In-plane surface plasmonics integrated with THz quantum cascade lasers for high collimation

We report planar integration of tapered Terahertz (THz) quantum cascade lasers (QCLs) with spoof surface plasmon (SSP) structures. The SSP structure consists of one plasmonic coupler and periodically arranged scatters. The resulting surface-emitting THz beam is highly collimated with a beam divergence as narrow as 3.6°×9.7°. As the beam divergence is inverse proportional to the light emission area, this low divergence indicates a good waveguiding property of the SSP structure, while the low optical background of the beam implies a high coupling efficiency of the THz wave from the laser cavity to the SSPs. Since all the structures are in-plane, this scheme provides a promising platform where the well-established SP techniques can be employed to engineer the THz QCL beam with high flexibilities.