Conical harmonic generation in isotropic materials

A novel class of nonlinear optical processes is described in which radiation at the nth harmonic is generated through the use of a (2n+1)-order nonlinearity. Utilizing an odd-order nonlinearity, this process allows for the generation and amplification of both odd- and even-order harmonics in isotropic materials. Additionally, this process can always be phase matched in normal-dispersion materials without the use of birefringence. Experimental results are presented in which conical third-harmonic emission is generated from a sapphire sample.     

二维材料高次谐波及光波电子学

超短超强激光驱动固体介质产生高次谐波辐射受到了人们越来越多的关注。固体高次谐波产生于亚光周期电子运动,它提供了在极短时间尺度上研究光与固体材料高阶非线性相互作用的全新视角。与原子分子相比,组成固体材料的原子晶格具有平移对称性,因此固体高次谐波会呈现出特有的规律。相较于块体材料,二维材料独特的电光特性可以为高次谐波产生带来全新的特点,文章重点介绍几种二维材料高次谐波及其在光波电子学方面的研究进展。     

BBO晶体的五倍频

本文报道了用BBO晶体对Nd:YAG激光器输出的1064nm光的五倍频,获得了213nm的紫外输出,266nm转换至213nm的转换效率达48%.并用数值积分的方法从理论上分析了1064nm与266nm的混频过程,得出的结论与实验结果相符.     

Second harmonic generation accompanying linear mode conversion in a laser-produced plasma

Second harmonic (SH) generation in a laser produced plasma (NdYAG laser with wavelength 1.06 μm, pulse duration 30 ps, intensity 10¹³Wcm^-2) was investigated experimentally as a function of polarization and angle of incidence of the laser beam. The results are in agreement with theoretical predictions for SH generation in the presence of linear mode conversion in the plasma.     

Second harmonic generation using partially coherent light

The second harmonic generation process is considered using partially coherent light formed by the superposition of a coherent field and a chaotic field. The conversion efficiency is derived. Expressions are also presented for the variance and for the probability density function of the second harmonic intensity and of the transmitted intensity at the fundamental wavelength.     

Light scattering lineshape in opaque materials

The wavevector distribution excited by light scattering in opaque materials is calculated and compared to recently observed asymmetrical Brillouin lineshapes.     

Enhancement of third harmonic generation in air filamentation using obstacles

The intensity of third harmonic emission in air filamentation disturbed by copper fibers and alcohol droplets has been investigated experimentally. Enhancement of the third harmonic emission up to more than one order of magnitude has been observed. The physical mechanism of third harmonic enhancement is attributed to suppression of the destructive interference by comparison of the experimental results and it is closely related to the type, size, and relative position of the obstacles.     

应用级联倍频方法提高倍频系统输出稳定性研究

提出应用级联倍频方法提高倍频系统输出稳定性,并就该方法的有效性进行了理论分析和模拟计算.分析和计算结果不但证明级联倍频方法能实现倍频系统稳定输出,而且还表明可以通过仔细调节第一块倍频晶体中波矢方向 k 与光轴间夹角、两块倍频晶体间的间隔,能调节改变实现倍频系统最稳输出时所需第二块晶体的理论计算长度,使之与第二块倍频晶体的实际加工长度一致,最终实现系统稳定倍频输出.级联倍频方法在实现高输出稳定性的同时能实现高的倍频转换效率,对应用于光参量啁啾脉冲放大系统的高稳定抽运源系统的设计建造具有重要参考意义. 关键词： 级联倍频 稳定倍频输出 光参量啁啾脉冲放大     

Optical harmonic generation in magnetic garnet epitaxial films near the fundamental absorption edge

Spectra of the second and third optical harmonics generated in epitaxial films of magnetic garnets were studied in the ranges 1.7–3.2 and 2.4–4.2 eV, respectively. A large magnetic contrast was revealed in second-optical-harmonic spectra, which reaches 100% at certain photon energies. By contrast, the symmetry-allowed magnetic contribution to spectra of the third optical harmonic was not found. While linear absorption in ferrite-garnet films grows by two orders of magnitude above the fundamental absorption edge at ～3.2 eV, the intensity of nonlinear spectra did not reveal any noticeable increase over this spectral range.     

Resonance processes during harmonic generation in plasmas using mid-infrared radiation

A few recently introduced approaches of the high-order harmonic generation in laser-produced plasmas are reviewed. We show how the tuning of odd and even high-order harmonics of ultrashort pulses along the strong resonance of laser-produced indium plasma using optical parametric amplifier of white-light continuum radiation (1250?1400 nm) allows observation of different harmonics enhanced in the vicinity of the transition of In II ions possessing high oscillator strength. We discuss various peculiarities and discuss the theoretical model of the phenomenon of tunable harmonics enhancement in the region of 62 nm using indium plasma. With the theoretical analysis, we present the approach allowing reproduce the experimental observations and characterize the dynamics of the resonant harmonic emissions. We also discuss the resonance enhancement of harmonics using mid-infrared radiation in the tin, chromium, and antimony plasmas and show the calculations of this process.     

Use of carbon-containing materials for efficient high-order harmonic generation of laser radiation

A systematic study of the high-order harmonic generation (HHG) of laser radiation in various carbon-containing plasma plumes (CCPPs) is presented. The materials studied are: graphite, boron carbide, C₆₀, polytetrafluoroethylene, polyethylene, carbon nanotubes, and soot. These studies show that CCPPs present the best plasma media for efficient lower order (from 9th to 19th) harmonic generation, while the harmonic cutoff restricted to the 29th order. The advantages of CCPPs for the harmonic generation are confirmed by comparison of the HHG conversion efficiency with that in Ag and In plasma plumes, where the highest conversion efficiency was reported. Use of two-color pump scheme allows further enhancement of harmonic yield from the CCPPs.     

Quadruple-frequency millimeter-wave generation using second-order rational harmonic mode-locking technique

A novel method for generating quadruple-frequency millimeter-wave （MMW） by using an actively mode-locked fiber ring laser is proposed and demonstrated. In this approach, the optical Mach-Zehnder intensity modulator （MZM） is biased to suppress the odd-order optical sidebands, the fiber laser operates in the second-order rational harmonic mode, and a fiber Bragg grating （FBG） notch filter is used to block the optical carrier. When the MZM is driven by a fixed radio-frequency （RF） source of 10 GHz, a stable MMW signal of 40 GHz with the phase noise better than -76 dBc/Hz at 1-kHz offset is generated.     

Molecular imaging using high-order harmonic generation and above-threshold ionization

Accurate molecular imaging via high-order harmonic generation relies on comparing harmonic emission from a laser-irradiated molecule and an adequate reference system. However, an ideal reference atom with the same ionization properties as the molecule is not always available. We show that for suitably designed, very short laser pulses, a one-to-one mapping from high-order harmonic frequencies to electron momenta in above-threshold ionization exists. Comparing molecular and atomic momentum distributions then provides the electron recollision amplitude in the molecule for enhanced molecular imaging. The method retrieves the molecular recombination transition moments highly accurately, even with suboptimal reference atoms.     

相位失配法实现倍频稳定输出的模拟分析

 对BBO晶体Ⅰ类相位匹配倍频过程和KTP晶体XY平面内的Ⅱ类相位匹配倍频过程进行数值模拟,结果表明：Ⅰ类和Ⅱ类相位匹配倍频过程,都可通过引入适量的相位失配量实现稳定倍频输出;随着相位失配量的增大,实现稳定倍频输出所需的晶体长度都变小,倍频稳定输出起伏变小,倍频转换效率有不同程度的下降;相对于XY平面内的Ⅱ类相位匹配倍频过程,BBO晶体的Ⅰ类相位匹配倍频过程对相位匹配偏离角更敏感,且稳定输出时倍频转换效率更低。     

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.     

Surface second harmonic generation of chiral molecules using three-coupled-oscillator model

Based on the three-coupled-oscillator molecular model we proposed, the relation between the second-order susceptibilities of a chiral film and the molecular hyperpolarizabilities is given. The effect of microscopic parameters on the second-order susceptibilities is simulated numerically and the difference between the efficiencies of s-polarized second-harmonic fields induced by the left- and the right-handed circularly-polarized fundamental beams is discussed. The theoretical basis for studying second-order nonlinear optical properties of the chiral molecular media with a tripod-like structure is provided in this paper.     

利用啁啾激光调制分子谐波信号

数值研究了啁啾激光驱动H_2~+辐射谐波的特点.结果表明,(1)在低强度和高强度激光驱动下,谐波光谱分别呈现红移和蓝移现象.引入正负向啁啾参数后,谐波频移和谐波辐射强度都可以得到调控.(2)谐波光谱在高强度激光驱动下呈现非奇次谐波.理论分析表明,非奇次谐波产生的原因是双H核频移不一致所导致的不对称相消干涉引起的.