A thresholdless laser is a laser where the threshold pump power is essentially zero. Such a device was proposed by Kobayashi et al. in 1982 . An essential point of that concept is that the spontaneous emission is forced to occur primarily into the spatial mode defined by the laser resonator (while normally most of it goes into other radiation modes). This is possible with a microcavity around the (microscopically small) gain medium, which modifies the mode structure of the environment of the gain medium. Even if there are several suitable modes, emission into the laser mode can be dominating if that mode has the highest Q factor.
Experimentally, some lasers with very low threshold powers based on this principle have been demonstrated, e.g. a vertical cavity surface-emitting laser with a threshold current of only 36 μA  and a photonic crystal nanolaser with a threshold pump power of 1.2 μW . Another technical approach is the single-atom laser , which was demonstrated in 2003, and indeed exhibited a zero threshold pump power. In that case, even more subtle quantum optics phenomena are behind the thresholdless behavior.
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|||J. M. Gérard and B. Gayral, “Strong Purcell effect for InAs quantum boxes in three-dimensional solid-state microcavities”, IEEE J. Lightwave Technol. 17 (11), 2089 (1999); https://doi.org/10.1109/50.802999|
|||I. Protsenko et al., “Quantum theory of a thresholdless laser”, Phys. Rev. A 59 (2), 1667 (1999); https://doi.org/10.1103/PhysRevA.59.1667|
|||J. McKeever et al., “Experimental realization of a one-atom laser in the regime of strong coupling”, Nature 425, 268 (2003); https://doi.org/10.1038/nature01974|
|||K. Nozaki et al., “Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser”, Opt. Express 15 (12), 7506 (2007); https://doi.org/10.1364/OE.15.007506|
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