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August 08, 2010

Designer spoof surface plasmon structures collimate terahertz laser beams

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The metamaterial patterns are directly sculpted on the highly doped GaAs facet of the device. Artificial coloring in the figure indicates deep and shallow micron scale grooves, which have different functions. The shallow "blue" grooves efficiently couple laser output into surface electromagnetic waves on the facet and confine the waves to the facet. Credit: Courtesy of the laboratory of Federico Capasso, Harvard School of Engineering and Applied Sciences

Nature Materials - Designer spoof surface plasmon structures collimate terahertz laser beams

Scientists from Harvard University and the University of Leeds have demonstrated a new terahertz (THz) semiconductor laser that emits beams with a much smaller divergence than conventional THz laser sources. The ability of metamaterials to confine strongly THz waves to surfaces makes it possible to manipulate them efficiently for applications such as sensing and THz optical circuits.

Surface plasmons have found a broad range of applications in photonic devices at visible and near-infrared wavelengths. In contrast, longer-wavelength surface electromagnetic waves, known as Sommerfeld or Zenneck waves are characterized by poor confinement to surfaces and are therefore difficult to control using conventional metallo-dielectric plasmonic structures. However, patterning the surface with subwavelength periodic features can markedly reduce the asymptotic surface plasmon frequency, leading to ‘spoof’ surface plasmons with subwavelength confinement at infrared wavelengths and beyond, which mimic surface plasmons at much shorter wavelengths. We demonstrate that by directly sculpting designer spoof surface plasmon structures that tailor the dispersion of terahertz surface plasmon polaritons on the highly doped semiconductor facets of terahertz quantum cascade lasers, the performance of the lasers can be markedly enhanced. Using a simple one-dimensional grating design, the beam divergence of the lasers was reduced from ∼180° to ∼10°, the directivity was improved by over 10 decibels and the power collection efficiency was increased by a factor of about six compared with the original unpatterned devices. We achieve these improvements without compromising high-temperature performance of the lasers.

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