B. Ratajska-Gadomska

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.

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.

Temperature evolution of the low-frequency optical Kerr effect spectra of liquid benzene in quasicrystalline approach

Theoretical calculations of low-frequency spectra, due to intermolecular collective modes in liquid benzene, are presented. The molecules in a liquid are assumed to be organized in instantaneous, quasicrystalline, short-range structures of the lifetime of order of 10−13 s, which perform rotational and translational vibrations. Those vibrations are responsible for the intermolecular part of the femtosecond optical Kerr effect response. The spectra, obtained for five different temperatures, show a good agreement with the experimental results of other authors. We prove that the low-frequency feature of the spectrum, which is evidently augmented at low temperatures, is associated with collective translational intermolecular vibrations.

Ultrafast optical Kerr effect spectroscopy of water confined in nanopores of the gelatin gel

We report on the investigation of a short-time collective dynamics of water confined in the pores of the gelatin gel, using the femtosecond optical Kerr effect spectroscopy. The ultrafast responses of water molecules obtained in bulk liquid and in three concentrations of gelatin gels are explained theoretically, both in a long time and in a short time regime, taking into account all molecular motions. We prove that the contribution of molecules involved in tetrahedral, strongly H-bonded structures stabilizing the gel network increases with the gel concentration. On the other hand the long-time relaxation of water molecules is significantly slowed down in the gel pores.

Homoclinic dynamics of the vibronic laser

Abstract Herewith we report on the nonlinear dynamics of the vibronic laser, it means the laser, in which the laser action is possible only due to phonons of the host crystal lattice. We have described such a system using five level model, in which the number of the coherently excited lattice phonons is included as one of the dynamical variables. We have shown that at certain values of control parameters, such as pump wavelengths and pump intensities, the system follows the homoclinic trajectory in the phase space and eventually passes to the chaotic state. It keeps traveling between two or more unstable fixed points. We have investigated the evolution of the homoclinic orbits with the change of the control parameters. The theoretical predictions have been proved by us experimentally in the short-cavity alexandrite laser. We demonstrate the homoclinic character of the laser output, occurring for different pump wavelengths and pump powers. Instead there are the regions of parameters for which we usually observe the stable CW action and pulsations can hardly be achieved.

Homoclinic orbits and chaos in the vibronic short-cavity standing-wave alexandrite laser

We present an experimental and a theoretical investigation of the unstable behavior of the laser-pumped short-cavity alexandrite laser. Its chaotic dynamics is achieved by an increase of the pump laser power. The route to chaos depends on the cavity length and the pump wavelength. The experimental results are explained by the theoretical model, which includes the host-lattice phonons’ dynamics in the standard laser equations. We conclude that the observed laser instabilities are due to the competition between the laser field and phonons in the host crystal. A linear-stability analysis yields the conclusion that the observed chaos has a homoclinic character.

A percolation approach to dye fluorescence quenching during the gelation process

The evolution of the stationary fluorescence yield of dyes embedded in a network of gelatin molecules is shown to reflect the kinetics of helix formation during the gelation process. A theoretical approach to the problem, applying the Flory-Stockmayer bond percolation model of the medium, is presented.

The role of stimulated Raman scattering in supercontinuum generation in bulk diamond

We report on experimental results of supercontinuum generation in bulk diamond. The spectrum of supercontinuum generated with 800 nm pump extends up to 600 nm towards short wavelengths. We present the numerical model explaining the phenomenon, in which the role of different nonlinear effects including stimulated Raman scattering is discussed. Unlike in other materials, in diamond the feature of supercontinuum due to stimulated Raman response is apparently visible.

Quantum theory of the vibronic solid-state laser

A quantum-mechanical description of a vibronic multimode standing-wave transition-metal ion laser is presented. The impurity ion levels are coupled both by the radiation field and by the phonons of the host lattice. A dynamic Heisenberg–Langevin set of equations for the material system and the photon and phonon operators, including radiative and nonradiative damping terms and quantum-stochastic forces, has been derived. The numerical solutions of those equations show the crucial role of the phonons in the laser dynamics. The competition between the photons and the phonons leads to regular stable self-pulsation for certain sets of parameters.