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The application of parity-time (PT) symmetry in optics, especially PT-symmetry breaking, has attracted considerable attention as a novel approach to controlling light propagation. Here, we report optical limiting by two coupled optical cavities with a PT-symmetric spectrum of reflectionless modes. The optical limiting is related to broken PT symmetry due to light-induced changes in one of the cavities. Our experimental implementation is a three-mirror resonator of alternating layers of ZnS and cryolite with a PT-symmetric spectral degeneracy of two reflectionless modes. The optical limiting is demonstrated by measurements of single 532-nm 6-ns laser pulses. At fluences below 10 mJ/cm2, the multilayer exhibits a flat-top passband at 532 nm. At higher fluences, laser heating combined with the thermo-optic effect in ZnS leads to cavity detuning and PT-symmetry breaking of the reflectionless modes. As a result, the entire multilayer structure quickly becomes highly reflective, protecting itself from laser-induced damage. The cavity detuning mechanism can differ at much higher limiting thresholds and include nonlinearity., 17 pages, 5 figures

Photonic and plasmonic devices rely on nanoscale control of the local density of optical states (LDOS) in dielectric and metallic environments. The tremendous progress in designing and tailoring the electric LDOS of nano-resonators requires an investigation tool that is able to access the detailed features of the optical localized resonant modes with deep-subwavelength spatial resolution. This scenario has motivated the development of different nanoscale imaging techniques. Here, we prove that a technique involving the combination of scanning near-field optical microscopy with resonant scattering spectroscopy enables imaging the electric LDOS in nano-resonators with outstanding spatial resolution (λ/19) by means of a pure optical method based on light scattering. Using this technique, we investigate the properties of photonic crystal nanocavities, demonstrating that the resonant modes appear as characteristic Fano line shapes, which arise from interference. Therefore, by monitoring the spatial variation of the Fano line shape, we locally measure the phase modulation of the resonant modes without the need of external heterodyne detection. This novel, deepsubwavelength imaging method allows us to access both the intensity and the phase modulation of localized electric fields. Finally, this technique could be implemented on any type of platform, being particularly appealing for those based on non-optically active material, such as silicon, glass, polymers, or metals.

Coorection

A complete control of the coupling between the modes of two photonic crystal microcavities is obtained by using a combination of several local tuning techniques. By a local and controlled modification of one cavity we are able to bring into resonance modes with a different polarization and spatial distribution, producing heteroatomic photonic molecules. Clear anticrossing is observed, denoting the mode hybridization which is in a very good agreement with finite difference time domain calculations.

We propose a time-dependent, spatially periodic photonic structure which is able to shift the carrier frequency of an optical pulse which propagates through it. Taking advantage of the slow group velocity of light in periodic photonic structures, the wavelength conversion process can be performed with an efficiency close to 1 and without affecting the shape and the coherence of the pulse. Quantitative Finite Difference Time Domain simulations are performed for realistic systems with optical parameters of conventional silicon technology. (c) 2006 Optical Society of America.

We study the dynamics of coupled dipolar oscillations in a Fermi-Bose mixture of $^{40}$K and $^{87}$Rb atoms. This low-energy collective mode is strongly affected by the interspecies interactions. Measurements are performed in the dynamical and quantum degenerate regimes and evidence the crucial role of the statistical properties of the mixture. At the onset of quantum degeneracy, we investigate the role of Pauli blocking and superfluidity for K and Rb atoms, respectively, resulting in a change in the collisional interactions., 13 pages, 6 figures. Sudmitted to the Journal of Optics B special issue on the 7th International Workshop on Atom Optics and Interferometry

We report on the achievement of simultaneous quantum degeneracy in a mixed gas of fermionic ^40 K and bosonic ^87 Rb. Potassium is cooled to 0.3 times the Fermi temperature by means of an efficient thermalization with evaporatively cooled rubidium. Direct measurement of the collisional cross-section confirms a large interspecies attraction. This interaction is shown to affect the expansion of the Bose-Einstein condensate released form the magnetic trap, where it is immersed in the Fermi sea., 5 pages, 4 figures, replaced one figure plus some changes