Betatron resonance crossing during the intense beam bunching

In the process of bunching an intense beam, the betatron oscillation frequency of particles changes due to a growth in their momentum rejection (in the presence of chromaticity) and changes in the Coulomb shift. These changes lead to the intersection of nonlinear betatron resonances. A mathematical model of the dynamics of ions in the process of bunching an intense beam has been built in order to assess the influence of transverse betatron resonances on the motion of particles. This model takes into account the influence of space charge on the longitudinal and transverse oscillations of particles in the presence of one- and two-dimensional betatron resonances. An analysis of the motion of particles near the one- and two-dimensional third-order resonances is given. Results of the numerical simulation of particle losses and emittance growth in the process of bunching an intense beam in the storage ring at the Institute for Theoretical and Experimental Physics (ITEP) are presented.     

Saturated-absorption spectroscopy: eliminating crossover resonances by use of copropagating beams

We demonstrate a new technique for saturated-absorption spectroscopy by use of copropagating beams that does not have the problem of crossover resonances. The pump beam is locked to a transition, and its absorption signal is monitored while the probe beam is scanned. As the probe comes into resonance with another transition, the pump absorption is reduced and the signal shows a Doppler-free dip. We use this technique to measure hyperfine intervals in the D2 line of 85Rb with a precision of 70 kHz and to resolve hyperfine levels in the D2 line of 39K that are less than 10 MHz apart.     

Stable coupled conjugate solitary waves in optical fibers

Four-wave mixing of an intense continuous-wave pump beam with an ultrashort soliton signal in an optical fiber is theoretically analyzed. A novel class of stable two-color coupled solitary waves is found. These vector parametric solitons represent the optimal frequency conversion of an ultrashort pulse.     

Stimulated Raman scattering of gaussian laser beam in relativistic plasma

This paper presents an investigation of Stimulated Raman Scattering of gaussian laser beam in relativistic Plasma. The pump beam interacts with a pre-excited electron plasma wave and thereby generate a back-scattered wave. Due to intense laser beam, electron oscillatory velocity becomes comparable to the velocity of light, which modifies the background plasma density profile in a direction transverse to pump beam axis. The relativistic non-linearity due to increase in mass of the electrons effects the incident laser beam, electron plasma wave and back-scattered beam. We have set up the non-linear differential equations for the beam width parameters of the main beam, electron plasma wave, back-scattered wave and derived SRS back-reflectivity by taking full non-linear part of the dielectric constant of relativistic plasma with the help of moment theory approach. It is observed from the analysis that self-focusing of the pump beam greatly affects the SRS reflectivity, which plays a significant role in laser induced fusion.     

Spatial and frequency manifestations of laser beam self-action effects in different schemes of inversionless amplification

Results of numerical simulation of coaxial propagation of an intense pump beam and a weak probe beam in a three-level medium with different types of energy diagrams in conditions of inversionless amplification are presented. The effect of the beam self-action and diffraction on the spatial and frequency characteristics of the probe wave were studied. These effects, in combination with linear absorption and refraction, give rise to complex spatial profiles of gain along the beam propagation path. The annular structure of the amplified beam and a small asymmetry of its frequency characteristics, resulting from the nontrivial gain profile over the beam cross section, were found and studied.     

Multiple internal resonances in MEMS arch resonators

Micromachined shallow arch resonant beams have attracted significant attention thanks to their rich dynamical behavior, inherent nonlinearities, and the potential to excite various internal resonances. Currently, there is a lack of comprehensive experimental studies for the various types of internal resonances in arches and particularly at the micro and nano scales. Here, we aim to investigate and identify different types of internal resonances of an initially curved beam, electrothermally actuated and electrostatically driven, by electrical characterization techniques. Upon changing the electrothermal voltage of silicon micromachined arches, the second symmetric natural frequency of an arch is adjusted to near twice, three times, and four times the fundamental natural frequency, which gives rise to 2:1, 3:1, and 4:1 autoparametric resonances between the two modes. These resonances are demonstrated experimentally. We show various frequency-response curves of the total response around the excitation frequency and highlight the contribution of each mode before, during, and after the internal resonances. Allan-deviation results are also shown indicating enhanced frequency stabilization of the arch oscillation when experiencing internal resonances. These studies motivate further research in this direction to exploit internal resonances of micromachined resonators for practical applications, such as sensors and mechanical amplifier.     

Development of Raman-shifted probe laser beam for plasma diagnosis using polaro-interferometer

Optical diagnostics of laser-produced plasma requires a coherent, polarized probe beam synchronized with the pump beam. The probe beam should have energy above the background emission of plasma. Though the second harmonic probe beam satisfies most of the requirements, the plasma emission is larger at the harmonic frequencies of the pump. Hence, at high intensities we need a probe beam at non-harmonic frequencies. We have set up a Raman frequency shifted probe beam using a pressurized hydrogen cell that is pumped by the second harmonic of the Nd glass laser that operates at only one Stokes line of 673.75 nm.     

非线性共振及束晕-混沌的场强参数特性

通过计算和分析周期聚焦磁场强度和表征束自生场强度的束导流系数的变化对强流粒子束运动特征的影响,系统研究了强流粒子束的非线性共振和束晕现象。采用庞加莱截面技术对束核包络振荡及其单粒子运动轨迹进行了数值模拟,清楚地展示了不同情况下束核包络非线性振荡以及对应的试验粒子空间分布。结果表明:束核自生场使得束核包络发生非线性振荡,随着束核自生场和聚焦磁场的增加,伴随着束核共振岛的出现,单粒子空间分布出现越来越多的束晕粒子。     

Direct measurement of free carrier nonlinearity in semiconductor-doped glass with picosecond pump-probe Z-scan experiment

We report the direct measurement of free carrier nonlinearity in a semiconductor-doped glass with picosecond pump-probe Z-scan experiment. A strong Z-scan signal from a weak and time delayed probe beam is observed. The probe beam Z-scan signal is comparable in magnitude to the Z-scan signal of the intense pump beam, clearly showing the dominance of the effective fifth order nonlinearity due to the pump beam generated free carriers in the overall nonlinear response of semiconductor-doped glass. The estimated magnitude of the fifth order nonlinearity is consistent with that obtained from earlier reported experiments.     

Vibrational quantum interference in Raman-induced Kerr effect: momentum conservation

The pump–probe Raman-induced optical Kerr effect (RIKE) of simple molecular liquids, studied with femtosecond laser pulses, exhibit long lasting beats ascribable to vibrational quantum interference (QI). While energy conservation entails vibrational resonances in RIKE, momentum conservation boils down to wave vector-matching in the pump and probe processes, which calls for the participation of a vibrational excitation wave. The refractive index dispersion around vibrational resonances is intimately related to the focusing angle of the pump (also probe) beam. The larger the focusing angle, the greater the excitation wave number, i.e. the more energetic the vibration in resonance; if the focusing angle is too small, energetic vibrations cannot be observed in vibrational QI, even if energy conservation is fulfilled. If the pump and probe beams are perfectly collimated, then all beams must be collinear in order to conserve momentum.     

Enhancement and modulation of terahertz radiation by multi-color laser pulses

We study theoretically intense terahertz radiation from multi-color laser pulse with uncommon frequency ratios. Comparing the two-color laser scheme, of which the uncommon frequency ratio should be set to be a specific value, we show that by using multi-color harmonic laser pulses as the first pump component, the lasers as the second pump component can be adjusted in a continuous frequency range. Moreover, these multi-color laser pulses can effectively modulate and enhance the terahertz radiation, and the terahertz yield increases with the increase of the wavelength of the uncommon pump component and is stable to the laser relative phase. Finally, we utilize the electron densities and velocities of ionization events to illustrate the physical mechanism of the intense terahertz generation.     

One — dimensional laser beam steering using frequency detuning in two — wave mixing with a BaTiO3 crystal

We demonstrate a method of one-dimensional laser beam deflection using frequency detuning in two-wave mixing. Energy exchange between the interfering beams in a photo-refractive BaTiO₃ crystal has been used for deflection of a pump beam into predetermined probe beam directions. A one-dimensional array of several beams is generated from a single probe beam, employing a piezo-mirror and beam splitter combination. Probe beams so produced are detuned by exciting the piezo-mirror with a periodic near-saw-tooth voltage so as to produce running fringes. However, stable holographic gratings are recorded by matching the frequency of the probe beam with that of a pump beam reflected from another vibrating piezo-mirror, thereby controlling the direction of beam deflection.     

三维自散焦介质中光束聚焦的最佳参量选择

对三维自散焦介质中由交叉相位调制效应诱导的光束聚焦过程进行了系统研究,讨论了抽运光振幅、抽运光-信号光初始偏移和抽运光-信号光波长比三个参量对信号光聚焦过程的影响。根据自散焦介质中强抽运光诱导弱信号光聚焦满足抽运光与探测光模型,对传输方程进行了优化,大大减少了数值模拟运算时间。数值方法采用分布傅里叶方法。数值模拟结果表明：抽运光越强、抽运光-信号光波长比越大,信号光聚焦程度越大;抽运光-信号光初始偏移存在一个最佳值,使得光束聚焦达到最佳效果。通过参量最佳值的选择,可提高光束聚焦的效率。     

Diode laser noise-spectroscopy of low-frequency atomic fluctuations in rubidium vapor

We have observed resonant features in the spectrum of the fluctuations of a linearly polarized diode laser beam transmitted through a rubidium vapor cell, corresponding to the evolution of the atomic spin in the presence of a constant magnetic field. The observed resonances occur at a noise frequency corresponding to twice the Larmor frequency of ground state rubidium atoms and are due to two-photon Raman processes involving the carrier frequency and the noise sideband. We observed noise resonances for frequencies of the order of one MHz via heterodyne detection. Due to nonlinear Faraday rotation, we detected emitted light with polarization orthogonal to the incident field. The influence of the laser light fluctuations on the transmitted light noise spectrum was investigated by using two diode laser sources with different spectral bandwidths. The observed features are in qualitative agreement with a semiclassical theoretical model that treats laser fluctuations up to first order.     

Optically tunable superprism effect in nonlinear photonic crystals

An analysis of the tunable superprism effect in a two-dimensional nonlinear photonic crystal is presented. We show that, by shifting the photonic bands of the crystal through the Kerr effect induced by a pump beam, one can tune the refraction angle of a transmitted signal beam over tens of degrees. We also demonstrate that the optical power required to tune the refracted angle is dramatically reduced if the frequency of the pump beam is close to a bandgap edge.     

激光感生双光栅中频谱分辨率的研究

研究了用于测量两束光频差的时间延迟激光感生双光栅的频谱分辨率，确定了当泵浦光具有高斯线型时四波混频信号的产生条件，此条件通过在可调谐的窄带激光和宽带激光间的拍频实验得到证实。     

掺铜KNSBN光折变晶体的光致吸收特性

掺铜KNSBN(钾钠铌酸锶钡)光折变晶体具有光致吸收(LIA)特性.实验表明,泵浦光的光强越强,波长越短,LIA系数就越大,探测光为e偏振态时的LIA系数是o光时的1.2倍.这种现象将被用来提高光场的空间或时间均匀性.     

Optical switch phenomenon in a self-defocusing medium

A spatial optical switch phenomenon caused by the induced focusing of a weak probe beam occurring in self-defoeusing nonlinear media is discussed theoretically. A weak beam is induced to focus when it copropagates with an intense pump beam under the conditions that the probe and pump beams peak at different positions and propagate in different directions. Due to the effect of cross-phase modulation, the weak beam can not only be focused but also be deflected. The phenomenon is discussed by numerically solving the coupled amplitude equations.     

Feasibility consideration for THz radiation by Raman scattering in a quadrupole channel

Considered is the stimulated Raman scattering of a pump wave by a mildly relativistic electron beam. The electron beam is assumed to be transported along an alternating quadrupole channel. Optimal matched beam solutions are obtained, and the requirement on the beam emittance is identified. Estimations suggest feasibility of the process for an intense THz radiation source.