"Phase transitions" in small systems: Why standard threshold definitions fail for nanolasers

Since the development of micro- and nanolasers, the question of laser threshold has been subject to debate. Different definitions have been used to try and establish its occurrence, often encountering major obstacles. We examine a set of common physical definitions which we apply to measurements taken in a micro-VCSEL. Their predictions not only clearly disagree, pointing to different pump values at which the laser should cross threshold, but they also correspond to autocorrelation values which demonstrate very low field coherence. A topological analysis of the rate equations, with average spontaneous emission added to the lasing mode, clearly identifies the contradictions and explains the origin of the discrepancies. Additional considerations help understanding the failure of the approach and highlight the path towards a unique and general definition of threshold in all lasers, irrespective of their sizes. A critical scrutiny of the assumptions made in the rate equations with spontaneous emission illustrates their strength and weaknesses and better defines the bounds within which their predictions hold. We remark in the conclusions how the main results of this paper could hold for other small systems.