Large static tuning of narrow-beam terahertz plasmonic lasers operating   at 78 K
  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. 
Chongzhao Wu, Yuan Jin, John L. Reno, and Sushil Kumar
physics.optics