W. Gadomski

Swept dynamics of a CO2 laser near threshold: Two- versus four-level model

Abstract The swept behavior of a single mode homogeneously broadened CO 2 laser has been characterized by applying a triangular modulation to an electro optic modulator inside the optical cavity. We observe a dynamical hysteresis effect depending on the modulation depth and on the sweep rate. Our experimental results can be explained in the framework of a four level molecular model which takes into account the coupling between the two resonant levels and the rotational manifold.

Laser with feedback: an optical implementation of competing instabilities, Shil’nikov chaos, and transient fluctuation enhancement

A CO2 laser with feedback shows different dynamic regimes depending on the dominant role of one or two of three coexisting unstable stationary points. These regimes have been characterized by statistical distributions of return times to a Poincare section at constant intensity. In particular, in the regime of Shil’nikov chaos the iteration maps of return times display a statistical spread owing to a transient fluctuation enhancement phenomenon peculiar to macroscopic systems, which is absent in low-dimensional chaotic dynamics.

Delayed bifurcation at the threshold of a swept gain CO2 laser

Abstract A single mode CO2 laser near threshold behaves differently depending on whether the threshold is reached by varying the pump or the cavity losses. Only in the first case we observe a delayed bifurcation. Our experimental results are explained by a four level molecular model which takes into account the coupling between the two resonant levels and the rotational manifold.

Water structure in nanopores of agarose gel by Raman spectroscopy

The evolution of water structure during the gelation process is examined in aqueous solution of agarose using Raman spectroscopy of the O-H stretching band. The measurements have been performed at room temperature for different concentrations of agarose, which yields different dimensions of nanopores in the network of the created gel. Our results show that water confined in the gel pores exhibits evident changes in the local order of molecules in comparison with bulk water and water in the sol state. During the sol-gel transition the number of molecules that participate in the regular tetrahedral H-bond structure increases, and the effect is stronger for higher concentration of the biopolymer.

Influence of confinement on solvation of ethanol in water studied by Raman spectroscopy.

Herewith we present the results of our studies on the effect of confinement on the solvation of ethyl alcohol in aqueous solutions using Raman spectroscopy of the O-H stretching band. Based on Gaussian-Lorentzian deconvolution of the O-H band Raman spectra we investigate the local structures created between water-water, water-alcohol, and alcohol-alcohol molecules, which are directly related to the solubility of the liquids. Comparison of the responses in bulk solutions and in solutions confined in the pores of the gelatin gel shows that for high ethanol concentrations solubility significantly increases with decrease of the pore sizes.

A femtosecond snapshot of crystalline order in molecular liquids

A theoretical model of ultrafast dynamics in the optical Kerr effect in molecular liquids is developed. It assumes that for short times there exist dynamic quasicrystalline structures including a central molecule and its nearest neighbors. The interaction of such structures with a femtosecond laser pulse leads to excitation of vibrational modes (local phonons) which are responsible for subpicosecond nonlinear polarizability of the liquid. The results of numerical calculations performed for benzene show a very good agreement with the experimental data. The lifetime of the dynamic quasicrystalline structures in benzene, at room temperature, is estimated to be about 200 fs.

Detailed insight into the hydrogen bonding interactions in acetone–methanol mixtures. A molecular dynamics simulation and Voronoi polyhedra analysis study

Voronoi polyhedra (VP) analysis of mixtures of acetone and methanol is reported on the basis of molecular dynamics computer simulations, performed at 300 K and 1 bar. The composition of the systems investigated covers the entire range from neat acetone to neat methanol. Distribution of the volume, reciprocal volume and asphericity parameter of the VP as well as that of the area of the individual VP faces and of the radius of the empty voids located between the molecules are calculated. To investigate the tendency of the like molecules to self-associate the analyses are repeated by disregarding one of the two components. The self-aggregates of the disregarded component thus turn into large empty voids, which are easily detectable in VP analysis. The obtained results reveal that both molecules show self-association, but this behavior is considerably stronger among the acetone than among the methanol molecules. The strongest self-association of the acetone and methanol molecules is found in their mole fraction ranges of 02-0.5 and 0.5-0.6, respectively. The caging effect around the methanol molecules is found to be stronger than around acetones. Finally, the local environment of the acetone molecules turns out to be more spherical than that of the methanols, not only in the respective neat liquids, but also in their mixtures.

Self-pulsations in phonon-assisted lasers

The observation of stable self-pulsing of a single-transverse-mode alexandrite laser with a Fabry–Perot cavity is reported. This process occurs many times above threshold and depends on the pump wavelength. The novel theoretical model, in which the dynamics of the host lattice phonons is taken into account, is shown to explain this phenomenon. The border of the Hopf bifurcation is found in the plane of two parameters: the intensity and the frequency of the pump laser. In the region where self-pulsations occur they are proved to be the result of the photon–phonon energy pulling.

Turn-on transient statistics and dynamics in a multimode, short-cavity laser

The statistics of turn-on delay times for a longitudinal mode and the total intensity from a multimode, standing-wave, short-cavity dye laser are measured and compared with numerical simulations of a multimode theory of laser dynamics. Differences between the measurements and the theory for the average and standard deviation of the turn-on time are due to transitory modes that decay as a result of frequency-dependent losses and gains. Associated with this phenomenon is a kink in the time evolution of the mode intensity that represents a transition in the dynamics from domination by independent growth of all modes to domination by competition between modes with differing net gains. This unequal competition increases the average and standard deviation of the mode turn-on time.

Influence of A D.C. electric field on the polarization of an intense laser beam in liquids

Abstract Analytical and numeric solutions of the nonlinear equation describing polarization changes of a strong laser beam propagating in liquid in the presence of a stationary electric field are derived. The polarization ellipse of the optical wave was found to oscillate along the propagation path. A new method of estimating the x(-ω,ω,ω,-ω)zyyz value is proposed.