Lasers are ubiquitous for information storage, processing, communications, sensing, biological research, and medical applications [1]. To decrease their energy and materials usage, a key quest is to miniaturize lasers down to nanocavities [2]. Obtaining the smallest mode volumes demands plasmonic nanocavities, but for these, gain comes from only single or few emitters. Until now, lasing in such devices was unobtainable due to low gain and high cavity losses [3]. Here, we demonstrate a plasmonic nanolaser approaching the single-molecule emitter regime. The lasing transition significantly broadens, and depends on the number of molecules and their individual locations. We show this can be understood by developing a theoretical approach [4] extending previous weak-coupling theories [5]. Our work paves the way for developing nanolaser applications [2, 6, 7] as well as fundamental studies at the limit of few emitters [5, 8, 9].
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