Vladimir V. Danilov

Optical limiting as result a of photoinduced electron transfer in hybrid systems with CdSe/ZnS quantum dots, C 60 , and Perylene

We investigated the interaction of an intense laser radiation with colloidal solutions containing CdSe/ZnS core shell quantum dots (QDs; mean size 3.4 nm), fullerene C(60), and Perylene. These materials would give rise to the photoinduced electron transfer and charge separation on the QDs and thus the optical limiting effect. Results confirm the intended aim, obtained by means of intermediate metastable products of reversible photochemical reactions, i.e., ion radicals of hybrid systems containing semiconductor nanoparticles.

Nonlinear optical limiters of laser radiation based on reverse saturable absorption and stimulated reflection

We presented wide range of laser-radiation optical limiters that are based on different physical principles, operating in 400-12000 nm spectral range with rate 100-1 ns and providing wide dynamic range of limiting required for eye and sensor protection. We propose and investigate a method for reduction of the optical limiting threshold due to the Bragg reflection in a system with distributed feedback. The limiting threshold with the Borman effect, accompanying by diffractional absorption suppress and stimulated reflection from the LC Bragg grating, is about (mu) J/cm2 for nano seconds pulse. We investigate nonlinear optical limiters of laser radiation based on fullerene-contained materials, operating on the basis of reverse saturable absorption (RSA) as well as stimulated nonlinear reflection of laser radiation. Fullerenes C60, C70 and C84 were studied with the second harmonic of Nd:YAG laser, 532 nm, single-mode radiation pulses of 30 ns duration and maximum energy 0.35 J. Definite thresholds of RSA limiting vary from 0.05 to 0.1 J/M2) depending on fullerene density and limiting scheme. Dynamic range of limiting sufficiently increases for multi-pass trains, the fluence decreases up to 10000 times. We elaborate 3000-12000 nm laser-radiation attenuators based on multi-layer interference containing vanadium-dioxide film. It is shown that these attenuators can operate due to both amplitude and diffraction effect. Attenuation is up to 1x103- 1x105 times for 1 MW/cm2 beams, operating time being 30-100 ns. For sensors in cooled optical systems, radiation attenuators can be used with vanadium oxides having the phase transition at cryogen temperatures.

Energy transfer and low-threshold optical limiting in the impurity liquid crystal systems

The advantages of use the problems of optical limiting of impurity mesomorphic systems are discussed. These advantages are caused by existence of the excitonic mechanism of the transport energy electronic excitation in liquid crystals. A dynamic range of optical limiting in such systems can be essentially extending using of two- photon absorption.

Intracavity wide-angle beam pointing in mid-IR gas lasers

The present work is the further evolution of our studies on wide-angle intracavity beaming in 3-10.6-micron gas lasers by space-time light modulators.

Photo dynamics of liquid crystal under the influence of IR-radiation pulse

rence of these phenomena has a low dependence on the sample temperature. Sending a linearly polarized light beam at normal incidence on the liquid crystal cell, reorientiation of the molecular director takes place when the intensity exceeds a critical value. This threshold intensity is seen to be dependent on the sample thickness and dopant concentration: moreover, the transition is usually of first-order type and is accompanied by very large hysteresis loops, showing a behaviour that is not observed in homeotropic samples made by pure nematic. During the experimental investigation other peculiar effects are observed optical phase locking and writing of the sample. Interesting results can also be reported in the case of optical reorientation of the mixture induced by a circularly polarized light beam. In particular, we have investigated the threshold behaviour when the circularly polarized light is normally incident on the sample for both the elicities of the beam (rightand lefthanded). For a right-handed circularly polarized incident beam, above the threshold the system is lead, by a firstorder transition, to a state corresponding to strong tilt distortion of the molecular director. A large hysteresis loop is associated to the transition (Fig. 1). On the contrary, for a left-handed circularly polarized beam, the system is lead into a nonstationary state characterized by slow oscillations of the molecular director (Fig. 2). These effects are strictly connected to the chirality of the cholesteric dopant CB15, which is right-handed; using the cholesteric CB15 (left-handed) as dopant agent, the effects are reversed. This suggests that, based on these observations, a simple experimental method can be envisaged, which is able to measure the unknown handedness of a chiral material provided that the material is soluble in the nematic host. Experimental observations are, finally, compared to theoretical predictions. The basic equations describing the interaction process of the electromagnetic field and the liquid crystal samples are derived taking into account the angular momentum exchange occurring between the incident light beam and the internal degrees of freedom of the liquid crystalline medium. The problem was then solved by direct numerical integration. The agreement with the experiment is satisfactory.

TEA-CO2 lasers with intracavity LC modulators

The abilities of TEA-CO2 laser controlling by LC modulators have been investigated. It is shown, that beam direction, beam profile, energy and time position of the pulse can be controlled by LC modulators. Some problems of interaction of radiation with LC are discussed.