The occurrence of the spontaneous synchronization of coupled oscillators is a well known phenomenon.
For example, in standard lasers, by employing specific devices like acousto-optics modulators, it is possible to couple orthogonal electromagnetic modes, make them synchronously oscillating and emitting short pulses.
On the other hand, in ultra-fast femtosecond oscillators, the mode-locking occurs because of the presence of a saturable absorber; as in the former case no disorder is present.
As reported in Nature Photonics, Marco Leonetti, Claudio Conti, and Cefe Lopez show that Random Lasers, i.e. lasers with disorder, may be driven through a mode-locking regime when changing the spatial distribution of the excited modes. Indeed, in random lasers, modes are naturally coupled because of the fact that the cavity is open and exhibit a sort of condensation above threshold. When tailoring the pump profile, and exciting modes with a different degree of coupling (from the weakly coupled regime for distant modes, to the strongly coupled one for overlapping modes), random lasers can be made oscillating either in an asynchronous or in a mode-locked regime.
Such an effect has strong influence on the spectral shape of the emitted radiation, showing that random lasers can be made strongly tunable and are engineerable.
This is the first evidence of such a mode-locking transition in the presence of disorder, may have a role in the development of novel kind of laser sources, and it is another step for the assessment of complexity in photonic systems.
The picture below shows an example of a laser cluster pumped by a shaped beam (courtesy of Marco Leonetti)
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