Three-halves harmonic emission from femtosecond laser produced plasmas (original) (raw)
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Production and Characterization of Femtosecond-Laser-Induced Air Plasma
Frontiers in Optics 2008/Laser Science XXIV/Plasmonics and Metamaterials/Optical Fabrication and Testing, 2008
The purpose of this research was to produce, characterize, and optimize laserinduced air plasma as a preliminary step in using the plasma as a source of terahertz radiation. An 800 nanometer, 50 femtosecond, 0.75 Watt, pulsed Ti:Sapphire laser system was used as the source infrared beam. A beam expander was used to expand the beam to a diameter of approximately 6.5 mm, and the beam was focused through a 25 mm focal length achromatic lens to produce laser-induced plasma in ambient air. A 40 kHz ultrasonic transducer was used to detect the plasma. A second harmonic generation crystal was placed within the beam expander to generate 400 nm blue light, enabling production of THz in the plasma via four wave mixing; a third-order nonlinear process.
Optics Letters, 2012
We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10 3 shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10 6 laser shots.
Third harmonic from air breakdown plasma induced by nanosecond laser pulses
Applied Physics B, 2018
Harmonic generation is a nonlinear optical effect consisting in frequency up-conversion of intense laser radiation when phase-matching conditions are fulfilled. Here, we study the mechanisms involved in the third harmonic (TH) generation process, the conversion efficiency, and the properties of TH radiation generated in air by focusing infrared linearly polarized nanosecond laser pulses at intensities of the order of TW/cm 2. By analyzing the emission from the air breakdown plasma, we demonstrate that filamentary breakdown plasma containing molecular nitrogen ions acts as an optical nonlinear medium enabling generation of TH radiation in the axial direction. The data reveal important properties of the TH radiation: maximum conversion efficiency of 0.04%, sinc 2 dependence of the TH intensity on the square root of the pump intensity, and three times smaller divergence and pulse duration of TH as compared to the pump radiation.
Time evolution of plasma afterglow produced by femtosecond laser pulses
Journal of Applied Physics, 2004
In this paper we investigate the time evolution of laser plasmas generated in atmospheric air by ultrashort ͑100 fs͒ 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.
Optical diagnostics of femtosecond laser plasmas
Science China-mathematics, 2001
Optical diagnostics of evolution of plasmas produced by ultrashort laser pulses is carried out using a femtosecond probing beam. The time sequence of plasma shadowgrams and interferograms are obtained. The filamentation instability in high-density region induces the local density modification. Large-scale toroidal magnetic fields confine plasma expansion in the transverse direction, resulting in the formation of a plasma jet. The plasma expansion along the target normal direction is found to scale as t1/2.
2009
The intensity enhancement of harmonics and change in harmonic profiles have been observed in a few plasma plumes (La, In, Mn) under variable phase modulation of Ti:sapphire laser pulses. It is seen that variation of relative harmonic intensities in the plateau region could be achieved by chirp variation of narrowband (ϳ10 nm bandwidth) radiation. The effect of the self-phase modulation (SPM) of broadband (ϳ20 nm bandwidth) laser radiation on the harmonic emission from nanoparticles, C 60 , and Ag plasma while passing through a glass slab is also studied. The observation of broadband harmonic emission and redshift in harmonic wavelengths for an unchirped laser pulse are explained in terms of SPM. The observation of blueshift of harmonic radiation with both positively and negatively chirped pulses was attributed to the different effects of SPM on the chirped pulses.
Electron densities, temperatures, and the dielectric function of femtosecond-laser-produced plasmas
Physical Review E, 1999
The optical properties of dense plasmas in the extreme ultraviolet spectral range are investigated experimentally and theoretically. A method is presented that enables measurement of electron densities in excess of 1024 cm-3. High-order harmonics are transmitted through a femtosecond-laser-produced plasma probing the optical density at several wavelengths. This technique has the advantage of providing direct access to ultrahigh electron densities,
Optical emission studies of plasma induced by single and double femtosecond laser pulses
Spectrochimica Acta Part B: Atomic Spectroscopy, 2009
Double-pulse femtosecond laser ablation has been shown to lead to significant increase of the intensity and reproducibility of the optical emission signal compared to single-pulse ablation particularly when an appropriate interpulse delay is selected, that is typically in the range of 50-1000 ps. This effect can be especially advantageous in the context of femtosecond laser-induced breakdown spectroscopy analysis of materials. A detailed comparative study of collinear double-over single-pulse femtosecond laser-induced breakdown spectroscopy has been carried out, based on measurements of emission lifetime, temperature and electronic density of plasmas, produced during laser ablation of brass with 450 fs laser pulses at 248 nm. The results obtained show a distinct increase of plasma temperature and electronic density as well as a longer decay time in the double-pulse case. The plasma temperature increase is in agreement with the observed dependence of the emission intensity enhancement on the upper energy level of the corresponding spectral line. Namely, intensity enhancement of emission lines originating from higher lying levels is more profound compared to that of lines arising from lower energy levels. Finally, a substantial decrease of the plasma threshold fluence was observed in the double-pulse arrangement; this enables sensitive analysis with minimal damage on the sample surface.
Radiation produced by femtosecond laser-plasma interaction during dielectric breakdown
Optics Letters, 2005
Optical breakdown by femtosecond and nanosecond laser pulses in transparent dielectrics produces an ionized region of dense plasma confined within the bulk of the material. This ionized region is responsible for broadband radiation that accompanies the breakdown process. Spectroscopic measurements of the accompanying light have been used to show that, depending on the laser parameters, the spectra may originate from plasma-induced second-harmonic generation, supercontinuum generation, or thermal emission by the plasma. By monitoring the emission from the ionized region, one can ascertain the predominant breakdown mechanism and the morphology of the damage region.