Optical Gain in Ultrathin Self-Assembled Bi-Layers of Colloidal Quantum Wells Enabled by the Mode Confinement in their High-Index Dielectric Waveguides

The authors acknowledge the financial support in part from NRF-NRFI2016-08 and in part from TUBITAK 115F279 and 117E713. H.V.D. gratefully acknowledges support from TUBA. O.E. acknowledges TUBITAK for the financial support through BIDEB 2211 program. The authors thank Mr. Mustafa Guler for TEM imaging of the as-synthesized CQWs and preparation of the TEM cross-sectional sample and Dr. Gokce Celik for her help on the ellipsometric measurements. This study demonstrates an ultra-thin colloidal gain medium consisting of bi-layers of colloidal quantum wells (CQWs) with a total film thickness of 14 nm integrated with high-index dielectrics. To achieve optical gain from such an ultra-thin nanocrystal film, hybrid waveguide structures partly composed of self-assembled layers of CQWs and partly high-index dielectric material are developed and shown: in asymmetric waveguide architecture employing one thin film of dielectric underneath CQWs and in the case of quasi-symmetric waveguide with a pair of dielectric films sandwiching CQWs. Numerical modeling indicates that the modal confinement factor of ultra-thin CQW films is enhanced in the presence of the adjacent dielectric layers significantly. The active slabs of these CQW monolayers in the proposed waveguide structure are constructed with great care to obtain near-unity surface coverage, which increases the density of active particles, and to reduce the surface roughness to sub-nm scale, which decreases the scattering losses. The excitation and propagation of amplified spontaneous emission (ASE) along these active waveguides are experimentally demonstrated and numerically analyzed. The findings of this work offer possibilities for the realization of ultra-thin electrically driven colloidal laser devices, providing critical advantages including single-mode lasing and high electrical conduction. Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) 115F279 117E713 BIDEB 2211 Turkish Academy of Sciences NRF-NRFI2016-08