Polarization Phenomena in Transparency and Absorption Effects Induced by the Field of Unidirectional Waves

Physical processes that form the spectra of saturated absorption and magnetic scanning resonances on an atomic transition with level momentum J = 1 in the field of unidirectional waves of arbitrary intensity under variation in their polarization directions are investigated by numerical simulation and analytically. It is shown that anomalies in nonlinear resonance spectra are determined by the polarization of the waves and the degree of openness of the atomic transition, while anomalies in the experimentally observed magnetic scanning spectra are attributed to the magnetic coherence induced by the fields directly on the levels of the lower state, rather than to its transfer from the excited states, as was assumed earlier.     

The influence of quantum interference effects on the resonance fluorescence spectra of a degenerate three-level atom

The resonance fluorescence spectra of a degenerate three-level atom of the V-type in the field of an intense monochromatic wave with an arbitrary polarization composition are investigated. Analytical expressions are derived for the resonance fluorescence spectra, and the angular distribution of spontaneous fluorescence of atoms is analyzed for the D-line emitted by vapors of alkali atoms. It is shown that the number of lines in the spectrum may decrease in the case of the linear polarization of spontaneous radiation. The radiation relaxation operator is obtained for the D-line of alkali metals in the case when an atom is near the metal surface. Interference effects for such systems are analyzed.     

Polarization phenomena in the transparency and adsorption effects induced by the field of counterpropagating waves

The physical processes that form the resonances of saturated absorption and magnetic scanning in the field of counterpropagating waves of an arbitrary intensity when their polarizations change are numerically simulated. The atomic transition with level moment J = 1 is used as an example to show that the anomalies in the experimental saturated absorption spectra are determined by the degree of opening of the atomic transition. In the case of magnetic scanning, the anomalies are caused by the magnetic coherence induced by the wave fields at the levels of the lower state rather than by its transfer from the excited states, as was proposed earlier.     

One- and two-photon resonance fluorescence of a three-level atom at high photon densities

We consider the possible physical processes that may arise in a three-level atom when only two of its levels interact with a strong electromagnetic field and when the atomic transition frequency is nearly equal to once and twice the frequency of the laser field, respectively. There have been found pronounced cooperative effects in the spectrum of the two-level system, which is in resonance with the laser field, arising from the presence of the third level. The excitation spectra describing the transitions from the first excited state into the second excited state and from that to the ground state consist, apart from the two central peaks, of two pairs of sidebands which are induced by the laser field of the neighbouring system. Detailed expressions of the spectral functions for the physical processes of one- and two-photon resonance fluorescence have been derived and discussed in the limit of high photon densities. The excitation spectrum of the low frequency modes has been considered and discussed in detail. It is found that quantum beats in spontaneous emission may appear in the spectra of the one- and two-photon resonance fluorescence arising from the interference between the two atomic transition frequencies and the frequency of the laser field. The importance of the low frequency modes that occur in the processes in question has been pointed out.     

Manifestation of higher polarization moments in the dipole emission under conditions of stepwise excitation and anisotropic collisions

The induction of higher polarization moments-the octupole orientation and the hexadecapole alignment—in the case of two-step excitation of atoms through dipole transitions and their manifestation in the polarization of dipole emission under conditions of anisotropic collisions are studied. For all channels of the two-step excitation process J ₀→J ₁ J, the efficiencies of production of higher polarization moments with integer and half-integer angular momenta from J=2 to J=7 through the absorption of linearly or circularly polarized light are calculated. The rates of collisional relaxation and interconversion of higher polarization moments, for both orientation and alignment, as functions of frequency detuning of the laser line from the center of the Doppler profile are calculated. The theory is illustrated by an example of two channels of excitation of the atomic state J=2 by circularly polarized light: J ₀=0→J ₁=1J=2 and J ₀=3→J 1=2 (or 3)→J=2. In the first case, the octupole orientation enhances or attenuates the signal of circular polarization depending on the laser frequency detuning. In the second case, it represents the only source producing orientation and circular polarization of the emitted light.     

Neon 3p-excitation by proton impact (100 keV-1 MeV): Cross sections and line polarization

The excitation of all ten levels of the neon 2p ⁵3p configuration by proton impact (100 keV-1 MeV) has been investigated by the spectral analysis of photon emission. Absolute emission cross sections have been obtained by calibration with a standard light source. Cascade effects are shown to be very important especially in the case ofJ=1 levels. For the 3p(¹ S ₀) level, which we studied additionally for electron impact excitation (100 eV-1 keV), our experimental cross sections are compared with the first Born approximation. Furthermore the polarization of sixteen 3p—3s emission lines has been measured as a function of impact energy. The energy dependence of the line polarization is characterized by theJ quantum numbers of the atomic states involved in the respective transition. An anomalous polarization of all lines originating from the decay ofJ=1 levels has been found and is referred to dominant cascade effects.     

On a thin structure of atomic spectra in an intense resonance electromagnetic field

The interaction of an atom with an intense resonance radiation field is considered. Quasi-energy levels are found which can differ essentially from the usual thin structure of atomic levels at moderate intensities. Spontaneous emission of such a system is considered and the interaction of an atom with a depolarized field is investigated in detail. In the latter case some specific shifts and broadening of emission or absorption lines depending essentially on the polarization and coherence of the resonance field are predicted.     

Effect of quantum interference processes on the angular distribution of the spontaneous radiation in the D line of alkali-metal vapors in a laser wave field

The resonance fluorescence of a degenerate V-type three-level atom in the field of an intense monochromatic wave with arbitrary polarization composition is investigated. The equations of motion, the general form of the radiation relaxation operator, and the analytical expressions for the angular distribution of the intensity of the spontaneous radiation from atoms, and the total intensity of the resonance fluorescence for such systems are obtained. The angular distribution of the spontaneous radiation from atoms for the D line of alkali-metal vapors is investigated. It is predicted theoretically that the intensity of the resonance fluorescence will decrease as the intensity of the pump wave increases in observations in a direction of the electric field vector of the laser wave.     

Influence of interference effects of spontaneous emission on the behavior and spectra of resonance fluorescence of a three-level atom in a strong wave field

An investigation is made of the influence of quantum interference processes accompanying radiative relaxation of excited states on the population dynamics, total intensity, and spectra of the resonance fluorescence of three-level V-type atoms. Analytic expressions are obtained for the total intensity and spectra of the resonance fluorescence taking into account the off-diagonal nature of the radiative relaxation operator. It is shown that quantum interference process can substantially alter the total spontaneous emission intensity of the atoms and the population dynamics of the atomic levels, as well as the resonance fluorescence spectra.     

Shift and asymmetry of the saturated absorption resonance in the field of counterpropagating elliptically polarized waves

The saturated absorption resonance in an atomic gas in the field of counterpropagating light waves with an arbitrary elliptical polarization is investigated. A conclusion about the peculiarities of the resonance shape is drawn from a general analysis of the symmetry of the problem. In particular, a new effect has been found: the polarization parameters of the light waves can lead to an asymmetry and shift of the resonance. These conclusions are corroborated by an approximate analytical solution for the F _g = 1 → F _e = 2 transition and by numerical calculations. Apart from their fundamental importance, the results obtained can have important applications in metrology (frequency and time standards).     

FT-EPR study on the sign of exchange interaction in triplet naphthalene-galvinoxyl encounter pair

Chemically induced dynamic electron polarization (CIDEP) created in the quenching of triplet naphthalene by galvinoxyl were investigated by time-resolved Fourier-transform electron paramagnetic resonance (FT-EPR) measurements with monitoring a free induction decay signal of a pulsed microwave irradiation. Transient FT-EPR spectra of galvinoxyl with CIDEP were observed in various nonpolar solvents with different viscosity. A transient FT-EPR signal phase shows remarkable dependence on the viscosity: FT-EPR signal phases were absorption and emission in the solvents with low and high viscosity, respectively. Time evolutions of the FT-EPR signal of galvinoxyl were well simulated by a model of the radical-triplet pair mechanism (RTPM) for CIDEP. A sign of theJ value in the triplet naphthalene-galvinoxyl system in various solvents were discussed on the basis of the sign rule in the RTPM and the transient FT-EPR signal phase. One of possible explanation for the solvent viscosity dependence of the transient FT-EPR signal phase was pressented on the basis of hypothetical model of theJ value.     

A new type of resonances in saturated absorption spectroscopy of 3-level systems

We report the experimental observation of new resonances in saturated absorption spectra of a J = 1 to J = 0 transition of Ne atoms in a static magnetic field. These resonances, which are distinct from the well-known Zeeman and cross-over resonances, result from the modification of stimulated Raman processes by the simultaneous resonant saturation of an optical transition. The light-shifts of the various resonances are also studied.     

<Emphasis Type="Italic">S</Emphasis>-matrix approach to the problem of high-harmonic generation in the field of intense laser wave

A method using expansion of the wave function in the basis of photonic and free atomic eigenstates is proposed for calculating the emission spectrum of an atom in a laser field. The wave function is constructed using the Kramers?Henneberger transformation so that the expression for the transition S matrix explicitly includes the nonlinear interaction with the laser field. The expansion coefficients are determined by the residual interaction, which depends on the coordinates of the classical free electron motion in the laser field. Resonances at the atomic transition frequencies explicitly arise in the emission spectrum when the residual interaction is considered in the first order. The numerical solution of the timedependent Schro¨ dinger equation for the hydrogen atom within the semiclassical approach is used to obtain emission spectra for laser pulses of different intensities and durations.     

Polarization,spectral, and spatial emission characteristics of chiral semiconductor nanostructures

A detailed study of the degree of circular polarization and the angular dependence of the emission spectra of an array of InAs quantum dots embedded in GaAs photonic nanostructures with chiral symmetry in the absence of an external magnetic field is carried out. A strong angular dependence of the spectra and the degree of circular polarization of radiation from quantum dots, as well as a significant effect of the lattice period of the photonic crystal on the radiation characteristics, is observed. The dispersion of photonic modes near the (±3, 0) and (±2, ±2) Bragg resonances is investigated in detail. The experimentally observed polarization, spectral, and angular characteristics of the quantum-dot emission are explained in the framework of a theory describing radiative processes in chiral photonic nanostructures.     

Spectral behavior study of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone

The photophysical properties such as singlet absorption, molar absorptivity, fluorescence spectra, fluorescence quantum yield (?_f) and transition dipole moment (μ₁₂) of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone (DMAPAPP) have been studied in different media. DMAPAPP exhibits a large red-shift in both absorption and emission spectra as the solvent polarity increases, indicating a large change in dipole moment of molecule upon excitation. The fluorescence quantum yield depends on the nature of the solvent. The absorption and emission spectra of DMAPAPP in dioxane–water mixture are also studied. The effect of different type of surfactants to determine their critical micellar concentration (CMC) and the microemulsion effect on the electronic absorption and emission spectra of DMAPAPP are recorded. The effect of acidity on the electronic absorption and emission spectra of DMAPAPP is studied to determine the pK_a and pK_a^? values.     

Features of Emission and Absorption of Indotricarbocyanine Dyes on Excitation to Higher Electronic States

The manifestation of ion pairs in the absorption and emission of fluorescence by symmetric indotricarbocyanine dyes with the same cation and anions Br, I, and BF₄ in ethanol, methylene chloride, and dichlorobenzene has been investigated. It is shown that the formation of ion pairs on passing to low-polarity solvents, along with the changes in the electronic absorption and emission spectra, manifests itself in the polarization and fluorescence excitation spectra. We were the first to establish that the presence of an equilibrium mixture of contact ion pairs and free ions in solutions has an effect on the position and probabilities of transitions not only for the first but also for higher excited singlet states of the molecules of polymethine dyes. The formation of ion pairs manifests itself more clearly when the spectral-luminescence properties of compounds in the shortwave spectral region are analyzed. It has been established that marked changes arise in the absorption, excitation, emission, and polarization fluorescence spectra, as well as in the lifetime and quantum yield of fluorescence in this spectral region when the solvent is changed.     

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.