  Here, we introduce and experimentally demonstrate optical amplification and
stimulated emission from a single optically pumped molecular tunneling junction
of a scanning tunneling microscope. The gap between a sharp gold tip and a flat
gold substrate covered with a self-assembled monolayer of
5-chloro-2-mercaptobenzothiazole molecules forms an extremely small optical
gain medium. When electrons tunnel from the molecules highest occupied
molecular orbital to the tip, holes are left behind. These can be repopulated
by hot electrons induced by the laser-driven plasmon oscillation on the metal
surfaces enclosing the cavity. Solving the laser-rate equations for this system
shows that the repopulation process can be efficiently stimulated by the gap
modes near field, TERS scattering from neighboring molecules acting as an
optical seed. Our results demonstrate how optical enhancement inside the
plasmonic cavity can be further increased by a stronger localization via
tunneling through molecules. We anticipate that stimulated emission from an STM
junction will advance our fundamental understanding of quantum plasmonics and
lead to new analytical applications. Furthermore, this concept represents the
basis for novel ultra-small, fast, optically and electronically switchable
devices and could find applications in high-speed signal processing and optical
telecommunications.
