  A new tuning mechanism is demonstrated for single-mode metal-clad plasmonic
lasers, in which refractive-index of the laser's surrounding medium affects the
resonant-cavity mode in the same vein as refractive-index of gain medium inside
the cavity. Reversible, continuous, and mode-hop-free tuning of ~57GHz is
realized for single-mode narrow-beam terahertz plasmonic quantum-cascade lasers
(QCLs), which is demonstrated at much more practical temperature of 78K. The
tuning is based on post-process deposition/etching of a dielectric
(Silicon-dioxide) on a QCL chip that has already been soldered and wire-bonded
onto a copper mount. This is a considerably larger tuning range compared to
previously reported results for terahertz QCLs with directional far-field
radiation patterns. The key enabling mechanism for tuning is a recently
developed antenna-feedback scheme for plasmonic lasers, which leads to
generation of hybrid surface-plasmon-polaritons propagating outside the cavity
of the laser with a large spatial extent. The effect of dielectric deposition
on QCL's characteristics is investigated in detail including that on maximum
operating temperature, peak output power and far-field radiation patterns.
Single-lobed beam with low divergence (<7deg) is maintained through the tuning
range. The antenna-feedback scheme is ideally suited for modulation of
plasmonic lasers and their sensing applications due to the sensitive dependence
of spectral and radiative properties of the laser on its surrounding medium.
