Two-dimensional photonic crystal band-edge laser using colloidal quantum dots as gain material

Abstract

Semiconductor-based lasers typically utilize multiple quantum wells (MQWs) for gain materials. Despite well-known advantages of MQW structure, emission wavelength is fixed in a narrow bandwidth, and device architecture become heavily dependent on substrate. This study focuses on solving such an issue by realizing highly tunable and less substrate-dependent laser device by combination of colloidal quantum dots and photonic crystal structure. Core-shell type colloidal quantum dots (CQDs) exhibit efficient photoluminescence with widely tunable bandgaps based on quantum confinement effect. Not only could such core-shell type CQDs be used to replace epitaxially grown semiconductor gain materials, but also be functionalized for brand-new concepts of optical devices and applications. CQDs form a deposited film simply by conventional spin-coating method. On the other hand, photonic crystal is a structure with periodic distribution of refractive index, in which periodicity dictates photonic band structure and photonic band gap. The useful feature has been used to fabricate lasers, waveguides and various other types of optical devices. In this study, properties of photonic band-edge mode are the focus of research, which slows the light in matter increasing the interaction in between. This dissertation demonstrates lasing emission from a two-dimensional (2D) photonic crystal (PC) backbone with densely packed CQDs embedded within the PC as a gain material. The PC slab consists of a silicon nitride film on silica substrate, forming an asymmetric slab waveguide. Numerical analyses based on finite-difference time-domain method show that photonic band structure of a simple square lattice could have an M-point band-edge mode in the air-band. An array of air-holes is fabricated into a Si3N4 film by e-beam lithography and reactive ion etching with various air-hole diameters. On top of the PC backbone, CQDs are spin-coated and cured, resulting in the air-holes of the PC backbone infiltrated by CQDs to provide optical gain for lasing action. Completed CQD-PC laser device is then optically pumped using a sub-nanosecond 532 nm pulsed laser The CQD- PC laser showed double band-edge mode laser operation with emission linewidth less than 1 nm at full-width-half-maximum.