Abstract: The amplitude, time, spectral, and energetic characteristics of a volume (diffuse) discharge at an elevated pressure (up to 5 atm) in a gap with an inhomogeneous electric field without an additional preionization source are studied. The concentration and temperature of electrons in the discharge plasma are mea-sured by spectral methods. The effective lifetime of the C 3Π u state of the nitrogen molecule at the trailing edge of the radiation pulse is shown to be determined by radiative and collisional quenchings. For transitions of the second positive system of nitrogen, a plasma discharge radiation power into the complete solid angle is obtained to be ∼120 kW, with a specific radiation power of up to ∼50 kW/cm3.

Optical properties of nanostructured layers on the surface of an underlying medium

Abstract: An analytical approach to the description of the optical characteristics of layers consisting of interacting nanoclusters is developed based on the integral equation method. Expressions for the fields inside and outside the system are obtained, the effective parameters of the structure elements are studied taking into account the mutual polarizing effect of nanoclusters and underlying medium, and the applicability conditions of the model proposed are determined. The possibility of controllable the tuning of the spectral properties of the system by changing its structural parameters is considered; it is shown that the reflection from the substrate surface can be decreased by depositing a nanostructured coating. Good agreement is obtained between the analytical calculation and the exact numerical solution of Maxwell’s equations.

Influence of terahertz laser radiation on the spectral characteristics and functional properties of albumin

Abstract: The exposure of albumin (transport protein of blood serum) to laser radiation with a frequency of 3.6 THz resulted in a change in the intensity of characteristic bands in UV absorption spectra and in circular dichroism spectra. These changes depend on the exposure duration and the laser radiation power and indicate conformational changes in protein molecules.

Compression as a method for increasing the informativity of optical coherence tomography of biotissues

Abstract: The efficiency of the mechanical compression of biotissues for improving the differentiation between pathological changes in the structure of a biotissue observed by the method of optical coherence tomography (OCT) is investigated. The effect of the compression in the OCT-images of samples of the human rectum affected by inflammation and carcinoma is studied ex vivo. It is shown that the use of compression makes it possible to differentiate between these pathological changes. To interpret experimental data, images of an inflamed part of rectum are modeled by the Monte Carlo method for different degrees of compression. The results of modeling agree qualitatively with the experimental data.

Luminescence peculiarities and optical properties of MgMoO4 and MgMoO4:Yb crystals

Abstract: Magnesium molybdate is considered as a promising material for cryogenic scintillation bolometers. The luminescence properties of MgMoO4 have been investigated on single crystals grown from melts of stoichiometric and nonstoichiometric compositions and on crystals doped with Yb3+ ions. Their optical properties are interpreted taking into account the anisotropy of the MgMoO4 crystal structure.

Effect of a dielectric shell of a silver nanoparticle on the spectral position of the plasmon resonance of the nanoparticle in photochromic glass

Abstract: The behavior of the spectral position of the absorption band of colloidal silver in photochromic glass is analyzed in relation to the variation in the refractive index of a thin layer of the medium (shell) that surrounds silver nanoparticles. It is shown that the appearance of a shell with a high refractive index leads to a long-wavelength shift in the plasmon absorption band of silver nanoparticles. These results make it possible to explain particular features of the absorption spectrum of photothermorefractive glasses.

Longitudinal coherence length of an optical field

Abstract: Longitudinal coherence of an optical field depending on the parameters of its frequency and angular spectra is considered. Expressions for the function and length of longitudinal coherence depending on the width of the frequency and angular spectra are obtained. The competitive influence of the angular and frequency spectra of the field on its longitudinal coherence is discussed. Experimental studies using a longitudinal shearing Michelson interferometer that proves theoretical results are performed.

Dynamics and Efficiency of the Photosensitized Singlet Oxygen Formation by Chlorin e 6 : The Effects of the Solution pH and Polyvinylpyrrolidone

Abstract: The effects of the solution pH and polyvinylpyrrolidone on the dynamics and efficiency of the formation of singlet oxygen 1O2 in buffer media (pH 6.3–8.5) photosensitized by chlorin e 6 are studied. It is demonstrated that the quantum yield of the 1O2 formation photosensitized by chlorin e 6 decreases with decreasing solution pH due to the aggregation of photosensitizer molecules. Polyvinylpyrrolidone facilitates the disaggregation of chlorin e 6, thus increasing its photosensitizing ability. For a complex of chlorin e 6 with this polymer, the luminescence kinetics of singlet oxygen is inverted, which should be taken into account in the determination of the lifetime of 1O2 in real biological systems.

Suppression of the spectral selectivity of two-layer phase-relief diffraction structures

Abstract: The method and results of studying the effect of relief depth on the attainable degree of reducing the spectral selectivity of two-layer diffraction structures based on plastics and glass. The possibilities of suppressing the spectral selectivity of double- and single-relief structures are compared.

Modeling of optical radiation energy distribution in plant tissue

Abstract: A three-dimensional mathematical model of interactions of optical radiation with plant tissue taking into account its structural inhomogeneity, spectral properties, and the effects of fluorescence is constructed. The developed model is implemented using the statistical Monte Carlo method for the Henyey-Greenstein phase function. The dependence of differential backscattering and fluorescence coefficients on the concentration of photosynthetic pigments (chlorophylls) is numerically studied. It is demonstrated that numerical characteristics agree with results of physical experiment. The approximate solution based on the expansion of the diffusion and fluorescence radiation fluxes into a series in terms of a small parameter is found. This expansion makes it possible to calculate the field of backscattered radiation with satisfactory accuracy and to qualitatively correctly describe the experimentally observed dependences of the fluorescence coefficient in the region of high chlorophyll concentration.

Slow light and saturable absorption

Abstract: Quantitative analysis of slow light experiments utilising coherent population oscillation (CPO) in a range of saturably absorbing media, including ruby and alexandrite, Er3+:Y2SiO5, bacteriorhodopsin, semiconductor quantum devices and erbium-doped optical fibres, shows that the observations may be more simply interpreted as saturable absorption phenomena. A basic two-level model of a saturable absorber displays all the effects normally associated with slow light, namely phase shift and modulation gain of the transmitted signal, hole burning in the modulation frequency spectrum and power broadening of the spectral hole, each arising from the finite response time of the non-linear absorption. Only where hole-burning in the optical spectrum is observed (using independent pump and probe beams), or pulse delays exceeding the limits set by saturable absorption are obtained, can reasonable confidence be placed in the observation of slow light in such experiments. Superluminal (“fast light”) phenomena in media with reverse saturable absorption (RSA) may be similarly explained.

Influence of light polarization on characteristics of a collinear acoustooptic diffraction

Abstract: The collinear acoustooptic diffraction of arbitrarily polarized radiation is studied. It is shown that the spectrum of diffracted light at the exit of an acoustooptic cell generally consists of four components, which have different frequencies and polarizations. Beats of these components lead to the modulation of the light passed through an analyzer installed at the exit of the system. Dependences of the amplitudes of components of the passed radiation on the frequency and power of an acoustic wave are studied for different orientations of the polarizer and analyzer.

Influence of IR radiation on the carotenoid content in human skin

Abstract: It is shown that the infrared irradiation decreases the content of β-carotene and lycopene carotenoids in human skin. A decrease in the content of β-carotene and lycopene may indicate that the IR radiation, as well as the UV radiation, is capable of forming free radicals in human skin. The investigations were performed in vivo using the technique of resonance Raman scattering developed by us for the noninvasive determination of antioxidant potential in skin.

Optical properties of the HfO 2 − x N x and TiO 2 − x N x films prepared by ion beam sputtering

Abstract: Optical characteristics of the HfO2 − xNx and TiO2 − xNx films obtained by reactive ion beam sputtering have been investigated by spectral ellipsometry. The chemical composition of the films was determined using X-ray photoelectron spectroscopy. The nitrogen content in the oxynitride films (determined by the N2/O2 ratio in the gas mixture during synthesis) reached ≈9 at % for TiO2 − xNx and ≈ 6 at % for HfO2 − xNx. It is found that the dispersion relations n(λ) and k(λ) for the TiO2 − xNx films change from those characteristic of titanium dioxide to those typical of titanium nitride with an increase in the nitrogen content from 0 to ≈9 at %. The optical parameters of the HfO2 − xNx films depend weakly on the nitrogen content in the range 0–6 at %.

Holographic prism as a new optical element: I. Principle of operation and experimental implementation

Abstract: A new multivalued measure of plane angles is proposed, which is based on a highly stable system of superimposed holograms recorded in a photochromic material. Their mutual position forms a reference set of angles, which is stored by the measure and can be reproduced by a reference laser. The methods for fabricating a holographic measure (recording and reproducing holograms forming it) are discussed. One of two possible modifications of such a measure is implemented on the basis of calcium fluoride crystals with color centers. The technique for preparing this measure (holographic prism) and its properties are described. Based on this prism, one can develop a new generation of angle-measuring or -setting instruments that simultaneously satisfy two contradictory requirements, i.e., mobility and high discreteness and accuracy of angular measurements.

Classical optical experiments and special relativity: A review

Abstract: Classical optical experiments that confirm the validity of special relativity are considered. Transformations of spatial coordinates and time that were proposed at different times for the passage from one inertial reference frame (IRF) to another and that differ from the classical Lorentz transformations are critically analyzed. It is shown that, although some of these transformations are capable of explaining the results of single classical optical experiments, in particular, the Michelson-Morley experiments, neither of them, except for the Tangherlini transformations, can explain the results of the entire set of these experiments. The discrepancy between the predictions of incorrect transformations and the results of the well-known experiments is caused by the absence of a clearly formulated procedure for synchronizing spaced clocks in a rest IRF (where the observer is located) and a moving IRF, which should be consistent with the transformation of time. A number of relativistic and quantum effects are indicated, which have been predicted but not yet detected, to a search for which efforts of physicists are directed, and which are convenient to describe with the help of the formalism of the Tangherlini transformations.

Spectral luminescence properties of transparent lead fluoride nanoglassceramics doped with erbium ions

Abstract: Transparent nanoglassceramics has been synthesized based on lead fluoride silicate glass doped with erbium ions. It is shown that heat treatment of lead fluoride silicate glass leads to the formation of nanoscale (16–40 nm) crystalline phase of lead fluoride. X-ray diffraction analysis and measurement of spectral luminescence properties have revealed that the erbium ions enter the crystalline phase of lead fluoride.

Negative circular polarization of InP QD luminescence: Mechanism of formation and main regularities

Abstract: The spectrum and kinetics of the circular polarization of InP quantum dot (QD) photoluminescence have been experimentally investigated under different conditions of optical excitation and at different bias voltages applied to the sample. It is established that, at a bias of about −0.1 V, the degree of photoluminescence polarization is negative and reaches −50% in limiting cases. It is concluded that the negative polarization is formed in QDs containing one recident electron per dot and is mainly caused by the optical orientation of the electron spin. It is shown that all experimentally observed regularities are well described in the framework of the model assuming the energy relaxation of photogenerated electron-hole pairs accompanied by the electron- hole spin flip-flop process.

Radiation reflection from inhomogeneities moving in a medium with a plasma dispersion law

Abstract: The dependence of the complex frequency of radiation reflected from an inhomogeneity moving in a medium on the frequency of the incident radiation is found for inhomogeneous wave regimes. Explicit expressions for the plasma dispersion law of the medium are presented. The complex Doppler effect, where one (real) frequency of the incident radiation corresponds to two complex frequencies of the reflected radiation, is demonstrated.

Generation of ultrabroadband terahertz radiation under optical breakdown of air by two femtosecond pulses of different frequencies

Abstract: Experimental results on the generation of terahertz (THz) electromagnetic radiation under ionization of air by femtosecond pulses at fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. The mean power of the generated THz radiation as a function of the time delay between two pulses of different frequencies is found to be quasi-periodical. Theoretically, we show that efficient generation of THz radiation is governed by the inertial part of the nonlinear response of the medium, which is determined by the dynamics of population of highly excited states with subsequent emission of electrons.

Backscattering of linearly and circularly polarized light in randomly inhomogeneous media

Abstract: The scattering of linearly or circularly polarized light from a semibounded randomly inhomogeneous medium is considered. A new technique for simulating the electromagnetic radiation transport using the Monte Carlo method is proposed, which makes it possible to avoid cumbersome calculation of Muller matrices. Expressions are obtained for the co- and cross-polarized components of backscattered light for incident light of arbitrary polarization. The coherent and incoherent backscattering components are calculated for arbitrary combinations of incident and scattered light polarizations. It is shown that the main contribution to coherent backscattering is from the co- and cross-polarized components for linearly and circularly polarized light, respectively. The backscattering from an optically active random medium is calculated.

Energy levels of rare-earth ions in Gd2O2S

Abstract: The energies of the ground 4fn levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd2O2S crystal has been determined. It is shown that the Pr3+, Tb3+, and Eu3+ ions can be luminescence centers in Gd2O2S. The levels of the Nd3+, Dy3+, Er3+, Tm3+, Sm3+, and Ho3+ ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce3+ ion is near the midgap, due to which Ce3+ effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd2O2S:Pr and Gd2O2S:Tb phosphors.

Subnanosecond spectrofluorimetry of new indolocarbazole derivatives in solutions and protein complexes and their dipole moments

Abstract: The spectral and temporal characteristics of new 6,12-dimethoxyindolo[3,2-b]carbazole, 5,11-dimethyl-6,12-dimethoxyindolo[3,2-b]carbazole, and 5,11-dihexyl-6,12-di(hexyloxy)indolo[3,2-b]carbazole fluorescence probes in organic solvents and protein complexes are studied The dipole moments of indolocarbazoles in 1,4-dioxane were measured by electrooptical absorption method The measured dipole moments have values within the range of (31–36) × 10−30 C m in the equilibrium ground state and increase to (48–56) × 10−30 C m after excitation The excited state lifetime of indolocarbazole derivatives increases with increasing polarity and viscosity of the environment The binding of indolocarbazoles with trypsinogen and human serum albumin increases the fluorescence intensity, changes the intensity ratio of fluorescence bands, and increases the average excited state lifetime of indolocarbazoles The analysis of the instantaneous fluorescence spectra and fluorescence decay parameters at different wavelengths revealed the existence of several types of probe binding sites in proteins It is found that the fluorescence characteristics of indolocarbazole derivatives depend on the conformation rearrangements of trypsinogen due to its thermal denaturation