We report experimental observations of the spectral modifications of ultrashort laser pulses caused by the generation of
terahertz (THz) radiation via phase-matched optical rectification in LiNbO<sub>3</sub>. The measured red shift and narrowing of the
laser pulse spectra is in a good agreement with the consideration of the THz generation processes in terms of difference
frequency mixing. Moreover, we propose and demonstrate a new method for evaluating the absolute energy of the THz
pulse generated via high-efficiency optical rectification. The method is based on analyzing the laser pulse spectral
modifications. Abilities to obtain >100% optical-to-THz quantum conversion efficiency are discussed.
Photopolymerization and photoinduced diffusion of the molecular oxygen in oxygen bearing C<SUB>60</SUB> films by (lambda) equals 459, 514 nm continuous laser radiation and by 100-fs, (lambda) equals 395, 410 nm pulses were compared by visual observation of photodarkening spots and by recording the Raman spectra, in the range 200 - 1700 cm<SUP>-1</SUP>, of the irradiated regions. It was found out that in the case of femtosecond pulses, the efficiencies of both photoinduced processes are significantly lower than those in the case of continuous radiation.
A new method of achromatic wavefront reconstruction by geometric-optical reflection of the reconstructing radiation from surfaces with constant phase difference between the object and reference waves is theoretically described and experimentally realized. Method's distinction from holographic one is discussed. Femtosecond laser pulses are used for recording in the experiment.
Relaxation of fullerenes in solids and solutions was investigated by femtosecond pump-probe technique at various experimental setups. Intensity of excitation pulse was varied at 10<SUP>10</SUP> - 10<SUP>12</SUP> W/cm<SUP>2</SUP>, energies of pump and probe quanta were changed from 1.57 to 3.14 eV, pulse duration -- from 60 to 350 fs. An increase of absorption was observed in solutions with a time scale of 1.2 ps for C<SUB>60</SUB> and 0.15 ps for C<SUB>70</SUB> after excitation of HOMO-LUMO transition. It was attributed to symmetry breaking of excited molecules. Observed for solid C<SUB>60</SUB> dependence of relaxation rate on the density of excited molecules and on the wavelengths of the pump and probe pulses is explained by 'hot' singlet-singlet annihilation the rate of which is proportional to R<SUP>-3</SUP>. Comparison of photoinduced absorption and bleaching decays allowed to conclude that only a small fracture of excited molecules produce localized charge carriers, mainly by excitonic annihilation proces and these carriers are responsible for long-lived residual absorption observed in photoinduced absorption kinetics. Difference absorption spectra observed near zero delay time were similar to electroabsorption ones superimposed on excited state absorption of C<SUB>60</SUB> film. This electroabsorption feature disappeared in phase with pumping pulse. This phenomenon is connected with Stark shift in strong electric field of the powerful pumping pulse.
The spectral dependence of ultrafast relaxation of excitations in a C<SUB>60</SUB> thin film was discovered with the help of femtosecond laser spectroscopy. The dependence can be explained by the relaxation of excitations in different energy bands. The temporal dependence of photoinduced darkening is characterized by peak at delays of the order of pulse duration (approximately 50 fs). The peak is replaced at greater delays by a decrease with characteristic times (tau) <SUB>1</SUB> equals 0.4 ps and (tau) <SUB>2</SUB> equals 30 ps, determined by the relaxation of electrons on intramolecular and intermolecular oscillations. The spectral dependence of two- photon absorption is investigated.