Near-field mapping of the emission distribution in semiconductor microdiscs

We have used a scanning near-field optical microscope to study the fluorescent light distribution in the near- and far-fields with two types of microdiscs, InGaP and GaN, fabricated in our laboratory. The InGaP microdisc has a radius of 2.5-5.0 microm, a thickness of 0.15-0.2 microm and a circular shape and the GaN disc has a radius of 5-8 microm with a thickness of 0.5-2 microm. Spontaneous emission enhancement in these microdiscs has been observed with emitting wavelengths of 650 and 550 nm respectively In both types of microdisc, the whispering-gallery mode (WCM) has been observed on the top surface using near-field optical and far-field microscopic methods. However, due to the different disc structures and optical confinements, the light distributions of the type types of disc are quite different. In the case of the InGaP disc, WGM is the dominant mode with a mixture of other modes. Interference-like ring intensities have been observed both inside the disc surface and outside, with a period ratio of 1:2. In addition, the propagating waves emitted from the side of the disc have been collected for the first time by using near-field optical microscopy. A theoretical calculation based on the theory of optical modes in microdisc lasers confirmed this observation. It also predicted the behaviour of the electric field distribution (transverse electric) inside and outside the disc, as well as the period of the wave propagation. In contrast, the near-field mapping of the GaN fluorescence showed not only a ring-like emission intensity along the circumference of the disc, an indication of WGM, but also an even intensity distribution inside the disc. This can be explained as the combination of the WGM with the Fabry-Perot mode of multi-reflection between the GaN layer and the substrate. The results also demonstrate the potential application of near-field optics to explore the light emission mode of a microdisc on a nanometre scale.