Combination of high-intensity femtosecond laser pulses for generation of time-dependent polarization pulses and ionization of atomic gas

Time-dependent polarization pulses are generated by combining two perpendicularly polarized, high-intensity laser pulses. The time evolution of the polarization state of the combined laser pulse is measured by the POLLIWOG technique. We observed changes in the polarization state while varying the relative delay. In order to investigate the effect of pulse combination on the ionization of atoms, the electron signals and the ion signals are measured by irradiating combinations of two perpendicularly polarized pulses or two parallel polarized pulses. With the two parallel polarized pulses, high-order fringe-resolved autocorrelations are obtained by measuring the time-integrated ion signals as a function of the time delay. When two perpendicularly polarized pulses are combined, the fringe period of the time-integrated electron signal as a function of the time delay is different from that of the time-integrated ion signal. This is due to the fact that the electron signal depends on the direction of the field vibration and the number of generated electrons. We also measured the electron energy distributions at different relative delays and confirmed that these depend on the polarization state of generated pulses.     

All-fiber passive mode-locked laser to generate ps pulses based in a symmetrical NOLM

We experimentally investigate an all-fiber passively mode-locked laser generating ps pulses. The experimental setup is a figure eight fiber laser configuration, including a power-symmetric Nonlinear Optical Loop Mirror (NOLM) with highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by polarization asymmetry between the counter-propagating beams in the loop. We used a Quarter-Wave Retarder in the loop to break the polarization symmetry. Using a polarizer beam-splitter cube as the NOLM output we got the best quality output pulses from the laser. At this output, we are monitoring the output pulse polarization component which is parallel to the input NOLM component. We achieved stable generation of ～25 ps pulses at the repetition frequency of 0.78 MHz with milliwatts average output power. The mode-locked laser ran in stable operation for hours.     

Manipulating Photoelectron Distributions for Rare Gas Atoms Ionized with Polarized Femtosecond Laser Pulses

Above-threshold ionizations of rare gas atoms excited by polarized femtosecond laser pulses are investigated. The photoelectron momentum spectra are obtained applying the strong-field approximation (SFA) theory. It is found that, distribution of the emitted photoelectrons varies with different polarizations of laser pulses. We have interpreted the relationship between the observed distribution and the laser polarization taking advantage of tunneling ionization theory and simple-man model. The polarization sensitivity indicates that one can easily manipulate the photoelectron distribution by controlling the polarization of the exciting pulse.     

PHOTOEXCITATION OF ATOM WITH ULTRASHORT LASER PULSES

The uncoupled basis set is used to describe the interaction between the ultrashort pulses and the Na atom. Several polarization configurations of laser pulses have been investigated, which produce different effects on the two-step photoexcitation process. The study shows that the final states of the transitions driven by ultrashort laser pulses have a strong dependence on the laser polarization configuration. A comparison of the two different basis sets shows that the uncoupled basis set may provide more information than the coupled basis set, such as angular distribution of the atom in the final states of the transitions.     

Interferences of ultrashort free electron wave packets

Interferences of free electron wave packets generated by a pair of identical, time-delayed, femtosecond laser pulses which ionize excited atomic potassium have been observed. Two different schemes are investigated: threshold electrons produced by one-photon ionization with parallel laser polarization and above threshold ionization electrons produced by a two-photon transition with crossed laser polarization. Our results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments.     

Polarimetric-Spectral-Sensitive Compressed Ultrafast Photography Enables Real-Time Intensity Visualization of Time-Varying Polarized Ultrafast Laser Pulses

Ultrafast laser pulses with complex spatiotemporal and polarization couplings have shown great application prospects in coherent control, particle acceleration, molecular dynamics excitation, and chiral or anisotropic detection. It is very important to observe the polarization-coupled ultrafast laser pulses for characterizing the performance of laser source and understanding the laser-matter interaction, but there is currently no detection technique that can completely characterize them, including the spatio-, spectral-, and polarimetric-temporal information. In this work, a polarimetric-spectral-sensitive has been compressed ultrafast photography (PS-CUP) to realize the real-time intensity visualization of time-varying polarized (TP) ultrafast laser pulses. PS-CUP can not only distinguish the polarization state of a TP pulse, but also realize the spatio-, spectral- and polarimetric-temporal measurement. To demonstrate the ability of polarization resolving detection, PS-CUP is used to successfully distinguish between the sinistral and dextral TP pulses and measure the angle and degree of linear polarization of a dextral TP pulse, respectively, and the experimental results are highly consistent with theoretical simulations. Owing to the unprecedented multidimensional detection ability, PS-CUP is prospected to provide a powerful tool for the diagnosis of various complex ultrafast laser pulses and the detection of ultrafast chiral optics.     

被动谐波锁模Er3+/Yb3+共掺双包层光纤环形腔激光器

 为了实现高重复频率和高功率的光脉冲,实验采用Er³⁺/Yb³⁺共掺双包层光纤作为增益介质,稳定的中心波长为976 nm的高功率半导体激光器作为泵浦源,利用非线性偏振旋转锁模技术,得到稳定的自起振锁模光脉冲。当泵浦功率为2.4 W时,激光器输出重复频率为8.829 MHz的连续锁模光脉冲,平均输出功率为52.5 mW,自起振锁模泵浦阈值功率为0.6 W,并观测到了稳定的高阶谐波锁模、调Q和调Q-锁模光脉冲输出。     

Effect of polarization on the structure of electromagnetic field and spatiotemporal distribution of <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>–</Superscript> pairs generated by colliding laser pulses

We have studied the production of electron–positron pairs due to polarization of vacuum in the presence of the strong electromagnetic field of two counterpropagating laser pulses. The structure of the electromagnetic field with the circular polarization has been determined using the 3D model of focused laser pulses, which was proposed by Narozhny and Fofanov. Analytic calculations have shown that the electric and magnetic fields are almost parallel to each other in the focal region when the laser pulses are completely transverse in the electric (E-wave) or magnetic (H-wave) field. On the other hand, the electric and magnetic fields are almost orthogonal when laser pulses consist of a mixture of E- and H-waves of the same amplitude. It has been found that although the latter configuration of colliding laser pulses has a much higher pair production threshold, it can generate much shorter electron–positron pulses as compared to the former configuration. The dependence of the production efficiency of pairs and their spatiotemporal distribution on the polarization of laser pulses has been analyzed using the structure of the electromagnetic field in the focal plane.     

Harmonic spectra bandwidth broadening with a new polarization gating

A new polarization gating is demonstrated by our principle-of-proof experiment, which is theoretically proposed to generate the isolated or double attosecond pulses with the multi-cycle driving laser pulse in the previous work [Optics Express. 20, 5196 （2012）]. In the experiment, the polarization gating is formed by two orthogonally-polarized linearly chirped multi-cycle laser pulses, and the spectral bandwidths of tile harmonics are broadened by more than two times, which agree with the previous theoretical work.     

Polarization dependence of the permanent deformation of silver nanoparticles in glass by ultrashort laser pulses

Glass containing spherical silver nanoparticles has been irradiated with single, intense, ultrashort laser pulses, with a wavelength corresponding to the surface plasmon resonance of the particles. After irradiation with linear polarization, dichroism is observed. Transmission electron microscopy studies reveal that these spectral changes are caused by deformation of the particles to anisotropic (in the TEM projection approximately ellipsoidal) shapes with an additional halo of small silver particles around the central one. The deformed particles are uniformly oriented with their longer axes perpendicular to the laser polarization. Using laser pulses with circular polarization, again a halo is formed around the particles, but the central particles remain spherical, and no dichroism was observed in the optical spectra. Received 30 November 2000     

Surface ripple changes during Cr film ablation with a double ultrashort laser pulse

Surface modification is investigated experimentally by varying the time separation of double femtosecond laser radiation and surface ripples by varying the time separation and polarization direction of double pulses train. Nanometer-sized particles are formed during resolidification of the molten region when the second pulse arrives within 10 ps and the molten material is ejected much after 10 ps. The ripple in the outer region remains oblique to the sum of the vector direction of the two pulses when the time delay is zero. With time delay ranging from 0.5 to 10 ps and different polarization directions of the laser radiation, the ripple generally aligned perpendicular to the polarization direction of the electric field with multiple pulses in the vicinity of ablation threshold is effectively eliminated without fragments at the edge. Furthermore, remnant ripples on irradiated area at higher energies with the same polarization direction are removed by irradiation at a lower energy with each different polarization direction of double pulse. Based on morphological observations for different time delays, possible mechanisms of ripple formations and eliminations are suggested.     

Scaling law and polarization of second harmonic emission from laser generated plasma

Measurements are described of the intensity and polarization of the second harmonic emission from a plasma generated by 1.054 μm, 35 ps laser pulses at oblique incidence. It is shown that the measured scaling cannot be explained by a simple power law and that at high intensities the degree of polarization of the second harmonic radiation does not depend on the polarization of the incident laser light.     

氧化石墨烯被动锁模掺镱光纤激光器多脉冲现象的实验研究

利用氧化石墨烯作为可饱和吸收体,在被动锁模全正常色散掺镱光纤激光器中研究了多脉冲的现象,在同一抽运功率不同偏振态下,实验获得了矩形脉冲谐波锁模、耗散孤子谐波锁模、准谐波锁模,脉冲峰值周期性调制,脉冲簇、脉冲束、混沌多重脉冲的多脉冲现象,插入激光腔内的2nm窄带滤波器具有限制增益带宽、对脉冲塑形、诱导多脉冲产生的作用,调节偏振控制器相当于改变腔内增益,是实现不同类型多脉冲现象的主要原因,本实验研究有利于加深对多脉冲动力学行为在正常色散区域氧化石墨烯锁模掺镱光纤激光器中的理解。     

High energy giant chirped passively mode-locked Yb3+-doped fiber laser with nanoseconds to picoseconds pulses

We report an all normal dispersion low repetition rate high energy passive mode-locked ytterbium-doped fiber laser with output pulses duration ranging from nanoseconds to picoseconds. The mode-locking mechanism of the laser is based on nonlinear polarization evolution and strong pulses shaping with a cascade long-period fiber grating bandpass filtering in highly chirped pulses. The laser generates highly stable pulses duration from 2.62 ns to 315 ps with a maximum pulse energy of 49.5 nJ and 2.5435 MHz repetition rate.     

Femtosecond pulse laser-induced self-organized nanogratings on the surface of a ZnSe crystal

Periodic nanostructures are observed on the surface of ZnSe after irradiation by a focused beam of a femtosecond Ti:sapphire laser, which are aligned perpendicular to the laser polarization direction. The period of self-organized grating structures is about 160 nm. The phenomenon is interpreted in terms of interference between the incident light field and the surface scattered wave of 800-nm laser pulses. With the laser polarization parallel to the moving direction we produce long-range Bragg-like gratings by slowly moving the crystal under a fixed laser focus. The nanograting orientation is adjusted by laser polarization and the accumulation effect. PACS 81.16.Rf; 78.67.-n; 33.80.Rv; 82.53.Mj; 81.16.-c     

保偏光纤中在不同频率区域拉曼效应和参量放大增益谱

根据考虑拉曼效应后的双折射光纤所满足的非线性相干耦合薛定谔方程, 推导出了当沿两个偏振轴入射两束不同波长的激光脉冲时所产生的增益表达式. 通过与入射相同频率的光脉冲所产生增益的对比, 在考虑拉曼效应的情况下, 讨论了入射不同频率光脉冲对增益谱的影响. 结果表明, 在正常色散区和反常色散区, 当输入两束激光脉冲频率不同时, 增益谱较输入相同频率激光脉冲时产生了明显的变化, 其外侧的斯托克斯部分和反斯托克斯部分增益峰, 随着群速度失配的增加强度明显加强、偏离中心频率, 可以用于提取太赫兹脉冲.当两偏振模处于不同色散区时, 增益谱与不考虑拉曼效应时也存在明显的不同, 增益谱的对称性遭到破坏, 斯托克斯部分的增益峰强度要明显高于反斯托克斯部分. 关键词： 不同频率区域 保偏光纤 拉曼效应 参量放大     

飞秒被动锁模环形腔掺Er3＋光纤激光器

在考虑增益、损耗、群速度色散、自相位调制、快速可饱和吸收体等各种参数同时作用情况下，分析了非线性偏振旋转效应自启动锁模机理，研究了腔体参数与锁模脉冲之间的关系，并给出飞秒被动锁模环形腔掺Er3＋光纤激光器实验原理。实验采用性能稳定的980nm半导体激光器作为抽运源，高掺杂短长度掺Er3＋光纤作为增益介质，利用非线性偏振旋转锁模技术，得到了稳定的飞秒自起振锁模光脉冲。抽运功率为23mW时，激光器输出锁模脉冲中心波长1552nm，3dB带宽为7.6nm，重复频率14.0MHz，平均输出功率0.43mW，自起振锁模泵浦阈值功率11.5mW,并观测到了稳定的高阶锁模脉冲输出。该激光器与报道过的相同结构光纤激光器相比,自起振泵浦阈值低、脉冲能量高、稳定性好，且频谱边带幅度小。     

Observation of dark pulse in a dispersion-managed fiber ring laser

The observation of dark pulse in a dispersion-managed fiber ring laser with net negative cavity group velocity dispersion (GVD) is reported. Both bright and dark pulses can be obtained in our fiber laser. When we appropriately adjust the cavity birefringence to achieve triple-wavelength mode-locked operation in the laser by rotating the polarization controller, the bright pulse could be switched to dark pulse. It is believed that the dark dispersion-managed (DM) pulse generation is caused by the linear and nonlinear intermodulation effects among the three wavelength pulses.     

Circularly polarized sub-1.5 cycle laser pulses at 1.8 μm

The generation of intense carrier-envelope-phase-stabilized sub-1.5 cycle circularly polarized pulses at 1.8 μm is reported. After changing the polarization of the pulses produced by an optical parameter amplifier to circular, selected nonlinear medium parameters are found to be able to expand the spectrum to supercontinuum (1,300–2,100 nm) with a extremely high transmittance (>65 %). Using such laser pulses, polarization control of terahertz emission is demonstrated.     

Self-assembled volume vortex grating induced by femtosecond laser pulses in glass

We presented a microfabrication process for optical volume vortex grating inside glass by femtosecond laser pulses. The self-trapped filament of femtosecond laser pulses can induce hundreds μm-long region refractive-index changes in glass. We realized the restructured optical vortex beams using a collimated He–Ne laser beam. The maximum first-order diffraction efficiency was about 19.6%. The volume vortex grating structure fabricated in glass is polarization dependent.