EPR and NMR line shapes in disordered ferroelectric crystals

A theory is developed for the magnetic resonance line shape in disordered ferroelectric crystals. In a study of the random fields responsible for inhomogeneous line broadening, nonlinear and spatial correlation effects are taken into account. It is shown that homogeneous broadening depends on the temperature and on the magnitude and orientation of the external magnetic field, as well as on the nonlinearity parameters of the ferroelectric material. The resonance line shape is calculated as the envelope of homogeneously broadened Lorentzian spin packets. Analytic expressions for I ₂(ω) and I ₃(ω) are introduced to account for nonlinearity and correlation effects of second and third order, respectively. Calculations are done for centrally symmetric crystals, as well as for those without a center of symmetry. It is shown that homogeneous broadening is important near the line peak, while the wings are determined mainly by inhomogeneous broadening. Manifestations of the predicted effects in the spectrum lines of disordered ferroelectric crystals are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 340–347 (February 1998)     

Experimental investigation of the oscillator strength of the exciton transition in GaAs single quantum wells

A technique that makes it possible to investigate the mechanisms of phase relaxation of excitons in GaAs single quantum wells has been developed using resonant reflection spectroscopy. The dependence of the oscillator strength of the exciton transition on the quantum well thickness has been measured in the thickness range 9.1–30.0 nm. It has been demonstrated that the oscillator strength with a high accuracy does not depend on the temperature in the range 8–90 K. The temperature dependence of the homogeneous broadening has been measured, and the inhomogeneous broadening of the resonance exciton line has been determined. A nonmonotonic dependence of the spectral broadening of the exciton line on the intensity of the resonant excitation at a temperature of 8 K has been revealed for the sample with a high-quality quantum well. It has been established that an increase in the excitation level by five orders of magnitude above the linear limit leads to an insignificant change in the oscillator strength of the exciton transition and to a multiple broadening of the spectral line profile.     

Modulation of optical FID and structure of absorption line due to superhyperfine interaction in ruby

Modulation of optical free induction decay (FID) and structure of R₁ absorption line due to superhyperfine interaction in ruby have been observed by using Stark-switching and Stark-sweeping techniques, respectively. The inhomogeneous broadening is effectively eliminated by a narrow optical hole burning. Frequency and phase characteristics of the FID signal have also been examined. The agreement between experimental results and theoretical calculations is quite satisfactory.     

Evolution of the ESR spectrum at the metal-insulator transition in quasi-one-dimensional systems

The evolution of ESR spectra in metals containing both conduction electrons and localized paramagnetic centers with inhomogeneous broadening of the magnetic resonance is treated theoretically. It is shown that such a spectrum can be effectively narrowed when a relaxation bottleneck is present for an arbitrary distribution of inhomogeneous broadening. The temperature dependence of the ESR spectrum in the polymeric phase of RbC₆₀ is investigated experimentally in the region of the metal-insulator transition. Application of the calculations to the experimental data demonstrates good agreement with the physical model if it is assumed that the given material is a quasi-one-dimensional system. Zh. éksp. Teor. Fiz. 113, 1048–1057 (March 1998)     

Laser-induced spectral broadening and hole-burning in rare earth doped crystals and coherent optical spectroscopy for frequency upconversion

The excitation spectra for linear fluorescence and for frequency upconversion are recorded in Er³⁺ doped crystal Er:YAG. An excitation induced spectral broadening and hole-burning are shown to appear at the resonant frequency of the linear fluorescence excitation spectrum, where the upconversion is simultaneously enhanced. Rate equation analysis is applied to model the optical pumping processes in the crystal. We present evidence that the induced resonance broadening and hole-burning are the result of excitation competition between the linear absorption and the excited state absorption. Femtosecond pulse pairs are applied to excite the linear and nonlinear frequency conversion, resulting in coherent controlled branching ratio between the linear and nonlinear frequency conversion. The experimental reasults are in good agreement with the numerical evaluation based on optical Bloch equation. PACS 78.47.+p; 42.65.Re; 78.20.Ci     

Kinematical interaction of optical excitations and consequences

The effects of the kinematical interaction of optical excitations were analyzed. The energies of the additional, kinematical excitations, arising due to the three-particle boson processes, were found and compared to the energies of the normal excitonic states. It was also shown that the sharp absorption peak, appearing in the low frequence domain of the optical spectra, arises due to the exciton-exciton interaction. The broadening of the kinematical levels was estimated and it was concluded that the anomally high broadening of optical levels cannot be attributed to the kinematical interaction of excitons. In the frequency range close to the resonance, Urbach's rule was reproduced. The theoretical values of Urbach's coefficient are in a good agreement with the experimental data.     

Optimization of the Q factor of the magnetic M x resonance under optical pump conditions

Optimization of parameters of the M _x resonance, which excited between Zeeman sublevels of a single HFS level of the ground state of an alkali metal in a vacuum cell in a circuit with a single beam for pumping and detection (M _x magnetometer circuit), has been carried out. A simple model taking into account all main factors controlling the resolving power of the M _x resonance (including spin-exchange broadening and absorption in a thick layer of the cell has been constructed. It is shown that the spin-resonance broadening of the resonance line is mainly determined by the requirements imposed on the optical thickness of the cell, which considerably restricts the realization of advantages of ultranarrow (<1 Hz) lines in magnetometry. The experiment confirming the efficiency of the model has been carried out.     

Thickness dependence of exciton absorption in pure GaAs crystals at the “prequantum” limit

The optical absorption of GaAs crystals with thicknesses d=0.4−4.4 μm is measured in the exciton-polariton resonance region at a temperature of 1.7 K. As the thickness is reduced, both a broadening of the exciton line and increased absorption with a negligible Stark shift are observed. The way the absorption spectra vary with crystal thickness is examined in terms of a competition between two regions for light-exciton interactions in the crystal: in the field of surface charges and electric-field free. Fiz. Tverd. Tela (St. Petersburg) 40, 869–871 (May 1998)     

A hybrid-phonon resonance in a quasi-two-dimensional nanostructure

The coefficient of absorption of electromagnetic radiation by a quasi-two-dimensional electron gas placed in an oblique magnetic field is found. The scattering of electrons by optical phonons is shown to lead to resonant absorption. The shape of the resonance peaks on the absorption curve is studied, and their doublet nature is demonstrated. Finally, the dependence of the resonance peaks on the angle between the magnetic field vector and the confinement plane is investigated. Zh. éksp. Teor. Fiz. 111, 1092–1106 (March 1997)     

Polariton reflectance spectra from thin ZnSxSe1−x layers

A study of reflectance spectra from thin ZnS_xSe_1−x solid-solution layers in the region of excitonic resonances is reported. It has been found that an increase in sulfur concentration in the layers increases the inhomogeneous broadening of the quantized polariton lines. It has been established that the inhomogeneous line broadening in a reflectance spectrum depends on the magnitude of exciton-photon mixing; it is small in the long-wavelength region where the photon component of the polariton is large, and large at short wavelengths where the mechanical component dominates. Fiz. Tverd. Tela (St. Petersburg) 40, 867–868 (May 1998)     

Filtration of partially coherent optical radiation by means of multiphonon Bragg diffraction

The filtering properties of three-phonon acousto-optical Bragg diffraction for increasing the degree of coherence of a partially coherent optical field are investigated by the example of multiphonon acousto-optical interaction in a TeO₂ single crystal. Two possible mechanisms of coherent scattering upon the formation of the highest orders are taken into account. It is shown that the three-phonon interaction is more efficient by at least a factor of two than any mode of one-phonon diffraction realized at the same frequency and the same acousto-optical interaction length. The experimental investigation, using partially coherent radiation of a He-Ne laser (λ = 0.63 μm) diffracted from a transverse acoustic wave propagating in a TeO₂ single crystal, on the whole confirmed the basic theoretical foundations.     

Influence of inhomogeneous exchange interaction on the moments of the ferromagnetic resonance lines in polycrystalline ferrites

From the viewpoint of the spin-wave approach to the theory of ferromagnetic resonance, the first four moments of the absorption line are calculated in this paper. It is shown that in addition to the magnetic dipole-dipole interaction, the inhomogeneous exchange interaction which has been neglected in previous papers exerts substantial influence on the asymmetry and peakedness of the ferromagnetic resonance line.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 75–78, July, 1979.     

Impurity absorption of light involving resonant states of shallow donors in quantum wells

The energy spectrum of localized and resonant states of shallow donors in heterostructures GaAs/Al_xGa_1?xAs with quantum wells is calculated. The widths of the resonant states belonging to the second size quantization subband are determined. It is shown that the width of a resonance level is mainly determined by the interaction with optical phonons. The spectrum of impurity absorption of light due to electron transitions from the ground state of the donor to the resonant states belonging to the second size quantization subband is calculated.     

Interference phenomena in Doppler broadened quantum transitions: Amplification of intense radiation without population inversion

We study the effect of nonlinear interference processes on quantum transitions in intense resonant electromagnetic fields with allowance for changes in level populations, relaxation processes, incoherent excitation, and Doppler broadening of the transitions, on the absorption, amplification, and refraction of the interacting fields. The theory is generalized to the case of nonlinear interference interaction of two intense fields in open and closed three-level quantum systems. Using the density matrix, we derive general expressions in the case of stationary interaction that make it possible to analyze the optical characteristics for various configurations of the interfering transitions by a simple substitution of parameters. The possibility of amplifying light without saturated population inversion in a resonant transition is discussed. We formulate the conditions for such amplification and use examples to show that under appropriate changes in the initial level populations and the intensity of the auxiliary light, the inversionless amplification coefficient does not decrease with increasing intensity of the amplified radiation. We also show that allowance for these accompanying processes greatly affects the choice of optimal conditions for interference in optical transitions. As an illustration we list the results of a numerical analysis of possible experiments. Zh. éksp. Teor. Fiz. 113, 445–470 (February 1998)     

Formation of the pulsed electron-electron double resonance signal in the case of a finite amplitude of microwave fields

A theoretical study of the effect of microwave (MW) fields of finite amplitude on the process of pulsed electron-electron double resonance (PELDOR) signal formation is carried out. It is shown that the behavior of the experimentally observed values can be described by four vectors of partial magnetizations whose motion is reduced to precession in effective magnetic fields. In the case of strong spin-spin interaction, the PELDOR effect can be observed when a sufficiently powerful MW field is applied at pumping frequency to affect both components of the Pake doublet. The possibility of a “two-frequency” spin echo to appear under the action of two pulses with different carrier frequencies in the system where the inhomogeneous broadening of the electron spin resonance line contour is mainly determined by the dipole-dipole interaction is demonstrated.     

Effect of four-photon interactions on coherent population trapping in Λ-systems

The resonance fluorescence spectrum of a Λ-system excited by two resonant light fields is calculated using a Markov analysis. Analytical formulas are derived in the strong-field limit within and beyond the rotating wave approximation. It is shown that the resonance fluorescence of the system does not vanish during coherent population trapping. Its spectrum consists of two multiplets which are similar to a triplet in the resonance fluorescence spectrum of a two-level atom and lie at the electronic transition frequencies, together with two triplets located at the frequencies of four-photon processes involving the optical excitation fields. The latter are fundamental in character and impose limits on the lower bound of the dephasing rate for the Raman resonance owing to the effect of radiative decay of the dipole transitions on the dynamics of the ground state. The effect of four-photon dephasing on the absorption spectrum of a Λ-system is analyzed and found to lead to a substantial reduction in the depth of a dip in the absorption spectrum which vanishes as the laser field strength is increased. Zh. éksp. Teor. Fiz. 113, 144–167 (January 1998)     

Transmission Spectra of a Layer of Active Atoms upon Inhomogeneous Broadening

Estimation of conditions for manifestation of bistability in the resonance curves of the transmission of a layer of active atoms with inhomogeneous broadening has been made. It is shown that hysteresis in the frequency dependence of such a layer is possible only under the conditions of the Stark shift of the resonant transition levels.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 271–274, March–April, 2005.     

Inhomogeneous and homogeneous broadening effects onnuclear resonance experiments

Inhomogeneous broadening of resonance lines causes a reduction of absorption in nuclear resonance experiments. This is particularly devastating to the observation of the Mössbauer effect in isomers like ¹⁰⁷Ag with a half-life of 44s.It has been suggested in the literature that homogeneous broadening can provide overlap of resonance lines and thus theMössbauer effect can be recovered. The analysis presented in this paper shows that this is not the case for Mössbauer experiments, unless the radiative width Γ_Γ also increases. Although overlap occurs when the lines are homogeneously broadened to a total width Γ_{_}H which exceeds the natural width Γ, and the inhomogeneous width Δ, the Mössbauer absorption remains low because it is proportional to the factor of Γ_γ/Γ _H.     

Broadening mechanisms for the adsorbate-induced resonance in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems

Adsorption of alkali atoms on the (1 1 1) and (1 0 0) noble metal surfaces has been shown recently to induce long-lived resonances located inside the surface projected band gap. However, the width of these resonances, as it appears in two-photon photo-emission experiments, is much larger than the inverse of their lifetime. We report on a theoretical study of some broadening mechanisms of these resonance lines in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems at low coverage, including the homogeneous natural line broadening and the inhomogeneous statistical broadenings due to the distribution of adsorption heights associated to the quantal vibration of the alkali adsorbate and to the lateral disorder of alkali adsorption on the surface. The inhomogeneous mechanisms are shown to induce a very large broadening of the resonance line, in quantitative agreement with experimental results. The most important broadening effect appears to be the effect of the distribution of alkali adsorption heights.