Toward the subdiffraction focusing limit of optical superresolution

A superresolving three-zone plate is applied to a Fresnel diffractive lens. It is shown that for radial incident polarization this combination produces a focal spot approaching superresolution allowed subdiffractive limit of 0.36lambda/NA for focusing. For media responsive to longitudinal field component only, our phase engineering scheme results in a focal spot size of 0.368lambda/NA. When used with a solid immersion lens, the scheme can generate the smallest focal spot available for passive optics.     

Collisional depolarization of fluorescence of polyatomic molecules in the gas phase by excited state quenchers

A new method of extracting information on the efficiencies of quenching of an excited state and collisional reorientation from the data on the fluorescence depolarization of molecular vapors by quenchers of the excited state is proposed. The method does not involve collisional cross sections and fluorescence lifetimes. From the experimental data on the depolarization of polarized luminescence of para-quaterphenyl and 2-(4′-dimethylamine) phenyl-5-phenyloxydiazole by oxygen and nitrogen, the ratios of the constants of the excitedstate quenching and orientation randomization (fluorescence depolarization) by oxygen are obtained. For these molecules, the probabilities of the excited-state quenching q and orientation randomization s per collision with oxygen molecules are determined (q=0.25±0.04 and s=0.13±0.04 for para-quaterphenyl and q=0.2±0.04 and s=0.21±0.04 for 2-(4′-dimethylamine)phenyl-5-phenyloxydiazole). The self-quenching of fluorescence of perylene vapors, with the probability 0.28 per collision, is found.     

Depolarization of luminescence of polyatomic molecules in the gas phase as a method of determining the efficiency of collisional transfer of angular momentum

The theory of collisional depolarization of luminescence of extended polyatomic molecules in rarefiedgases is considered. The interrelation between the frequency of collisions, the relaxation time of the angular momentum, and the cross section of the luminescence depolarization is established, and the dependence of these parameters on the efficiency of an abrupt change in the angular momentum is calculated. The use of the theory of collisions of solids in the Enskog approximation made it possible to take into account the effect of the shape and mass of colliding molecules on the degree of depolarization. It is established that, in terms of this theory, there exists a limiting efficiency of an abrupt change in the angular momentum, which, however, does not attain the value proposed in the model of strong collisions (Jdiffusion). The dependence of the depolarization of luminescence of 1,4-di-(2-5-p-tolyloxazolyl) benzene molecules on the concentration of a buffer gas (argon) is measured. It is found that about five collisions with Ar atoms are required for randomization of the angular momentum of these molecules.     

Pulse compression without chirp control and frequency detuning by high-order coherent Raman scattering in impulsively excited media

It is shown that phase-locked pulses as short as 3 fs can be generated by coherent scattering in impulsively excited Raman media without the necessity of external phase control. The underlying mechanism, temporal characteristics, spectra, phase relations, physical limitations owing to competition processes, and precompensation of dispersion by the hollow waveguide window are studied analytically and numerically without the use of the slowly varying envelope approximation and with a global approach to dispersion. Additionally, the large frequency shifts in both the Stokes and anti-Stokes directions of as much as half the carrier frequency raise the possibility of generating widely tunable ultrashort pulses.     

Coherent-absorber mode locking of solid-state lasers

We study solid-state laser mode locking in the self-induced transparency regime of an intracavity absorber. Depending on the absorber dephasing, pulse energy, and resonator dispersion, we find stable coherent 2pi -pulse operation, pulse splitting, oscillatory pulse shapes, and smooth transition to incoherent saturable-absorber mode locking.     

Crystal and molecular structures of <Emphasis Type="Italic">trans</Emphasis>-2-(4′-dimethylaminobenzylideneacetyl)-5,5-dimethylcyclohexane-1,3-dione

The crystal and molecular structures of a cyclic β-triketone, namely, trans-2-(4′-dimethylaminobenzylideneacetyl)-5,5-dimethylcyclohexane-1,3-dione (I), are determined using X-ray diffraction analysis. It is established that the compound in the crystalline state exists in a diketo-enol form stabilized by intramolecular hydrogen bonds. The specific features of the structure and the physicochemical and fluorescence properties of the compound are discussed.