  The discovery of the spontaneous mode-locking of lasers, i.e., the
synchronous oscillation of electromagnetic modes in a cavity, has been a
milestone of photonics allowing the realization of oscillators delivering
ultra-short pulses. This process is so far known to occur only in standard
ordered lasers with meter size length and only in the presence of a specific
device (the saturable absorber). Here we demonstrate that mode-locking can
spontaneously arise also in random lasers composed by micronsized laser
resonances dwelling in intrinsically disordered, self-assembled clusters of
nanometer-sized particles. Moreover by engineering a novel mode-selective
pumping mechanism we show that it is possible to continuously drive the system
from a configuration in which the various excited electromagnetic modes
oscillate in the form of several, weakly interacting, resonances to a
collective strongly interacting regime. By realizing the smallest mode-locking
device ever fabricated, we open the way to novel generation of miniaturized and
all-optically controlled light sources.
