Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences
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Vortex Ti:Sapphire laser by using an intracavity spot-defect spatial filter
No full textNational Natural Science Foundation of China [61275206, 61475166, 61405226]; Shanghai Natural Science Fund [14ZR1445200]We demonstrated an LG(01)-mode vortex Ti:Sapphire laser by utilizing an intracavity spot defect for spatial filtering. At the 140-mu m diameter of spot defect, the power of vortex laser output reached 135 mW, and the slope efficiency of laser was 17.7%. The effects of the sizes of spot defects on the transverse laser modes and laser powers were also discussed. (C) 2017 Elsevier Ltd. All rights reserved
Harmonic generation at air-soil interface by femtosecond laser filament
No full textKey Project from the Bureau of International Cooperation Chinese Academy of Sciences [181231KYSB20160045]; 100 Talents Program of the Chinese Academy of Sciences; State Key Laboratory of High Field Laser Physics; [XDB16]We observe the third-harmonic generation and second-harmonic generation together with element fluorescence from the interaction of a femtosecond laser filament with a rough surface sample (sandy soil) in non-phase-matched directions. The harmonics prove to originate from the phase-matched surface harmonics and air filament, then scatter in non-phase-matched directions due to the rough surface. These harmonics occurr when the sample is in the region before and after the laser filament, where the laser intensity is not high enough to excite the element fluorescence. The observed harmonics are related to the element spectroscopy, which will benefit the understanding of the interaction of the laser filament with a solid and be helpful for the application on filament induced breakdown spectroscopy
Laser-driven fast electron ionization wave propagation in a dielectric target
No full textNational Natural Science Foundation of China [11425418, 11405244, 11127901, 61521093]; Shanghai Natural Science Funds [14ZR1444800]; Strategic Priority Research Program [XDB16]; Open Foundation of the National Key Laboratory of Shock Wave and Detonation PhysicsUltrafast shadowgraphy with sub-picosecond resolution is applied to investigate the propagation of laser-driven fast electron beams inside a dielectric target. Time-resolved expansion of an ionization sphere caused by hot electron transportation in the target is observed. An abnormal absorption area with a width of approximately 10 mu m is observed near the ionization front and is consistent with the one-dimensional electron transport model. The observed distortions of the edge diffraction fringes near the surface of the target are analyzed to qualitatively retrieve the phase shift caused by the ionization front. A simplified three-dimensional model is proposed to analyze the underlying physics and indicates that the valley in the ionization front appears due to the unstable propagation of hot electrons in a self-induced fountain electric field, which is induced by the collective effect of the free electrons and the ions. Published by AIP Publishing
Measurements of nonlinear refraction in the mid-infrared materials ZnGeP2 and AgGaS2
No full text100 Talents Program of CAS; National Natural Science Foundation of China [61475169, 61521093, 11127901]; Youth Innovation Promotion Association CASAccurate values of the nonlinear index of refraction n(2) for the nonlinear optical crystals ZnGeP2 and AgGaS2 were measured using the Z-scan technique. The results were obtained using similar to 100-fs mid-infrared pulses from an OPA-DFG system driven by the 800-nm laser. The measured values of the AGS agree well with the theoretical values based on the two-band model for a direct band gap material. We confirmed the validity of the measured values for the indirect semiconductor ZGP by comparing the results with those obtained at similar to 1300 nm in a previous study. We have proved the broadening due to GVD is negligible in our experiments. We believe these nonlinear measurements of AGS and ZGP are meaningful for the analysis and design of optical systems in the mid-infrared region
Pure Even Harmonic Generation from Oriented CO in Linearly Polarized Laser Fields
No full textNational Natural Science Foundation of China [11274326, 61521093, 61405222, 11127901]; Chinese Academy of Sciences [XDB16]The first high harmonic spectrum containing only the odd orders was observed in experiments 30 years ago. However, a spectrum containing pure even harmonics has never been observed. We investigate the generation of pure even harmonics from oriented CO molecules in linearly polarized laser fields employing the time-dependent density-functional theory. We find that the even harmonics, with no odd orders, are generated with the polarization perpendicular to the laser polarization when the molecular axis of CO is perpendicular to the laser polarization. The generation of pure even harmonics reveals a type of dipole acceleration originating from the permanent dipole moment. This phenomenon exists in all systems with permanent dipole moments, including bulk crystal and polyatomic molecules
Dick Effect in the Integrating Sphere Cold Atom Clock
No full textNational Natural Science Foundation of China [11604353]; Youth Innovation Promotion Association of Chinese Academy of SciencesThe Dick effect is an important factor limiting the frequency stability of sequentially-operating atomic frequency standards. Here we study the impact of the Dick effect in the integrating sphere cold atom clock (ISCAC). To reduce the impact of the Dick effect, a 5MHz local oscillator with ultra-low phase noise is selected and a new microwave synthesizer is built in-house. Consequently, the phase noise of microwave signal is optimized. The contribution of the Dick effect is reduced to 2.5x10(-13) tau(-1/2) (tau is the integrating time). The frequency stability of 4.6x10(-13) tau-(1/2) is achieved. The development of this optimization can promote the space applications of the compact ISCAC
Direct loading of atoms from a macroscopic quadrupole magnetic trap into a microchip trap
No full textNational Natural Science Foundation of China [11604348]We demonstrate the direct loading of cold atoms into a microchip 2-mm Z-trap, where the evaporative cooling can be performed efficiently, from a macroscopic quadrupole magnetic trap with a high loading efficiency. The macroscopic quadrupole magnetic trap potential is designed to be moveable by controlling the currents of the two pairs of anti-Helmholtz coils. The cold atoms are initially prepared in a standard six-beam magneto-optical trap and loaded into the macroscopic quadrupole magnetic trap, and then transported to the atom chip surface by moving the macroscopic trap potential. By means of a three-dimensional absorption imaging system, we are able to optimize the position alignment of the atom cloud in the macroscopic trap and the microchip Z-shaped wire. Consequently, with a proper magnetic transfer scheme, we load the cold atoms into the microchip Z-trap directly and efficiently. The loading efficiency is measured to be about 50%. This approach can be used to generate appropriate ultracold atoms sources, for example, for a magnetically guided atom interferometer based on atom chip
Effect of two-center interference on molecular ionization in multiphoton ionization regime
No full textNational Key program for Science and Technology Research and Development [2016YFA0401100]; National Basic Research Program of China [2013CB922201]; National Natural Science Foundation of China [11304329, 11334009, 11425414, 11374197, 11447114, 11504215]Using solution of the full three-dimensional time-dependent Schrodinger equation (TDSE) in prolate spheroidal coordinates, we investigate the orientation dependence of ionization of H-2(+) in near-infrared laser fields. It is found that, the ionization probability decreases as a function of the alignment angle in tunneling ionization regime, while it ascends with the increase of orientation angle in multiphoton ionization regime for the internuclear distance R=2 a.u. Furthermore, the result obtained by the length gauge strong-field approximation theory is in qualitative agreement with that calculated by the TDSE but the radiation gauge strong-field approximation and molecular ADK theories fail to reproduce the TDSE result. Analysis indicates that the above intriguing feature can be ascribed to the interference between the partial electron wave packets emitted from different molecular cores, which becomes evident at low laser intensity due to increased width of the initial mechanical momentum of the photoelectron at ionization moment. In addition, when the internuclear distance increases to R=4 a. u., the ionization yields decrease vs alignment angle in both tunneling and multiphoton regimes since the electron wavefunction of the 1 sigma(g) orbit is more concentrated in the molecular axis than that of R=2 a.u
Performance of smoothing by spectral dispersion with consideration of the gain characteristic of Nd:glass amplifier
No full text采用数值方法研究了钕玻璃放大器的增益特性对高功率激光系统中光谱色散匀滑单元性能的影响.分析结果表明,入射光中心波长与放大器增益曲线中心波长不一致时,焦斑强度分布会受到一定的影响,且该影响随放大倍数增大趋于明显,而两个波长一致时,强度分布变化较小.靶面焦斑整体的辐照均匀性则主要取决于经过相位调制后的激光束的带宽,放大器的增益特性对其空间功率谱及均匀性无明显的影响.所得结论为光谱色散匀滑单元在激光系统中的实际应用提供了重要的理论参考.National Natural Science Foundation of China [11204043, 11374067]; Open Fund of Key Laboratory for High Power Laser Physics of Chinese Academy of Sciences [SG-001103]A key issue in developing a high-power laser driver, which can be used for inertial confinement fusion and laser produced plasma experiments, is to obtain uniform irradiation on the target surface, thus a number of spatial or temporal techniques have been proposed for laser beam smoothing. A scheme combining a lens array with the technique of smoothing by spectral dispersion (SSD) is being explored in the SG-II Laser Facility located in Shanghai Institute of Optics and Fine Mechanics. As the laser system involves a variety of optical elements, their influences have to be considered in the implementation of such a scheme. The Nd: glass amplifier is one of the most important parts of the system, and the phase-modulated laser beam will propagate through it along the long light path when SSD is employed. In this paper, the performance of uniform irradiation of the target pattern is studied based on two-dimensional simulations when the gain characteristic of the amplifier is taken into account. The major factors, such as the small signal gain profile of the amplifier, the amplification factor, the bandwidth of the phase-modulated laser beam and the difference between the central wavelength of the laser and the central wavelength of the amplifier gain curve, are analyzed in detail. The numerical results show that when the central wavelength of the incident beam is different from the central wavelength of the amplifier gain curve, intensity distribution of the target pattern will be affected to a degree depending on the amplification factor; while these two wavelengths are very close to or identical with each other, variation in the intensity distribution is trivial. The symmetry of the phase-modulated laser spectrum will be destroyed due to the gain characteristic of the amplifier, especially when the bandwidth is relatively wide. However, the slight asymmetry does not result in significant influence on the spatial power spectrum nor uniformity of the target pattern, even in the case where the central wavelength of the incident beam is different from that of the amplifier gain curve. The reasons would be 1) the gain curve of the amplifier is actually quite flat within the laser bandwidth, and 2) with the technique of SSD, all spectral components contribute to the target intensity distribution within an average time. The analysis indicates that the performance of uniform irradiation of the target pattern depends mainly on the bandwidth of the phase-modulated laser beam. A wider bandwidth can always generate better irradiation when it is within a certain range, say no more than 0.3 nm, but beyond this range, the nonuniformity tends to remain at a level about 0.25-0.3. Multistage Nd: glass amplifiers will be employed in the practical laser driver, and the case investigated in this paper involves only one stage for simplicity. The conclusion obtained in this paper is important for implementing the technique of SSD in the laser system
Compensation for the setup instability in ptychographic imaging
No full textNational Natural Science Foundation of China [61675215]The high-frequency vibration of the imaging system degrades the quality of the reconstruction of ptychography by acting as a low-pass filter on ideal diffraction patterns. In this study, we demonstrate that by subtracting the deliberately blurred diffraction patterns from the recorded patterns and adding the properly amplified subtraction to the original data, the high-frequency components lost by the vibration of the setup can be recovered, and thus the image quality can be distinctly improved. Because no prior knowledge regarding the vibrating properties of the imaging system is needed, the proposed method is general and simple and has applications in several research fields