Abstract
A new tuning mechanism is demonstrated for single-mode metal-clad plasmonic lasers, in which the refractive-index of the laser’s surrounding medium affects the resonant-cavity mode in the same vein as the refractive-index of gain medium inside the cavity. Reversible, continuous, and mode-hop-free tuning of ∼57 GHz is realized for single-mode narrow-beam terahertz plasmonic quantum-cascade lasers (QCLs), which is demonstrated at a much more practical temperature of 78 K. 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 the 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 (<7°) 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.
Highlights
Plasmonic lasers that utilize metal-clad cavities confine electromagnetic energy in the form of surface-plasmon-polaritons (SPPs) at subwavelength dimensions
A static-tuning technique based on post-process dielectric deposition was recently demonstrated for terahertz quantum-cascade lasers (QCLs) with third-order DFB operating at ∼10 K with a tuning of ∼5 GHz.[25] (The tuning could be extended while the QCL is in operation, but only temporarily, by the deposition of solid nitrogen through multiple-cycle condensation to ∼25 GHz in a liquid-helium cryostat.) Along similar lines, we demonstrate here a greatly enhanced continuous tuning range of ∼57 GHz for singlemode QCLs emitting at 2.8 THz and operating at 78 K, achieved by deposition of silicon-dioxide post-fabrication, and with significantly improved beam profiles
In contrast to all previous tuning results for terahertz QCLs that have been demonstrated at temperatures close to that of liquid-helium, this work achieves large tuning for QCLs operating in a liquid-nitrogen cooled dewar since the tuning mechanism does not impact the gain of the QCL sensitively
Summary
Plasmonic lasers that utilize metal-clad cavities confine electromagnetic energy in the form of surface-plasmon-polaritons (SPPs) at subwavelength dimensions. (Received 24 July 2016; accepted 30 November 2016; published online 19 December 2016)
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