1,721,017 research outputs found
Comment on "Direct observation of laser filamentation in high-order harmonic generation"
No full textWe argue for a different physical interpretation of the results given in the recent Letter by Painter et al. [Opt. Lett. 31, 3471 (2006)] in which an elongated TI:saphire beam with two distinct waists is considered as direct evidence of laser filamentation. As the pulse power is well below the critical power for self-focusing, the authors pleaded for new examination of the n(2) value for He. A three-dimensional numerical modeling, using the published n(2) value for He, reproduces very well the measured data and invalidates the filamentation hypothesis. (c) 2007 Optical Society of America
Synchronized generation of bright high-order harmonics using self-guided and chirped femtosecond laser pulses
No full textBright harmonic generation in a long Ne gas jet achieved using self-guided and chirped femtosecond laser pulses has been analysed. Three-dimensional modelling was performed to look into the details of laser pulse propagation, harmonic generation and attosecond pulse characteristics. The theoretical analysis and the comparison to the experimental results showed emitted harmonics form a bright attosecond pulse train with good synchronization in time. In the space domain the synchronization is especially good in the self-guided region.This work was supported by the Korea Science and Engineering Foundation through the Creative Research Initiative Program
Dynamical medium depletion in high-order above-threshold ionization with few-cycle laser pulses
No full textThe influence of dynamical medium depletion in high-order above-threshold ionization (ATI) in left/right asymmetry of photoelectron energy spectra is analyzed. Based on a classical analysis of high-order ATI electrons produced by few-cycle laser pulses, calculated asymmetry maps of electron spectra reproduce very well the experimental results reported in Lindner [Phys. Rev. Lett. 92, 113001 (2004)], utilized for determining the Guoy phase shift of few-cycle laser pulses. The anomalous behavior of the high-energy part of the ATI electron spectra is, then, fully understood in terms of earlier medium depletion occurring in the leading edge of the laser pulse. In order to correctly reproduce the experimental findings a physical temporal envelope of the laser pulse, which only vanishes at the infinity, plays a crucial role.The authors (C.H.N and V.T.) acknowledge the support of
the Science and Technology Ministry of Korea through the
Creative Research Initiative Progra
Diffusional behaviour of essential oil components in active packaging polypropylene films by Multiple Head-Space-Solid Phase Microextraction-Gas Chromatography (MHS-SPME-GC-MS)
No full textEssential oils (EOs) are promising substances to be incorporated in food packaging materials as antioxidant and/or antimicrobial agents in active packaging systems. Once incorporated into the materials, their constituent compounds diffuse through the material. However, there is little information about the diffusion behaviour of EOs in plastics, and this is one of the major drawbacks in the design of active packaging relying on EOs as active agents. This study presents a new method for studying the diffusion of basil, citronella, oregano and rosemary EOs into plastic films. Diffusion assays of EOs from a coated polypropylene (PP) film into a stack of virgin PP films were conducted by the Moisan method using up to 10 layers of PP. Direct quantification of diffused analytes from the plastic sheets was performed by multiple headspace solid phase microextraction–gas chromatography–mass spectrometry. The fitting of experimental data for the main EO components with a Fickian model gave diffusion and partition (between active and virgin PP) coefficients, which are discussed under the light of some intrinsic factors contributing to the diffusion process. The results demonstrate the dependence of diffusion and partition coefficients on the initial concentration of EO components in the coated films, on their molecular weight and on their polarity. In particular, for similar molecular weight and initial concentration, a higher polarity is responsible for a faster diffusion, whereas lower polarity determines the retention of the EO component in the coating layer
Single Isolated attosecond pulse from multicycle lasers
No full textWe propose a new experimental scheme to produce clean isolated pulses lasting a few hundreds of attoseconds. It is based on high harmonics generation and uses the polarization gating technique combined with the ionization dynamics and the spatial filtering provided by the three-dimensional field propagation. The proposed method is easy to implement, robust against laser parameter fluctuations, and shows to be effective up to a 25 fs pulse duration
Temporal structure analysis of bright high-order harmonics obtained using self guided and chirped laser pulses
No full textHigh-order harmonic generation by chirped and self-guided femtosecond laser pulses. I. Spatial and spectral analysis
No full textFor the analysis of high-order harmonics generated in a long gas jet by intense chirped femtosecond laser pulses, calculations of the laser propagation and harmonic generation were performed in terms of a nonadiabatic three-dimensional model. The self-guided propagation, observed at the conditions of bright harmonic generation, was confirmed by the model calculations. When using negatively chirped pulses, the calculated distribution of the harmonic field is spatially and spectrally confined, being generated on axis with a narrow spectral profile. The positively chirped pulses generate broad spectral distribution on axis, and narrow off axis, but in the latter case with a large emission angle. The estimation of harmonic beam divergence agreed well with experimentally measured data, showing the lowest divergence at the conditions of the brightest harmonic generation. Spectral, temporal, and spatial modifications of the propagated laser pulse are found to influence decisively the single-atom response and ultimately the harmonic field, providing a coherent picture of harmonic generation.The research was supported by the Ministry of Science and Technology of Korea through the Creative Research Initiative Program. V.T. acknowledges support from KOSEF Brain Pool Program
High-order harmonic generation by chirped and self-guided femtosecond laser pulses. II. Time-frequency analysis
No full textWe present a time-dependent analysis of high-order harmonics generated by a self-guided femtosecond laser pulse propagating through a long gas jet. A three-dimensional model is used to calculate the harmonic fields generated by laser pulses, which only differ by the sign of their initial chirp. The time-frequency distributions of the single-atom dipole and harmonic field reveal the dynamics of harmonic generation in the cutoff. A time-dependent phase-matching calculation was performed, taking into account the self-phase modulation of the laser field. Good phase matching holds for only few optical cycles, being dependent on the electron trajectory. When the cutoff trajectory is phase matched, emitted harmonics are locked in phase and the emission intensity is maximized.The research was supported by the Korea Science and Engineering Foundation sKOSEFd through the Creative Research Initiative Program
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