Valery L. Bogdanov

Light quenching of tetraphenylbutadiene fluorescence observed during two-photon excitation.

We observed the steady-state and time-resolved emission of tetraphenylbutadiene (TPB) whea excited by simultaneous absorption of two photons (514 to 610 nm). The intensity initially increased quadratically with laser power, as expected for a two-photon process. At higher laser powers the intensity increases in TPB were subquadratic. The intensity and anisotropy decay times of TPB were unchanged under the locally intense illumination. Importantly, the time zero anisotropy of TPB was decreased under conditions where the intensity was subquadratic. Furthermore, the subquadratic dependence on incident power was not observed for two-photon excitation of 2,5-diphenyloxazole (PPO), for which the incident wavelength does not overlap with the emission spectrum. These results are consistent with stimulated emission (light quenching) of TPB at high laser intensities. The phenomenon of light quenching may be important for other fluorophores used in biochemical research, particularly for the high local intensities used for two-photon excitation.

Spectral holography of pico- and nanosecond laser pulses

Recording and reconstruction of pico- and nanosecond pulses were accomplished with spectral holography. The developing of spectral holography in nanosecond time-domain was provided while using high-spectral-resolution equipment based on Fabry-Perot etalon with the side entrance/exit. The recognition of shaped nanosecond light pulses was also realized.

Resonant four-photon scattering and dynamics of excited molecules

Resonant four-photon scattering and excited molecules depopulation/rotational diffusion are considered. Scattering pulse is generated due to three ultrashort pulses interacting with molecules. First pulse prepares excited molecules and two coincident probe pulses, which have a delay in time relative to excitation pulse and in resonance with absorption band of excited molecules, control a transient nonlinear susceptibility. High sensitivity of proposed nonlinear optical measurements to orientational motion of excited molecules is shown.

Transient third-order optical nonlinearity of organic molecules

Transient third-order nonlinearity is induced by electronic resonances of excited molecules. Intensity of a signal, which is generated due to transient nonlinear susceptibility, strongly depends on a relaxation pathway and rate of high excited molecular electronic states, and on molecular dynamics in a low-lying excited state. Experimental and theoretical data, concerning a transient susceptibility dependence on dynamics of excited polyatomic organic molecules (ultrafast dephasing and depopulation processes, orientation anisotropy, time-dependence of transient absorption spectrum), are discussed. Obtained results demonstrate a high sensitivity of transient third order susceptibility to dynamics of excited molecules.

Backscatter anisotropy and the shape of retroreflected lidar pulse from sea water

In several LIDAR sounding experiments, certain anomalies in the registered pulse shape were observed, such as the time delay between maxima of pulse of sea-water response and surface- reflected pulse, and unexpectedly high level of signals from isolated sea-water layers. These peculiarities cannot be explained with the traditional theoretical treatment of the echo-signals. In this paper, a new explanation of the return signals shape anomalies is presented. Our approach is based on the accounting of the angular anisotropy of the scattering in the back hemisphere and the geometrical conditions of the typical LIDAR experiments. We present computational data and laboratory measurement results related to enhanced backscattering by large coated particles suggested to model the optical properties of some algal cell species (diatoms, and so forth). The model calculations of the LIDAR signal shape show that such particles may be responsible for an anomalously high level of LIDAR signals from isolated sea-water layers.

Angular anisotropy and kinetics of light backscattering from discrete random media

The results of experimental study and theoretical treatment of the evolution of the backscattered light pulse in statistically homogeneous disordered medium are presented. Scattering kinetics was measured for water solutions of the latex microparticles with time-correlated photon-counting setup and ML Ar -laser. It is shown that backscattered pulse arrived to photo detector with essential delay with respect to the pulse reflected from the surface of the scattering medium. This delay is due to angular anisotropy of the scattering. Backscattering of non-coherent diffuse component of radiation propagating in forward direction develops in inner layers of the medium and determines the value and concentration dependence of the backscattered pulse delay. 1.

Four-photon spectroscopy of excited molecules: relaxation pathways and rates from high-excited states of 1,4-diphenylbutadiene molecules

The dispersion of the third-order optical susceptibility was measured for excited 1 molecules. The pathways and rates of relaxation for high-excited electronic states were determined. 1.