A circular dielectric grating for vertical extraction of single quantum dot emission

Abstract

We demonstrate a nanostructure composed of partially etched annular trenches in a suspended GaAs membrane, designed for efficient and moderately broadband (%5 nm) emission extraction from single InAs quantum dots. Simulations indicate that a dipole embedded in the nanostructure center radiates upward into free space with a nearly Gaussian far field, allowing a collection efficiency >80% with a high numerical aperture (NA ¼ 0.7) optic and with %12Â Purcell radiative rate enhancement. Fabricated devices exhibit a %10% photon collection efficiency with a NA ¼ 0.42 objective, a 20Â improvement over quantum dots in unpatterned GaAs. A fourfold exciton lifetime reduction indicates moderate Purcell enhancement. Efficient extraction of single photons emitted by individual semiconductor epitaxial quantum dots (QDs) is a necessity for many applications in spectroscopy and classical and quantum information processing.1 As epitaxially grown QDs are embedded in semiconductor material, total internal reflection of the emitted light at the semiconductor-air interface and radiation divergence can typically lead to <1% collection efficiencies even with high numerical aperture (NA) optics. Photonic structures such as micropillar cavities can provide both QD radiative rate Purcell enhancement and a far-field radiation pattern that can be effectively collected, 2 but require precise spectral tuning of the cavity resonance to the QD emission line. In contrast, vertically oriented etched nanowires 3 are spectrally broadband structures that have recently been shown to provide large free space collection efficiencies, albeit without Purcell enhancement and with an involved fabrication process. Broadband operation not only relaxes the spectral alignment requirement, which may impose strict constraints in fabrication tolerances, but also is a necessity in spectroscopic applications in which simultaneous detection of various spectrally separate transitions is desired. Here, we present an approach for efficient free space extraction of QD emission using a suspended circular grating. This structure requires a simple nanofabrication procedure and supports a relatively broad (few nanometer) optical resonance with a directional, nearly Gaussian far-field, which allows efficient free space photon collection. Simulations predict a collection efficiency of %53% (80%) into a NA ¼ 0.42 (0.7) optic. In fabricated devices, we report a % 10% single QD photoluminescence (PL) collection efficiency into a NA ¼ 0.42 objective, a %20Â improvement compared to QDs in unpatterned bulk GaAs. A fourfold reduction in QD lifetime is also observed, indicating moderate radiative rate enhancement.Our nanostructure (Fig. 1) consists of a circular dielectric grating with radial period K that surrounds a central circular region of radius 2K, produced on a suspended GaAs slab of thickness t ¼ 190 nm. The GaAs slab supports single TE and TM polarized modes (electric or magnetic field parallel to the slab, respectively). The grating is composed of ten partially etched cir...

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