Toward Quantum Semiconductor Nanocavities

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The goal of this project is to fabricate high-Q (~10,000), small-volume (cubic wavelength or smaller) 3D semiconductor nanocavities. Single-oxide-aperture microcavities have yielded Q = 2,000 with a diameter of 2 microns. The Q will be increased by fabricating a nanocavity with multiple aluminum-oxide apertures (Dennis Deppe, U. Texas - Austin), thus combining the advantages of etched pillars and oxide apertures, or by using a very small volume photonic-crystal nanocavity (Axel Scherer, Caltech). The Q will be measured by detecting the linewidth of photoluminescence of the quantum dots grown into the center of the nanocavity spacer. Professor Deppe, who has much experience with the growth of InGaAs quantum dots, will grow the material for both types of nanocavities. During this grant the emphasis will be shifted from interface-fluctuation dots at 750 nm to the more strongly confined self-assembled quantum dots of InGaAs at 1300 nm. 1.3 mm is an attractive wavelength not only because of its obvious importance for optical communications but also because the ratio of vacuum Rabi splitting to photon escape rate from the cavity is more than twice as large there. The longer wavelength increases the feature size of a wavelength-in-the-material cavity, making it easier to fabricate. The stronger confinement promises higher temperature operation. Finally instruments for characterizing 1.3-mm samples have been acquired by the PI.Minimum-volume, high-Q nanocavities are the natural nanotechnological limit for the shrinking size of light-emitting diodes and VCSEL's. Such structures are also of current interest to quantum information processing as sources of single photons on demand or for quantum entanglement. The Q/volume ratio can be lower for a useful single-photon turnstile than for strong coupling, so the values of Q and volume obtained will determine the emphasis upon single-photon source versus strong coupling. Radiative coupling between a single quantum dot and a nanocavity mode would be seen as a narrowing of the cavity linewidth as the cavity mode is temperature scanned through the dot's lowest energy transition in a dewar with very small temperature dependence of the sample position. Strong coupling would result in double-peaked transmission or luminescence, and absorption of a single photon would already change the absorption spectrum for the next probe photon.