Experimental And Applied Study Of Femtosecond Laser-induced Air Plasma

Ultrashort pulses with duration from ps (10-12s) to fs (10-15s) and millijoule pulse energy can possess extremely high peak intensity and introduce many nonlinear phenomena during the interaction with all kinds of materials. Among them, femtosecond laser-induced air ionization is a topic of fundamental importance and is inevitably involved in many ultrafast-optics-related applications. The generation of plasma and its spatial-temporal evolution, the characteristics of plasma radiation and the nonlinear interaction of plasma with femtosecond laser pulses are the central issues of this thesis. It is speculated that Bremsstrahlung radiation from femtosecong laser-induced air plama may involve a stimulation process and thus provide optical gain for light amplification.In the meantime, the interaction of laser-induced plasma with solid target has found a broad range of applications. On the microscopic scale, the micro-nano-fabrication and crystallization of amorphous silicon membrane with femtosecond lasers have technically attractive advances. On the macroscopic scale, ultrashort pulse laser ablative propulsion has the potential to be an important alternative in space travel as a new type of light propulsion.The main contents and results in this thesis are listed as follows:1. For the first time, we report the observation of a novel phenomenon of the generation of super-luminescent jet light beams emanating from the micro air plasma induced by a relatively tight focused near-infrared femtosecond laser pulses. A systematic study of the key characteristics of such jet beams and its generation machanism are conducted. Single or double jet-like optical beams, being slightly divergent and coexisting with severely distorted conical emission of colored speckles, are obtainable only when the focal lens of proper f-number is slightly tilted or shifted. In addition to the apparent difference between SJBs and the conventional optical filaments (OFs) in both spatial and spectral characteristics, the two also differ substantially in the underlying generation mechanisms.(In particular, the different constituents of the nonlinear phase shift in the two cases also set them apart.) It is analyzed and confirmed that the four-wave mixing (FWM) process contributes most to the generation of such jet-like beams. The Stokes and anti-Stokes waves with distinct1064nm and547nm peak wavelengths obtained from the phase matching calculation are in good agreement with the experimentally recorded spectrum. The anti-Stokes wavelength at547nm also well explains the observed brilliant yellow color of the jet beams. As a new type of coherent optical beams from disrupted conical emission, these unique super-luminescent jet-like beams can propagate over a long distance in air after proper collimation, and very likely they may find some applications in remote sensing in future.2. By using electron density rate equation, we have analyzed the time evolution and spatial distribution of electron density of the air plasma induced by tightly-focused50fs laser pulses with100mm focal length lens. It reveals that for the chosen laser parameters the air can be totally ionized well before the pulse peak comes to the focusing point. The corresponding distribution of plasma related refractive index has been then obtained from the electrons density calculation, and it is found that there exists a saturation region in the central focal area where the refractive index of plasma becomes constant (～0.9923). Moreover, a simple plasma-air interface refraction model is proposed to explain the different spectral distribution and optical power dependent of the divergence of the "nonlinearly diffracted beam", namely, the divergence angle first increases fast and then stabilizes at a certain value more than twice of the ordinary diffraction angle.3. We have built a transient spectrum measurement system including a fast gated spectrometer. By this system we are able to record the dynamic spectra of a single pulse induced air plasma over a selectable time window. Plasma temperature of5500 K is determined by fitting the continuum spectra of plasma emission with the well-known Planck’s blackbody emission formula at2ns after air ionization. Typical lifetime of the micro air plasma is determined to be～5.5ns based on the spectrally integrated and time-resolved spectral measurements. Evolution characteristics of both continuum and line spectra of the plasma are examined for various laser pulse energies associated with different pulse durations. Spatially resolved spectroscopic measurements have shown that compared with the spectra obtained from the plasma section closer to the focal lens (4X objective), the spectra from the other part of the plasma channel, namely away from the focal lens, has more pronounced oxygen and argon ionic lines.4. Temporal and spectral characteristics of air plasma generated by dual collinearly-propagating50femtosecond laser pulses are experimentally studied under different focusing conditions. In the case of relatively shallow focusing with a plano-convex lens of100mm focal length, enhancement of plasma emission is clearly observed for the whole inter-pulse delay range of0-8ns. In great contrast, air plasma generated with a10X objective (tightly focused) presents enhanced bremsstrahlung emission only when the inter-pulse delay is less than0.5ns, and for a longer inter-pulse delay of8ns, the delayed pulse passes through the transient vacuum channel due to the first pulse induced air plasma expansion without further inducing air ionization. This phenomenon may be utilized in dynamic spatial filtering of ultrashort and intense laser pulses without employing any hard apertures and vacuum hardwares.5. We have experimentally demonstrated that because of intensity clamping, when the laser peak power is higher than the critical power for self-focusing, further increase of the laser power cannot result in corresponding increase of the laser ablation rate of an aluminum foil placed in air. The ablation rate will approach a stabilized value once the incendent power is above a certain value. Also, the experimental technique implemented in our work may be used in measuring the self-focusing critical power and the nonlinear refractive index. The thus measured self-focusing critical power and the nonlinear refractive index are7.2GW and1.3×10-19cm2/W respectively under standard atmospheric pressure. While for the one atmosphere of argon gas, the corresponding values are3.8GW and2.6×10-19cm2/W respectively.6. We have accomplished for the first time the experiments of optical filament propulsion. In the propulsion of microbeads by femtosecond laser filament formed by111cm focal length lens, it is found that when placed at the position where the strongest filament fluorescence exists, the microbead does not obtain the maximal ablation rate as well as the maximal propelled distance. Moreover, the position where the maximal ablation rate exists is just consistent with that where the maximal propelled distance of the microbead occurs. Through numerical simulations of optical filamentation formation and adoping the two temperature model, it is confirmed that the actual ablation volumn plays a dominant role in the laser filamentation propulsion. Even in the optical filament scheme where there exists much longer air ionization channel, the thrust generated by the laser ablation of the target material remains the main origin of impulse force. In other words, optical filamentation propulsion is still a type of laser ablative propulsion.7. A shadowgraphic method is proposed to measure the impulse coupling coefficients of microbeads propelled vertically with femtosecond laser pulses. This method can directly record the flying time of beads with high sensitivity. In the experiments of single millijoule femtosecond laser propulsion of1.4milligram iron beads, impulse coupling coefficient of～5dyne/W is obtained. This shadowgraphic method is also employed to investigate the impulse coupling coefficients of iron beads with different pulse energies and pulse widths. For50fs laser pulses, when the pulse energy is changed from0.86mJ to1.62mJ, the generated impulse coupling coefficient decreases, which may be attributed to the dominate material removal mechanism of atomization occurred at laser fluence much higher than the optimum value. For laser pulses with fixed pulse energy of0.86mJ, as the pulse duration is increased from50fs to8ps, the generated impulse coupling coefficient doesn’t achieve maximum value at50fs, it first increases sharply, reaching a maximum around500fs, and then decreases at much slower pace, approximately maintaining at a relatively high value. This is caused by the change of the ablation mechanism involved from atomization to phase explosion.