定义
阈值功率几乎为零的激光器。
无阈值激光器是阈值泵浦功率基本上为零的激光器。这种装置是由小林等人在1982年提出的[1]。这个概念的一个基本点是，自发发射被迫主要发生在激光谐振器定义的空间模式中（而通常大部分进入其他辐射模式）。这可以通过（微观上小的）增益介质周围的微腔来实现，该微腔修改了增益介质环境的模式结构。即使有几种合适的模式，如果激光模式的Q因子最高，则发射到激光模式可能会占主导地位。
通过实验，已经证明了一些基于该原理的阈值功率非常低的激光器，例如阈值电流仅为36 μA [4]的垂直腔面发射激光器和阈值泵浦功率为1.2 μW的光子晶体纳米激光器[8]。另一种技术方法是单原子激光器[7]，它在2003年被证明，并且确实表现出零阈值泵浦功率。在这种情况下，更微妙的量子光学现象是无阈值行为的背后。
参考文献
[1] T. Kobayashi et al., “Novel-type lasers, emitting devices, and functional optical devices by controlling spontaneous emission”, presented at the 46th Fall Meeting of the Japanese Applied Physics Society, 1982, paper 29a-B-6 (in Japanese)
[2] E. Yablonovitch and T. J. Gmitter, “Inhibited spontaneous emission in solid state physics and electronics”, Phys. Rev. Lett. 58 (20), 2059 (1987), doi:10.1103/PhysRevLett.58.2059
[3] H. Yokoyama and S. D. Brorson, “Rate equation analysis of microcavity lasers”, J. Appl. Phys. 66 (10), 4801 (1989), doi:10.1063/1.343793
[4] D. L. Huffaker and D. G. Deppe, “Improved performance of oxide-confined vertical-cavity surface-emitting lasers using a tunnel injection active region”, Appl. Phys. Lett. 71, 1449 (1997), doi:10.1063/1.119933
[5] 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), doi:10.1109/50.802999
[6] I. Protsenko et al., “Quantum theory of a thresholdless laser”, Phys. Rev. A 59 (2), 1667 (1999), doi:10.1103/PhysRevA.59.1667
[7] J. McKeever et al., “Experimental realization of a one-atom laser in the regime of strong coupling”, Nature 425, 268 (2003), doi:10.1038/nature01974
[8] K. Nozaki et al., “Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser”, Opt. Express 15 (12), 7506 (2007), doi:10.1364/OE.15.007506
