Application of coherent four-wave mixing for two-dimensional mapping of the spatial distribution of excited atoms in a laser-produced plasma

An experimental technique for the two-dimensional mapping of the relative populations of excited states of atoms and ions in a low-temperature plasma of optical breakdown is developed on the basis of coherent four-wave mixing (FWM) with hyper-Raman resonances. The possibility to extract information concerning the spatial distribution of the populations of excited atomic and ionic states from two-dimensional maps of the FWM intensity allowing for phase mismatch and one-photon absorption is considered. Conditions when FWM in a plasma occurs in the phase-matched regime and is not subject to a considerable influence of one-photon absorption are experimentally determined.     

Superresolution vibrational imaging by simultaneous detection of Raman and hyper-Raman scattering

We have developed a superresolution vibrational imaging method by simultaneous detection of Raman and hyper-Raman scattering. Raman and hyper-Raman images obtained with the same laser spot carry independent information on the sample spatial distribution, owing to different signal dependence (linear in Raman and quadratic in hyper-Raman) on the incident light intensity. This information can be quantitatively analyzed to recover the incident light intensity distribution at the focal plane. A superresolution vibrational image is then derived by the constrained deconvolution of the images by the obtained incident light intensity distribution. This method has been applied to a TiO? nanostructure and the obtained superresolution image was compared with a scanning electron microscopy image. The spatial resolution achieved by the present method is evaluated to be 160 nm, which is more than twice better than the diffraction limited resolution.     

Investigation of molecular dynamics in β-carotene using femtosecond pump-FWM spectroscopy

We have carried out two different pump four-wave mixing experiments, combining an initial pump excitation and a subsequent four-wave mixing probe process, on the photosynthetic pigment β-carotene to reveal different aspects of its molecular dynamics after photoexcitation. Firstly, the pump degenerate four-wave mixing (pump -DFWM) technique, in which the DFWM is resonant with the S₁- to S _n -transition of β-carotene, is used to monitor the events following excitation of the system. The transient shows a peculiar shape and is seen to depend on the energy of the initial pump pulse as well as on the concentration of the solute in the solvent. Secondly, pump coherent anti-Stokes Raman scattering (pump-CARS) is used to elucidate the excited state vibrational dynamics of β-carotene. This technique gives access to the dynamics of both ground and excited electronic states with vibrational selectivity.     

Measurements of the electron concentration and conductivity of a partially ionized inert gas plasma

A theoretical model for the resonant hyper-Raman scattering in semiconductor crystals is presented. The Wannier excitons are considered as intermediate states. The expressions for the resonant hyper-Raman scattering cross section are obtained. The theoretical model developed allows taking into account different mechanisms of the exciton-photon and exciton-phonon interactions.     

等离子体中受激Raman效应引起的近简并四波混频

本文研究等离子体中由于受激Raman散射激发起来的电子等离子体波引起的近简并四波混频。文中解析地导出此情况下等离子体介质的三阶非线性极化率,并讨论其位相匹配条件。通过求解偶合波方程得到位相复共轭反射率,并详细研究该反射率与泵浦光和探针光的频率差以及与相互作用长度的关系。     

Molecular enhanced four-wave parametric generation in sodium atom

A number of resonances have been observed in the excitation spectrum of the uv resonance lines of sodium. The mechanism proposed includes absorption by molecular states, energy transfer to excited atomic levels and four-wave parametric generation enhanced by radiative decay of the excited states.     

Two-phonon hyper-Raman scattering and polariton spectra in rutile crystal

The first observation of both two-phonon and polariton hyper-Raman scattering in rutile crystal is reported. The effect of two-phonon states on A_u-polariton dispersion has been found.     

Four-wave mixing of optical and microwave fields

We demonstrate degenerate four-wave mixing involving both optical and microwave fields. This four-wave mixing process, with fields that differ in frequency by 5 orders of magnitude, results from stimulated Raman scattering of the optical field from an atomic ground-state Zeeman coherence in warm rubidium vapor, which is induced and maintained by the microwave field.     

关于超Raman效应中散射强度的计算

本文根据辐射的量子理论,对超Raman散射强度进行了计算。从理论上直接断言:所有具有振动红外活性的Raman介质都同时具有振动超Raman活性。由于偶极跃迁几率只对反Stokes超Raman散射线有贡献,所以反Stokes线与Stokes线的强度公式不对称,前者含有更丰富的频谱。并且对于低波数频移,即使在室温条件下,两者也几乎有同数量级的强度。最后讨论了超Raman散射中的共振效应。 关键词：     

Application of quantum mechanical method to analysis of the intensity distribution in spectra of resonant hyper-Raman scattering of low-symmetry molecules

The quantum-mechanical method of calculating the relative intensities of lines in the spectra of resonant hyper-Raman scattering of polyatomic molecules in the Herzberg-Teller approximation is tested with respect to low-symmetry molecules. The method makes it possible to describe resonant Raman and hyper-Raman scattering spectra, as well as vibronic absorption spectra, from the same viewpoint based on a common set of parameters. The particular features of the implementation of the method are discussed based on the calculation of the spectra of resonant hyper-Raman scattering of chlorobenzene and adenine, and the expedience of the application of the method is illustrated. Satisfactory agreement is obtained between calculation results and available experimental data.     

The evaluation of temporal electronic structures of nonresonant Raman excited virtual state of thiourea

An algorithm has been introduced to calculate molecular bond polarizabilities of thiourea, which supply essential electronic information about the nonresonant Raman excited virtual states. The main dynamical behaviour of the excited virtual states of thiourea is that the Raman excited electrons tend to flow to the N-H bonds and C-N bonds from the S-C bonds because of the electronic repulsion effect. The difference in Raman excited electron relaxation time of thiourea under 514.5-nm and 325-nm excitations has been observed, which quantitatively shows that the Raman scattering process is dependent on the wavelength of the pumping laser. Finally, the distribution of the electrons at the final stage of relaxation is given out through the comparison between the bond electronic densities of the ground states and the bond polarizabilities after deexcitation.     

Quantum entanglement of Fock states with perfectly efficient ultraslow single-probe photon four-wave mixing

We propose a method to achieve quantum entanglement of two Fock states with perfectly efficient, ultraslow propagation enhanced four-wave mixing. A cold atomic medium is illuminated with a two-mode cw control laser to produce coherent mixtures of excited states. An ultraslowly propagating, single-photon quantum probe field completes the four-wave mixing with 100% photon flux conversion efficiency, creating a depth dependent entanglement of two Fock states. We show that at a suitable propagation distance, a maximum entangled state is created with a single-photon wave-packet state that has 50% probability of being in each of two product-type Fock states.     

Bond polarizability calculations of hyper-Raman scattering in sodium, potassium and rubidium halides

Bond polarizability calculations extended for hyper-Raman scattering by the optical LO and TO zone-centre phonons in sodium, potassium and rubidium halides are presented. The effect of the crystal ionicity on both of the independent hyper-Raman tensor components is taken into consideration. Contributions of rotation of crystals bonds and their stretching to hyper-Raman scattering are determined. For LO phonons the electrooptic part of the hyper-Raman tensor is found. The results obtained are compared, when possible, with experimental and theoretical data reported in the literature.     

斯托克斯和反斯托克斯拉曼位移差与时间反演对称性

首先列出若干Stokes和anti-Stokes位移不对称的事例,并在此基础上指出,如果将anti-Stokes散射严格定义为Stokes散射的时间反演行为,则对于初态或末态具有Kramers简并的体系,Stokes和anti-Stokes散射的拉曼位移可以不同.这是因为时间的正过程和反演过程各对应于不同的跃迁途径,其位移大小的差别与简并能级的分裂值相关.     

On stability of Raman and degenerate hyper-Raman scattering

The problem of the stationary generation by Raman and degenerate hyper-Raman scattering is solved by means of two methods: stationary solution of the generalized Fokker-Planck equation and linear analysis of stability of the Langevin equations. The conditions of stability and the corresponding stationary values for amplitudes of the Stokes, anti-Stokes or output laser modes are obtained for particular cases. A phenomenon analogous to the second order phase transition can occur for Stokes or anti-Stokes interaction of Raman scattering above the threshold value of the pump amplitude. The anti-Stokes interaction of degenerate hyper-Raman scattering can show bistable behaviour above the second threshold value of the pump amplitude.The author would like to express his sincere thanks to Dr. J. Peina for comments and stimulating discussions.     

Thermal loss mechanism in the generation of multifrequency laser emission via stimulated Raman scattering and four-wave Raman mixing studied by photothermal refraction spectroscopy

The heat produced in conjunction with the processes of stimulated Raman scattering and four-wave Raman mixing in hydrogen was measured by photothermal refraction spectroscopy. Many vibrational, rotational, and vibrationally shifted rotational Raman lines are exclusively/simultaneously generated by changing the polarization of the laser beam and the hydrogen pressure. Thermal loss occurs predominantly from vibrational Raman scattering, which can be ascribed to a large Raman shift frequency of 4155 cm^-1 for the vibrational transition. In contrast to stimulated Raman scattering, little or no thermal loss is observable during the process of four-wave Raman mixing. Received: 12 April 1999 / Revised version: 12 July 1999 / Published online: 20 October 1999     

Hyper-Raman microspectroscopy: a new approach to completing vibrational spectral and imaging information under a microscope

We have developed hyper-Raman scattering microspectroscopy and applied it to a microcrystal of all-trans-beta-carotene. The hyper-Raman spectrum of all-trans-beta-carotene exhibits a Raman-inactive but infrared-active vibrational mode at 1564 cm(-1). Hyper-Raman imaging of a microcrystal was performed with this band. Infrared-active vibrational imaging was achieved with a spatial resolution much higher than that of conventional infrared microscopy. The combination of Raman and hyper-Raman spectroscopy opens up a new scope for high-spatial-resolution vibrational microspectroscopy that is not restricted by the selection rule.     

Strengthened Hyper-Raman Lines with the Coherent Superposition State

High-order harmonic generations from a one-dimensional Coulomb potential atom are calculated with the initial state prepared as a coherent superposition between its ground and first excited states. When the energy difference of the two states is small, we can choose proper laser pulse such that the first excited state can be excited only to other bound states instead of being ionized. We show that only the hyper-Raman lines are observable instead of the harmonics. The energy difference of the ground and the first excited state can be deduced from the highest peak of the hyper-Raman lines. We further show that the similar results can be obtained by using a combination of two laser pulses with different frequencies interacting with the atom initially at the ground state.     

Theoretical investigation of ozone production in negative corona discharge

The purpose of this work is to highlight the chemical kinetics in plasma of pure oxygen. A comprehensive model of ozone generation in wire-to-cylinder negative corona discharge is presented. The model combines the physical processes in the discharge with the chemistry of ozone formation. It is based on an extensive reaction scheme including the major electronic and ionic processes. The importance of excited atomic and molecular states is demonstrated. The obtained results show clearly that the surface material affect quite well the ozone decomposition.