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exposed to an intense laser pulse
摘要： We investigate the role of the rotational excitation in the coherent and unidirectional B 2Σ+u emission of N2+ at 391 nm called air lasing generated when N2 is irradiated with intense femtosecond near-infrared laser pulses at 800 nm. We simulate the time-dependent population transfer process of N2+, which is assumed to be generated suddenly in an intense ultrashort laser pulse, by including vibrational and rotational degrees of freedom, and reveal that, when the light field intensity is below ≈ 4 × 1014 W cm?2, the population inversion can be achieved in N2+ only between rotationally highly excited levels in the vibrational ground (v' = 0) state of the B 2Σ+u state and those in the vibrational ground (v'' = 0) state of the electronic ground X 2Σ+g state, so that the lasing at 391 nm is realized even when the total population in the v' = 0 state of the electronically excited B 2Σ+u state is not inverted with respect to the total population in the v'' = 0 state of the electronic ground X 2Σ+g state.
关键词： air lasing,N2+,rotational excitation,population inversion,femtosecond laser pulses
更新于2025-09-23 15:21:01
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - All-Fiber 2.07 μm Distributed Feedback Laser Based on π-Phase-Shifted FBG Inscribed in Heavily Doped Holmium Fiber by Femtosecond Laser Pulses
摘要： Distributed-feedback (DFB) fiber laser is a versatile source of low-noise single-frequency radiation that finds applications in spectroscopy, optical communications and sensing devices. Depending on the active medium, different lasing wavelengths can be obtained: 1.03–1.12 μm for Yb, 1.53–1.62 μm for Er, and 1.73–1.93 μm for Tm-doped fibers. Special fiber Bragg grating (FBG) with a length of 20–70 mm and phase shift in the structure, directly written in an active medium, serves as distributed feedback cavity of this type of laser. Commonly, special UV photosensitive fibers and phase masks inscription technique are used for an FBG fabrication. Alternative femtosecond (fs) point-by-point technique [1] has a number of advantages as compared to UV-based ones: 1) wide variety of active fibers can serve as a host material for an FBG, thanks to the nonlinear mechanism of fs pulses absorption, 2) FBG period can be easily tuned by controlling fs pulses frequency and velocity of fiber translation during the inscription process, 3) each fiber grating “pitch” can be written independently, which means that phase shift with an exact value and position along FBG can be introduced. Despite the number of publications dealing with holmium fiber lasers [2], there no publications devoted to DFB ones. At the same time, such type of laser sources has a great potential as a master oscillator in all-fiber coherent beam combination systems [3]. We report on the creation of holmium-doped DFB fiber laser with a wavelength of 2.07 μm operating in single-frequency regime (Fig. 1a). The laser is based on custom-made single-mode holmium-doped fiber having absorption coefficient about 43 dB/m at 1.125 μm, which is the wavelength of pump Yb fiber laser. 42-mm π-phase-shifted FBG was directly inscribed in the active fiber by femtosecond IR laser pulses and served as distributed feedback cavity of the laser. Lasing threshold was reached at 350 mW pump power, and output power exceeded 36 mW at pump power of 4.8 W (Fig. 1b). Single-polarization mode operation was observed with linewidth of ~10 kHz. Additionally to the room-temperature operation regime we will present and discuss the laser operation regimes at cryogenic temperature (77 K), as well as high-temperature (>400 K). To the best of our knowledge, it is the first realization of all-fiber DFB laser based on holmium active medium.
关键词： femtosecond laser pulses,holmium-doped fiber,single-frequency radiation,Distributed-feedback fiber laser,π-phase-shifted FBG
更新于2025-09-23 15:19:57
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Enhancement of photocatalytic activity by femtosecond-laser induced periodic surface structures of Si
摘要： Laser induced periodic surface structures (LIPSS) represent a kind of top down approach to produce highly reproducible nano/microstructures without going for any sophisticated process of lithography. This method is much simpler and cost effective. In this work, LIPSS on Si surfaces were generated using femtosecond laser pulses of 800 nm wavelength. Photocatalytic substrates were prepared by depositing TiO2 thin films on top of the structured and unstructured Si wafer. The coatings were produced by sputtering from a Ti target in two different types of oxygen atmospheres. In first case, the oxygen pressure within the sputtering chamber was chosen to be high (3 × 10–2 mbar) whereas it was one order of magnitude lower in second case (2.1 × 10–3 mbar). In photocatalytic dye decomposition study of Methylene blue dye it was found that in the presence of LIPSS the activity can be enhanced by 2.1 and 3.3 times with high pressure and low pressure grown TiO2 thin films, respectively. The increase in photocatalytic activity is attributed to the enlargement of effective surface area. In comparative study, the dye decomposition rates of TiO2 thin films grown on LIPSS are found to be much higher than the value for standard reference thin film material Pilkington ActivTM.
关键词： photocatalytic dye decomposition,silicon,femtosecond laser pulses,TiO2 thin film,laser induced periodic surface structures,nanoripples
更新于2025-09-23 15:19:57
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Transient optics of gold during laser irradiation: From first principles to experiment
摘要： Intense femtosecond laser pulses can induce dramatic changes on different properties of materials. Laser induced changes of the optical properties are particularly relevant, since they can lead to a modi?cation of the amount of energy that the material absorbs from the laser pulse. In noble metals, changes of re?ectivity upon femtosecond laser illumination are expected to be strong due to the excitation of d electrons. In this work we perform measurements of the re?ectivity of laser excited gold in the infrared and in the ultraviolet range, respectively. We ?nd a remarkable dependence of the re?ectivity on laser ?uence, which is in turn different in both ranges of photon energy. In order to understand the behavior of the re?ectivity in laser excited solids and to explain our measured re?ectivity curves we develop a theoretical scheme in the framework of the two-temperature model with the electronic temperature as the key parameter. Our approach is based on all-electron calculations of the interband contribution to the re?ectivity and a careful determination of the intraband, Drude-like terms and a realistic model for the space and time resolved energy transfer from a Gaussian laser pulse into the electronic system. We obtain very good agreement between experiment and theory and identify the main mechanisms for re?ectivity changes as a function of laser ?uence.
关键词： intraband,Drude-like terms,interband contribution,electronic temperature,Gaussian laser pulse,re?ectivity,two-temperature model,femtosecond laser pulses,optical properties,gold
更新于2025-09-23 15:19:57
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Direct imaging of direction-controlled molecular rotational wave packets created by a polarization-skewed double-pulse
摘要： High‐precision, time‐resolved Coulomb explosion imaging of rotational wave packets in nitrogen molecules created with a pair of time‐delayed, polarization‐skewed femtosecond laser pulses is presented, providing insight into creation process and dynamics of direction‐controlled wave packet. To initiate unidirectional rotation, the interval of the double‐pulse was set so that the second, polarization‐tilted pulse hit the molecules at the time when molecules were aligned or antialigned along the polarization vector of the first pulse. During the revival period of the rotational wave packet, pulse intervals around both the full and half revival times were used. The observed molecular wave packet movies clearly show signatures of quantum rotation such as angular localization (alignment), dispersion, and revival phenomena during the unidirectional motion. The patterns are quite different depending on the pulse interval even when angular distribution at the second pulse irradiation is similar. The observed interval‐dependence of the dynamics was analyzed on the basis of the real‐time images, with an aid of numerical simulations, and the creation process of the packets were discussed. We show the observed image patterns can be essentially rationalized in terms of rotational period and alignment parameter. Because the double‐pulse scheme is the most fundamental in the creation of direction‐controlled rotational wave packets, this study will lead to more sophisticated control and characterization of directional molecular motions.
关键词： rotational wave packets,Coulomb explosion imaging,quantum rotation,femtosecond laser pulses,unidirectional rotation
更新于2025-09-23 15:19:57
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Proof-of-Principle Experiment for Nanoparticle-Assisted Laser Wakefield Electron Acceleration
摘要： Laser electron accelerators have paved the way to build compact electron accelerators, but the quality of the electron beam should be improved to enable efficient use in various specialized applications. We demonstrate a proof-of-principle experiment for nanoparticle-assisted laser wakefield acceleration (NA LWFA) to improve the quality of the electron beam. Nanoparticles are generated through laser ablation of an aluminum target and introduced into a helium plasma to trigger the injection of electrons into the nonlinear plasma wake excited by 70 TW femtosecond laser pulses. We observe a significant enhancement of the electron beam energy, energy spread, and divergence compared with the case of self-injection. For instance, the best quality electron bunches present a maximum energy of up to 340 MeV, with a relative energy spread of 4.7% and a vertical divergence of 5.9 mrad. The initial results on NA LWFA are very promising and motivate further theoretical and experimental research into developing nanoparticle-assisted laser wakefield acceleration.
关键词： femtosecond laser pulses,electron beam quality,plasma wake,nanoparticle-assisted,laser wakefield acceleration
更新于2025-09-19 17:13:59
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Femtosecond Laser-Induced Periodical Nanomodification of Surface Composition
摘要： Nanoscale periodical chemical composition modification of different two-component compound materials GaAs, CdTe, W0.5C and TiC was realized for the first time through multi-shot femtosecond laser irradiation of their surfaces. The resulting surface modification represents periodical nanoscale ripples with different chemical composition in valleys and ridges. The underlying mechanism is related to a sequence of elementary processes, such as laser excitation surface plasmon-polariton, their interference with the incident laser radiation, inhomogeneous surface heating and incongruent cavitation-like material removal. The latter mechanism is responsible for higher evaporation rate of the components with higher volatility, resulting in growth of surface concentration of low-volatility components.
关键词： ablation,surface modification,nanostructures,femtosecond laser pulses
更新于2025-09-19 17:13:59
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[IEEE 2019 IEEE 8th International Conference on Advanced Optoelectronics and Lasers (CAOL) - Sozopol, Bulgaria (2019.9.6-2019.9.8)] 2019 IEEE 8th International Conference on Advanced Optoelectronics and Lasers (CAOL) - Nonlinear compression of ultra-high-power laser pulses
摘要： Nowadays the peak power of PW lasers is limited by the damage threshold of optical compressor diffraction gratings. A promising way to overcome the difficulty and increase peak power is to use an additional pulse shortening after optical compressor without significant energy loses. The discussed method is called CafCA – Compression after Compressor Approach. CafCA possesses three undisputed merits: simplicity and low cost, negligible pulse energy loss, and applicability to any high-power lasers without changing their optical schemes. The key idea of the approach is to broaden spectrum of powerful laser pulses exactly after grating compressor and correct spectral phase with help of reflected dispersion devices (chirped mirrors in particular). The idea has been known since the end of 1960s but its application to lasers with a peak power higher 1 TW has been restricted by a number of physical reasons. The reasons and ways of its overcoming are discussed in the paper. We analyze the recent experimental results that demonstrate the CafCA efficiency in the range up to 200 TW.
关键词： temporal compression,self-phase modulation,PW-level femtosecond laser pulses
更新于2025-09-19 17:13:59
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Ultrasensitive SERS-Based Plasmonic Sensor with Analyte Enrichment System Produced by Direct Laser Writing
摘要： We report an easy-to-implement device for surface-enhanced Raman scattering (SERS)-based detection of various analytes dissolved in water droplets at trace concentrations. The device combines an analyte-enrichment system and SERS-active sensor site, both produced via inexpensive and high-performance direct femtosecond (fs)-laser printing. Fabricated on a surface of water-repellent polytetrafluoroethylene substrate as an arrangement of micropillars, the analyte-enrichment system supports evaporating water droplet in the Cassie–Baxter superhydrophobic state, thus ensuring delivery of the dissolved analyte molecules towards the hydrophilic SERS-active site. The efficient pre-concentration of the analyte onto the sensor site based on densely arranged spiky plasmonic nanotextures results in its subsequent label-free identification by means of SERS spectroscopy. Using the proposed device, we demonstrate reliable SERS-based fingerprinting of various analytes, including common organic dyes and medical drugs at ppb concentrations. The proposed device is believed to find applications in various areas, including label-free environmental monitoring, medical diagnostics, and forensics.
关键词： SERS,direct laser processing,femtosecond laser pulses,medical drugs,superhydrophobic textures,plasmonic nanostructures,analyte enrichment
更新于2025-09-16 10:30:52
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - High Conversion Efficiency in a System “Nonlinear-Optical Crystal Partially Filling the Cross Section of a Rectangular Waveguide”
摘要： It is shown that terahertz radiation (0.1-3 THz) can be effectively generated via optical rectification of femtosecond laser pulses in a system of a “nonlinear-optical crystal partially filling the cross section of a rectangular metal waveguide”. The numerical calculations for the phase-matching in LiNbO3, ZnTe, DAST and GaSe nonlinear optical crystals have been performed. The influence of THz radiation absorption on the efficiency of generation of coherent THz radiation in the system has been investigated.
关键词： optical rectification,femtosecond laser pulses,phase-matching,nonlinear-optical crystal,terahertz radiation
更新于2025-09-16 10:30:52