The nonlinear optical response of carbon nanotubes (CNTs) to the interaction with intense ultrashort laser pulses was studied theoretically and experimentally. A full quantum-mechanical theory for harmonics generation from a single-walled CNT has been developed, using the quantum kinetic equations for π-electrons with both intraband and interband transitions taken into account. In the regime of strong driving fields, a non-perturbative approach with the numerical solution of the quantum kinetic equations in the time domain was used to calculate the density of the axial electric current in CNTs. The results of this theory are compared to experiments performed on samples of multi-walled CNTs, using pulses of 160 fs generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The experimental results show indeed an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields of ~ 10<sup>10</sup> W/cm<sup>2</sup>, in very good agreement with the theoretical predictions. The interaction of CNTs with strong laser fields results not only in the generation of harmonics, but also in the generation of a broad spectral background. Generation of a continuous background in the vicinity of the third-harmonic of the laser field was also obtained from the quantum-mechanical calculations, however, with lower intensities than observed experimentally. Possible explanations for this discrepancy will be discussed.
Amorphous and hydrogenated (a-SiC:H) as well as crystalline silicon carbide are widespread materials for optoelectronic applications. In this paper, we studied the effect of laser/RF plasma jet treatment of a-SiC:H thin films deposited by Plasma Enhanced Chemical Vapor Deposition, on Si wafers. A Nd:YAG laser ((lambda) equals 1.06 micrometers , t<SUB>FWHM</SUB> equals 14 ns, E<SUB>0</SUB> equals 0.015 J/pulse) was used with a fluence of 4 mJ/cm<SUP>2</SUP> incident on the sample, the number of pulses being varied. Plasma treatments were performed in a plasma jet generated by a capacity coupled RF discharge in N<SUB>2</SUB>. Different analysis techniques were used to investigate the films, before and after the irradiation: X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. We followed the modification of their structure and composition as an effect of the laser/plasma treatment. A comparison with the excimer and also with the RF treatments was performed.
High quality AlN, GaN, ZnO and PZT thin films were deposited on different substrates by laser ablation of metallic or PZT targets in nitrogen or oxygen reactive atmosphere. A YAG laser ((lambda) equals 1.06 micrometers , t<SUB>FWHM</SUB> equals 10 ns, 0.3 J/pulse) was used as laser source. The target collector distance was varied in the range 3 - 8 cm, the gas pressure in the range 10<SUP>-3</SUP> - 10<SUP>-1</SUP> mbar, the collectors were heated at temperatures between 200 and 400 degree(s)C. The influence of the process parameters on the physical properties of the deposited films was analyzed. The profile of atoms inside the film was evidenced by SIMS analysis. Depth profile XPS and AES confirm the presence of Al-N, Ga-N and Zn-O bonds inside the films. X-ray diffraction studies revealed different crystalline orientation depending on deposition conditions. The recorded spectra show the crystalline orientation of the layers, at collector's temperatures lower than the best results reported in literature. Optical absorption spectroscopy studies in the UV and visible range evidenced a high transparency (over 80% transmission) for the AlN, GaN and ZnO deposited films. For PZT a particular result consists in the piezoelectricity of the deposited films `as received', without any subsequent thermal or electric treatment.
Crystalline GaN thin films have been deposited by laser ablation of liquid Ga target in nitrogen reactive atmosphere. A Nd-YAG laser (l equals 1.06 mm, t<SUB>FWHM</SUB> equals 10 ns) able to deliver an energy of 0.35 J/pulse was used as laser source. The nitrogen pressure was varied in the range of 10<SUP>-2</SUP> - 10<SUP>-1</SUP> mbar. As substrates sapphire plates, heated below 300 degree(s)C, were used. The characteristics of the deposited films were evidenced by different techniques as XPS, SIMS, X-ray diffraction, optical absorption spectroscopy. The Nls region of the XPS spectrum contains as main peak the one centered at 397.3 eV and corresponding to Ga-N bond. From the distance between the photoelectron Ga 3d peak and the Auger Ga LMM peak, the calculated Auger parameter of 1083.9 eV corresponds to the one reported in literature for GaN compound (1084.05 eV). SIMS profiles corresponding to N and Ga in depth- distribution carried out the presence of layers of the order of 130 - 150 nm, with uniform distribution of Ga and N inside the layer. Both techniques evidenced an oxygen contamination below 5%. XRD recorded spectra show the presence of a strong peak assigned to (002) GaN crystalline orientation. Optical absorption spectroscopy studies in the UV and visible range evidenced a high transparency for the deposited films.