Formation of conical emission of supercontinuum during filamentation of femtosecond laser radiation in fused silica

The formation of conical emission of supercontinuum during filamentation of femtosecond laser pulses with central wavelengths in a wide range is studied experimentally, numerically, and analytically. The frequency-angular intensity distribution of the spectral components of conical emission is determined by the interference of supercontinuum emission in a filament of a femtosecond laser pulse. The interference of supercontinuum emission has a general character, exists at different regimes of group velocity dispersion, gives rise to the fine spectral structure after the pulse splitting into subpulses and the formation of a distributed supercontinuum source in an extended filament, and causes the decomposition of the continuous spectrum of conical emission into many high-contrast maxima after pulse refocusing in the filament. In spectroscopic studies with a tunable femtosecond radiation source based on a TOPAS parametric amplifier, we used an original scheme with a wedge fused silica sample. Numerical simulations have been performed using a system of equations of nonlinear-optical interaction of laser radiation under conditions of diffraction, wave nonstationarity, and material dispersion in fused silica. The analytic study is based on the interference model of formation of conical emission by supercontinuum sources moving in a filament.     

飞秒激光成丝过程中由等离子体光栅引起的超连续谱增强与转移

在飞秒宽带激光脉冲成丝过程中, 由于双光束干涉效应而形成的空间等离子体光栅, 研究了空气中飞秒宽带激光脉冲成丝所产生的超连续谱增强效应, 并进一步观测到泵浦光脉冲成丝产生的超连续谱能量向较弱的未成丝的探测光脉冲转移, 有效拓展探测光脉冲带宽的新现象. 通过基于布拉格衍射特性的理论计算较好地解释了实验结果.     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

基于固体介质的倍频程连续光谱产生的研究进展

超快激光经过透明介质时由于非线性作用光谱会得到展宽,甚至能够产生超过一个倍频程的相干超连续光谱,这样的光源能够压缩得到几个甚至单个光周期的超短脉冲,并在现代超快科学的各个领域得到了广泛应用.实验中已经在气体、液体和固体中都观测到了光谱的展宽,目前较为成熟的方法是使用充满惰性气体的空芯光纤和具有高非线性效应的固体材料展宽光谱.但空芯光纤由于芯径限制无法用于高能量激光脉冲的光谱展宽,而固体材料又容易被高功率密度的脉冲激光损坏.随着激光技术的发展其脉冲能量不断提高,一种新的、利用多片薄固体介质实现光谱展宽的方式被提出.多片薄的非线性介质可以实现光谱展宽的逐片累积,而且避免了激光在介质中因自聚焦产生过高功率密度带来的损坏.目前使用这种方法已经在实验上得到了近毫焦尔量级的倍频程光谱,覆盖了近紫外到中红外的整个区域,并实现了脉冲压缩.本文简要回顾了超快激光在固体中光谱展宽的发展历程,概述了新型薄片固态介质产生超连续光谱的原理,对近年来使用此新方法的实验进行了简要分析,并对其发展前景进行了展望.     

温度对飞秒激光脉冲在NaCl溶液中成丝产生的超连续谱的影响

利用飞秒激光脉冲在NaCl溶液中成丝,研究了溶液温度对飞秒激光成丝过程中产生的超连续谱的影响.发现在激光脉冲能量较低时,溶液温度对超连续谱的影响几乎可以忽略;而在激光脉冲能量较高时,随着NaCl溶液温度的升高,超连续谱呈现出被压缩的趋势.此外,在激光脉冲能量较高的情况下, NaCl中会产生大量的气泡.通过分析,得出了飞秒激光在溶液中成丝产生的气泡是影响超连续谱发射的主要因素.     

Supercontinuum generation under filamentation driven by intense femtosecond pulses in supercritical xenon and carbon dioxide

It is found that supercritical fluids are a unique source of multioctave supercontinuum radiation, which is generated upon filamentation of an intense femtosecond laser pulse. If the laser pulse power significantly exceeds the critical power of self-focusing, then a supercontinuum with a width of three and a half spectral octaves (from 350 to 2000 nm) is generated in supercritical xenon. The red wing of supercontinuum generated in supercritical carbon dioxide has the form of a plateau in the range from 1400 to 1900 nm, while the blue wing of the spectrum is almost completely attenuated.     

Supercontinuum generation of ultrashort laser pulses in air at different central wavelengths

Supercontinuum generation by femtosecond filaments in air is investigated for different laser wavelengths ranging from ultraviolet to infrared. Particular attention is paid on the role of third-harmonic generation and temporal steepening effects, which enlarge the blue part of the spectrum. A unidirectional pulse propagation model and nonlinear evolution equations are numerically integrated and their results are compared. Apart from the choice of the central wavelength, we emphasize the importance of the clamped intensity reached by self-guided pulses, together with their temporal duration and propagation length as key players acting on both supercontinuum generation of the pump wave and emergence of the third harmonic. Maximal broadening is observed for large wavelengths and long filamentation ranges.     

飞秒激光离焦抽运熔融石英产生超连续白光的实验研究

利用飞秒激光在熔融石英介质中传输产生能量达数毫焦、波长范围覆盖400-900 nm 且光谱分布较为均匀的超连续白光,实验过程中将熔融石英介质离焦放置以避免被击穿. 研究了入射激光能量以及介质离焦距离对超连续白光特性的影响. 结果表明采用高能量的入射激光脉冲离焦抽运介质的方法能够有效避免介质 击穿损伤并提高超连续白光脉冲的能量输出.     

Control of femtosecond filamentation by field-free revivals of molecular alignment

We show that the filamentation dynamics of a femtosecond laser probe pulse can be readily controlled by properly matching it to the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation with an advancing ultrashort pump pulse. Several features of the filamentation process including supercontinuum generation, the length of the plasma channel generated in the wake of the filament, the associated secondary radiations and the multiple filamentation pattern are all easily modified by tuning the cross phase modulation induced by the field-free revivals of molecular alignment, through the delay between the pump and the probe pulses. We show that molecular alignment can also be used to generate conical waves with extremely short intensity spike called shocked X-waves and to further tune the frequency of a few-cycle laser pulse in the wake of a self-guided intense filament.     

Spectral modulation and supercontinuum generation assisted by infrared femtosecond plasma grating

Spectral modulation and supercontinuum generation of a probe pulse is investigated by using the plasma grating induced by the interference of two infrared femtosecond laser pulses. The dependences of the supercontinuum generation from the probe pulse on the time delay, the relative polarization angle between the probe pulse and the two-pump pulses, and the input probe pulse energy are investigated. The far-field spatial profiles of the three pulses are measured with different time delays and relative polarization angle, and the core energy of the probe pulse as functions of the time delay and relative polarization angle are also shown.     

Ultrafast Broadband Nonlinear Spectroscopy of a Colloidal Solution of Gold Nanoparticles

JETP Letters - The generation of supercontinuum radiation at the filamentation of femtosecond laser pulses (800 nm) with a supercritical gigawatt peak power in pure water and colloidal solutions of...     

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.     

激光大气成丝中的非线性光频转换

超快强激光在光学介质（如空气）中传播时由于克尔自聚焦效应和等离体散焦效应动态平 衡会发生一种独特的非线性激光成丝现象。激光成丝过程会诱导一些独特的物理现象,如非线性 光频转换产生超连续光谱、等离子体诱导高压放电、锥形辐射等,在大气传感、天气控制等研究 领域具有重要的应用前景。本文针对飞秒激光大气成丝过程中与传输介质相互作用所诱导的非线 性发光过程,介绍了激光大气成丝所产生的超连续光谱（白光）激光、谐波产生和太赫兹波辐射 三种非线性光频转换现象,并着重探讨了太赫兹波辐射的物理机理、研究现状和应用前景。     

Long-range spectrally and spatially resolved radiation from filaments in air

The distance-resolved spectral intensity distribution of the backscattered light from long filaments generated in air using ultra-short and intense laser pulses is presented. A clean fluorescence spectrum from N₂ molecules and ions, which is produced by the high peak intensity inside the plasma filament of the fundamental pulse, was clearly resolved from the backscattered supercontinuum. The supercontinuum generated by both the fundamental and the third-harmonic pulses developed progressively and became fully developed only at the end of the filamentation.     

飞秒平顶光束经微透镜阵列在熔融石英中的成丝及其超连续辐射

实验研究了平顶激光光束经微透镜阵列在熔融石英中成丝的演化以及超连续辐射的产生,并进一步与高斯光束的成丝和超连续辐射进行了对比研究.分别对这两种光束的多丝传输进行了横向和纵向成像.结果表明,使用平顶光束可以获得更为均匀的多丝分布,成丝的起点也更为一致;尤其重要的是,相对于高斯光束,平顶光束可以使用更高的入射激光脉冲能量而不会造成介质的损伤,从而可以获得更高脉冲能量和更高转换效率的超连续辐射.     

Femtosecond laser pulse filamentation versus optical breakdown in H<Subscript>2</Subscript>O

The competition between femtosecond laser pulse induced optical breakdown and femtosecond laser pulse filamentation in condensed matter is studied both experimentally and numerically using water as an example. The coexistence of filamentation and breakdown is observed under tight focusing conditions. The development of the filamentation process from the creation of a single filament to the formation of many filaments at higher pulse energy is characterized systematically. In addition, strong deflection and modulation of the supercontinuum is observed. They manifest themselves at the beginning of the filamentation process, near the highly disordered plasma created by optical breakdown at the geometrical focus. Received: 9 July 2002 / Revised version: 15 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/6562-623, E-mail: wliu@phy.ulaval.ca     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

块状介质超连续谱光谱稳定性的理论优化

用变分法讨论了飞秒脉冲在块状透明介质中超连续谱的产生，结合频谱展宽与非线性相移的关系，讨论了块状介质的长度、抽运光的功率、光束口径、透镜的焦距以及介质与透镜的相对位置对超连续谱光谱宽度和光谱稳定性的影响，结果表明入射功率为几倍自聚焦阈值功率时，超连续谱光谱稳定性最好。为实验上获得稳定的超连续谱光谱输出提供了理论指导。     

掺镱硼酸钙氧钇飞秒激光器及在拉锥光纤中产生跨倍频程超连续光

跨倍频程超连续光谱的产生是光学频率梳系统中测量载波包络相移频率的关键.本文采用拉锥单模光纤作为非线性光谱展宽介质, 将半导体激光(LD)抽运的掺镱硼酸钙氧钇(Yb:YCOB)振荡器输出的飞秒激光耦合到该拉锥光纤中, 通过飞秒激光在光纤中发生的相位调制、四波混频等非线性效应将光谱展宽至超过倍频程的范围.振荡器输出的飞秒激光脉冲宽度为130 fs, 中心波长为1052 nm, 重复频率为76.8 MHz, 平均功率为620 mW, 耦合进单模拉锥光纤后获得了光谱覆盖范围从550 nm至1350 nm的跨倍频程超连续光谱, 最大输出平均功率为323 mW, 耦合效率达到52%.为进一步实现全固态飞秒激光光学频率梳提供了重要基础.     

Ultrabroadband continuum generated in air (down to 230 nm) using ultrashort and intense laser pulses

We present results on supercontinuum generation extended up to 230 nm in air during the propagation of a powerful femtosecond laser pulse. The broad supercontinuum generated in air is contributed by self-phase modulation and self-steepening of the fundamental laser pulse, the third-harmonic pulse and their interaction. In particular, the strong interaction between the fundamental and the third-harmonic pulses leads to broad and efficient continuum generation of the third-harmonic pulse itself. The spectrum of the third-harmonic generated in air extends over several tens of nm and overlaps with the shorter wavelength extent of the fundamental continuum. PACS 42.65.Ky; 42.65.Jx; 52.35.Mw