Transient Optical Gain Spectroscopy in Dense Colloidal Quantum Dot Films and Multicolor Low-Threshold Laser Application

Colloidal quantum dots have emerged as efficient fluorescent materials with widely tunable bandgap covering full visible spectrum as a result of quantum confinement effect. Studies of amplified spontaneous emission and lasing from semiconductor nanocrystals have been reported. However, large losses due to non-radiative multi-excitonic Auger recombination have severely hindered the progress of laser research. In this talk, the role of single exciton states as dominant optical gain mechanism in densely packed II-VI semiconductor quantum dots via transient gain spectroscopy will be discussed. This single exciton gain can effectively suppress the Auger process and enable dense films to reach the gain threshold at a very low excitation level. It is suggested that optimizing the Stokes shift together with narrow inhomogeneous broadening of the colloidal quantum dots play a key role in empowering the single exciton states.

Next, I will talk about the development of the surface-emitting distributed feedback (DFB) lasers from the closely packed colloidal quantum dot films by using a compact solid-state laser whose pulse duration exceeds the Auger decay time (~100 ps). Solid films with periodic nano-grating patterns show 2nd-order DFB lasing action at 120, 280, and 330 Â µJ/cm2 of optical pumping energy densities for red, green and blue lasers, respectively. Highly monochromatic (less than 1 nm of full-width-half-maximum) single mode lasing and measured far-field profiles for all three colors demonstrate high degree of spatial coherence in the output beams of the DFB lasers. Further development towards practical quantum dot laser applications can be achieved by proper heat management and improvement in pump efficiency.

Kwangdong Roh was born in Seoul, South Korea. He received his B.S. degree from Seoul National University in 2008, and started his graduate studies in the Department of Physics at Brown University in 2009. At Brown, under the guidance of Professor Arto Nurmikko, he studied optical properties focusing on dynamics of excitons for optical gain mechanisms in colloidal quantum dots and fabrications of optically pumped lasers. He is expected to obtain his Ph. D. degree in September 2015. His research interests also include development of optoelectronic devices such as light-emitting diodes, lasers, photovoltaic cells and photodetectors from semiconductor nanocrystals, organic/inorganic materials, and optical metamaterials.