Making Semiconductor Single Photon Emitters Faster and Brighter
Demory, Brandon
2017
Abstract
Single photon emitters are critical resources for quantum science and technologies. More specifically, on-demand single-photon emission (SPE) from semiconductor quantum dot (QD) structures is crucial for quantum cryptography and low-power communications. Group III-nitride (III-N) QDs are a high interest QD solution because of their potential for SPE beyond cryogenic temperatures, enabling a more practical system. However, in contrast to the III-arsenic QDs which possess a radiative lifetime typically <1ns, the operating speed for isolated III-N QDs is limited to only tens of megahertz, due to the strong piezoelectric field in strained III-N heterostructures, causing longer radiative lifetimes on the order of tens of nanoseconds. Coupling the emission from III-N QD structures to an optical cavity such as a photonic crystal or a metallic cavity can reduce the radiative lifetime and simultaneously improve the emission intensity. For shorter wavelength III-N QDs, a metallic cavity is a better enhancement solution than a dielectric cavity because of the large fabrication tolerance, broad spectral enhancement window, and reduced fabrication complexity. In addition to improving the emission process, increasing the extraction efficiency of generated photons is desired for device integration. Light extraction from semiconductor QDs is limited by the high index contrast between the air and the semiconductor. Moreover, the far field emission pattern of the exiting emission is broad and not ideal for further optical coupling. Integrating lenses and reflectors with the QD structure increases the emission extraction efficiency and condenses the far field pattern, creating a more ideal far field pattern for optical coupling into an optical waveguide. In this work, our aim is to develop conventional methods for improving semiconductor QDs’ speed and brightness by taking advantage of wafer-scale fabrication techniques. First, a self-aligning silver film cavity was investigated for enhancement of the spontaneous emission from III-N QDs. Using e-beam evaporation, the silver film encased the QD pillar without additional post processing. With the appropriate film thickness, the silver localized surface plasmon resonance is tuned to overlap the QD emission wavelength. Optical spectroscopy measurements of the QDs before and after the cavity showed an order of magnitude reduction in the emission lifetime, simultaneously coupled with an order of magnitude increase in the emission intensity. Additionally, the QDs become uniform in quantum efficiency from the improved radiative process caused by the high cavity quantum efficiency. Next, we demonstrate through similar optical spectroscopy measurements on a dense array of QDs, that emitter lifetimes on the order of tens of picoseconds can be achieved by changing the geometry of the silver film cavity and removing the top of the silver film. To improve the QD light extraction efficiency and light collection, an integrated parabolic nano lens and reflector system was designed. The SiN integrated optics resulted in a four to six times improvement in the collectable emission compared to the bare QD pillar, with ~80% of the far field emission pattern within the 0.5NA zone, which is more ideal for waveguide coupling. Furthermore, the same lens is applied to a multicolor GaN/InGaN light-emitting diode (LED), improving the light extraction efficiency to 70% across the visible spectrum. Lastly, we propose in future work a multi-layered open top cavity structure capable of reducing the emitter lifetime below 10 picoseconds. These results contribution to the foundations for room temperature, high speed, directional, on-chip, single photon emitters.Subjects
III-nitride quantum dots silver film cavity Quantum dots Parabolic nanolens exciton-localized surface plasmon coupling
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