Investigation On Intense Femtosecond Laser Induced Planar Filamentation For Spectral Imaging

Femtosecond laser filamentation is referred to a filament-shape intensity clamping zone formed by the propagation of femtosecond laser pulses in transparent media.Femtosecond laser filament has several unique properties such as high intensity,remote propagation distance without diffraction,and plasma conductivity,which make it promising in many applications in areas such as laser guiding of lightning,combustion diagnostics,air lasing and remote sensing of atmospheric trace molecules.In atmospheric sensing,filament-induced nonlinear spectroscopy(FINS)has been used for probing several small hydrocarbons in atmosphere such as methane and acetylene;however,there are very rare studies reporting the detections of large organic molecules.In addition,the filament has only a typical diameter of about 100 micros,and thus could not be used to detect two dimensional concentrations distributions of trace molecules.This thesis is mainly devoted to the investigations on femtosecond filament-induced molecular spectra and planar filament-induced spectral imaging.By using FINS,we first measure nonlinear fluorescence spectra of iodine molecules and several large organic molecules,showing the feasibility of FINS in application to probing phase change and distinguishing large organic molecules.We then concentrate on the studies on the planar filaments formation and spatial distributions induced by intense femtosecond laser pulses in solvent because the critical power is low and thus it is easier to produce filaments in solvent.This will help develop two-dimensional spatial imaging method by using planar filaments.The contexts and the innovation of this thesis can be summarized as follows:1.By irradiating iodine vapor molecules,produced from the iodine sublimation,with a high-intensity laser pulse generated from femtosecond laser filamentation in the atmosphere,we obtained the characteristic spectral band of iodine molecules at 341.6 nm(the D-X~1?_g~+transition).We observed that the characteristic spectrum can be very clean after eliminating the air spectrum by using time-resolved measurements.We found that the intensity of the spectral band at 341.6 nm varies as functions ofheating temperature of the iodine sample and the distance of the filament to the solid iodine sample,showing the dependence of the concentration of iodine molecules on the temperature and sublimation path during the sublimation process of iodine sample.Based on the above results,we demonstrate the feasibility of filament-induced nonlinear spectroscopy in applications to detecting phase change of matter.2.We demonstrated the feasibility of femtosecond filament-induced nonlinear spectroscopy for probing large organic molecules by recording the fingerprint spectra of fomic acid,ethanol,n-propyl alcohol,isopropyl alcohol,acetic ether,and butanone molecules.By analyzing the fingerprint spectra of these molecules,we obtained the ratio of different spectral bands.We subsequently show the possibility of distinguishing the two isomer molecules,i.e.,n-propyl alcohol,and isopropyl alcohol,by using filament-induced nonlinear spectroscopy,which provide a new way of differentiating organic isomer molecules.3.We investigated femtosecond laser filamentation in Rhodamine B dye-doped methanol solvent by focusing femtosecond laser beam with a cylindrical lens,and showed the spatial distributions of the formed filaments by observing the fluorescence(~610 nm)of Rhodamine B dye molecules induced by the filaments.The solvent was chosen because the critical power for self-focusing in solvent is low,leading to an easy generation of filamentation.We first observed the spatial distribution of multiple filaments from side and top of the propagation direction of the laser beam,and found that planar distributions of multiple filaments can be formed.We subsequently checked planar multifilaments by using laser etching technique,in which we employed the high-intensity filaments to ablate a silver layer spin-coated on a silicon substrate and imaged them by a microscope as well as a scanning electron microscope.By examining the damaged profiles on the silver layer,the distributions of the generated multiple filaments can be obtained.It was found that the generated multiple filaments are lined in a plane with the width of a typical filament diameter.The achievement of planar multifilaments provides a way to sense,such as,two-dimensional concentration distribution of specific species in the atmosphere and combustion fields by using filament-induced nonlinear spectroscopy.