Far-field observation of the radial profile of visible whispering-gallery modes in a single microdisk based on Si-nanocrystal/SiO2 superlattices

Abstract

The radial profile of visible whispering gallery modes (WGM) from a single microdisk based on silicon nanocrystals (Si-nc) and SiO2 superlattices was observed. Using thermal evaporation, the active layer, which consists of 30 pairs of ∼3nm thick Si-ncs and ∼4nm thick SiO2 layer, was fabricated on Si substrate. Si-ncs with diameters of 3nm, which are well defined by SiOx layer thickness, were formed by high temperature annealing at 1100°C for 60min under N2 environment. After standard photolithography and dry etching procedure, a microdisk with 8.8μm diameter on a silicon pedestal was successfully obtained. We calculated the expected radial profiles of the WGMs by solving the Maxwell equations using appropriate boundary conditions. Comparison with finite-difference time-domain (FDTD) simulation depicts similar radial profiles of the WGMs. Using a confocal microphotoluminescence setup, light emission from the top of a single disk was analyzed depending on the detection position. Thanks to the birefringence of nature of Si-nc/SiO2 superlattices, well-isolated sharp TE mode WGMs could be detected from the top, without using polarizers. Irrelative to detection position, broad Si-nc background luminescence is consistently found. And as the detection point was moved from the center to the outside of the disk, WGMs fields were strongly decreased especially for the detection at the outside. Taking experimental circumstances into account, the radial profile of WGMs field was estimated and was then compared to the experimental WGM profile. The expectation was consistent with experimental results confirming the confinement of WGMs fields within a disk. Although FDTD simulation reveals that the geometrical Q factor can reach >105, the maximum Q factor we observed was 2.5×103. This implies that the scattering and absorption losses must be suppressed in order to enhance the microdisk performance. Simulations show significant WGM field at the top/bottom surface of the disk and visible imperfections of the disk surface was confirmed by scanning electron microscopy, and are expected to produce considerable surface scattering loss limiting the overall disk cavity Q factor. However, such nondirectional surface scattering allowed us to successfully detect WGMs from the top of the disk.