We demonstrate the use of a near-field probe to map the angular dependence of high-Q whispering-gallery modes in fused-silica microspheres. The mapping is performed by placing a micrometer-sized tip formed on the end of a monomode fiber into the evanescent f ield at the microsphere surface, causing light to be coupled from the microsphere resonance into the fiber guided mode. The light output of the fiber is then measured while the tip is moved to different points on the microsphere surface. We have used this method to investigate the lifting of spherical degeneracy in the system.

For many years twin beams originating from parametric down-converted light beams have aroused great interest and attention in the photonics community. One particular aspect of the twin beams is their peculiar intensity correlation functions, which are related to the coincidence rate of photon pairs. Here we take advantage of the huge bandwidth offered by two-photon absorption in a semiconductor to quantitatively determine correlation functions of twin beams generated by spontaneous parametric down-conversion. Compared with classical incoherent sources, photon extrabunching is unambiguously and precisely measured, originating from exact coincidence between down-converted pairs of photons, travelling in unison. These results strongly establish that two-photon counting in semiconductors is a powerful tool for the absolute measurement of light beam photon correlations at ultrashort timescales.

The thickness of ( 110) and ( 200) sectors in truncated single crystals of linear polyethylene grown from dilute n-octane solution at 95 °C was measured by atomic force microscopy (AFM) in tapping mode. The (110) sector was found to be 1.1 nm thicker than the (200) sector. This can be explained by a somewhat smaller tilt angle of the chains with respect to the basal plane of the crystal. The tilt of the chains in ( 110) and ( 200) sectors, obtained by electron diffraction, is 22°and 30°, respectively. This implies that the length of the stem, i.e., the length of the straight part of the chains between two consecutive folds, is identical in both sectors of the truncated single crystals. The melting of individual LPE truncated single crystals was revisited by AFM. The melting temperatures of the ( 200) and ( 110) sectors are 124.9 and 125.9 ( 0.3 °C, respectively. The lamellar thickness distribution calculated from AFM pictures clearly indicates that reorganization into thicker lamellae takes place during heating. The melting of mats of filtration of truncated LPE single crystals has also been investigated by time-resolved small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The melting curve of LPE single crystals generally exhibits two endotherms. The low and the high melting temperature endotherms are characteristic of single crystals formed from dilute solution and reorganized thicker lamellae during the heating ramp, respectively. Time-resolved SAXS measurements data indicate that two populations of lamellar crystals coexist between 122 and 128 °C. The maximum value observed for the lamellar thickness of the recrystallized material is 30 nm, i.e., twice the initial lamellar thickness of single crystals obtained from dilute solution. There is good agreement between the thicknesses of the crystalline layers obtained from the SAXS data and the values calculated from the Gibbs-Thomson relationship using experimental values of the peak temperature of the two endotherms.

Nonlinear propagation experiments in GaAs photonic crystal waveguides (PCW) were performed, which exhibit a large enhancement of third order nonlinearities, due to light propagation in a slow mode regime, such as two-photon absorption (TPA), optical Kerr effect and refractive index changes due to free-carriers generated by TPA. A theoretical model has been established that shows a very good quantitative agreement with experimental data and demonstrates the important role that the group velocity plays. These observations give a strong insight into the use of PCWs for optical switching devices.

We demonstrate the efficient excitation of high-Q whispering-gallery modes in near-spherical fused-silica microparticles in the size range 60-450 microm by the use of an eroded monomode optical fiber. When the sphere is placed in the evanescent field of the guided fiber mode, light is resonantly coupled from the fiber into the microparticle. We report a broadening of resonance modes and a shift of the resonance central frequency as the coupling strength is increased by reduction of the gap between the sphere and the fiber.

Two-photon counting (TPC) interferometry has been realized by measuring the electrical current due to two-photon absorption in the space charge layer of a semiconductor detector located at the output port of an interferometer. We apply this technique to study the correlation properties of twin beams issued from parametric fluorescence. We describe in details how the different second-order correlation functions (interbeam, intrabeam) can be extracted at the femtosecond timescale from raw data. The values of these correlation functions determined by our experiments are in excellent agreement with theory. More precisely, extrabunching in twin beams is unambiguously demonstrated and theoretically described using two models: a comprehensive multimode quantum optics model and a simpler classical stochastic approach. Given the high brightness of our twin-beam source, both theories yield similar results. Finally, convenient analytical expressions of the correlation functions were derived from both theories, expressions in which we have been able to relate specific terms to accidental and exact coincidences between photons. Two-photon interferometry thus determines to which extent twin photons are twin. This technique should become a useful tool for future quantum optics developments. the delay between the two paths [25, 27, 28]. Since the SHG is a nearly instantaneous process, the temporal resolution of this photon correlation experiment can be as good as a few fs. However, SHG in crystals has a very limited spectral acceptance. Consequently only the coincidences between photons belonging to the same pair [26], i.e. those which are phased-matched, will be detected: As stated by Dayan [26], SHG "post-selects" photons of one pair. Appearing as an advantage in these experiments [24, 25, 27, 28], this exclusive sensitivity to exact coincidences between twin photons actually prevents a complete investigation of the degree of correlation of such photon beams. More precisely, the amount of exact coincidences between twin photons cannot be rated relatively to the amount of accidental coincidences originating from the chaotic nature of each of the beams (the signal and the idler ones). The effect of exact vs accidental coincidence (i.e. the simultaneity of twin photon creation) thus cannot be easily explored. Contrary to SHG or resonant two-photon absorption (TPA) in atoms [23, 38-40], multi-photon processes in semiconductors-occurring between continua of energy-is not limited by phase matching or resonance conditions [41]. In 2009, it was experimentally demonstrated that two-photon counting (TPC) in semiconductor detector allows the study of second-order correlations of broadband chaotic continuous-wave (CW) sources down to the µW level [42] and also permits measurements of second-order correlation and cross correlation functions of twin beams [43, 44]. Moreover, this technique displays a unique capability to quantify the amount of pairs of twin photons compared to accidental coincidences. The purpose of this paper is to present...

We study the photon correlation properties of broadband parametric down-converted light. The measurement of the photon correlation is carried out thanks to a modified Hanbury Brown-Twiss interferometer based on two photon absorption in GaAs detector. Since this method is not affected by the phase matching conditions of the detecting apparatus (so called "final state post-selection"), the detection bandwidth can be extremely large. This is illustrated by studying, with the same apparatus, the degree of second order coherence of parametric light in both degenerate and non-degenerate cases. We show that our experiment is able to determine the coherent as well as the incoherent contributions to the degree of second order coherence of parametric light with a time resolution in the fs range scale.

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