A computational model, which describes EM field formation in a pulsed laser from a randomly generated initial
spontaneous field inside the laser cavity has been developed. The model is based on a two-dimensional fast
Fourier transform and describes a real laser system taking into account a lensing and a diaphragm effect of the
laser rod. The laser cavity is described by five effective planes, which represent different laser cavity elements-the back and the front mirror, the Q-switch element and the laser rod. At each plane the EM field is calculated in
real space and propagation between the planes is achieved in Fourier space by multiplication with an appropriate
phase factor. The computational time needed for simulation of a realistic pulse formation is in order of minutes.
The model can predict the shape and the integral energy of the pulse, its transverse profile at different distances
from the front mirror (including near and far field) and beam divergence. The results of the model were found
to be in good agreement with measured parameters for a Q-switched ruby laser system running in stable as well
as unstable cavity configurations. The temporal shape of a laser pulse was measured and calculated not only for
the ruby laser, but also for a Nd:YAG laser. It was found that FWHM of a pulse produced by ruby laser is three
times longer than FWHM of a pulse produced by Nd:YAG laser.
Polymer dispersed liquid crystal was studied by dynamic light scattering. In the experiments where no electric field is applied the system exhibits additional slow dynamics to ne observed in bulk nematics. This slow dynamics spreads out over several decades of time and gradually disappears with increasing electric field. It can be attributed to the rotational diffusion of the liquid crystal droplets in random potential as was confirmed by model computations. The random potential is due to the irregularities in the shape of the droplets. While the measured time intensity correlation function does not depend on the scattering vector in the system where no field is applied due to the multiple scattering, its dependence in the electric field shows expected size effects. In confined systems with typical size d there are no eigenmodes of the orientational fluctuations with wave vectors less than minimal wave vector qmin equals (pi) /d. We observed quadratic dependence of the inverse relaxation time on the scattering vector down to certain scattering vector, below which the relaxation time remains constant. The size calculated form this minimal scattering vector is in agreement with the average droplet size obtained from scanning electron microscope photographs and atomic force microscope images of our sample. The electric field changes the temperature behavior of the inverse relaxation time near the nematic-isotropic phase transition which increases with the temperature when electric field is applied, but decreases when the field is absent.
We have studied the order parameter dynamics close to the SmA-SmC*A phase transition in homeotropic cells of 4-(1-ethylheptyloxycarbonyl) phenyl-4'-alkylcarbonyloxy biphenyl-4-carboxylate by photon correlation spectroscopy. The order parameter fluctuations in the antiferroelectric SmC*A phase can be decomposed into the fluctuations of the phase and the amplitude of the molecular tilt angle. Considering the unit cell to consist out of two adjacent layers, one can describe these fluctuations with two ferroelectric modes and two antiferroelectric modes. Using photon correlation spectroscopy we measured both ferroelectric phase modes in the backward scattering geometry. This is the first simultaneous observation of these modes. The temperature dependence of the relaxation rates of these modes gives the coupling coefficients of the ferroelectric and antiferroelectric order parameters, whereas the dispersion relation leads to the values of the diffusivity coefficients for antiferroelectric and ferroelectric phase modes in the SmC*A phase.
Measured population dynamics of erbium metastable level in a Yb:Er:phosphate glass following the excitation with a Nd:glass laser is reproduced by a theoretical model based on rate equations. To our knowledge, this is the first model to include frequency hole burning of inhomogeneously broadened pumping transition of ytterbium ion. The model explains also the previously observed dependence of pumping efficiency on Nd:glass laser pulse length.
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