Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences
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Photoemission study of the electronic structure of valence band convergent SnSe
No full textNational Key R&D program of China [2017YFA0305400]; Chinese Academy of Science-Shanghai Science Research Center [CAS-SSRC-YH-2015-01]; Engineering and Physical Sciences Research Council [EP/M020517/1]; National Natural Science Foundation of China [11674229, 11227902]; Science and Technology Commission of Shanghai Municipality [14520722100]IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance. The multiple close-to-degenerate (or "convergent") valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band structure. In this paper, however, using angle-resolved photoemission spectroscopy, we perform a systematic investigation of the electronic structure of SnSe. We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe
Parasitic lasing in large aperture Ti: sapphire chirped pulse amplifier
No full textNational Natural Science Foundation of China (NSFC) [61378030, 61521093]We research some properties of parasitic lasing (PL) in the Ti: sapphire chirped pulse amplifier with the crystal diameter of 100 mm. The evolutionary process from the spontaneous emission to the PL and its influence on amplified output energy, spectrum, and beam profile are experimentally measured. The threshold of PL in the crystal is 22 J, and the output signal can still keep rising with the pump when the pump energy is below 38 J. The PL has no obvious impact on the output spectrum and beam profile besides the energy
Collisionless Shock Acceleration of High-Flux Quasimonoenergetic Proton Beams Driven by Circularly Polarized Laser Pulses
No full textMinistry of Science and Technology of the People's Republic of China [2016YFA0401102]; National Natural Science Foundation of China [11705260]; Shanghai Natural Science Foundation [17ZR1434300]; Strategic Priority Research Program of the Chinese Academy of Sciences [XDB16]We present experimental studies on ion acceleration using an 800-nm circularly polarized laser pulse with a peak intensity of 6.9 x 10(19) W/cm(2) interacting with an overdense plasma that is produced by a laser prepulse ionizing an initially ultrathin plastic foil. The proton spectra exhibit spectral peaks at energies up to 9 MeV with energy spreads of 30% and fluxes as high as 3 x 10(12) protons/MeV/sr. Two-dimensional particle-in-cell simulations reveal that collisionless shocks are efficiently launched by circularly polarized lasers in exploded plasmas, resulting in the acceleration of quasimonoenergetic proton beams. Furthermore, this scheme predicts the generation of quasimonoenergetic proton beams with peak energies of approximately 150 MeV using current laser technology, representing a significant step toward applications such as proton therapy
Monte Carlo approach to calculate ionization dynamics of hot solid-density plasmas within particle-in-cell simulations
No full textGerman Academic Exchange Service (DAAD); China Scholarship Council (CSC)A physical model based on a Monte Carlo approach is proposed to calculate the ionization dynamics of hot-solid-density plasmas within particle-in-cell (PIC) simulations, and where the impact (collision) ionization (CI), electron-ion recombination (RE), and ionization potential depression (IPD) by surrounding plasmas are taken into consideration self-consistently. When compared with other models, which are applied in the literature for plasmas near thermal equilibrium, the temporal relaxation of ionization dynamics can also be simulated by the proposed model. Besides, this model is general and can be applied for both single elements and alloys with quite different compositions. The proposed model is implemented into a PIC code, with (final) ionization equilibriums sustained by competitions between CI and its inverse process (i.e., RE). Comparisons between the full model and model without IPD or RE are performed. Our results indicate that for bulk aluminium at temperature of 1 to 1000 eV, (i) the averaged ionization degree increases by including IPD; while (ii) the averaged ionization degree is significantly over estimated when the RE is neglected. A direct comparison from the PIC code is made with the existing models for the dependence of averaged ionization degree on thermal equilibrium temperatures and shows good agreements with that generated from Saha-Boltzmann model and/or FLYCHK code
Enhancing the electron acceleration by a circularly polarized laser interaction with a cone-target with an external longitudinal magnetic field
No full textNational Key Research and Development Program of China [2016YFA0401101]; National Natural Science Foundation of China [11475030, 11375032, 11575035, 11435011]The propagation of left-hand (LH-) and right-hand (RH-) circularly polarized (CP) lasers and the accompanying generation of fast electrons in a magnetized cone-target with pre-formed plasmas are investigated. In this work, the strength of external magnetic field is comparable to that of the incident laser. Theoretical analyses indicate that the cut-off density of LH-CP laser is larger than that without an external magnetic field. When the external magnetic field normalized by the laser magnetic field is larger than the relativistic factor, the RH-CP laser will keep on propagating till the laser energy is depleted. The theoretical predictions are confirmed by two-dimensional particle-in-cell simulations. Simulation results show that in the presence of external longitudinal magnetic field, the energies and yields of fast electrons are greatly enhanced for RH-CP laser. Besides, the coupling efficiency of laser energy to energetic electrons for RH-CP laser is much higher than that for LH-CP laser and without external magnetic field. Furthermore, detailed simulation results perform an enhancement of the incident laser absorption with increasing external magnetic field. Published by AIP Publishing
Two-Dimensional SnS: A Phosphorene Analogue with Strong In-Plane Electronic Anisotropy
No full textNational Natural Science Foundation of China [11504234]; Science and Technology Commission of Shanghai Municipality [15QA1403200]; ShanghaiTech University; Strategic Priority Research Program of the Chinese Academy of Sciences [XDB04030000]We study the anisotropic electronic properties of two-dimensional (2D) SnS, an analogue of phosphorene, grown by physical vapor transport. With transmission electron microscopy and polarized Raman spectroscopy, we identify the zigzag and armchair directions of the as-grown 2D crystals. The 2D SnS field-effect transistors with a cross Hall-bar structure are fabricated. They show heavily hole-doped (similar to 10(19) cm(-3)) conductivity with strong in-plane anisotropy. At room temperature, the mobility along the zigzag direction exceeds 20 cm(2) V-1 s(-1), which can be up to 1.7 times that in the armchair direction. This strong anisotropy is then explained by the effective mass ratio along the two directions and agrees well with previous theoretical predictions. Temperature-dependent carrier density determined the acceptor energy level to be similar to 45 meV above the valence band maximum. This value matches a calculated defect level of 42 meV for Sn vacancies, indicating that Sn deficiency is the main cause of the p-type conductivity
Image grating metrology using phase-stepping interferometry in scanning beam interference lithography
No full textLarge-sized gratings are essential optical elements in laser fusion and space astronomy facilities. Scanning beam interference lithography is an effective method to fabricate large-sized gratings. To minimize the nonlinear phase written into the photo-resist, the image grating must be measured to adjust the left and right beams to interfere at their waists. In this paper, we propose a new method to conduct wavefront metrology based on phase-stepping interferometry. Firstly, a transmission grating is used to combine the two beams to form an interferogram which is recorded by a charge coupled device(CCD). Phase steps are introduced by moving the grating with a linear stage monitored by a laser interferometer. A series of interferograms are recorded as the displacement is measured by the laser interferometer. Secondly, to eliminate the tilt and piston error during the phase stepping, the iterative least square phase shift method is implemented to obtain the wrapped phase. Thirdly, we use the discrete cosine transform least square method to unwrap the phase map. Experiment results indicate that the measured wavefront has a nonlinear phase around 0.05 [email protected]. Finally, as the image grating is acquired, we simulate the print-error written into the photo-resist
Interference pattern period measurement at picometer level
No full textTo produce large scale gratings by Scanning Beam Interference Lithography (SBIL), a light spot containing grating pattern is generated by two beams interfering, and a scanning stage is used to drive the substrate moving under the light spot. In order to locate the stage at the proper exposure positions, the period of the Interference pattern must be measured accurately. We developed a set of process to obtain the period value of two interfering beams at picometer level. The process includes data acquisition and data analysis. The data is received from a photodiode and a laser interferometer with sub-nanometer resolution. Data analysis differs from conventional analyzing methods like counting wave peaks or using Fourier transform to get the signal period, after a preprocess of filtering and envelope removing, the mean square error is calculated between the received signal and ideal sinusoid waves to find the best-fit frequency, thus an accuracy period value is acquired, this method has a low sensitivity to amplitude noise and a high resolution of frequency. With 405nm laser beams interfering, a pattern period value around 562nm is acquired by employing this process, fitting diagram of the result shows the accuracy of the period value reaches picometer level, which is much higher than the results of conventional methods
High-density grating pair for displacement measurement
No full textA novel method of displacement measurement based on a high density grating pair is proposed. When a laser beam is incident normal to a closely placed high density grating pair, efficiencies of transmission diffraction orders will change periodically along with the relative displacement of the two gratings in the grating period direction. The period of efficiency changing is equal to the grating period, thus measurement of displacement in the grating period direction can be achieved by detecting the power of diffraction orders
Analysis of the noise in backprojection light field acquisition and its optimization
No full textNational Natural Science Foundation of China (NSFC) [61327902, 61377005]; Recruitment Program of Global Youth Experts; Research Grants Council of the Hong Kong Special Administrative Region, China [HKU714/13]Light field reconstruction from images captured by focal plane sweeping can achieve high lateral resolution comparable to the modern camera sensor. This is impossible for the conventional micro-lenslet-based light field capture systems. However, the severe defocus noise and the low depth resolution limit its applications. In this paper, we analyze the defocus noise in the focal-plane-sweeping-based light field reconstruction technique, and propose a method to reduce the defocus noise. Both numerical and experimental results verify the proposed method. (C) 2017 Optical Society of Americ