Optical parametric oscillation with distributed feedback in cold atoms

There is currently a strong interest in mirrorless lasing systems, in which the electromagnetic feedback is provided either by disorder (multiple scattering in the gain medium) or by order (multiple Bragg reflection). These mechanisms correspond, respectively, to random lasers and photonic crystal lasers. The crossover regime between order and disorder, or correlated disorder, has also been investigated with some success. Here, we report one-dimensional photonic-crystal lasing (that is, distributed feedback lasing) with a cold atom cloud that simultaneously provides both gain and feedback. The atoms are trapped in a one-dimensional lattice, producing a density modulation that creates a strong Bragg reflection with a small angle of incidence. Pumping the atoms with auxiliary beams induces four-wave mixing, which provides parametric gain. The combination of both ingredients generates a mirrorless parametric oscillation with a conical output emission, the apex angle of which is tunable with the lattice periodicity

Density of states governs light scattering in photonic crystals

We describe a smooth transition from (fully ordered) photonic crystal to (fully disordered) photonic glass that enables us to make an accurate measurement of the scattering mean free path in nanostructured media and, in turn, establishes the dominant role of the density of states. We have found one order of magnitude chromatic variation in the scattering mean free path in photonic crystals for just $\sim 3$ shift around the band-gap ($\sim 27$ nm in wavelength)

The mode-locking transition of random lasers

The discovery of the spontaneous mode-locking of lasers(1,2), that is, the self-starting synchronous oscillation of electromagnetic modes in a cavity, has been a milestone of photonics allowing the realization of oscillators delivering ultrashort pulses. This process is so far known to occur only in standard ordered lasers and only in the presence of a specific device (the saturable absorber). We engineer a mode-selective pumping of a random laser formed by a self-assembled cluster of nanometric particles. We show that the random laser can be continuously driven from a configuration exhibiting weakly interacting electromagnetic resonances(4,5) to a regime of collectively oscillating strongly interacting modes(6,7). This phenomenon, which opens the way to the development of a new generation of miniaturized and all-optically controlled light sources, may be explained as the first evidence of spontaneous mode-locking in disordered resonators