空气中等离子光栅诱导探测光丝三次谐波辐射放大的实验研究

对探测光丝的不同位置与等离子光栅相互作用和探测光丝作用到等离子光栅不同位置引起三次谐波的增强进行了实验研究.研究发现,探测光丝的三次谐波信号强度对于探测光丝不同位置与等离子光栅相互作用和等离子光栅内部钳制的激光强度具有极强的依赖关系.与等离子光栅相互作用,三次谐波信号与等离子光栅基波信号的相位匹配与否是解释探测光丝三次谐波信号强度变化的关键.控制探测光丝以小角度与等离子光栅相互作用是实现探测光丝三次谐波信号有效放大的最佳途径.     

基于角色散的非共线匹配宽带三倍频技术

宽带光打靶可以有效降低激光等离子体相互作用过程中非线性效应。提出一种基于角色散的非共线匹配宽带三倍频方案,利用宽带基频与窄带二倍频的非共线和频产生宽带三倍频,和频过程中通过特殊设计的渐变光栅实现不同频率的基频光束以特定角度入射,补偿了波长差异引入的位相失配使得全波段满足位相匹配条件。理论模拟表明,采用KDP晶体Ⅱ类位相匹配,将中心波长为1058 nm、带宽10 nm的宽带基频光与526.5 nm的二倍频光进行非共线匹配和频,可以实现高效宽带三倍频转换。     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

晶体面形畸变对三倍频转换效率的影响

在惯性约束聚变(ICF)领域,为了满足物理实验需求,需要采用短波长的紫外激光打靶。目前,国内外高功率固体激光装置普遍采用谐波转换的方式来获得三倍频紫外激光。高效的频率转换必须满足相位匹配条件,而晶体的反射面形畸变过大对准直精度和倍频效率均匀性都会产生不利影响。通过实验验证了晶体准直光斑的质量退化主要来源于晶体因夹持和重力导致的反射面形畸变,证明了改进晶体的夹持方式可以有效改善晶体面形,提高准直精度和准直光斑质量,并显著提升三倍频转换效率。     

Near-field optics with photonic crystals

Finite-difference time-domain (FDTD) analysis of light propagation in a defect mode of a two-dimensional photonic band-gap (PBG) structure demonstrates that the light field is localized in such a structure within areas with subwavelength (λ/10) sizes. FDTD simulations reveal efficient formation of an evanescent wave at the output of such a PBG structure, permitting the subwavelength resolution to be achieved in the near field. A probe object with a size less than the wavelength of incident light is shown to perturb the near-field distribution behind the PBG structure and to change the signal detected in the far-field zone. The field intensity distribution inside a PBG structure is also sensitive to the presence of a probe object, offering a way to control the light field localized in defect modes of PBG structures. Received: 9 August 1999 / Accepted: 18 October 1999 / Published online: 10 November 1999     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Efficient output coupling of intracavity high-harmonic generation

We demonstrate a novel technique for coupling extreme-ultraviolet (XUV) harmonic radiation out of a femtosecond enhancement cavity. We use a small-period diffraction grating etched directly into the surface of a dielectric mirror. For the fundamental light, this element acts as a high reflector. For harmonic wavelengths, it acts as a diffraction grating, coupling XUV radiation out of the cavity. Using this method, we observed the third through twenty-first odd harmonics with a dramatic increase in usable power over previous results of high-harmonic generation at high repetition rates.     

相对论圆偏振激光与固体靶作用产生高次谐波

超短超强激光与固体靶表面等离子体相互作用可以通过高次谐波的方式产生从极紫外到软X射线波段的相干辐射,获得飞秒甚至阿秒量级的超短脉冲,可用于观测原子或分子中的电子运动等超快动力学过程.本文实验研究了相对论圆偏振飞秒激光与固体靶相互作用的高次谐波产生过程,实验结果表明,在较大入射角下,圆偏振激光也可以有效地产生高次谐波辐射.通过预脉冲控制靶表面的预等离子体密度标长,发现高次谐波的产生效率随密度标长的增加而单调下降.进一步通过二维粒子模拟程序,分析了激光的偏振以及预等离子体密度标长对高次谐波产生的影响,很好地解释了实验观测结果.     

等离子体光栅靶的表面粗糙度对高次谐波产生的影响

极紫外光和软X射线由于其波长和脉冲持续时间极短,可用于超快物理过程和物质微观结构的探测.最近几年,研究人员发现激光和等离子体相互作用可以产生持续时间极短(阿秒)且相干性较好的高次谐波辐射,其波长可接近甚至达到水窗波段.然而,实验研究指出,理论上应出现的一些谐波在实验中并没有出现.本文针对超短超强激光与非理想条件下的等离子体光栅靶相互作用产生高次谐波的物理过程进行了理论分析和粒子模拟.研究结果表明,等离子体光栅的周期性结构对于高次谐波的频谱和辐射角分布存在显著调制效果.光栅靶表面粗糙度直接影响光栅的光学调制效果,改变高次谐波的频谱分布和辐射角分布.理想光栅条件下,满足光栅匹配条件的特定阶数谐波明显获得增强,且辐射张角集中在平行靶面的方向.靶表面粗糙度的出现,导致光栅匹配条件失效,高次谐波能量向各阶分散且辐射张角逐渐偏离靶表面方向.研究结果较好地解释了实验中观测到的谐波频谱分布,为进一步的研究提供了一定参考.     

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。     

Coherent harmonic generation on UVSOR-II storage ring

Among the attractive coherent light sources resulting from the interaction between femtosecond lasers and relativistic electron beams, simultaneous Coherent Synchrotron Radiation (CSR) in the THz region, slicing and UV-VUV Coherent Harmonic Generation (CHG) can be achieved on synchrotron radiation facilities. Recently, a Ti:Sa laser at high repetition rate (1 kHz) has been seeded in the optical klystron of the Free Electron Laser at UVSOR-II (Okazaki, Japan). In this paper, the experimental set-up allowing delivery of sub picosecond UV pulses from CHG, and TeraHertz radiation from CSR is described. We further focus on the third coherent harmonic (266 nm) generated. The expected typical characteristics of this radiation, predicted by both numerical and analytical models recalled here, are experimentally verified and several studies of the influence of the seed laser on the output CHG intensity are reported. Such experiment enables UVSOR-II facility to produce in parallel short pulses at two different colors, synchronized at high repetition rate with one single infrared laser: a unique set-up of great interest for the facility users.     

Design of High-Efficiency Diffraction Gratings Based on Rigorous Coupled-Wave Analysis for 800nm Wavelength

We report on the design of a high diffraction efficiency multi-layer dielectric grating with wide incident angle and broad bandwidth for 80Ohm. The optimized grating can achieve 〉 95% diffraction efficiency in the first order at an incident angle of 5° from Littrow and a wavelength from 77Ohm to 83Ohm, with peak diffraction efieieney of 〉 99.5% at 80Ohm. The electric field distribution of the optimized multi-layer dielectric grating within the gratings ridge is 1.3 times enhancement of the incidence light, which presents potential high laser resistance ability. Because of its high-effieieney, wide incident, broad bandwidth and potential high resistance ability, the multi-layer dielectric grating should have practical application in Ti：sapphire laser systems.     

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.     

Photonic band-gap enhanced second-harmonic generation in a planar lithium niobate waveguide

Enhanced second-harmonic generation (SHG) conversion efficiency was theoretically predicted in waveguide geometry with coupling to a one-dimensional grating photonic band gap (PBG). We report a series of experiments using samples made with lithium niobate. A waveguide was fabricated near the surface by applying the proton-exchange technique. The characteristics of waveguide modes were determined by several techniques: prism coupling, diffraction, and Cherenkov radiation. The WKB method was used to analyze the results. Ultraviolet laser lithography was applied to make PBG gratings on the sample. We further investigated Cherenkov second-harmonic generation (CSHG), i.e., SHG radiated into the substrate, under the condition of a band-edge PBG resonance in the waveguides. The SHG inside planar waveguides was also experimentally investigated. We fabricated waveguides with multiple pump modes and found that the second mode was more efficient in enhancing the second harmonic signal. This result is explained by our model. Several samples were investigated in detail; the highest conversion efficiency of CSHG with a PBG was enhanced around 50 times above the CSHG signal without a PBG. A numerical model was constructed with parameters calculated from our sample characterization data to interpret the experimental results.     

High-order optical harmonic generation from solid surfaces

During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the density drops to the vacuum level in a distance much shorter than the wavelength. This sharp plasma-vacuum boundary performs an oscillatory motion in response to the electromagnetic forces of the intense laser light. It is shown that the generation of reflected harmonics can be interpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the reflected frequency spectrum correspond to odd and even harmonics of the laser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic generation is strongly enhanced in the relativistic regime of laser intensities.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

High fluence laser ablation of aluminum targets: Time-of-flight mass analysis of plasmas produced at wavelengths 532 and 355 nm

We report on Time-of-Flight Mass Spectrometry (TOFMS) analysis of plasmas produced in laser ablation of Al targets. We used both the second (532 nm) and third (355 nm) harmonic of a Nd: YAG laser system, carrying out the investigation in a regime of relatively high laser fluence (up to 70 J/cm²), where the production of ionized species in the plume is maximized. We present TOF mass spectra of ions in the laser-produced plasma, and a detailed analysis of the relative abundance of different charged species as a function of the laser fluence. The presence of single, doubly and triply ionized Al atoms has been observed and the fluence threshold for their production is reported. We also studied the total ion and electron yield at different laser fluences, its saturation above specific energy densities, and singly ionized cluster-ions produced in the laser plasma.     

Anomalous behavior of group velocity and index of refraction in a defect photonic band gap structure

In the present paper, we have made an analysis to observe the effect of introduction of defect on dispersion relation, group velocity, and effective group index in a conventional photonic band gap (PBG) structure. The study shows that inside the PBG materials group velocity and effective group index becomes negative in both types (conventional as well as defect PBG structure) of structure at a certain range of frequencies. Also, near the edges of the bands it attains very high values of index of refraction. A defect PBG structure gives a very unique feature that group velocity becomes exactly zero at a particular value of frequency and also becomes several hundred times greater than the velocity of light which is not attainable with the conventional PBG structure. Defect PBG structures with such peculiar characteristics are seen in lasing without inversion, in construction of perfect lens, in trapping of photon and other optical devices.     

Quasinondestructive holographic recording in photochromic LiNbO3

We have observed quasinondestructive holographic storage with a continuous-wave laser at lambda = 532 nm in near-stoichiometric LiNbO3 doped with Tb and Fe. This crystal showed an exceptionally long grating decay time and also exhibited a fast color change upon exposure to ultraviolet (UV) light. It was demonstrated that the grating recorded from the UV-exposed, colored state can be continuously read out over 9 h at a reading intensity as high as 8 mW/cm(2). In addition, the written grating could be easily erased with UV illumination which returned the crystal back to the original colored state.     

HARMONIC EMSSIONS FROM LASER-PRODUCED PLASMAS

Haimonic veneration as a result of resonant absorption in laser-roduced plasmas is investigated by considering the steepened plasma density gradients in the vicinity of the critical density. We obtained an efficiency of the second harmonic emission which is in better agreement with experiments than previous theories. The present calculation has also been extended to including the efficiency of the third harmonic emission, the results obtained indicate that at sufficiently high laser intensities, the third harmonic emission generated by the resonant absorption can also be observed.     

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.