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Pressure dependence of high order harmonic generation in nitrogen molecular gas and atmospheric air
摘要： The effect of the variation of the gas pressure on the high harmonic generation (HHG) from nitrogen molecular gas (N2) and atmospheric air with ultrashort intense laser pulses is measured. The optimum pressure in the interaction region of a gas jet for maximizing the HHG yield is determined for both gases. Enhancement of the HHG output and its extension to higher harmonic orders are observed around the optimum pressure value of 0.33 bar. Theoretical calculations based on one-dimensional models explain this effect and provide reasonable agreement with experimental results.
关键词： Femtosecond laser,Conversion efficiency,High-order harmonic generation (HHG),Phase matching,XUV radiation,Pressure optimization
更新于2025-09-23 15:23:52
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Efficiency control of high-order harmonic generation in gases using driving pulse spectral features
摘要： The low conversion efficiency of high-order harmonic generation (HHG) in gases is an insurmountable limitation for many applications, where a considerable photon flux is an instrumental prerequisite. We present a study of the HHG conversion efficiency dependence on the driving laser intensity and analyze the conditions for optimal phase-matching in a loose focusing configuration and long generation medium using a Ti:Sapphire laser. Moreover, by determination of the influence of plasma effects on the driving laser pulse, we observe a correlation between the HHG conversion efficiency and the blueshift of the fundamental pulse. The maximal HHG conversion efficiency is achieved just before the driving laser spectrum is considerably affected by the plasma. Similar behavior is observed in HHG for different noble gases. In this respect, the appearance of a plasma-induced spectral shift in the driving laser might serve as an indication of a substantial loss of HHG conversion efficiency. Consequently, our findings can be exploited to obtain essential information about the laser-plasma interaction process during HHG and can pave the way for a more convenient control of optimal HHG conditions.
关键词： plasma effects,high-order harmonic generation,conversion efficiency,driving laser intensity,spectral blueshift,phase-matching
更新于2025-09-23 15:21:01
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Invited Article: Emission of intense resonant radiation by dispersion-managed Kerr cavity solitons
摘要： We report on an experimental and numerical study of temporal Kerr cavity soliton dynamics in dispersion-managed fiber ring resonators. We find that dispersion management can significantly magnify the Kelly-like resonant radiation sidebands emitted by the solitons. Because of the underlying phase-matching conditions, the sideband amplitudes tend to increase with increasing pump-cavity detuning, ultimately limiting the range of detunings over which the solitons can exist. Our experimental findings show excellent agreement with numerical simulations.
关键词： phase-matching conditions,temporal Kerr cavity solitons,dispersion-managed fiber ring resonators,pump-cavity detuning,resonant radiation sidebands
更新于2025-09-23 15:21:01
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Optimal Phase-Matching Strategy for Beam Scanning of Sub-Arrayed Phased Arrays
摘要： The design of phased array antennas generating two functional beams is here addressed. The first/primary beam is fixed and it is generated at the element level through the complex (amplitude and phase) weighting coefficients, while the second/secondary one is synthesized at the sub-array level by aggregating the array elements into non-overlapped clusters and using additional phase shifters at the clusters outputs. Towards this end, the sub-array configuration is determined by means of an ad-hoc iterative method aimed at reducing the unavoidable quantization lobes, while the synthesis of the sub-array phase coefficients, once the excitations at the element level have been set, is cast as a phase-matching problem whose analytic solution is optimal since it guarantees the minimum least-square error with respect to a reference phase distribution. A set of representative numerical results is reported and discussed to validate the proposed design approach and to point out its features and potentialities when addressing the synthesis of both pencil and shaped-beam power patterns.
关键词： Beam Scanning,Contiguous Partition Method,Sub-Arraying,Phase Matching,Active Electronically Scanned Array (AESA)
更新于2025-09-23 15:21:01
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Progress toward third-order parametric down-conversion in optical fibers
摘要： Optical ?bers have been considered an optimal platform for third-order parametric down-conversion since they can potentially overcome the weak third-order nonlinearity by their long interaction length. Here we present, in the ?rst part, a theoretical derivation for the conversion rate both in the case of spontaneous generation and in the presence of a seed beam. Then we review three types of optical ?bers and we examine their properties in terms of conversion ef?ciency and practical feasibility.
关键词： optical ?bers,phase matching,nonlinearity,photon triplet states,third-order parametric down-conversion
更新于2025-09-23 15:19:57
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Spectral splitting and phase matching of the macroscopic high-order harmonic generation in intense laser fields
摘要： We theoretically investigate the macroscopic high-order harmonic generation in argon gaseous medium by numerically solving the three-dimensional macroscopic propagation equation, and the results show that the harmonic spectral and spatial profiles of harmonics are gradually splitting with the increasing of the driving laser intensity. This splitting is mainly due to the distortions which the driving field suffers during propagation and the consequence is also on phase matching. To illustrate the physical mechanisms of harmonic splitting, a theoretical analysis of the phase matching is presented. The harmonic spectra from different focus positions reveal that the split is dependent on the focus-gas-jet relative position. Moreover, we demonstrate that the spectral splitting of high harmonics can hardly be observed for neon gas jet due to the high ionization energy.
关键词： phase matching,high-order harmonic generation,intense laser fields,macroscopic propagation,spectral splitting
更新于2025-09-23 15:19:57
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Arrays of Carbon Nanotubes in a Field of Continuous Laser Radiation
摘要： A numerical analysis of the phase-matching conditions during the incidence of one or two counterpropagating laser beams on an ordered array of single-walled carbon nanotubes (CNTs) is performed. The conditions for the generation of slow surface plasmon waves of the terahertz (THz) and far infrared range propagating along the nanotubes of the irradiated array are determined. It is shown that the plasmon frequency can be controlled by changing the angle of incidence of laser radiation on the structure under study. Thus, it is possible to fulfill the condition of longitudinal resonance, in which each array nanotube is a dipole antenna radiating at the plasmon frequency. In this case, the array forms a system of a large number of in-phase emitters, which allows increasing the efficiency of conversion of laser radiation into THz radiation in comparison with a single nanoantenna.
关键词： phase-matching conditions,terahertz radiation,carbon nanotubes,laser radiation,plasmon waves
更新于2025-09-23 15:19:57
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Widea??Bandgap Organic Crystals: Enhanced Opticala??toa??Terahertz Nonlinear Frequency Conversion at Neara??Infrared Pumping
摘要： Enhanced terahertz (THz) wave generation is demonstrated in nonlinear organic crystals through refractive index engineering, which improves phase matching characteristics substantially. Unlike conventional low-bandgap nonlinear organic crystals, the newly designed benzimidazolium-based HMI (2-(4-hydroxy-3-methoxystyryl)-1,3-dimethyl-1H-benzoimidazol-3-ium) chromophore possesses a relatively wide bandgap. This reduces the optical group index in the near-infrared, allowing better phase matching with the generated THz waves, and leads to high optical-to-THz conversion. A unique feature of the HMI-based crystals, compared to conventional wide-bandgap aniline-based crystals, is their remarkably larger macroscopic optical nonlinearity, a one order of magnitude higher diagonal component in macroscopic nonlinear susceptibility than NPP ((1-(4-nitrophenyl)pyrrolidin-2-yl)methanol) crystals. The HMI-based crystals also exhibit much higher thermal stability, with a melting temperature Tm above 250 °C, versus aniline-based crystals (116 °C for NPP). With pumping at the technologically important wavelength of 800 nm, the proposed HMI-based crystals boost high optical-to-THz conversion efficiency, comparable to benchmark low-bandgap quinolinium crystals with state-of-the-art macroscopic nonlinearity. This performance is due to the excellent phase matching enabled by decreasing optical group indices in the near-infrared through wide-bandgap chromophores. The proposed wide-bandgap design is a promising way to control the refractive index of various nonlinear organic materials for enhanced frequency conversion processes.
关键词： organic crystals,terahertz,refractive index engineering,phase matching,nonlinear optics
更新于2025-09-23 15:19:57
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[IEEE 2018 IEEE International Ultrasonics Symposium (IUS) - Kobe, Japan (2018.10.22-2018.10.25)] 2018 IEEE International Ultrasonics Symposium (IUS) - Suppression of Propagation Losses in TC SAW Resonators Using Thin Plates of LiTaO3 Bonded to Quartz Substrates
摘要： Suppression of Propagation Losses in Thin-Film Lithium Niobate Optical Waveguides
关键词： Lithium Niobate,Thin-Film,Optical Waveguides,Quasi-Phase Matching,Propagation Losses
更新于2025-09-19 17:15:36
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[IEEE 2018 4th IEEE International Conference on Emerging Electronics (ICEE) - Bengaluru, India (2018.12.17-2018.12.19)] 2018 4th IEEE International Conference on Emerging Electronics (ICEE) - Observation of visible light flashes in high power near infrared narrow linewidth polarization maintained fiber laser caused by Stimulated Brillouin Scattering triggered cascaded Raman Scattering
摘要： We report a surprising observation of visible light flashes in the fusion splice point between the seed stage to the power amplifier in a 500 W narrow-linewidth, polarisation-maintaining Ytterbium-doped fiber laser. The flashes were observed to have a strong correlation with the backward pulses generated due to the onset of Stimulated Brillouin Scattering (SBS) in the laser. We believe these visible flashes appear as a result of second harmonic generation of higher order Raman Stokes components of the 1064nm laser signal. The Raman components are generated by the back-propagating, high peak power SBS pulses at threshold. The Raman Stokes in the core of the fiber generates their respective second harmonic counterparts in the cladding through a Cherenkov-type phase matching process.
关键词： Brillouin Scattering,Fiber laser,Cherenkov phase matching,Raman Scattering
更新于2025-09-19 17:13:59