Graphene Nanogap Junctions & Surface Plasmon Lasers

In order to make a laser, you need three ingredients: gain and feedback. Gain is typically achieved by (electronically or optically) pumping a gain medium, which is this case is a simple optically pumped semiconductor below a thin-film layer of gold. However, what makes this laser special is the way in which it achieves its feedback. Instead of relying on a light-based cavity, this laser creates feedback by relying on the backscattering of surface plasmons on metal-hole arrays. Simply put, the surface plasmons are simultaneously excited with the semiconducting gain layer and will propagate along the metal-semiconductor interface. The metal layer, however, contains two square arrays of patterned holes, which act as scattering points to the surface plasmons. In fact, with an appropriately chosen geometry of the holes, the surface plasmons can be reflected specularly from these holes. It is the back-reflection between these two hole arrays, spaced 50 um apart, that provides the feedback that guarantees the lasing process in this surface plasmon laser.  The hole arrays therefore effectively act as distributed Bragg reflectors (DBRs) for this 'double-slit' surface plasmon laser.

E.W. de Vos et al., Surface plasmon laser with two hole arrays as cavity mirrors. Optica 6, 92 (2019)