Harmonic generation from silicon membranes at visible and ultraviolet wavelengths.

Nonlinear silicon photonics offers unique abilities to generate, manipulate and detect optical signals in nano-devices, with applications based on field localization and large third order nonlinearity. However, at the nanoscale, inefficient nonlinear processes, absorption, and the lack of realistic models limit the nano-engineering of silicon. Here we report measurements of second and third harmonic generation from undoped silicon membranes. Using experimental results and simulations we identify the effective mass of valence electrons, which determines second harmonic generation efficiency, and oscillator parameters that control third order processes. We can then accurately predict the nonlinear optical properties of complex structures, without introducing and artificially separating the effective χ(2) into surface and volume contributions, and by simultaneously including effects of linear and nonlinear dispersions. Our results suggest that judicious exploitation of the nonlinear dispersion of ordinary semiconductors can provide reasonable nonlinear efficiencies and transformational device physics well into the UV range.

Harmonic generation in the opaque region of GaAs: the role of the surface and magnetic nonlinearities.

Phase-locked second and third harmonic generation in the opaque region of a GaAs wafer is experimentally observed and analyzed both in transmission and reflection. These harmonic components, which are generated close to the surface, can propagate through an opaque material as long as the pump is tuned to a region of transparency or semitransparency and correspond to the inhomogeneous solutions of Maxwell's equations with nonlinear polarization sources. We show that measurement of the angular and polarization dependence of the observed harmonic components allows one to infer the different nonlinear mechanisms that trigger these processes, including not only the bulk nonlinearity but also the surface and magnetic Lorentz contributions, which usually are either hidden by the bulk contributions or assumed to be negligible. The experimental results are compared with a detailed numerical model that takes into account these different effects, including for the first time combined linear and nonlinear material dispersions in a nonlinear Lorentz oscillator model of the bulk nonlinearities. Our results suggest that the intensity of the second harmonic signal generated by the surface can be more intense than the signal generated by the bulk. These findings have significant repercussions and are consequential in nanoscale systems, which are usually investigated using only dispersionless bulk nonlinearities, with near-complete disregard of surface and magnetic contributions and their microscopic origins.

Comparative analysis of ferroelectric domain statistics via nonlinear diffraction in random nonlinear materials.

We present an indirect, non-destructive optical method for domain statistic characterization in disordered nonlinear crystals having homogeneous refractive index and spatially random distribution of ferroelectric domains. This method relies on the analysis of the wave-dependent spatial distribution of the second harmonic, in the plane perpendicular to the optical axis in combination with numerical simulations. We apply this technique to the characterization of two different media, Calcium Barium Niobate and Strontium Barium Niobate, with drastically different statistical distributions of ferroelectric domains.

18F-FDG PET/CT in the detection of a primary radiation-induced metastatic angiosarcoma / 18F-FDG PET/TC en la detección de angiosarcoma primario radioinducido metastásico

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Flat focusing mirror.

The control of spatial propagation properties of narrow light beams such as divergence, focusing or imaging are main objectives in optics and photonics. In this letter, we propose and demonstrate experimentally a flat focusing mirror, based on an especially designed dielectric structure without any optical axis. More generally, it also enables imaging any light pattern in reflection. The flat focusing mirror with a transversal invariance can largely increase the applicability of structured photonic materials for light beam propagation control in small-dimension photonic circuits.

Calcificaciones pulmonares metastásicas en un hiperparatiroidismo terciario / Metastatic pulmonary calcifications in tertiary hyperparathyroidism

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Enhanced efficiency of the second harmonic inhomogeneous component in an opaque cavity.

In this Letter, we experimentally demonstrate the enhancement of the inhomogeneous second harmonic conversion in the opaque region of a GaAs cavity with efficiencies of the order of 0.1% at 612 nm, using 3 ps pump pulses having peak intensities of the order of 10 MW/cm(2). We show that the conversion efficiency of the inhomogeneous, phase-locked second harmonic component is a quadratic function of the cavity factor Q.

The role of ferroelectric domain structure in second harmonic generation in random quadratic media.

We study theoretically and numerically the second harmonic generation in a nonlinear crystal with random distribution of ferroelectric domains. We show that the specific features of disordered domain structure greatly affect the emission pattern of the generated harmonics. This phenomena can be used to characterize the degree of disorder in nonlinear photonic structures.

Third-harmonic generation via broadband cascading in disordered quadratic nonlinear media.

We study parametric frequency conversion in quadratic nonlinear media with disordered ferroelectric domains. We demonstrate that disorder allows realizing broadband third-harmonic generation via cascading of two second-order quasi-phase matched nonlinear processes. We analyze both spatial and polarization properties of the emitted radiation and find the results in agreement with our theoretical predictions.

Cavity solitons in two-level lasers with dense amplifying medium.

Local-field effects are known to induce bistability in dense optical media. We examine theoretically whether this property is preserved in broad-area cavities, and show that bistability between the homogeneous lasing and nonlasing states of the system persists provided a Fourier filtering technique is used to prevent off-axis emission. The resulting bistability gives rise to spatial light localization in the form of cavity solitons, which exhibit a particularly large degree of plasticity as a function of the characteristics of the addressing beam. This is the simplest laser able to sustain cavity solitons.

Two-photon cavity solitons in active optical media.

We show that broad-area cascade lasers with no absorbing intracavity elements support the spontaneous formation of two-dimensional bright localized structures in a dark background. These cavity solitons consist of islands of two-photon emission embedded in a background of single-photon emission. We discuss the mechanisms through which these structures are formed and interact, along with their properties and stability.

Amplification of near-resonant signals via stochastic resonance in a chaotic CO2 laser

We show experimentally and numerically that a chaotic CO2 laser with modulated losses operating in the region of an intermittency resulting from the band-merging crisis can serve as an amplifier of near-resonant signals, i.e., signals with a frequency close to the first subharmonic frequency, via deterministic stochastic resonance. The mechanism underlying stochastic resonance in this case is a synchronization of the random switching events between two chaotic repellers after the band-merging crisis with near-resonant signals at the detuning frequency. We demonstrate experimentally that the gain factor in chaos is larger than near the first period-doubling bifurcation by a factor of 2. Numerical results obtained in a two-level rate-equation model are in good agreement with the experimental ones.

Suppression of radiation in a momentum-nonconserving nonlinear interaction.

We have considered the quadratic nonlinear radiation from a thin dipole sheet in front of a mirror. Radiation at the second-harmonic (SH) frequency on incidence of a fundamental field can be inhibited or enhanced independently of the SH field intensity stored between the nonlinear layer and the mirror. We have shown that this apparent contradiction can be fully understood only if the quadratic nonlinear interaction includes terms with a momentum mismatch equal to the magnitude of the SH field wave vector, such as the term that accounts for transfer of energy from the reflected SH field back to the incident fundamental.

Observation of azimuthal modulational instability and formation of patterns of optical solitons in a quadratic nonlinear crystal.

We report what is believed to be the first experimental demonstration of the azimuthal self-breaking of intense beams containing a vortex phase dislocation into sets of optical spatial solitons in a quadratic nonlinear material. The observations were performed in a KTP crystal.