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Background
The question of threshold in nanolaser has been debated since the early 1990's, when it was recognized, on the basis of rate equations, that the "break" in the input-output laser characteristics (or the "S-shaped" curve, in logarithmic scale) would disappear to give rise to a strictly linear dependence (what was called the "thresholdless laser").
Since the "kink" is a clear and useful indicator of threshold crossing -- i.e., the transition from incoherent to coherent emission -- fundamental and practical questions arose immediately: how can one know when the laser operates above threshold? (in other words: when is its output phase coherent and when are its fluctuations reduced)?
Much work has been devoted to answering these questions, with very interesting fundamental contributions (e.g., P.R. Rice and H.J. Carmichael, Photon statistics of a cavity-QED laser: A comment on the laser-phase-transition analogy, Phys. Rev. A 50, 4318 (1994)), but from a practical point of view the questions still stand.
In the meantime, such devices have been built [M. Khajavikhan et al., Thresholdless nanoscale coaxial lasers, Nature 482, 204 (2012)] (technology is far ahead of fundamental physics in this domain!) but the only evidence for "lasing" rests on linewidth narrowing.
Is there a nanolaser threshold?
Innovative approaches are necessary to find an operative and practical solution for determining the nature of the emission and the laser "threshold" (which should be probably seen as a threshold region, with different, evolving features as a function of pump).
In collaboration with the Laboratoire de Photonique et de Nanostructures (Marcoussis, France), we have developed one first approach based on a phase-space technique (cf. illustration below) which can be used for pulsed pumping [1].
Below threshold Above threshold
In our group, we are currently working to gain further insight in the dynamical evolution of a laser in the threshold region, both numerically (nano- and meso-lasers) and experimentally (meso-lasers).
1] X. Hachair et al., Identification of the stimulated-emission threshold in high-β nanoscale lasers through phase-space reconstruction, Phys. Rev. A 83, 053836 (2011). Preprint on http://arxiv.org/pdf/1005.1538v2.pdf
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