Shaken Granular Lasers

Granular materials have been studied for decades, driven by industrial and technological applications. These very simple systems, composed of agglomerations of mesoscopic particles, are characterized, in specific regimes, by a large number of metastable states and an extreme sensitivity (e.g., in sound transmission) to the arrangement of grains; they are not substantially affected by thermal phenomena, but can be controlled by mechanical solicitations. Laser emission from shaken granular matter is so far unexplored. Here we provide experimental evidence that laser emission can be affected and controlled by the status of the motion of the granular material; we also find that competitive random lasers can be observed. We hence demonstrate the potentialities of gravity-affected moving disordered materials for optical applications, and open the road to a variety of novel interdisciplinary investigations, involving modern statistical mechanics and disordered photonics.

Figure 1

Experimental setup: (a) sketch of the granular sample placed on a vibrating woofer and a vertical translational stage; the light beam is also shown. (b) Transmission of a 1064 nm continuous-wave beam for different amplitudes $a$ of the driving force; above a critical amplitude, the transmission is independent on the value of $a$ for an increasing range in $z$, denoting the transition to a gaseous region.(c) As in (b) with vertical log scale.

Figure 2

Spectra of shaken granular lasers. (a)–(f) Emission at pumping energy of 1.6 mJ (left column), and 3.7 mJ (right column), for the bottom region [$z=3\mathrm{mm}$, panels (a),(b)], the central region [$z=7\mathrm{mm}$, panels (c),(d)] and the top region [$z=13\mathrm{mm}$, panels (e),(f)]. (g) Single-shot spectra at energy 5.5 mJ, $z=13\mathrm{mm}$ liquid region, and $a=0.2$. The insets show a sketch of the sample and the arrow indicates the position of the beam with respect to the bottom. The spectra are arbitrarily shifted in the vertical direction.

Figure 3

Competition of random lasers. In the liquid region $z=13\mathrm{mm}$, peak intensity for the RL at 600 nm (a) and at 620 nm (b) versus the oscillation amplitude $a$; the ratio of peak at 620 nm and that at 600 nm is shown in panel (c).