Pulse shaping via modulation of resonant absorption

Propagation of monochromatic linearly polarized plane electromagnetic wave through resonantly absorbing anisotropic medium with frequency-modulated response is studied analytically and numerically. Frequency modulation is assumed to be provided by means of either mechanical vibration of a solid sample or modulation of quantum transition frequency by a driving low-frequency electromagnetic field. Possibility of generation of train of pulses with polarization of the incident field as well as with orthogonal polarization is shown. For each polarization, optimal combinations of values of four parameters that provide maximal ratio of peak pulse intensity to the average output intensity is found numerically. Possible realization of this resonant method of pulse shaping in laser crystal Dy²⁺:CaF₂ is discussed.     

Resonant Processes of Scattering with Allowance for Different Populations of Magnetic Sublevels

The effect of different populations of the magnetic sublevels of the ground state on the propagation of polarized radiation through resonant media is studied theoretically. The nonlinear refractive indices for circularly and linearly polarized waves in media with arbitrary angular momenta j ₁ and j ₂ are found. The influence of the coherence between the magnetic sublevels on multiphoton effects is analyzed. It is shown that, as a result of elastic Rayleigh scattering, energy transfer from one component of the wave polarization to the other takes place. The picture of induced four-photon parametric scattering of a weak wave in the presence of a strong wave also changes.     

Conventional and collision photon echo in ytterbium vapor

The polarization properties of the photon echo generated by two linearly polarized pulses of resonant radiation at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition of ¹⁷⁴Yb are investigated. A complicated polarization behavior of the photon echo versus an angle between the polarization vectors of the excitation pulses is revealed in a mixture of ytterbium vapor with inert gas. For the angles ranging from 0° to 75°, a conventional echo with its linear polarization coinciding with the second excitation pulse dominates and the echo amplitude decreases with an increasing angle. For the angles ranging from 75° to 89°, the photon echo is elliptically polarized. Finally, for an angle of 90°, the conventional echo disappears and the collision echo becomes linearly polarized along the first excitation pulse.     

Raman-Nath acoustooptical diffraction in antiferromagnet α-Fe2O3 induced by modulation of the polarizations of optical normal modes

Raman-Nath acoustooptical diffraction was experimentally revealed in easy-plane antiferromagnet α-Fe₂O₃. The diffraction is due to linear modulation of the polarizations of the optical normal modes propagating along trigonal axis C₃ of the crystal due to oscillations of the antiferromagnetism vector L caused by magnetoelastic interaction. The dependence of the diffraction parameters on a dc magnetic field H _⊥ applied in the basal plane was studied. The dependence obtained is in good agreement with the field dependence of the exchange enhancement coefficient of magnetoelastic coupling in fields higher than that at which hematite transforms to a single-domain state (H _D ～ 1.8 kOe). for a sound flux power of about 1 W/cm² in a sample, the maximum intensity of the diffracted wave at H _⊥ = H _D is about 0.9% of the input light intensity and its polarization is perpendicular to that of the incoming linearly polarized wave. The results obtained agree qualitatively with the theory of acoustooptical diffraction in antiferromagnets and confirm the antiferromagnetic mechanism of diffraction in the experimental geometry under study.     

Optical properties of polarization-dependent geometric phase elements with partially polarized light

The behavior of geometric phase elements illuminated with partially polarized monochromatic beams is investigated both theoretically and experimentally. The element discussed in this paper is composed of wave plates with π-retardation and a space-variant orientation angle. We found that a beam emerging from such an element comprises two polarization orders; right-and left-handed circularly polarized states with conjugate geometric phase modification. This phase equals twice the orientation angle of the space-variant wave plate comprising the element. Apart from the two polarization orders, the emerging beam coherence polarization matrix includes a “vectorial interference matrix” which contains information concerning the correlation between the two orthogonal, circularly polarized portions of the incident beam. In this paper we measure this correlation by a simple interference experiment. In addition, we found that the equivalent mutual intensity of the emerging beam is modulated according to the geometric phase induced by the element. Other interesting phenomena concerning propagation will be discussed theoretically and demonstrated experimentally. The experiment made use of a spherical geometric phase element that was realized by use of a space-variant subwavelength grating illuminated with CO₂ laser radiation of 10.6 μm wavelength.     

Determination of the parameters of rotational and translational diffusion of a resonant medium by nonlinear polarization spectroscopy

A complete theoretical interpretation of the experimental data on the transformation of the polarization state of a weak wave in a layer of resonant medium (methane) depending on the intensity of high-power counterpropagating wave is presented in terms of the vector theory of radiation-matter interaction. A method for the experimental determination of the coefficients of rotational and translational gas diffusion is developed based on the theoretical calculation of two types of dichroism and birefringence and the use of experimental data on the change in the probe wave polarization at saturation of the resonant transition by the intensity of a counterpropagating wave with two different (linear and circular) field polarizations. This method also takes into account low backscattering of the strong wave field from optical path inhomogeneities.     

Experimental and numerical study of self-induced transparency in a neon absorber

Self-induced transparency on the neon transition 2s ₂(J=1)–2p ₄(J=2) is studied both experimentally by investigating the propagation of 3-nsec laser pulses at 1.15 µm in an absorber discharge and theoretically by a numerical integration of Bloch's equations and the wave equation.—The application of linearly and circularly polarized light corresponds to the interaction with a quasi-nondegenerate and degenerate transition, respectively. Pulse shapes, delays, and transmittancevs. input peak intensity are found in quantitative agreement with the calculated data. While with linear light polarization all SIT characteristics clearly appear, with circular polarization the pulse break-up and the oscillation of transmittance due to optical nutations are washed out.—The homogeneous linewidth derived from pulse delay data agrees with the value from conventional measurements. The results prove SIT useful as a quantitative spectroscopic method.Work supported by the Deutsche Forschungsgemeinschaft     

Experimental and numerical study of self-induced transparency in a neon absorber

Self-induced transparency on the neon transition 2s ₂(J=1)–2p ₄(J=2) is studied both experimentally by investigating the propagation of 3-nsec laser pulses at 1.15 µm in an absorber discharge and theoretically by a numerical integration of Bloch's equations and the wave equation.—The application of linearly and circularly polarized light corresponds to the interaction with a quasi-nondegenerate and degenerate transition, respectively. Pulse shapes, delays, and transmittancevs. input peak intensity are found in quantitative agreement with the calculated data. While with linear light polarization all SIT characteristics clearly appear, with circular polarization the pulse break-up and the oscillation of transmittance due to optical nutations are washed out.—The homogeneous linewidth derived from pulse delay data agrees with the value from conventional measurements. The results prove SIT useful as a quantitative spectroscopic method.     

The RHIC polarized source upgrade

The RHIC polarized H^? ion source is being upgraded to higher intensity and polarization for use in the RHIC polarization physics program at enhanced luminosity RHIC operation. The higher beam intensity will allow reduction of the longitudinal transverse beam emittance at injection to AGS to reduce polarization losses in AGS. There is also a planned RHIC luminosity upgrade by using the electron beam lens to compensate the beam-beam interaction at collision points. This upgrade is also essential for future BNL plans for a high-luminosity electron-proton (ion) Collider eRHIC. The basic limitations on the high-intensity H^? ion beam production in charge-exchange collisions of the neutral atomic hydrogen beam in the Na-vapor jet ionizer cell were experimentally studied.     

Laser-induced Alignment and Coulomb Explosion of CO2

实验研究了CO₂分子在飞秒强激光脉冲作用下的动力学过程,包括分子取向,隧穿电离和库仑爆炸,激光强度从1×10¹³W/cm²变化到6×10¹⁴W/cm². 当激光强度小于分子的电离阈值时,CO₂分子的非绝热转动激发形成一个相干转动波包,波包演化导致分子沿激光电场方向取向. 激光脉冲结束后,分子取向可以周期性地再现,利用另一束激光可以对取向结构进一步进行修饰. 当激光强度大于分子     

Doppler-free intracavity polarization spectroscopy using elliptically polarized light

The optically induced polarization change was studied in the case of a standing elliptically polarized wave. Using a Zeeman-tuned gas laser at 3.39 μm with an internal methane cell we have demonstrated the influence of the dichroism on the nonlinear absorption signals.     

Polarization effects at two-beam interaction on reflection holographic gratings in sillenite crystals

The polarization structure of the signal wave that interacts with a circularly polarized pump wave on the reflection holographic gratings in the (112)-, (111)-, and (110)-cut Bi₁₂TiO₂₀ (BTO) crystals is experimentally and theoretically analyzed. An analytical expression for the vector field of the signal wave that is valid for an arbitrary orientation of the cubic refractive crystal is derived using approximation of the nondepleting circularly polarized pump wave. The contribution of the absorption component of the reflection hologram into the counter interaction is estimated for the crystals under study using the approximation of the experimental data with theoretical curves. Based on the experiments with incoherent quasi-monochromatic radiation, it is demonstrated that the real BTO crystals exhibit weak induced linear birefringence.     

Second harmonic generation in nanoscale films of transition metal chalcogenides: Taking into account multibeam interference

Second harmonic generation is studied in structures containing nanoscale layers of transition metal chalcogenides that are two-dimensional semiconductors and deposited on a SiO₂/Si substrate. The second harmonic generation intensity is calculated with allowance for multibeam interference in layers of dichalcogenide and silicon oxide. The coefficient of reflection from the SiO₂-layer-based Fabry–Perot cavity is subsequently calculated for pump wave fields initiating nonlinear polarization at every point of dichalcogenide, which is followed by integration of all second harmonic waves generated by this polarization. Calculated second harmonic intensities are presented as functions of dichalcogenide and silicon oxide layer thicknesses. The dependence of the second harmonic intensity on the MoS₂ layer thickness is studied experimentally in the layer of 2–140 nm. A good coincidence of the experimental data and numerical simulation results has been obtained.     

Polarization properties of coherent VUV light at 125 nm generated by sum-frequency four-wave mixing in mercury

The polarization of the VUV light generated by four-wave sum-frequency mixing process ω₄=2ω₁+ω₂ in mercury vapor at room temperature is analyzed in detail. Due to the specific two-photon transition used to enhance the nonlinear process, the polarization of the VUV wave is shown to be identical to the polarization of the wave at the frequency ω₂. In particular, circularly polarized VUV is observed with degree of circular polarization exceeding 0.99. Received: 11 July 1999 / Revised version: 8 October 1999 / Published online: 8 March 2000     

Experimental study of the resonance radiation group delay in degenerate systems

The dependence of the polarization-and intensity-modulation group delay on the polarization of electromagnetic wave was studied experimentally for different transitions between the hfs components of the ⁸⁷Rb D₁ absorption line. It was found that the polarization-modulation delay strongly depends on the degeneracy structure of resonant transition and, in the general case, on the ellipticity of light-wave polarization. It is demonstrated that the polarization-modulation delay does not occur for the transitions not involving dark states. The polarization delay was studied as a function of the polarization ellipticity angle. The intensity-modulation delay was measured for the resonance radiation to show that it is observed for all ⁸⁷Rb D₁-line transitions and is independent of polarization.     

Spin–orbit stiffness of the spin‐polarized electron gas

In a spin‐polarized electron gas, Coulomb interaction couples the spin and motion degrees of freedom to build propagating spin waves. The spin wave stiffness S_sw quantifies the energy cost to trigger such excitation by perturbing the kinetic energy of the electron gas (i.e. putting it in motion). Here we introduce the concept of spin–orbit stiffness, S_so, as the energy necessary to excite a spin wave with a spin polarization induced by spin–orbit coupling. This quantity governs the Coulombic enhancement of the spin–orbit field acting of the spin wave. First‐principles calculations and electronic Raman scattering experiments carried out on a model spin‐polarized electron gas, embedded in a CdMnTe quantum well, demonstrate that S_so = S_sw. Through optical gating of the structure, we demonstrate the reproducible tuning of S_so by a factor of 3, highlighting the great potential of spin–orbit control of spin waves in view of spintronics applications. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Lyman alpha (1216 Å) generation in krypton

We measured the nonlinear susceptibilities of Kr up to fifth order via nonresonant third-harmonic generation (THG) of a frequency-doubled dye laser at 3648 Å. In agreement with theory the optimum intensity conversion efficiency saturates at 10^-5 in pure Kr. The phase matching of the third harmonic and fundamental wave breaks up owing to intensity-dependent changes of refractive indices. Phase matching in high-density Kr/Ar mixtures raised the 3648 Å to 1216 Å third harmonic conversion obtained in pure Kr by more than two orders of magnitude, as predicted by theory but hitherto never experimentally confirmed. With laser intensities of 5 × 10⁹ W/cm² at 3648 Å we measured conversion efficiencies of 1.4 × 10^-4 limited by gas breakdown. Taking into account the Lyman alpha absorption of Kr₂ and KrAr molecules in the high-density regime, a conversion efficiency of 10^-3 is expected from extrapolating the experimental results to Kr/Ar mixtures of up to 3 × 10³ torr. It is limited by self-focusing, assuming the minimum threshold intensity of gas breakdown is 10¹¹ W/cm², as predicted by theory.     

Optical orientation of nuclei in nitrogen alloys GaAsN at room temperature

The intensity and giant circular polarization of edge luminescence in a longitudinal magnetic field have been measured in nitrogen alloys GaAsN under circularly polarized pumping. It has been found that these dependences are shifted with respect to zero field by a value B _eff. The magnitude of the internal field B _eff increases with the pumping intensity and reaches saturation (≈250 G) at large excitation densities. The saturation of the B _eff field with growth of pumping indicates that this is a field of nuclei, polarized dynamically due to hyperfine interaction with optically oriented deep paramagnetic centers, rather than a field of exchange interaction created on the center by spin-polarized photo-excited conduction electrons. The short time of nuclear polarization by electrons (<15 μs), measured under modulation of circular polarization of the exciting light with high frequency, points to a small number of nuclei undergoing hyperfine interaction with an electron localized at a center.     

Second harmonic generation from alkali metals

Nonlinear optical constants of the alkali metals, responsible for the second harmonic generation, are calculated using the Drude model. It is found that the electric quadrupole term of the nonlinear polarization is larger than that of a previous theory of Jha by a major factor of ω²_p/ω². By considering a more realstic electron density distribution, we also obtain an additional dipole term due to symmetry breakdown at the surface. The new nonlinear polarization gives the reflected second harmonic intensity in good agreement with experiments.     

Acousto-optic diffraction in hematite due to the magnetoelastic modulation of the polarizations of optical modes

Acousto-optic diffraction in the Raman-Nath regime is observed in the α-Fe₂O₃ easy-plane antiferromagnet. This diffraction is caused by the magnetoelastic modulation of the polarizations of normal optical modes propagating along the trigonal axis of the crystal. The diffraction parameters are analyzed as functions of the static magnetic field applied in the basal plane. The intensity of the diffraction wave in fields on the order of the monodomainization field of the samples (H _D ≈ 1.8 kOe), at an acoustic flux power of I _S ～ 7.0 W/cm², was approximately 0.11% of the incident light intensity and its polarization was perpendicular to the polarization of the linearly polarized incident wave. It is shown that the observed intensity is comparable with the intensity of the acousto-optic diffraction in nonmagnetic materials. The results are well described by the theory of acoustooptic diffraction in antiferromagnets {E. A. Turov, Zh. Éksp. Teor. Fiz. 112, 1464 (1997) [JETP 85, 797 (1997)]} and confirm its basic conclusions.