Abstract
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 $R=1--3\text{ }\ensuremath{\mu}\text{m}$ and quality factors $Q\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{3}$) 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 $1.1--1.3\text{ }\ensuremath{\mu}\text{m}$. 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.
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