Two-color second-order correlations of the light scattered near-resonantly by a quantum dot were measured by means of spectrally filtered coincidence detection. The effects of filter frequency and bandwidth were studied under monochromatic laser excitation, and a complete two-photon spectrum was reconstructed. The two-photon spectrum exhibits a rich structure associated with both real and virtual two-photon transitions down the “dressed states” ladder. Photon pairs generated via virtual transitions are found to violate the Cauchy-Schwarz inequality by a factor of 60. Our experiments are well described by the theoretical expressions obtained by del Valle et al. [ Phys. Rev. Lett. 109, 183601 (2012)] via time- and normally-ordered correlation functions.

We analyze the light scattered by a single InAs quantum dot interacting with a resonant continuous-wave laser. High-resolution spectra reveal clear distinctions between coherent and incoherent scattering, with the laser intensity spanning more than four orders of magnitude. We find that the fraction of coherently scattered photons can approach unity under sufficiently weak or detuned excitation, ruling out pure dephasing as a relevant decoherence mechanism. We show how spectral diffusion shapes spectra, correlation functions, and phase coherence, concealing the ideal radiatively broadened two-level system described by Mollow.

We report a Franson interferometry experiment based on correlated photon pairs generated via frequency-filtered scattered light from a near-resonantly driven two-level semiconductor quantum dot. In contrast to spontaneous parametric down-conversion and four-wave mixing, this approach can produce single pairs of correlated photons. We have measured a Franson visibility as high as 66%, which goes beyond the classical limit of 50% and approaches the limit of violation of Bell’s inequalities (70.7%).

We demonstrate the Purcell spontaneous-emission enhancement effect for Raman scattering from gases in a $\sim$10-$\mu$m-long Fabry-Pérot microcavity with a mode volume of $\sim$100 $\mu$m${}^3$ and a peak finesse of 50 000. The cavity increases the pump laser intensity by resonant recirculation while simultaneously enhancing the rate of Raman scattering into a separate cavity mode by modification of the density of states of the intracavity electromagnetic vacuum field. The result is an overall enhancement of $\sim$10${}^7$ compared to free-space emission into the same solid angle as the cavity, enabling trace gas sensing requiring only minute sample quantities.

A simple but highly effective method for enhancement of spontaneous Raman scattering from gases is described. It employs a multimode blue laser diode that receives feedback from a near-concentric bidirectional multipass cavity in such a way as to generate a circulating power on the order of 100 W for a sample volume of 10 m m 3 . The feedback, provided via a volume Bragg grating, reduces the laser bandwidth to 4 c m − 1 . Spectra of spontaneous Raman scattering from ambient atmospheric air, detected collinearly with the pump, were recorded with a limit of detection below 1 ppm. The performance of the setup is further illustrated through detection of acetone, methane, and isotopic carbon dioxide in breath.

We report on resonant light scattering experiments in which a single InAs quantum dot is exposed to two independently tunable continuous-wave lasers while the scattered light is analyzed background free, with high spectral and temporal resolution. In contrast to the well-known monochromatic case exhibiting Rabi oscillations, in this bichromatic case the additional field introduces oscillations at half the difference of the laser frequencies and harmonics thereof, persisting beyond the natural lifetime. The familiar “dressed-states” ladder now contains an infinite number of levels per manifold. With suitable averaging, the rich resulting spectra and second-order correlations can be reproduced accurately using the Bloch equations and the quantum regression theorem.DOI: http://dx.doi.org/10.1103/PhysRevB.89.155305Published 7 April 2014Received 24 December 2013Revised 25 March 2014©2014 American Physical Society