Abstract-Phase-locked terahertz plasmonic laser array with 2 W output power in a single spectral mode

Yuan Jin, John L. Reno, Sushil Kumar

. Longitudinal phase-locking scheme for subwavelength metallic cavities. (a) The scheme that allows for phase-locked operation of multiple parallel-plate subwavelength metallic cavities. The objective of the scheme is to enhance radiation from a long ridge cavity by splitting it into several shorter microcavities, which increases the number of radiating end facets when the microcavities are under phase-locked operation. A specific periodic arrangement of the microcavities and slit-like apertures in the top metal layer of the cavities establishes single-sided SPPs in the surrounding medium of the cavities, which leads to phase-locked operation of the microcavities. The enhanced radiation in the surface normal direction is primarily due to a larger number of radiating end facets for the microcavity array. A secondary contribution of radiation is the slit-like apertures within the microcavities. (b) A specific design of the multicavity QCL array for phase-locked operation. The distance between neighboring microcavities is equal to the wavelength of the single-sided SPPs (𝜆SPP${\lambda }_{\mathrm{S}\mathrm{P}\mathrm{P}}$) that are established in the surrounding medium. Each microcavity is of length 3×𝜆SPP$3\times {\lambda }_{\mathrm{S}\mathrm{P}\mathrm{P}}$ and has two slit-like apertures in its top metal layer with an inter-aperture spacing of 𝜆SPP${\lambda }_{\mathrm{S}\mathrm{P}\mathrm{P}}$. An illustration of the standing wave of the electric field corresponding to the lowest-loss resonant mode under phase-locked operation is given for both the vertical (𝐸𝑧$E_z$) and in-plane (𝐸𝑥$E_x$) components of the field. The radiating sites in the microcavity array include the end facets of the microcavities as well as the slit-like apertures, each of which has the same phase for the 𝐸𝑥$E_x$ field, which leads to radiation in the surface normal direction.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-6-708

Plasmonic lasers suffer from low output power and divergent beams due to their subwavelength metallic cavities. We developed a phase-locking scheme for such lasers to significantly enhance their radiative efficiency and beam quality. An array of metallic microcavities is longitudinally coupled through traveling plasmon waves, which leads to radiation in a single spectral mode and a diffraction limited single-lobed beam in the surface normal direction. We implemented our scheme for terahertz plasmonic quantum-cascade lasers (QCLs) and measured peak output power in excess of 2W$2\mathrm{W}$ for a single-mode 3.3THz$3.3\mathrm{T}\mathrm{H}\mathrm{z}$ QCL radiating in a narrow single-lobed beam, when operated at 58K$58\mathrm{K}$ in a compact Stirling cooler. We thereby demonstrated an order of magnitude increase in power and thirty-times higher average intensity for monolithic single-mode terahertz QCLs compared to prior work. The number of photons radiated from the cavity outnumber those absorbed within its claddings and semiconductor medium, which constitutes >50%$>50\mathrm{\%}$ radiative efficiency and is significantly greater than that achieved for previous single-mode mid-infrared or terahertz QCLs.

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