Terahertz microcavity lasers with subwavelength mode volumes and thresholds in the milliampere range
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(Color online) (a) Scanning electron microscope image of the fabricated microdisks. The sidewalls are not vertical and their slope depends on the crystal orientation, as shown in the top-right schematic drawing. (b) Schematic device geometry before the removal of the top doped layers. The metal top contact does not cover the whole top surface of the microcylinder. Radii of the fabricated microdisks: . Radii of the deposited metal: . The hatched regions are the portions of layer that are removed to increase the resonator factor.
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(Color online) (a) (LVI) characteristics at and in cw mode of a typical small-size terahertz microdisk. The is . Inset: emission spectrum at for an injection current of . The detection was performed with a He-cooled silicon bolometer. (b) LVI characteristics at and in cw mode of a typical medium-size terahertz microdisk. The is . Inset: emission spectrum at for an injection current of . (c) LVI characteristics at several temperatures and in pulsed mode ( at ) of a typical large-size terahertz microdisk. The at is and the devices operate up to . Inset: emission spectra for several injection currents.
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(Color online) Numerical simulations of the modes in the microcylinder devices with the smallest diameter. The 3D simulations are performed within a finite-element approach (Ref. 18). For the 2D simulations we have mapped the 3D problem onto a 2D one using the axial symmetry of the system, as described by Oxborrow in Ref. 19. [(a)–(d)] 3D simulations of the modes with radial numbers and azimuthal numbers , 2, 3, and 4. The vertical component of the electric field on a horizontal section is plotted. The dotted circles correspond to the extension of the metal contact. The mode is bound to the metal and the resonance frequencies—reported on the panel legends—are spaced of . The lasing mode is , (d). (e) Axisymmetric simulation of the (, ) mode prior removal of the top doped layer. The electric field magnitude is plotted. The mode is guided by the top layer up to the resonator edge. Inset: close-up of the mesa edge with a three-times larger color scale. It shows that the white region in the main picture is not a numerical divergence but just out of scale. (f) Axisymmetric simulation of the (, ) mode after the removal of a part of highly doped layer. The mode is now bound to the top metallic contact. Inset: close-up of the mesa edge with a three-times larger color scale.
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