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[(a) and (b)] Illustrations of the formation of freestanding quantum dot microtubes on GaAs substrates. It is evident that the presence of a sinusoidal corrugation around the inside edge of the mesa leads to freestanding microtubes with an engineered geometry. [(c) and (d)] Illustrations of the substrate-on-substrate transfer of freestanding microtube ring resonators from GaAs to Si substrates. The GaAs wafer is first placed directly on top of a Si substrate with the presence of a solvent. With the removal of the GaAs wafer, the microtubes then register on the Si substrate due to the gravitational force induced by the solvent in and around the tube.
(a) SEM image of a freestanding quantum dot microtube fabricated on GaAs substrate, evidenced by the presence of an etched GaAs mesa. (b) SEM image of a freestanding quantum dot microtube transferred on a clean Si substrate that is free of any etched pattern. (c) SEM image of a quantum dot microtube on Si. The sinusoidal geometry of the microtube surface is evident.
(a) PL spectrum of InGaAs/GaAs quantum dot microtube ring resonators on Si under a pump power of at 77 K. Quantum dot emission was measured from the freestanding and corrugated region (solid line) and also the region without any corrugation (dashed line). The calculated azimuthal mode numbers are also shown. (b) A detailed view of the axial field distributions associated with the azimuthal mode number 40. The calculated results (dotted vertical lines) using a photonic quasi-Schrödinger equation are also shown for comparison. Illustrations of the level spacing in a parabolic potential and the two nondegenerate modes induced by the presence of inside and outside edges around the tube are shown in the lower and upper insets, respectively.
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