G. Orriols

Molecule formation and energy transfer processes in a vapor with high density of 3P-excited sodium atoms

Abstract A rich fluorescence spectrum extending between 4000 and 8200 A has been observed whenever sodium vapor is excited by dye laser light tuned to the 3 2 S → 3 2 P transition. Molecule formation due to collisions between excited and unexcited atoms is manifested by the presence of an emission band of sodium in the spectral range 4160–4570 A.

Signal damping of second-harmonic generation in poled soda-lime silicate glass

We report second-harmonic generation in poled microscope slides of soda-lime silicate glass. During poling a sodium precipitate appears on the glass surface on the cathode side. Evidence of sodium motion through the sample helps us to understand the inducing mechanism of the second-order nonlinearity in the poled glass. We found decay of the second-harmonic signal over time. The decay rate increases with the ambient temperature, while the residual signal depends on the poling temperature.

Double fitting of Maker fringes to characterize near-surface and bulk second-order nonlinearities in poled silica

An experimental analysis of the distribution and thickness of the bulk nonlinearity induced in poled silica is reported. The second-order susceptibility decreases exponentially from the anodic interface. Maker fringe patterns showing a double structure are interpreted in relation to the presence of two nonlinear profiles, one concentrated near the anodic surface and another extending into the bulk of the sample. The Maker fringe theory is properly generalized and a double fitting technique reproducing well the experimental results is used to characterize the induced nonlinearities. The dependence of the second-harmonic signal on the poling temperature is given, which is different from that of sol-gel silica.

Bulk and near-surface second-order nonlinearities generated in a BK7 soft glass by thermal poling

Bulk second-order nonlinearity was generated in BK7 glass at a higher temperature and with a longer poling time than near-surface second-order nonlinearity. The temporal decay of the bulk second-order nonlinearity was slower than that of the near-surface second-order nonlinearity. The thickness of the near-surface nonlinear layer increased with poling time. Poled BK7 glass was also measured by x-ray photoelectron spectroscopy. Depletion of Na at the anodic surface and its accumulation at the cathodic surface was observed. At the cathodic surface, a higher-energy peak near O (1s) appeared, which shows peroxy-radical defects. At the anodic surface, a lower-energy peak near Si (2p) appeared, which may be attributed to E′ centers or to two-coordinated Si defects. The mechanisms of generation of these defects and of the second-order nonlinearities are discussed.

Dressed-atom approach for probe spectroscopy in Doppler-broadened three-level systems with standing-wave saturator

The three-level probe spectroscopy of Doppler-broadened transitions in the presence of a standing wave pump laser is investigated through a diagrammatic method. A probe-response diagram is defined where the resonance positions versus the probe laser frequency and absorber velocity are plotted. The dressed atom description provides a convenient method for deriving the probe response diagram and calculating the position of the Doppler-free structures on the absorption spectrum, including the ac Stark shifts. We have interpreted probe spectra on the basis of the probe response diagram, presenting new features and giving a detailed physical interpretation of all the coherent effects appearing in the spectra. Population effects are not included in the dressed system analysis and are calculated on the basis of a semiclassical treatment, but the probe response diagram provides a complete interpretation of the involved phenomena.

Cavity optimisation by separating functions in thermal optical bistability

Abstract Cavity optimisation in thermal optical bistability is analysed by considering bistable etalons with the absorption localized in one of the reflective coatings. The alternatives of front and back mirror absorption are theoretically compared and experimentally demonstrated with different organic liquids used as thermally-sensitive optical spacer. Steady-state bistable loops with submilliwatt powers are reported.

Homoclinic phenomena in opto-thermal bistability with localized absorption

Abstract Time dynamics of a family of opto-thermal nonlinear devices is described by means of a system of linear partial differential equations subjected to a nonlocal and nonlinear boundary condition and a rich variety of homoclinic phenomena is numerically found. Linear-stability analysis shows that the effective dynamical dimension is determined by the device structure, i.e. by the number of layers between two mirrors, and then it may be easily varied. A variety of local and global bifurcations observed in bilayer systems are described in detail, showing that the dynamics is in effect two-dimensional except for subtle features appearing in a gluing bifurcation where two homoclinic connections occur almost simultaneously. Complex behaviour is shown to occur in the case of trilayer systems, with a very similar dynamics to the one of the well-known Rossler model of third-order ordinary differential equations. Two different families of aperiodic phase portraits are described in detail and their association with homoclinic connections to saddle invariant sets of different configurations is pointed out. The occurrence of complex dynamics is demonstrated by means of first-return 1D maps obtained in proper Poincare sections.

Laser irradiation of a three-level gas system: Continued-fraction theory and applications

The general steady-state solution of the density matrix equations for a Doppler broadened three-level system irradiated by two resonant standing-wave laser beams of arbritrary intensity is analyzed. The solution is expressed in a matrix continued fraction form, that involves 4×4 matrices in important configurations and is convenient for numerical computations. Some representative cases including the absorption spectra for a probe laser of arbitrary intensity, the Doppler-free multiphoton resonances and the optically pumped lasers are analyzed numerically in connection with previous experimental investigations.

Equivalent low-order model for a nonlinear diffusion equation

Abstract We present an equivalent low-order model for a simple PDE system that exhibits interesting low-dimensional dynamics with a rich variety of homoclinic phenomena and whose effective dimension may be gradually increased by means of system parameters. The system is a linear heat equation subject to a nonlinear and nonlocal boundary condition and the reduction procedure is based on a finite element method. We will show that both the PDE and ODE systems have indentical stationary solution with a very similar linear stability behaviour and exhibit also very similar dynamics, at least within parameter ranges corresponding to physical devices.

New phenomena in Doppleron resonances

Abstract In an inhomogeneously Doppler broadened three-level system we analyze the absorption of a probe laser when a strong standing-wave laser pumps a transition sharing a level with the probe transition. Doppleron resonances occur for a pump field tuned off the resonant frequency. Their radiative shifts and strengths versus the pump field intensity are analyzed. The contribution of coherent and incoherent processes are separated. A large signal created by the pump field spatial dependence for the stationary molecules appears at the center of the probe absorption Doppler profile. A comparison is made with an experiment by Reid and Oka (Phys. Rev. Lett. 38 (1977) 67).