The interaction of an ensemble of atoms with common vacuum modes may lead to an enhanced emission into these modes. This phenomenon, known as superradiance, highlights the coherent nature of spontaneous emission, resulting in macroscopic entangled states in mundane situations. The complexity of the typical observations of superradiance, however, masks its quantum nature, allowing alternative classical interpretations. Here we stress how this picture changed with the implementation ten years ago of a new process for single-photon generation from atomic ensembles. We present then the last piece of evidence for the superradiant nature of such a process, reporting the observation of an accelerated emission of the photon with a rate that may be tuned by controllably changing the number of atoms in the ensemble. We hope such an investigation will help open up a new bottom-up approach to the study of superradiance.

We report on the storage of orbital angular momenta (OAM) of light via the phenomenon of coherent population oscillation (CPO) in cold cesium atoms. The experiment is performed using a delayed four wave mixing configuration, where the transverse optical information of a probe field carrying OAM associated with its azimuthal phase dependence is stored in the CPO of Zeeman sublevels of the hyperfine transition F=3→F'=2 of the cesium D2 line. We also demonstrate experimentally the simultaneous storage and retrieval of different OAM states propagating along different directions in space, leading to algebraic operations with OAM and, therefore, opening the possibility of multiplexing OAM states.

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We demonstrate and interpret a technique of laser-induced formation of thin metallic films using alkali atoms on the window of a dense-vapour cell. We show that this intriguing photo-stimulated process originates from the adsorption of Cs atoms via the neutralisation of Cs + ions by substrate electrons. The Cs + ions are produced via two-photon absorption by excited Cs atoms very close to the surface, which enables the transfer of the laser spatial intensity profile to the film thickness. An initial decrease of the surface work function is required to guarantee Cs + neutralisation and results in a threshold in the vapour density. This understanding of the film growth mechanism may facilitate the development of new techniques of laser-controlled lithography, starting from thermal vapours.

We present a new experimental procedure, based on the use of disordered light patterns, to improve the efficiency of random lasing and stimulated Raman scattering (SRS), which appear to be simultaneous and coupled at high intensities. A dual-regime random laser (RL) was achieved by using silver nanowires as scattering elements to induce an optical feedback in cavity-free lasers that use a Raman dye as the gain medium. It is shown that in the SRS-RL coupling regime, the random modulation of the pump intensity profile promotes efficient amplification of coherent Stokes photons in small randomly distributed regions, which simulate the formation of transverse laser modes. As a consequence, the spatial coherence of radiation emitted by the SRS-RL source can be tuned by managing the speckle contrast and the random wavevector distribution, corresponding to the parameters that define the degree of disorder introduced in the light pulses that pump the random lasing medium. Measurements of the 2D spatial intensitycorrelation function of the emitted fields were performed to evaluate the efficiency of disordered light to control the spatial coherence of SRS-RL.