Terahertz plasmonic laser radiating in an ultra-narrow beam

TL;DR

This paper introduces a novel antenna-feedback technique for plasmonic lasers that achieves highly directional terahertz emission with the narrowest beam reported, enhancing far-field coupling and practical applicability.

Contribution

It presents a new antenna-feedback scheme for plasmonic lasers, enabling single-mode operation with ultra-narrow beams and improved far-field radiation coupling, demonstrated in terahertz quantum-cascade lasers.

Findings

Achieved beam divergence as small as 4x4 degrees in terahertz QCLs.

Demonstrated single-mode operation with enhanced far-field radiation.

Broken the integer-limit of periodicity for radiative-field enhancement.

Abstract

Plasmonic lasers (spasers) generate coherent surface-plasmon-polaritons (SPPs) and could be realized at subwavelength dimensions in metallic cavities for applications in nanoscale optics. Plasmonic cavities are also utilized for terahertz quantum-cascade lasers (QCLs), which are the brightest available solid-state sources of terahertz radiation. A long standing challenge for spasers is their poor coupling to the far-field radiation. Unlike conventional lasers that could produce directional beams, spasers have highly divergent radiation patterns due to their subwavelength apertures. Here, we theoretically and experimentally demonstrate a new technique for implementing distributed-feedback (DFB) that is distinct from any other previously utilized DFB schemes for semiconductor lasers. The so-termed antenna-feedback scheme leads to single-mode operation in plasmonic lasers, couples the resonant SPP mode to a highly directional far-field radiation pattern, and integrates hybrid SPPs in surrounding medium into the operation of the DFB lasers. Experimentally, the antenna-feedback method, which does not require the phase matching to a well-defined effective index, is implemented for terahertz QCLs, and single-mode terahertz QCLs with beam divergence as small as 4 x 4 degree are demonstrated, which is the narrowest beam reported for any terahertz QCL to-date. Moreover, in contrast to negligible radiative-field in conventional photonic band-edge lasers, in which the periodicity follows the integer multiple of half-wavelength inside active medium, antenna-feedback breaks this integer-limit for the first time and enhances the radiative-field of lasing mode. The antenna-feedback scheme is generally applicable to any plasmonic laser with a Fabry-Perot cavity irrespective of its operating wavelength, and could bring plasmonic lasers closer to practical applications.