Optical characterization of graphene in vacuum ultraviolet spectral region & spectroscopic studies of colliding laser plasmas (Al, Si)

The aim of this research is to investigate and explore new innovative material(s) and techniques regarding development and improvement of vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) optics and sources; for the advancement of EUV and VUV technological areas like space exploration (e.g. observation and spectroscopic diagnostics of the solar corona) and EUV lithography (e.g. advancement and minimization of integrated electronic circuits (ICs)). The research work was primarily focused on the investigations of the optical and structural properties of graphene (mono and few-layer) deposited on SiO2/Si substrate in VUV spectral region by exploiting different diagnostic techniques, based on reflection and polarimetric measurements. The study was addressed starting from silicon dioxide deposited on silicon (SiO2 / Si), which works as the substrate for graphene samples. The optical properties of SiO2/Si were thoroughly investigated at the hydrogen Lyman–alpha line (121.6 nm) by employing the tabletop EUV-VUV polarimetry facility located at CNR-INF Padova. An approach based on the combined use of reflectometry with polarimetry technique was used to find out the reliable values of the optical constants. The results show the potential of the approach and it was demonstrated in this study that the optical constants retrieved by using ellipsometric parameters; ratio (ρ), and phase shift (), are more reliable than the retrieved one using least square fitting of the reflectivity. Moreover, it was found that SiO2 behaves as a phase retarder by introducing a phase difference between the s- and p- polarization components of the incoming light. The phase differences observed was 18° to 160° depending on the incidence angle. Using the similar experimental technique, the ellipsometric parameters (phase shift (ϕ), ratio (ρ)) of graphene (1LG/SiO2/Si) sample were also investigated and compared with that of SiO2/Si to see the effect of the graphene as capping layer. It was found that 1LG on top of SiO2 improves optical throughput and despite having atomic thickness it affects the polarimetric properties of the underlying substrate. Further, detailed optical properties of mono (1L) and tri-layer (3L) of commercial graphene grown on (SiO2/Si) substrate were studied at hydrogen Lyman alpha by using laboratory based (at CNR-IFN, Padova) and synchrotron light-based (at BEAR beamline, Elettra synchrotron) EUV-VUV reflectometer setups. Angular reflectance measurements of graphene samples along with bare substrate were performed by taking into account the light polarization. Distinguishable optical performance was observed for both samples (1LG and 3LG) in spite of the ultra-thin thickness of the films. Optical anisotropy with the axis of symmetry nearly perpendicular to the surface and coherently related to the p-orbitals structural orientation has been experimentally demonstrated. Anisotropic “effective optical constants” corresponding to “effective thickness” were retrieved by simulating the interaction of the electromagnetic wave with the structure of the sample. Furthermore, the reliability of the derived optical constants was tested qualitatively by deducing surface differential reflectance (SDR) from the reflectance measurements. Another very interesting effect induced by graphene is the shift of the pseudo-Brewster angle with respect to what was observed for the substrate. The downshift of the pseudo-Brewster angle was observed for both samples 1LG (-1.5°), and 3LG (-5°), with larger shift for an increasing number of layers. However, in literature an upshift in the Brewster angle is reported but for different spectral region. AFM, XPS and Raman spectroscopies were used to study surface morphology, quality of graphene coatings, and to estimate the thickness/ number of layers. To the best of our knowledge, these remarkable optical properties of graphene at VUV spectral region was determined for the first time and results are of considerable interest for VUV optics advancement. The last part of the thesis is about the study of the stagnation layer formed at the collision front of two colliding plasmas by employing time resolved spectroscopic technique. Time evolution and dynamics of the Al-Al, Al-Si colliding plasmas studied and compared in the case of flat and wedge targets. It was observed that in case of wedge target the overall emission from stagnation layer was more intense and higher ionization states of (Al and Si) appeared earlier in time having higher intensity compared to the flat target. The time evolution of the electron number density was also studied and it was observed that wedge target results in a relatively higher electron number density