F. Pi

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.

Nanocalorimetric analysis of the ferromagnetic transition in ultrathin films of nickel

We report on in situ heat capacity measurements (370–800K) using quasiadiabatic ultrafast differential scanning nanocalorimetry in thin films (1–200nm) of Ni grown by electron beam evaporation. The heat capacity shows a broad peak with a rounded maximum that is attributed to the decrease of long-range interactions in the ferromagnetic to paramagnetic phase transition of Ni. The calorimetric data exhibit a reduction of the Curie temperature as the thickness of the films (or the average grain size) decreases. The magnitude of the jump in specific heat at TC scales with the number of surface or interface atoms.

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.

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.

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.

Rössler chaos in opto-thermal bistable devices

Abstract It is experimentally shown that the reflected light of a high-contrast low-finesse Fabry-Perot etalon with a partially absorbing input mirror and a trilayer spacer of alternatively opposite thermooptic materials exhibits dynamical oscillations that correspond to the folded chaotic bands of the well-known Rossler model. A third-order system of equations is introduced to describe the response of the nonlinear device and some numerical simulations are reported in good agreement with the experimental results.

Homoclinic bifurcations in thermo-optical bistability with localized absorption

Abstract We present numerical and experimental evidence of homoclinic bifurcations in opto-thermal bistable cavities with an absorbing mirror and a transparent bilayer spacer of opposite thermo-optic materials.

Thermally induced optical multistability with a beam splitter in the degenerate two-beam interferometer configuration

Abstract Thermally induced optical multistability has been obtained with light of arbitrary wavelength in a cavity structure with strong interference effects on the input mirror absorption. The system constitutes the degenerate version of the recently introduced two-beam interferometric bistable devices based on a beam splitter yielding in-phase interference outputs.