High-spatial-resolution near-field photoluminescence and imaging of whispering-gallery modes in semiconductor microdisks with embedded quantum dots

Near-field photoluminescence (NPL) spectroscopy with a spatial resolution below the light diffraction limit was used to study the optical fields of whispering-gallery modes (WGMs) in GaAs and AlGaAs microdisks (radii R=1--3 μm and quality factors Q~5×10³) containing InAs quantum dots (QDs). WGMs with azimuthal mode numbers $m=10--36$ and radial mode numbers $l=1--3$, together with low-Q Fabry-P\'erot modes and other non-WGMs, have been resolved in NPL spectra in the range of 1.1--1.3 μm. We found that spectrally resolved NPL intensity images of the WGMs reveal m azimuthal maxima around the disk circumference, which correspond to the period of the field amplitude. Using finite-difference-time-domain (FDTD) calculations, we show that the amplitude imaging arises from the interference between the WGMs and a background field formed by low-Q modes. We show further that such interference reflects the inhomogeneity of the radiation decay of the QD ensemble in the MD. We demonstrate a direct measurement of WGM quantum numbers in microdisks by using NPL imaging, which shows good agreement with FDTD calculations.