Selective reflection combined with Fabry--Perot effects from two-level atoms confined between two dielectric walls

The coefficient of selective reflection at oblique incidence from two-level atoms confined between two dielectric walls is calculated in this paper. It is found to be related to the transient behaviour of atoms after colliding with the wall and the distribution of the field inside the vapour corresponds to L / \lambda , with L the thickness of the film and \lambda the incident wavelength. We find that the sub-Doppler structure is manifest both for normal incidence and small angle oblique incidence. It is feasible to detect the real part of selective reflection in several cases that have not been achieved before.     

Level- and line-crossing experiments in selective reflection from mercury vapour

When the technique of magnetic depolarization is applied on selective reflection of the 2537 Å resonance line from mercury vapour Doppler- and pressure-broadened level-crossing curves are received. From the pressure dependence of the damping constant, determined as: Г_c=(5.3±0.5) · 10^?9 ·N sec^?1 cm³ the cross section of resonance broadening is derived for 280 °C as: σ=(1550±150) · 10^?16 cm². Besides the zero-field level-crossing effect an additional interference effect is observed when a mixture of different isotopes is used. This line-crossing effect allows the determination of isotope shift and hyperfine structure and gives a new possibility for studying foreign gas pressure-broadening. Experiments were performed with the same apparatus in selective reflection as well as in forward-scattering. The experimental level- and line-crossing curves principally agree with curves calculated according to a theory of Series when the incident spectral distribution is considered.     

反射型体全息光栅对超短脉冲激光光束衍射的性质

在光栅记录介质色散效应的影响下,拓展Kogelnik的耦合波理论研究了反射型体全息光栅对偏振方向垂直和平行于入射面的超短脉冲激光光束衍射的性质。结果表明对于记录在LiNbO3光折变晶体中的体全息光栅,在色散效应、光栅参量和入射条件的共同影响下,光栅对偏振方向垂直于入射面的超短脉冲激光光束衍射的光谱带宽大于对偏振方向平行于入射面的超短脉冲激光光束衍射的光谱带宽,在不考虑光栅介质色散效应的影响时,它们都变大。进一步给出了衍射光的光谱带宽及光栅的衍射效率随入射脉冲的光谱带宽与光栅的光谱带宽比值的变化情况。     

Statistical and coherence effects in nonlinear optical parametric up conversion. II. Incoherent interaction

Parametric up conversion (PUC) by incoherent nonlinear optical mixing of second-order coherent strong pumping radiation and another weak chaotic input radiation with a finite spectral width is treated in the second part of this paper. The efficiency of PUC is calculated in the second approximation of the iterative method. It is shown that in the case of perfect phase matching the efficiency of PUC decreases with increase of both the spectral width of the input radiation and the dispersion of medium, whilst at considerable values of phase mismatch the reverse effect appears. The spectral distribution of the resulting sum-frequency radiation is calculated in the first approximation of the iterative method. There is a general tendency to narrowing of the spectral distribution of the generated radiation in the course of PUC. Moreover, when there is a phase mismatch present, a spectral shift of the central maximum of generated radiation towards the blue or red region, according to the signs of the phase mismatch and the typical dispersion coefficient, appears in the later phases of the PUC.     

Double and negative reflection of cold atoms in non-Abelian gauge potentials

Atom reflection is studied in the presence of a non-Abelian vector potential proportional to a spin-1/2 operator. The potential is produced by a relatively simple laser configuration for atoms with a tripod level scheme. We show that the atomic motion is described by two different dispersion branches with positive or negative chirality. As a consequence, atom reflection shows unusual features, since an incident wave may split into two reflected ones at a barrier, an ordinary specular reflection, and an additional nonspecular one. Remarkably, the latter wave can exhibit negative reflection and may become evanescent if the angle of incidence exceeds a critical value. These reflection properties are crucial for future designs in non-Abelian atom optics.     

Wavelength dependence four-wave mixing spectroscopy in a micrometric atomic vapour

This paper presents a theoretical study of wavelength dependence four-wave-mixing (FWM) spectroscopy in a micrometric thin atomic vapour. It compares three cases termed as mismatched case Ⅰ, matched case and mismatched case Ⅱ for the probe wavelength less, equal and greater than the pump wavelength respectively. It finds that Dickenarrowing can overcome width broadening induced by Doppler effects and polarisation interference of thermal atoms, and high resolution FWM spectra can be achieved both in matched and mismatched wavelength for many cases. It also finds that the magnitude of the FWM signal can be dramatically modified to be suppressed or to be enhanced in comparison with that of matched wavelength in mismatched case Ⅰ or Ⅱ. The width narrowing and the magnitude suppression or enhancement can be demonstrated by considering enhanced contribution of slow atoms induced by atom-wall collision and transient effect of atom-light interaction in a micrometric thin vapour.     

Quantum interference effect on low-energy He atomic beam scattering at the surface of He II

The specular reflection coefficient for low-energy ⁴He atoms incident on the free surface of superfluid ⁴He is calculated as a single particle motion coupled to the ripplon field in an effective surface potential. We find a characteristic dip in the reflection coefficient as an interference effect for the truncated surface potential.     

Transformation of electromagnetic radiation by rapidly moving inhomogeneities of a transparent medium

A theory of reflection and transmission of electromagnetic radiation by inhomogeneities of the parameters of a static transparent medium moving at the velocity of light is developed. Expressions are obtained for the Doppler frequency shift of radiation; it turns out that, under the condition of pronounced frequency dispersion, the frequency of incident radiation corresponds to two frequencies of reflected radiation (complementary waves). It is found that, as the velocity of an inhomogeneity tends to the phase velocity of radiation in the medium, the reflection and transmission coefficients of radiation by the inhomogeneity indefinitely increase. It is shown that the electromagnetic radiation frequency may increase severalfold, with a transformation coefficient of about unity, due to the Doppler shift by the inhomogeneities of a nonlinear medium that are induced by pulses (solitons) of intense counterpropagating radiation.     

Radiation loss of electrons under scattering by a Thomas-Fermi atom

A universal theory and calculation results for the bremsstrahlung of electrons on complex atoms are presented. The theory accounts for the dynamic polarization of the core in the energy range from 0.5 to 10 keV, which is characteristic of radiation energy losses in a hot plasma with heavy ions. The treatment is based on the statistical atom model and the quasi-classical approximation of the incident electron. The model accounts for the penetration of the incident electron into the atomic core, which affects the relationship between the polarization and static radiation channels. The contribution of the polarization channel in both the spectral and the total radiation loss of electrons at various frequencies, nucleus charges, and energies of the incident particle is analyzed. It is shown that the contribution of the polarization channel is comparable with that of the static channel (which was calculated elsewhere) in a wide range of parameters. The results obtained are in a reasonable quantitative agreement with the detailed quantum-mechanical calculation carried out for individual atoms.     

X-ray Bragg diffraction in asymmetric backscattering geometry

We observe three effects in the Bragg diffraction of x rays in backscattering geometry from asymmetrically cut crystals. First, exact Bragg backscattering takes place not at normal incidence to the reflecting atomic planes. Second, a well-collimated (approximately 1 microrad) beam is transformed after the Bragg reflection into a strongly divergent beam (230 microrad) with reflection angle dependent on x-ray wavelength--an effect of angular dispersion. The asymmetrically cut crystal thus behaves like an optical prism, dispersing an incident collimated polychromatic beam. The dispersion rate is approximately 8.5 mrad/eV. Third, parasitic Bragg reflections accompanying Bragg backreflection are suppressed. These effects offer a radically new means for monochromatization of x rays not limited by the intrinsic width of the Bragg reflection.     

Thin film on metal: Resonance effects in light diffraction at the edge

The theory of light diffraction at the impedance step is developed which allows for an additional surface wave. Reflection and transmission coefficients for a surface wave are determined as well as radiative losses at the diffraction of the surface polaritons (SP). Coefficients of transformation of the incident bulk radiation energy to the SP energy are calculated. Resonance effects in the mirror reflection of bulk radiation are investigated. Possible application of the predicted effects for the study of optical properties of thin films and transition layers are discussed.     

Anomalous laser energy absorption associated with resonance saturation

Laser energy absorption measurements have been undertaken in an experiment involving the transmission of a pulse of laser radiation through sodium vapour. The wavelength of the laser was tuned to overlap the 589 nm resonance transition of the sodium atoms. Although simple radiation transport appears to account for the attenuation of the laser beam at low values of incident laser irradiance, anomalously large absorption has been observed at high values of incident laser energy. We suggest that this anomalous absorption of laser energy can be regarded as evidence of superlastic electron heating and subsequent ionization.     

Density profiles and collective excitations of a trapped two-component Fermi vapour

We discuss the ground state and the small-amplitude excitations of a degenerate vapour of fermionic atoms placed in two hyperfine states inside a spherical harmonic trap. An equations-of-motion approach is set up to discuss the hydrodynamic dissipation processes from the interactions between the two components of the fluid beyond mean-field theory and to emphasize analogies with spin dynamics and spin diffusion in a homogeneous Fermi liquid. The conditions for the establishment of a collisional regime via scattering against cold-atom impurities are analyzed. The equilibrium density profiles are then calculated for a two-component vapour of ⁴⁰K atoms: they are little modified by the interactions for presently relevant values of the system parameters, but spatial separation of the two components will spontaneously arise as the number of atoms in the trap is increased. The eigenmodes of collective oscillation in both the total particle number density and the concentration density are evaluated analytically in the special case of a symmetric two-component vapour in the collisional regime. The dispersion relation of the surface modes for the total particle density reduces in this case to that of a one-component Fermi vapour, whereas the frequencies of all other modes are shifted by the interactions.     

The peculiarities of radiation of chiral photonic crystals with isotropic defect layer

The peculiarities of defect modes and reflection spectra of chiral photonic crystals (CPC) with an isotropic defect for various thicknesses of the defect layer were investigated. The peculiarities of the spectra of photonic density of the state (DOS) and those of the intensity at the center of the defect were investigated, too. It was shown that there is one more possibility of tuning of laser emission - by the change of the defect layer thickness - in CPCs doped with laser dyes (with resonance atoms) and with an isotropic defect. It was shown that the given system can work as a tunable narrow-band filter (or mirror) with tunable wavelength width and tunable location of frequency of the complete transmittance (reflection) band. The radiation peculiarities of these systems were discussed. It was shown that in certain conditions an anomalous strong radiation can be observed for defect modes, and low-threshold laser generation is possible, again, for these modes. It was also shown that radiation suppression is possible for defect modes. The peculiarities of group velocity, group velocity dispersion and non-reciprocity were studied.     

Parametric Doppler effect in the case of oblique incidence in a medium with a plasma dispersion law

The parametric Doppler effect that occurs upon reflection of a plane monochromatic wave obliquely incident on an inhomogeneity of the refractive index moving in a medium with a frequency dispersion (homogeneous wave regimes) is analyzed. For a plasma-type frequency dispersion, the dependences of the frequency and angle of reflection on the frequency of the incident wave and on the angle of incidence are found.     

Resolution enhancement in noise spectrum by using velocity selective optical pumping in cesium vapor

We demonstrate experimentally that the resolution of amplitude noise spectrum in Cs atomic vapor can be enhanced by narrowing the absorption using velocity selective optical pumping technique. It is found that the steep atomic dispersion accompanied by high absorption leads to more conversion of laser phase noise to amplitude noise, when the field propagates throughout the atoms, and meanwhile the spectral resolution is improved. The effect of optical pumping intensity on the spectrum resolution is experimentally discussed, and a theoretical explanation for this phenomenon is given, which shows that the phase-to-amplitude noise conversion is directly proportional to the dispersion of medium.     

Classical theory of non-degenerate sum-frequency generation by incoherent nonlinear optical mixing of coherent and chaotic radiations II. Coherent and chaotic input radiations — Efficiency and spectral distribution

Sum-frequency generation by incoherent nonlinear optical mixing of one coherent and one chaotic, mutually uncorrelated input radiations in a dispersive medium is treated in this paper. The efficiency of the process is calculated in the second approximation of the iterative method. It is shown that for perfect phase matching or small phase mismatch of interacting waves and small spectral width of chaotic input radiation the efficiency of incoherent sum-frequency generation can be enhanced compared with the coherent interaction due to the difference between group velocities of sub-frequency radiations. On the other hand, for greater spectral width the efficiency of the process decreases with increasing spectral width of chaotic input radiation. In the case of considerable phase mismatch the efficiency of sum-frequency generation first decreases, but then increases with increasing spectral width of chaotic input radiation. The spectral distribution of the resulting sum-frequency radiation is calculated in the first approximation of the iterative method. There is a general tendency to narrowing the spectral distribution of generated radiation in the course of the process. Moreover, when there is phase mismatch present, a spectral shift of the maximum of generated radiation towards blue or red region, according to the signs of the phase mismatch and the typical dispersion coefficient, appears in the later phases of the sum-frequency generation.This work was partially supported by Research Project C.P.B.P. 01. 07.     

透射型光折变体全息光栅对超短脉冲激光光束衍射的特性

在Kogelnik耦合波理论的基础上,考虑光栅记录介质的色散效应的影响,研究了光折变体全息光栅对不同偏振状态的超短脉冲激光光束衍射的性质,讨论了高斯型入射脉冲激光光束的谱宽与光栅的有效衍射谱宽之比不同时,衍射和透射光束的光谱宽度、时间宽度、波形和衍射效率的变化。结果表明,光栅的有效衍射谱宽受光栅参量及入射条件的影响,对衍射性质的影响很大,且在考虑光栅记录介质的色散效应时减小。当入射脉冲的偏振方向垂直于入射面时,光栅的有效衍射谱宽大于偏振方向平行于入射面的情形,衍射效率在入射脉冲宽度较大时小于偏振方向平行于入射面的情形;谱宽比较大时,衍射光束的时间分布曲线产生展宽和变形,且比偏振方向平行于入射面的情形展宽和变形得更加明显。     

Transformation of intensity fluctuations in nonlinear reflection of light

The transformation of intensity fluctuations accompanying the nonlinear resonance reflection of light from a glass-resonant gas interface is discussed. The illumination of the interface at angles close to the critical angle for total internal reflection is found to have certain advantages over orthogonal incidence. At the maximum of the strongest resonance, the reflected light intensity amounts to more than 50% of the incident light. In these conditions, a squeezing of the amplitude (photon) fluctuations is observed in the radiation generated by a semiconductor laser-nonlinear selective reflector system. A simple semiclassical theory adequately describes some important characteristics of this system: the shape of the reflection resonances, the strength of the saturation fields, and so on.     

Level- and line-crossing in forward scattering and selective reflection in a transverse magnetic field

LC-experiments in forward scattering and selective reflection are mostly performed in a longitudinal magnetic field. We have extended the investigations to a transverse field receiving the following results. The zerofield level- and line-crossing shows a different shape. From the line-crossing the isotope shift between Hg 200 and 202 was determined and the applicability of this method discussed. A narrowing of the width of the level-crossing in selective reflection was observed at low pressure.