Chaotic dynamics in coupled resonator sequences.

Optically induced thermal and free carrier nonlinearities in silicon micro-ring resonator influence their behavior. They can be either deleterious by making them instable and by driving their resonances out of the designed wavelengths, or enabler of different applications. Among the most interesting one, there are optical bistability and self induced oscillations. These lead to all optical logic, signal modulation, optical memories and applications in neural networks. Here, we theoretically and experimentally demonstrate that when many resonators are coupled together, thermal and free carrier nonlinearities induce also chaos. The chaotic dynamics are deeply analyzed using experimentally reconstructed phase space trajectories and the tool of Lyapunov exponents.

Thermo-optical bistability with Si nanocrystals in a whispering gallery mode resonator.

We report on the observation of optical bistability in an integrated planar microresonator with embedded silicon nanocrystals (Si-ncs). The phenomenon originates from the thermo-optical modulation of the silica-embedded Si-ncs refractive index, which in turn alters the spectral position of the resonator mode. The estimated thermo-optical coefficient of the Si nanocrystalline material, dn/dT≈2.92×10(-5)> K(-1), is an order of magnitude lower than that of bulk silicon. Both time-resolved pump-and-probe experiments and numerical simulations confirm that the silica host is responsible for the heat dissipation from the resonator. Moreover, a negligible Q-factor degradation at pump powers as high as 100 mW, along with the absence of a fast component in time-resolved measurements, confirm the minute contribution from excited carriers effects. These observations, combined with the already published large third-order nonlinearities of Si-ncs (an order of magnitude larger than in bulk Si), make this system an outstanding candidate for low-power on-chip nonlinear comb generation.

Oscillatory vertical coupling between a whispering-gallery resonator and a bus waveguide.

We report on a theoretical and experimental study of the optical coupling between a whispering-gallery type resonator and a waveguide lying on different planes. In contrast to the usual in-plane geometry, the present vertical one is characterized by an oscillatory behavior of the effective coupling as a function of the vertical gap. This behavior manifests itself as oscillations in both the resonance peak waveguide transmission and the mode quality factor. An analytical description based on coupled-mode theory and a two-port beam-splitter model of the waveguide-resonator vertical coupling is developed for arbitrary phase-matching conditions and is successfully used to interpret the experimental observations.

Colloidal crystal thin films grown into corrugated surface templates.

The influence of patterned surfaces on the formation of one- and two-dimensional colloidal crystals is analyzed. We have used the corrugated surface of a digital versatile disc (DVD) for template surface processing. When the sphere diameter is on the order of the groove width of patterned substrates, a rich variety of particle decorations appear. However, if particle size is much larger than template patterns, large domains of particle ordering are formed.

Layering transitions in confined colloidal crystals: the hcp-like phase.

This paper investigates the sequence of morphological transitions in a nearly hard sphere arrangement confined in a wedge cell. A model that shows smooth transitions between the different particle orderings for a small number of layers is proposed. In this model, both the buckling and the (100) hexagonal close packed (hcp) phases are particular cases of a much more general particle arrangement tendency that we call hcp-like ordering. This phase, which does not correspond to any known close packed ordering, is able to adopt packing arrangements commensurate with the cell thickness. More striking, the hcp-like phase adapts itself to the progressive changes of the cell thickness by a smooth change in the interlayer spacing. We present hcp-like orderings up to six layers and a complete sequence of transformations between two and four layers. Finally, a packing model of the transition from two to three layers is also presented.

Faceting and commensurability in crystal structures of colloidal thin films.

This Letter investigates the influence of finite size effects on the particle arrangement of thin film colloidal crystals. A rich variety of crystallographic faceting with large single domain microcrystallites is shown. Optical reflectance experiments together with scanning electron microscopy permit the identification of the crystal symmetry and the facet orientation, as well as the exact number of monolayers. When the cell thickness is not commensurable with a high symmetry layering, particles arrange themselves in a periodic distribution of (111)- and (100)-orientated face centered cubic (fcc) microcrystallites separated by planar defects. These structures can be described as a fcc ordering orientated along a vicinal surface, modified by a periodic distribution of fcc (111) stacking faults.