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Pulse-length effects in strong-field ionization of atoms by co-rotating and counter-rotating bicircular laser pulses
摘要： We study strong-?eld ionization of H and selected rare-gas atoms by solving the three-dimensional time-dependent Schr?dinger equation in the single-active-electron approximation with particular attention to the effect of pulse length on the total ionization yields (TIYs) produced by counter-rotating bicircular (CRBC) lasers compared to co-rotating bicircular (COBC) lasers. To this end, TIYs, photoelectron momentum distributions, and above-threshold ionization spectra are presented for various pulse durations and peak intensities. For pulses containing ?ve or more cycles of the fundamental near-infrared laser ?eld, the CRBC/COBC ratio of TIYs is affected due to ionization via excited states and multiphoton ionization channels. For pulses containing less than ?ve cycles, the CRBC/COBC ratio of TIYs depends on both the frequency combination and number of optical cycles: With the 1ω-2ω frequency combination, the ratio of TIYs is enhanced (reduced) for pulses with an odd (even) number of optical cycles. The 1ω-3ω frequency combination was considered for pulses containing two or three optical cycles, and for both pulse lengths the CRBC/COBC ratio of TIYs is less than unity.
关键词： photoelectron momentum distributions,bicircular laser pulses,time-dependent Schr?dinger equation,above-threshold ionization spectra,strong-?eld ionization
更新于2025-09-11 14:15:04
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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) - Diamond Functionalization by Ultrafast Laser Pulses
摘要： The extreme material properties of diamond make it an attractive candidate for many technological applications. However, these same material properties often prove disruptive to the machining of diamond into functional devices. Ultrashort laser pulses focused inside diamond offer a potential new route for the functionalisation of diamond, offering a range of fabrication modalities on a single platform. Focusing beneath the surface of diamond, there is sufficient intensity from ultrashort laser pulses for non-linear absorption of light, which can lead to modification of the diamond lattice. The non-linearity of the process confines the modification to the focal volume of the laser, without effecting the diamond surface or surrounding regions. At high pulse energies (above ~100nJ), the laser deposits sufficient energy to break down the diamond lattice and induce a graphitic phase. Tracing the diamond through the laser focus allows the creation of conductive graphitic wires embedded inside the diamond in 3D. The processing becomes deterministic and the wire conductivity increases when the fabrication is carried out at high numerical apertures (NA = 1.4) [1]. The large refractive index mismatch at the diamond surface generates a depth dependent spherical aberration which should be corrected using adaptive optics. The laser written electrical wires can be used to fabricate radiation sensors, as shown in Fig. 1(a), which benefit from the radiation hardness of diamond. The advances in laser fabrication allow the realisation of new detector geometries. The graphitic inclusions fabricated by ultrashort pulse lasers beneath the surface of diamond exert stress on the surrounding diamond, which in turn modifies the optical properties of the crystal. By designing stress fields, it is possible to fabricate optical waveguides [2] and waveguide Bragg gratings [3] inside the diamond. In a different fabrication regime, it is also possible functionalise the diamond using ultrashort pulses without causing breakdown of the diamond lattice. We have shown that with a single ultrashort pulse it is possible to generate an ensemble of vacancies at the laser focus inside the diamond. A thermal anneal process heals any damage to the lattice and is successful in forming highly coherent isolated single nitrogen vacancy (NV) colour centres [4, 5]. We have recently demonstrated improvements to the method using a laser based localised anneal with fluorescence feedback to write arrays of NV centres with near unity yield and in-plane positioning accuracy of 40nm [6].
关键词： functionalisation,diamond,graphitic wires,ultrafast laser pulses,NV centres,optical waveguides
更新于2025-09-11 14:15:04
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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) - Generation of a Few Cycle Terahertz Pulse in Aperiodically Poled Lithium Niobate by Sequence of Pump Pulses
摘要： It is shown that chirped aperiodically poled lithium niobate crystal is capable to generate powerful THz pulses with controllable number of THz field oscillations (from nearly single- to multi-cycles), when it is pumped by sequence of femtosecond laser pulses with chirped delays between adjacent pulses. The peak electric field strength of about 0.3 MV/cm (for non-focused THz beam) is predicted if intensity of each pump pulse in the sequence is 20 GW/cm2.
关键词： Optical rectification,Aperiodically poled lithium niobate,Terahertz pulse,Femtosecond laser pulses
更新于2025-09-11 14:15:04
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Temporal evolution of photo-induced thermal strain in InSb probed by ultra-short laser produced Cu Kα x-rays
摘要： Development and characterization of a wire target based kHz rep rate Cu Kα x-ray source using a Ti:sapphire laser system and its use in time resolved x-ray diffraction (TXRD) of the InSb (111) sample are presented. The observed Kα x-ray photon flux is ～3.2 × 109 photons sr?1 s?1 at a laser intensity of ～3.5 × 1016 W cm?2. TXRD signal from the InSb (111) crystal pumped by an ultrashort Ti:sapphire laser pulse (fluence ～ 13 mJ cm?2) shows a lattice expansion due to heating on a multipicosecond time scale. The crystal gradually cools down and recovers at ～1.5 ns after the laser excitation. The observed strain variation in the crystal matches well with the simulated results. The study of full recovery of the sample will be helpful for the development of InSb based devices.
关键词： Cu Kα x-ray source,time resolved x-ray diffraction,ultrashort laser pulses,thermal strain,InSb
更新于2025-09-11 14:15:04
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Plasmonic layer-selective all-optical switching of magnetization with nanometer resolution
摘要： All-optical magnetization reversal with femtosecond laser pulses facilitates the fastest and least dissipative magnetic recording, but writing magnetic bits with spatial resolution better than the wavelength of light has so far been seen as a major challenge. Here, we demonstrate that a single femtosecond laser pulse of wavelength 800 nm can be used to toggle the magnetization exclusively within one of two 10-nm thick magnetic nanolayers, separated by just 80 nm, without affecting the other one. The choice of the addressed layer is enabled by the excitation of a plasmon-polariton at a targeted interface of the nanostructure, and realized merely by rotating the polarization-axis of the linearly-polarized ultrashort optical pulse by 90°. Our results unveil a robust tool that can be deployed to reliably switch magnetization in targeted nanolayers of heterostructures, and paves the way to increasing the storage density of opto-magnetic recording by a factor of at least 2.
关键词： femtosecond laser pulses,plasmon-polariton,storage density,magnetic recording,all-optical magnetization reversal
更新于2025-09-11 14:15:04
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Polarized Positron Beams via Intense Two-Color Laser Pulses
摘要： The generation of ultrarelativistic polarized positrons during the interaction of an ultrarelativistic electron beam with a counterpropagating two-color petawatt laser pulse is investigated theoretically. Our Monte Carlo simulation, based on a semiclassical model, incorporates photon emissions and pair productions, using spin-resolved quantum probabilities in the local constant field approximation, and describes the polarization of electrons and positrons for the pair production and photon emission processes, as well as the classical spin precession in between. The main reason for the polarization is shown to be the spin asymmetry of the pair production process in strong external fields, combined with the asymmetry of the two-color laser field. Employing a feasible scenario, we show that highly polarized positron beams, with a polarization degree of ζ ≈ 60%, can be produced in a femtosecond timescale, with a small angular divergence, ～74 mrad, and high density, ～1014 cm?3. The laser-driven polarized positron source raises hope for providing an alternative for high-energy physics studies.
关键词： polarized positron beams,ultrarelativistic electron beam,local constant field approximation,spin-resolved quantum probabilities,high-energy physics,Monte Carlo simulation,two-color laser pulses
更新于2025-09-11 14:15:04
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Stochastic electron acceleration in relativistic laser pulse and stationary periodic electric and magnetic fields
摘要： The mechanisms of stochastic electron acceleration in relativistic laser pulses and stationary periodic electric and magnetic fields are investigated by employing a new Hamiltonian approach. The new Hamiltonian is the dephasing rate between the electron and laser pulse such that it is time independent when the stationary fields are absent. The physics underlying stochastic electron motion is clearly revealed, and the conditions for triggering stochastic instability are obtained by finding the Chirikov-like mapping. It demonstrates that if the amplitudes of the stationary fields exceed some threshold values, the Hamiltonian can be randomly changed, and thus, net energy transfer between electrons and the laser radiation are possible. The maximum electron energy gained from the stochastic motion has a weak dependence on the amplitude of stationary fields and can significantly exceed the vacuum ponderomotive energy. All these analytical results have been confirmed by the numerical simulations.
关键词： Chirikov-like mapping,stationary periodic electric and magnetic fields,Hamiltonian approach,relativistic laser pulses,stochastic electron acceleration
更新于2025-09-11 14:15:04
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[Laser Institute of America ILSC? 2019: Proceedings of the International Laser Safety Conference - South Kissimmee, Florida, USA (March 18–21, 2019)] International Laser Safety Conference - Simulated supercontinuum generation in the human eye
摘要： Femtosecond laser pulses of sufficiently high intensity are prone to undergo a number of nonlinear effects while propagating in media. The increasing use of high intensity, femtosecond laser systems underscores the need to understand the retinal hazards generated by nonlinear optical effects, the generation of supercontinuum. Current laser safety standards such as ANSI Z136.1 for pulse wavelengths of 1200 nm - 1400 nm have been determined from experimental studies using pulse durations longer than 100 fs and linear pulse simulations. The combination of strong absorption, broad bandwidth, and dispersive effects makes standard nonlinear pulse simulation methods, based on the slowly varying envelope approximation, unsuitable for the study of near-infrared pulses in biological tissues. To model retinal hazards, we leverage an existing model for linear ultrafast pulse propagation that does not rely on an envelope approximation and simulate spectral broadening in water. Using one-dimensional simulations incorporating the effect of self-focusing, we validate the model using previous experiments for white-light supercontinuum generation in water. We then simulate propagation of 10 fs - 1 ps, 1200 nm - 1400 nm pulses at the current ANSI MPE limit for pulses under 10 ps.
关键词： nonlinear effects,supercontinuum generation,femtosecond laser pulses,self-focusing,ANSI Z136.1,retinal hazards
更新于2025-09-11 14:15:04
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Fast repetition rate fs pulsed lasers for advanced PLIM microscopy
摘要： Simultaneous metabolic and oxygen imaging is promising to follow up therapy response, disease development and to determine prognostic factors. FLIM of metabolic coenzymes is now widely accepted to be the most reliable method to determine cellular bioenergetics. Also, oxygen consumption has to be taken into account to understand treatment responses. The phosphorescence lifetime of oxygen sensors is able to indicate local oxygen changes. For phosphorescence lifetime imaging (PLIM) dyes based on ruthenium (II) coordination complexes are useful, in detail TLD1433 which possesses a variety of different triplet states, enables complex photochemistry and redox reactions. PLIM is usually reached by two photon excitation of the drug with a femtosecond (fs) pulsed Ti:Sapphire laser working at 80 MHz repetition rate and (time-correlated single photon counting) (TCSPC) detection electronics. The interesting question was whether it is possible to follow up PLIM using faster repetition rates. Faster repetition rates could be advantageous for the induction of specific photochemical reactions because of similar light doses used normally in standard CW light treatments. For this, a default 2p-FLIM–PLIM system was expanded by adding a second fs pulsed laser ("helixx") which provides 50 fs pulses at a repetition rate of 250 MHz, more than 2.3 W average power and tunable from 720 nm to 920 nm. The laser beam was coupled into the AOM instead of the default 80 MHz laser. We demonstrated successful applications of the 250 MHz laser for PLIM which correlates well with measurements done by excitation with the conventional 80 MHz laser source.
关键词： oxygen consumption,Ru(II) complexes,phosphorescence lifetime microscopy,Multiphoton microscopy,fast repetition rate excitation,short laser pulses
更新于2025-09-11 14:15:04
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Effects of hole-boring and relativistic transparency on particle acceleration in overdense plasma irradiated by short multi-PW laser pulses
摘要： Propagation of short and ultraintense laser pulses in a semi-infinite space of overdense hydrogen plasma is analyzed via fully relativistic, real geometry particle-in-cell (PIC) simulations including radiation friction. The relativistic transparency and hole-boring regimes are found to be sensitive to the transverse plasma field, backward light reflection, and laser pulse filamentation. For laser intensities approaching I (cid:2) 1024 W/cm2, the direct laser acceleration of protons, along with ion Coulomb explosion, results in their injection into the acceleration phase of the compressed electron wave at the front of the laser pulses. The protons are observed to be accelerated up to 10–20 GeV with densities around a few times the critical density. The effect qualitatively depends on initial density and laser intensity, disappearing with the initial density increase or intensity decrease.
关键词： particle acceleration,hole-boring,overdense plasma,laser pulses,relativistic transparency
更新于2025-09-11 14:15:04