Efficient light collection from a site-controlled quantum dot formed on the apex of a pyramid

Abstract

FDTD simulation predicts a single mode coupling efficiency of 48 %. FDTD simulation predicts that extraction efficiency of QDs on pyramids in upper and lower direction is improved. Its realization for a site-controlled III-nitride pyramidal QDs is expected to have a potential application in quantum communication, quantum cryptography, .and quantum simulation.

Semiconductor quantum dots (QDs) as a quantum emitter can generate single photons which can deliver quantum information. In particular, III-N semiconductor QDs have potential advantages of scalability and current-driven operation at room temperature because of their small size, p-type doping and large valence band offset energy. Despite the advantages, QDs still struggle in achieving unity extraction efficiency, which is a prerequisite for their applications. For example, Stranski-Krastanov growth mode QDs grown in planar structures have difficulties in extracting light due to total internal reflection. Even though epitaxially grown three dimensional pyramidal structures exhibit the increase in light extraction, it is still required to have higher efficiency to applications. In this thesis, two approaches based on finite-difference time-domain (FDTD) simulations are proposed as upper and lower side light extraction. First, upward light extraction is enhanced by using an immersion lens. An immersion lens on top of site-controlled QDs, fabricated by two photon polymerization, extracts and focuses emitted light from QDs. FDTD simulation predicts 8 times increase a light intensity and 3 times increase in light extraction efficiency at 450 nm emission wavelength and numerical aperture of 0.5. Second, downward light extraction is enhanced by coupling between the source and an optical fiber with a metal mirror. A metal layer along pyramidal QDs directs the emitted light downwards. A dielectric layer between the metal layer and pyramids is introduced to reduce metal absorption. The emitted light of 450 nm wavelength is then coupled with an 800 nm diameter tapered optical fiber