Curcumin is the main constituent of curry. In its ground state it shows chemo-preventive, chemo-therapeutic and anti-inflammatory effects. For its immunostimulating action it has been considered for the development of drugs suitable for treating AIDS and cystic fibrosis. Further biological action is induced in curcumin by photoactivation: in suitable environmental conditions electronically excited curcumin can act as a singlet oxygen generator. Moreover, cytotoxicity is enhanced by light exposure and antibacterial effects are photosensitized. This work is aimed to understand the photobiological action of curcumin by elucidating the deactivation mechanisms of its first excited singlet state. In particular we find evidence of the role of tautomerization in the excited state by measuring fluorescence lifetimes and quantum yields for such compound dissolved in solvents of different polarity and H-bonding capability. Degradation quantum yield and singlet oxygen generation efficiency were also measured in acetonitrile and methanol. The results emphasize the strong dependence of the deactivation processes from the environment. The deactivation phenomenology can be fully explained by postulating intramolecular proton transfer in thecisenol conformer to be the leading non-radiative deactivation pathway.

We demonstrate sub-shot-noise photon-number correlations in a (temporal) multimode mesoscopic (∼ 10 3 detected photons) twin-beam produced by ps-pulsed spontaneous non-degenerate parametric downconversion. We have separately detected the signal and idler distributions of photons collected in twin coherence areas and found that the variance of the photon-count difference goes below the shot-noise limit by 3.25 dB. The number of temporal modes contained in the twin-beam, as well as the size of the twin coherence areas, depends on the pump intensity. Our scheme is based on spontaneous downconversion and thus does not suffer from limitations due to the finite gain of the parametric process. Twin-beams are also used to demonstrate the conditional preparation of a nonclassical (sub-Poissonian) state.

Experimental reconstructions of photon number distributions of both continuous-wave and pulsed light beams are reported. Our scheme is based on on/off avalanche photodetection assisted by maximum-likelihood estimation and does not involve photon counting. Reconstructions of the distribution for both semiclassical and quantum states of light are reported for single-mode as well as for multimode beams.

We address the generation of fully inseparable three-mode entangled states of radiation by interlinked nonlinear interactions in χ (2) media. We show how three-mode entanglement can be used to realize symmetric and asymmetric telecloning machines, which achieve optimal fidelity for coherent states. An experimental implementation involving a single nonlinear crystal where the two interactions take place simultaneously is suggested. Preliminary experimental results showing the feasibility and the effectiveness of the interaction scheme with seeded crystal are also presented.

Avalanche p–n photodiodes with uniform breakdown over the junction area are known to be capable of single-photon detection. An experimental study has been performed on the temporal resolution of these single-photon avalanche diodes (SPADs) in measurements of the shape of ultrashort light pulses. By using an active-quenching circuit, pulsed operation of the device has been obtained in accurately controlled conditions, avoiding spurious effects met in previous passive-quenching circuits. Laser pulses with durations down to 150 ps FWHM have been used; the results obtained show that the resolution is remarkably better than this value. Performances and limitations of SPADs are discussed; temporal resolutions of a few tens of picoseconds may be expected.

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We demonstrate the possibility of a self-consistent characterization of the photon-number statistics of a light field by using photoemissive detectors with internal gain simply endowed with linear input/output responses. The method can be applied to both microscopic and mesoscopic photon-number regimes. The detectors must operate in the linear range without need of photon-counting capabilities.Comment: To be published in "Journal of Modern Optics

We present a description of the operation of a multi-pixel detector in the presence of non-negligible dark-count and cross-talk effects. We apply the model to devise self-consistent calibration strategies to be performed on the very light under investigation.Several concepts and technologies have been proposed that lead to the development of detectors such as visible-light photon counters (VLPC) [18], superconductive transition edge sensors (TES) [19], time-multiplexed detectors [20][21][22], hybrid photodetectors (HPD) [23,24] and Silicon photomultipliers (SiPM) [25]. Irrespective of the concept and design features, these detectors may in general be classified in terms of photon detection efficiency, spectral response, time development of the signal, dead time, and notable photon number resolving capability. As of today, the ideal detector has yet to appear and the optimal choice is application specific. This paper focuses on SiPMs, detectors featuring unique characteristics that are achieved by a rapidly evolving technology. Silicon photomultipliers consist of a high density (by now limited to ∼ 2000 cells/mm 2 ) matrix of diodes with a common output. Each diode (or cell) is operated in a limited Geiger-Mueller (GM) regime, in order to achieve gains at the level of 10 6 . Quenching mechanisms are implemented to avoid establishing self-sustaining discharges. These detectors are sensitive to single photons triggering GM avalanches and can be endowed with a dynamic range well above 100 photons/burst. The photon detection efficiency (PDE) depends on the sensor design and specification, but it may well exceed 60%. Moreover, SiPM are genuine photon-number resolving detectors in that they measure light intensity simply by the number of fired diodes. Compactness, robustness, low cost, low operating voltage, and power consumption are also added values against traditional photodetectors. On the other hand, SiPMs are affected by significant dark count rates (DCR), associated to cells fired by thermally generated charge carriers. Moreover, the GM avalanche development is known to be associated to the generation of photons [26], which may in turn trigger secondary avalanches and result in relevant cross-talk. Whether DCR and cross-talk may be directly measured, it is clear that they are folded in the detector response to any signal and need to be modelled to properly assess the statistical properties of the light field being investigated. This paper reports the experimental validation of two models, on the way to a self-consistent characterization of the SiPM response. Experimental set-upThe detector response to a weak light field is shown in Fig. 1(a), featuring the sensor output signal after a high-bandwidth amplifier with a gain of 50. The different bands in the image, obtained in persistency mode, correspond to samples with different numbers of triggered cells, i.e. different numbers of detected photons. The photon-number resolving properties are also clear in Fig. 1(b) that shows the corresponding spectrum as obtained ...