Optical microcavities that contain single quantum dots have promising applications in quantum cryptography as sources of single photons. The realisation of efficient devices relies on the ability to fabricate electrically-pumped, high Q factor (Q>2000), wavelength-sized microcavities. In this work two approaches-oxide confined and micropillar structures-are compared by optical simulation.
The modification of the spontaneous emission-Purcell factor and emission coupling efficiency-in such devices is treated semiclassically here, assuming the weak coupling regime. Hence, the spontaneous emission rate and direction may be computed using the effective mode volume, resonant wavelength, and quality factor of the optical modes in the microcavity. In the context of this work, the optical modes of rotationally symmetric microcavities are determined by solving Maxwell's vectorial wave equation in the frequency domain employing vectorial finite elements, subject to an open boundary, taking into account diffraction and radiation of electromagnetic waves. Consequently, the spontaneous emission properties of realistic microcavities without any restrictions regarding structure and size may be investigated.
The optical mode solver is first calibrated with measured electroluminescence spectra of an oxide confined microcavity structure with oxide diameters ranging from 2.4 um to 0.7 um. Excellent agreement is achieved between measurements and simulations, which assures the predictive capability of the optical mode solver. For oxide confinements with diameters smaller than 1 um strong degradation of the Q factor and, hence, the Purcell factor is observed. Excessive diffraction losses are identified as the main cause of this effect in the present design. Furthermore, the advantages of micropillar structures with respect to this issue are demonstrated.
We describe the realization and characterization of nanoscale light emitters comprising one or few semiconductor quantum dots (QDs) in the active region. These devices are intended for use as single-photon sources in fiber-based quantum cryptography systems. The epitaxial growth of low-density QDs emitting at 1300 nm is described, and emission from single QDs is demonstrated through micro-photoluminescence measurements. The realization of QD light-emitting diodes (LEDs) having a submicrometer active area is reported, based on oxidized current apertures. Finally, the integration of nanoscale current injection with high-quality factor optical cavities is described, in view of obtaining efficient single-photon LEDs.