Analysis of factors determining the azimuthal and radial order of lasing mode in III–V quantum dot disk microlasers

Abstract

Microlasers supporting whispering gallery modes attract great interest due to their potential use in photonic integrated circuits, as they have a size just an order of magnitude larger than their emission wavelength and exhibit good thermal and dynamic properties. It is usually assumed that in disk microlasers, a whispering gallery mode of the highest azimuthal order and first radial order is the lasing one. However, in this work, we argue that it is of much lower azimuthal order and much higher radial order. We provide both experimental evidence in the form of far-field radiation patterns measured for disk microlasers with 10, 15, and 25 μm radii and theoretical analysis of the modal gain. We investigate how surface recombination of carriers leads to depletion of the outer region of the disk and, thus, to lowering of the gain. This leads to a preference for higher radial order modes. At the same time, modes that are localized too close to the center have smaller vertical confinement factors, which we prove with numerical modeling showing increased leakage of the mode's electric field to the substrate. When both aforementioned factors are considered, we arrive at an estimation of the radial and azimuthal mode order that agrees with the experiment.