| Laser-material interaction has been an interesting problem. Since the first laser was invented by T. H. Maiman, a physicist of USA, in 1960, laser technique has shown its vital force and has been introduced to almost every field of natural science. The achievement of the study of"laser-material interaction"is an important symbol of the development of laser technique. Laser-material interaction is one of the most important tasks of physics not only in theoretic fields but also in practical application such as laser processing, preparation of materials, manufacture of military weapons and so on. In recent years, the rapid development of the high energy ultrashort laser provide people with extreme experimental conditions such as unprecedented power density (>1020 W/cm2 ), field intensity (>1011 V/cm ) and time precision (10-15 s) etc which will lead to a lot of created study and breakthrough.Silicon is the main material of microeleetronic device,which has an indirect energy bandage and is therefore highly inefficient as a light source,Photoelectronic devices are always expected to be linked with silicon-based microelectronic technology. Although III-V group semi-conductor matetial, such as GaAs, even is suitable for photoelectronic device, it is much hoped to be fabricated by based-silicon material to carry out silicon-based photoelectronic integration due to present much-mature based-silicon material. The study of nanometer silica of one dimension has been paid close attention by scientists in semicondutor and namo fields.When the power density of the incident laser pulse exceeds the damage threshold of the material, the surface of the irradiated material instantaneously reaches a temperature higher than the material's vaporization temperature due to photon and multi-photon absorption and other absorption mechanisms. Thus the material will be melted, vaporized, even ionized within several nanoseconds, causing explosion of the surface and formation of the dense plasma. The laser-induced plasma ejecting from the target surface carries both mass and energy that can be used to grow thin films. At present, pulsed laser deposition (PLD) has been one of the most popular methods employed to prepare high-quality thin films, such as oxides, nitrides, semiconductors and superconductors. In order to find the optimum experimental condition and to control the process of the growth of thin films, it is necessary to investigate the properties of the plasma. Unfortunately, very limited works have been focused on the study of fundamental laser plasma parameters, such as the electron density, the electron temperature, the plasma velocity, changing with the delay time and laser power density, and the ejecting particles spatial distribution. In fact, these results are not only essential for understanding the laser material interaction process and clarifying the local behavior of the expanding plasma, but also enabling us to optimize the condition of PLD.Spectral diagnosis was used to detect the plasma, obtaining the time-resolved and time-of-flight spectra, further acquiring the electron density, the electron temperature, and the velocity of Si plasma. Besides, a study on the emission mechanism of laser plasma and the law of its evolution has been done, and the line spectral shift has been quantitatively analyzed as well. The main work and results are as follows:1. The study on the emission mechanism of laser plasma and the law of its evolution with the delay time was done. The intensity of continuum spectra in short-band decreased more rapidly than that in long-band. This is because the mechanism of the continuum spectra is bremsstrahlung and recombination emission. But the bremsstrahlung dominates only in high temperature. The emission mechanism of line spectrum is collision excitation, especially the excitation of high power electron. 2. The continuum spectra last longer and its intensity is larger at the pressure of 1 atm than at 1 Pa,which is due to the contribution of the ionized ambient to the electron density and the bound effect of the ambient. It is found for the first time that the intensity of the continuous spectra increases rapidly, reaching the maximal intensity at delay time of 20 ns, then decreases slowly. It is explained by bremsstrahlung and recombination.3. It is also found for the first time that the lifetime of nitrogen ion is much different from that of silicon ion. The most plausible reason is their generation mechanism is different. The nitrogen ion are mainly generated from the air breakdown which undergoes a cascade avalanche process,while the silicon ion can be induced by laser irradiation and collision ionization. In addition, the appearance times of maximal signal of the ion spectrum can be explained by their lifetimes of upper energy level and appearance times of maximal signal of continuous spectra.4. The maximum of the intensity of continuous spectra, electron density and electron temperature can be got at the distance of 0.3mm from the target.5. From theoretical analysis and Gaussian fit, as well as Lorentz fit of the line spectra of Si plasma, it is confirmed that the line broadening results mainly from Stark broadening mechanism. The line shifts are different for different spectrum, but their ratio is almost constant under the same experimental condition. The line shift observed in our experiment was explained with the shielding effect of the plasma. Also the line shift becomes large as the increase of electron density.6. The velocity of the plasma plume was got from the time-of-flight spectra. Their quantity magnitude was 1.0×103 m/s and 1.0×104 m/s at background pressures of 1 atm and 3 Pa respectively. Also the velocity decreased more rapidly at the pressure of 1 atm than that at 3 Pa. The velocity was affected greatly by the ambient pressure, contrasting with the effect of the laser energy. Then the velocity of Si plasma was computed. However, because the ambient pressure in our experiment is 3 Pa, the calculation corresponds vacuum, therefore the calculated value of velocity is not fit very well with that of experiment. |
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