On-demand Single-photon Source with Optimal Fiber Interface for Semiconductor Quantum Dots

Abstract

Non-classical behaviors like path superposition, the no-cloning theorem and quantum entanglement enables us to overcome classical limitations in quantum communications, quantum sensing and quantum simulations. Single-photon sources, one of the non-classical light, is basic buliding blocks for those quantum technologies. Various kinds of single-photon sources have been found and developed such as trapped ion, solid-state quantum emitter including defects in ctystals and semiconductor quantum dots. Among them, semiconductor quantum dots (QDs) are attractive sources because they can emit bright and indistinguishable single-photons and provide entangled photon pairs through unique electronic energy structures. In addition, optical properties of single-photons from the semiconductor QDs can be manipulated by engineering refractive index of the surrounding host materials. It can be done by fabricating structures such as waveguides or optical cavities, using existing semiconductor foundry. To implement this fascinating semiconductor QDs in a practical way, integrating them into optical fiber platforms is one of the useful application for long-distance information processing between distributed quantum nodes. However, since QDs must be grown inside the host semiconductors, the high refractive index of the host materials limits light extraction efficiency into air due to total internal reflection. Although this problem can be solved by designing appropriate structures, extracted photons are hard to coupled with optical fibers because typical fiber has low numerical aperture (NA) about 0.12 ~ 0.14. So, the semiconductor structures must be designed to have not only high extraction efficiency but also high directionality for mode matching with fibers. Here, we presented our reseaches for inverstigating QD-fiber interfaces and demonstrating fiber-integrated QD devices in different ways. Moreover, we improved QD-fiber platform to have various functionalities including pre-characterization or wavelength tuning by incorporating QD to V- groove fiber array. We newly designed photonic cavity named hole-circular Bragg grating (hole-CBG) for high extraction efficiency and fiber-coupling efficiency. Instead of periodic vacant rings in typical CBG structures, vacant hole arrays replaced the vacnt rings reducing refractive index contrast in radial direction. The reduced index contrast in radial direction results in enlarged mode volume, followed by vertical beam emission with increased mode overlap with optical fiber. From finite difference time domain (FDTD) simulations, we optimized hole-CBG strcuture and the cavity shows a fiber-coupling efficiency of 53% in single-mode fiber (NA = 0.12) with high Purcell factor above 100. The fiber- coupling efficiency even increased up to 60% in air-suspended fiber-coupled scheme. We fabricated the hole-CBG on InAs/InP QD and integrated it on optical fiber via polydimethylsiloxane (PDMS) microstamp. Then we performed a QD spectroscopy to characterize the hole-CBG on chip and fiber- integrated QD devices. From the experiments, we verified their single-photon nature using Hanbury Brown and Twiss interfereometer and calculated collection and fiber-coupling efficiency. The hole- CBG (QD-fiber device) show a clear anti-bunching signal with high collection (fiber-coupling) efficiency of up to 30% (9%). From this thesis, we demonstrated newly designed cavity for the optimal QD-fiber interface so called hole-CBG, and further, proposed fiber-integrated QD device using pigtailed optical fiber and multi-functional scalable single-photon source platform using V-groove fiber array. Our approach paves the way for on-demand quantum light sources based on solid-state QDs which allow alignment-free, stable, plug-and-play operation.