In this paper we investigate the time evolution of laser plasmas generated in atmospheric air by ultrashort (100fs) laser pulses. The detected quantity is the time integrated photon yield emitted by the plasma, which monotonically depends on the amount of energy transferred by the laser pulses to the plasma. We study the effect of a preionizing pulse on the efficiency of plasma generation by a second “probe” pulse and demonstrate that preionization results into a considerable increase of the overall photon yield emitted by the plasma. An explanation of this phenomenon relies on the fact that the larger the electron density experienced by the probe pulse, the more effective the energy transfer from the probe pulse to the residual plasma, the more intense is the light from the plasma. With this concept in mind and by relying on a pump-probe technique, we also measure the total photon yield emitted by the plasma produced by the combination of the two pulses, as a function of their relative delay time. We observe a considerable increase in the plasma brightness for delay times much longer than the laser pulse duration. This phenomenon is associated with an increase of the electron density even after the end of the pump pulse, due to secondary electron-impact ionization originating from highly-energetic primary photoelectrons, and to superelastic electron-molecule collisions. We also develop a simplified model describing the time evolution of the electron and ion densities and the electron temperature. From the calculated time evolution of these quantities produced by a single laser pulse, we can predict with a good approximation the main features of the plasma generated by an ultrashort laser pulse.
The far-field intensity pattern of laser beams diffracted by axicons is extensively characterized both theoretically and experimentally. The regular structure of the pattern, consisting of high-contrast fringes, is explained. The experimental results have been interpreted by representing the diffracted field as generated by an extended virtual source shaped as a circle centered on the optical axis of the incident laser beam. The simulations include modifications to the diffraction pattern arising from the laser radiation diffraction limit at the axicon tip, and they reproduce well the measured intensity profile at different distances from the axicon.
We demonstrate a novel concept, to the best of our knowledge, for reproduction of the target movement without the frame-by-frame display technique elaborated by the Lumiere brothers. The specially designed elements—apodizing filters with axially quadratic transmittance of radiation, are used to continually record a target position change in time. The recording system deals with nonmatrix detectors and requires only one-step conversion of light energy into an electrical signal to monitor the target under study. The concept makes it possible to develop new techniques for night vision in the near-infrared and thermal spectra. The path and speed of a bouncing and receding ball in the computer screen animation are recorded and reproduced in 3D.
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