Dissertation Defense

Making Semiconductor Single Photon Emitters Faster & Brighter

Brandon DemoryPhD Candidate

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 beneficial 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 the cryogenic temperature range, 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 a micro-cavity such as a photonic crystal or a metallic cavity can reduce the radiative lifetime and simultaneously improve the emission intensity brightness. 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. In this work, a self-aligning silver film cavity was investigated for enhancement of the spontaneous emission from GaN/InGaN QDs. Using conventional film E-beam evaporation techniques, the silver film encases the QD pillar without additional post processing. With the appropriate film thickness, the silver localized surface plasmon resonance (LSPR) is tuned to overlap the QD emission wavelength. Optical spectroscopy measurements of the QDs before and after the cavity show an order of magnitude reduction in the emission lifetime, simultaneously coupled with an order of magnitude increase in the emission intensity brightness. Next, an integrated parabolic nano lens and reflector system for a nano pillar light-emitting diode (LED) was designed. The SiN parabolic nano lenses are fabricated using E-beam lithography, resist reflow, and dry etching. The integrated optics results in a four to six times improvement in the collectable emission compared to the bare LED, with ~80% of the far field emission pattern within the 0.5NA zone. The narrow far field pattern is ideal for waveguide coupling.

Sponsored by


Faculty Host

Prof. Pei-Cheng Ku and Prof. Hui Deng