Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated. The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma. As the third harmonic pulse has higher group velocity than that of fundamental pulse, therefore, it moves faster than the fundamental pulse. It gets slipped out of the domain of fundamental pulse and its amplitude saturates. Phase matching condition is satisfied by applying wiggler magnetic field,which provides additional angular momentum to the third harmonic photon to make the process resonant. Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.     

耦合对相对论效应下的三次谐波的影响

研究了相对论效应引起的场量的非线性对谐波辐射源的影响；在入射光为长脉冲激光并计及各阶谐波间的耦合时，解析研究了三次谐波的振幅、频率、相位的变化及增长与饱和特性并计算了转化率，结果表明谐波间的耦合使幅值和转化率减少。     

Harmonic generation by an interaction of laser with an array of anharmonic carbon nanotubes

A scheme of second and third harmonic generation from laser irradiated anharmonic carbon nanotubes (CNTs) embedded in silica is proposed. An intense ultra short laser pulse interacting with the array of CNTs displaces its electrons. When this displacement becomes comparable to the radius of electrons of CNTs, the restoring force of electrons varies nonlinearly with the displacement. As a result, plasmon resonance gets broadened and the large resonance absorption of the laser by the electrons of CNTs occurs. The power conversion efficiency of harmonic generation due to the anharmonicity of CNTs is much higher than that due to the ponderomotive force nonlinearity. The effect of amplitude modulated parameter on the amplitude of second and third harmonic is studied. The high amplitude of generated harmonics can be realized with the amplitude modulated laser as compared to ordinary unmodulated laser.     

二次谐波中的饱和效应和乘性噪声

研究了单模激光中产生的二次谐波的统计性质,用全饱和激光理论导出了二次谐波光子计数ｍ的几率分布函数、二次阶乘矩和三次阶矩的表达式,同实验值比较,理论与实验在阈值附近符合得很好。研究结果表明,激光场基波和二次谐波的涨落随空腔衰减常数的增大而减小;当激光场中含有乘性噪声时,乘性噪声增强基波的涨落,同时抑制二次谐波的涨落。     

Resonant third harmonic generation of a short pulse laser in plasma by applying a wiggler magnetic field

We examine the effect of wiggler magnetic field on pulse slippage of short pulse laser-induced third harmonic generation in plasma. The process of third harmonic generation of an intense short pulse laser in plasma is resonantly enhanced by the application of a magnetic wiggler. The laser exerts a ponderomotive force at second harmonic driving density oscillations. The second harmonic oscillations coupled with electron velocity at the laser frequency, produces a non-linear current, driving the third harmonic. Third harmonic pulse generates in the fundamental pulse domain. However, the group velocity of the third harmonic wave is greater than the fundamental wave. Hence, the third harmonic pulse saturates strongly and moves forward from the fundamental pulse at shorter distance than the second harmonic pulse.     

Spatial distribution of the interacting Waves’ amplitudes under third harmonic generation in a negative-positive refractive medium

The process of third harmonic generation in a cubically nonlinear medium that is negatively refractive at the fundamental frequency and positively refractive at the third harmonic frequency is considered. For the stationary case, the amplitude distribution was obtained for waves interacting inside a sample for different values of phase mismatch. In the periodic regime of generation, it is shown that the amplitude of the fundamental wave inside the medium can exceed its value at the input, which is impossible in the standard case of harmonic generation in a medium positively refractive at both frequencies. The influence of energy losses in the sample on the spatial distribution of waves’ amplitudes of both frequencies has been studied.     

Efficient collinear frequency tripling of femtosecond laser with compensation of group velocity delay

This paper demonstrates an approach that negative uniaxial crystal has a relative anomalous dispersion effect which can compensate group velocity delay, and applies this approach to nonlinear frequency conversion of an ultrafast laser field. High efficiency of the third harmonic generation is experimentally fulfilled by adopting a collinear configuration of doubing-compensation-tripling system. Through finely adjusting the incident angle and optical axis direction of the compensation plate, it obtains ultraviolet (UV) output energy of 0.32~mJ centered at 270~nm with spectral bandwidth of 2~nm when input beam at 800~nm was 70~fs pulse duration and 6~mJ pulse energy which was extracted from Ti:sapphire laser system by a diaphragm, corresponding to an 800-to-270~nm conversion efficiency of 5.3% and a factor-of-1.6 improvement in the third harmonic generation of UV band in comparison with a general conventional configuration. Furthermore, when the full energy of 18~mJ from a Ti:sapphire laser system was used and optimized, the UV emission could reach 0.83~mJ.     

Theory of a laser-plasma method for detecting terahertz radiation

A theory is developed for calculating the spectrum and the shape of a terahertz wave packet from the temporal profile of the energy of the second harmonic of the laser field generated during nonlinear interaction of laser and terahertz pulses in an optical-breakdown plasma. The spectral and temporal characteristics of the second-harmonic envelope and a terahertz pulse are shown to coincide only for short laser pulses. For long laser pulses, the second-harmonic spectral line shifts to the red and its temporal profile is determined by the time integral of the electric field of terahertz radiation.     

Spectral modulation of third-harmonic generation by molecular alignment and preformed plasma

We demonstrate spectral modulation of third-harmonic generation from molecular alignment effects. The third harmonic spectrum is broadened or narrowed under different influences of cross-phase modulations originating from various molecular alignment revivals. Furthermore, the spectrum and spatial distribution of the generated third harmonic pulse change dramatically in the presence of a preformed plasma. Under the influence of a preformed plasma, a narrower third harmonic spectrum is observed, and the conical third-harmonic pulse increases while the axial part decreases. The investigation provides an effective method to modulate the spectral characteristic and spatial distribution of third-harmonic generation from intense femtosecond filament.     

BEPCII横向束流反馈系统的梳状滤波器

BEPCII是一个多束团、大流强的装置, 由于高频腔的高次模和电阻壁阻抗等因素，不可避免地会出现束流不稳定性。BEPCII中采用束流反馈系统来抑制束流不稳定性。横向束流反馈系统主要包括前端电子学、信号处理电子学、反馈器件和放大器等几个部分。梳状滤波器是信号处理电子学的重要部件，利用两根长短不同的电缆以及一个功分器和合成器构成了一种简单有效的梳状滤波器，其梳状深度达到-41 dB，使用这种梳状滤波器的横向反馈系统成功地抑制了束流中出现的不稳定性。     

Ionization of a hydrogen atom from its ground state by a coherent bichromatic laser field

We study the “coherent phase control” between the three-photon ionization by a fundamental laser field and the one-photon ionization by its third harmonic for a hydrogen atom in its ground state. The relative phase δ of the harmonic field with respect to the fundamental laser radiation “modulates” the interference between the two ionization channels, which is important near the crossing points between the ionization rates of the two individual processes. Numerical results for the total ionization rate and for the angular distribution of the photoelectrons as a function of the phase δ are presented for frequencies located in the vicinity of the atomic resonances corresponding to the absorption of two laser photons. Received 31 August 2000 and Received in final form 6 February 2001     

Modeling of helicopter-borne tunable TEA CO2 DIAL system employment for detection of methane and ammonia leakages

The characteristics of a helicopter-borne lidar based on tunable TEA CO₂ laser and its third harmonic designed for remote detection of methane and ammonia leakages from pipelines are analyzed numerically. The spectral range near 3 μm was shown to be most promising for remote sensing of methane emissions. Parameters of radiation of the tunable pulse-periodic mini-TEA CO₂ laser and generators of harmonics to be utilized in the helicopter-borne differential absorption lidar are estimated. Emissions of different gas intensities are analyzed for possible detectability at a distance of up to 1 km. The use of the third harmonic of the TEA CO₂ laser allows methane emissions from a pipeline to be detected and measured with mean measurement error from 10% to 15% for methane concentrations varying from the background level to the explosion-hazardous one. The optimal pair and possibilities of the ammonia remote sensing on the base of the first harmonic of TEA CO₂ laser were determined as well.     

Spectral and temporal analysis of ultrashort ultraviolet pulses generated by high-intensity laser radiation in the atmosphere

We report direct spectral and temporal characterization of ultrashort ultraviolet (UV) pulses resulting from third-harmonic generation by high-intensity Ti: sapphire laser radiation in the atmosphere. The experimental technique implemented in this work enables the measurement of the pulse width and the chirp of the UV field, as well as the electron density of the plasma produced by laser pulses. The bandwidth of the third harmonic generated in our experiments by laser pulses with an initial pulse width of 45 fs supports transform-limited UV pulses as short as 30 fs.     

Characterization of the third-harmonic radiation generated by intense laser self-formed filaments propagating in air

We perform laboratory experiments to study ultraviolet radiation generated by intense self-formed laser filaments produced by propagating high-power femtosecond laser pulses in air. The laser used in the experiment is a 0.5 TW Ti:sapphire system with the center wavelength at 800 nm. The observed ultraviolet emission occurs in the form of the third harmonic and frequency-upshifted radiation from the fundamental. We present direct characterization of the generated harmonic and frequency-upshifted radiation, including transverse imaging and spatially resolved spectral measurements.     

Impact of spatial modulation on the third harmonic wave propagation and far-field focal spot in frequency conversion system

Third harmonic generation (THG) of high power laser is discussed in KDP crystals utilized in frequency conversion systems of large laser facilities. The far-field focal spot of the third harmonic wave is presented based on numerical simulation of the nonlinear coupled-wave equations, in which the walk-off and paraxial diffraction are taken into account and the electric field of the fundamental wave (1053 nm, 1ω) pulse is phase and amplitude modulated in spatial domain. Impact of the modulation depth and frequency on the focal spot energy, the side lobes location and conversion efficiency are analyzed in detail. The results show that the side lobes location is equivalently determined by the modulation frequency of both phase modulation and amplitude modulation, while the energy-concentration is decreased mostly because of the 1ω modulation depth. Relatively, the phase modulation plays a more important role than the amplitude modulation in decreasing main lobe energy for different reasons. The phase modulation makes the energy flowing from the main lobe to side lobes, while amplitude modulation not only makes the energy flowing but also decreases tripling efficiency significantly.     

Efficiency enhancement of a two-beam free-electron laser using a nonlinearly tapered wiggler

A nonlinear and non-averaged model of a two-beam free-electron laser (FEL) wiggler that is tapered nonlinearly in the absence of slippage is presented. The two beams are assumed to have different energies, and the fundamental resonance of the higher energy beam is at the third harmonic of the lower energy beam. By using Maxwell's equations and the full Lorentz force equation of motion for the electron beams, coupled differential equations are derived and solved numerically by the fourth-order Runge-Kutta method. The amplitude of the wiggler field is assumed to decrease nonlinearly when the saturation of the third harmonic occurs. By simulation, the optimum starting point of the tapering and the slopes for reducing the wiggler amplitude are found. This technique can be applied to substantially improve the efficiency of the two-beam FEL in the XUV and X-ray regions. The effect of tapering on the dynamical stability of the fast electron beam is also studied.     

Generation of an isolated sub-30 attosecond pulse in a two-color laser field and a static electric field

We theoretically investigate high-order harmonic generation(HHG) from a helium ion model in a two-color laser field,which is synthesized by a fundamental pulse and its second harmonic pulse.It is shown that a supercontinuum spectrum can be generated in the two-color field.However,the spectral intensity is very low,limiting the application of the generated attosecond(as) pulse.By adding a static electric field to the synthesized two-color field,not only is the ionization yield of electrons contributing to the harmonic emission remarkably increased,but also the quantum paths of the HHG can be significantly modulated.As a result,the extension and enhancement of the supercontinuum spectrum are achieved,producing an intense isolated 26-as pulse with a bandwidth of about 170.5 eV.In particular,we also analyse the influence of the laser parameters on the ultrabroad supercontinuum spectrum and isolated sub-30-as pulse generation.     

The study of high-order harmonic generation using ellipticity-modulated laser pulse

The high-order harmonic generation using ellipticity-modulated laser field is investigated experimentally and theoretically. The laser field with time-varying ellipticity is obtained using two quarter wave plates. The harmonic spectral intensity peaks appear periodic for the localization induced by the ellipticity modulation of the laser pulse. We calculate the single-atom response to the driving laser field using the Lewenstein model and compare the calculated and measured harmonic spectra generated with ellipticity-modulated driving laser pulses. The theoretical simulation and the experiment results agree well with each other.     

Half-cycle pulses in the mid-infrared from a two-color laser-induced filament

Four-wave mixing (FWM) of femtosecond near-infrared laser pulses and its second harmonic in the filamentation regime is shown to give rise to ultrashort field waveforms in the mid-infrared with pulse widths as short as a half of the field cycle and produce ultrabroadband supercontinuum spectra stretching from the mid-IR to the terahertz region. Generation of 7-fs pulses centered at 4.35 μm is demonstrated by a two-color filamentation experiment, where the 25-fs, 800-nm fundamental-wavelength output of a Ti: Sapphire laser is mixed with its second harmonic. The spectral and temporal properties of the mid-IR waveforms, as well as their emission pattern, are consistent with the FWM scenario of frequency conversion generalized to include the Kerr effect and ionization-induced refractive-index modulation.