Extraction efficiency enhancement of single InAs quantum dot emission through light scattering on the Au nanoparticles

Abstract

Single semiconductor quantum dots (QDs) have been considered as the promising solid-state single photon sources. To obtain bright quantum sources, the key issue is to enhance extraction efficiency of the QD emission, which is challenging since QDs normally emit isotropically in a high refractive index material. In this article, we investigate the influence of Au nanoparticles on the QD photoluminescence (PL) extraction efficiency based on the techniques of optically positioned QDs and single QD emission detection. The InAs QD samples studied are grown using the molecular beam epitaxy on a (001) GaAs substrate. The sample consists of, in sequence, a 200 nm GaAs buffer layer, a 100 nm AlAs sacrificed layer, a 30 nm GaAs, a QD layer, and a 100 nm GaAs cap layer. The QD sample is mounted in a cryostat cooled down to 5 K, and excited by illumination of a 640 nm diode laser (CW or pulsed with a repetition frequency of 80 MHz). Excitation laser beam is focused to an approximately 2 μm spot on the sample using a microscope objective (NA : 0.5) which is mounted on a nanocube XYZ piezo nanopositioning stage with a scanning range of 100×100×100 μm3. The QD PL is collected using the same objective and measured using a 0.5 m focal length monochromator equipped with a silicon charge-coupled device (CCD). The PL decay measurements are performed using a silicon avalanche photodiode (APD) and a time-correlated single-photon counting (TCSPC) board.#br#In order to study the influence of different environments surrounding the QDs on the spontaneous emission rate and the extraction efficiency, the same QD emissions are measured under the conditions that: (1) A typical QD is at first chosen and optically positioned and then its emission is measured. (2) A GaAs layer containing the QDs is lifted off from the as-grown sample by an AlAs sacrificed layer and placed on the Au film with or without Au nanoparticles. (3) Optical measurements are carried out to obtain the QD emission intensity. This technique enables us to compare the same QD emission intensity for the as-grown QD sample, which is placed on the Au film or on the Au nanoparticles.#br#In summary, it is found that the measured QD emission intensity increases up to 6 times that of the original for the QD placed on the Au nanoparticles, otherwise it is only doubled for the QD placed on the Au film. The time-resolved PL measurements show that the QDs have nearly the same decay time for the QDs in different environments, implying that the QD spontaneous emission rate has not been changed. Therefore, the enhanced PL is due to the increase of extraction efficiency. The physical mechanism underlying the Au nanoparticles-induced PL enhancement is attributed to the trapped QD emission light within the sample and scattered again by Au nanoparticles and collected by the microscopy objective.