4,323 research outputs found
Discontinuous electrolyte systems for improved detection of biologically active amines and acids by capillary electrophoresis with laser-induced native fluorescence detection
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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
Prediction of beam tilting effects using an enhanced ray launching model in the urban area
No full textThis paper presents an effective propagation area method that enhances the computation efficiency of the existing ray-launching model. Using the effective propagation area method, the effective propagation region to be considered can be constructed from a given geographical database, and buildings that have a substantial effect on the total received power can be considered in a very efficient manner. In order to verify the predicted accuracy of the proposed model, the received power has been predicted for the real microcellular environment and compared with the published measurement, It is observed from the prediction results that the proposed model shows considerable improvement in computation speed and good agreement with the measurements. In addition, the effect of beamtilting on the path loss in an urban area is analyzed using the proposed model. It has been observed that beam downtilting can effectively confine the signal tea the desirable home cell with reduced transmitting power, narrow half-power beam width (HPBW), and suitable fixed antenna height.X114sciescopu
Improvement of the computation efficiency for a ray-launching model
No full textAn enhanced ray-launching model is proposed to improve the efficiency and the speed of computation of propagation characteristics. To obtain the improvements, two methods are used: the effective-propagation-area method and the dominant-corner extraction method. The effective-propagation-area method is a method which restricts the propagation region to be considered when the wave propagation characteristics are predicted. The dominant corner-extraction method is used to diminish the amount of the diffraction ray computation by considering only the particular building corners which have a significant influence on the received power. The received power and the RMS delay spread are predicted for microcellular environments using the proposed model and compared with the results of the general ray-launching model. It is observed from the prediction results that the proposed model shows the considerable improvement in computation speed, and good agreement with the general ray-launching model.X116sciescopu
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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