Near-field mapping of the emission distribution in semiconductor microdiscs

We have used a scanning near-field optical microscope to study the fluoresc ent light distribution in the near- and far-fields with two types of microd iscs. InGaP and GaN, fabricated in our laboratory The InGaP microdisc has a radius of 1.5-5.0 mu m, a thickness of 0.15-0.2 mu m and a circular shape and the GaN disc has a radius of 5-8 mu m with a thickness of 0.5-2 mu m. S pontaneous emission enhancement in these microdiscs has been observed with emitting wavelengths of 650 and 550 nm respectively, In both types of micro disc, the whispering-gallery mode (WGM) has been observed on the top surfac e using nearfield optical and far-field microscopic methods, However, due t o the different disc structures and optical confinements, the light distrib utions of the type types of disc are quite different, In the case of the In GaP disc, WGM is the dominant mode with a mixture of other modes. Interfere nce-like ring intensities have been observed both inside the disc surface a nd 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 th e theory of optical modes in microdisc lasers confirmed this observation. I t also predicted the behaviour of the electric field distribution (transver se 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 s howed not only a ring-like emission intensity along the circumference of th e disc, an indication of WGM, but also an even intensity distribution insid e the disc. This can be explained as the combination of the WGM with the Fa bry-Perot mode of multi-reflection between the GaN layer and the substrate. The results also demonstrate the potential application of near-field optic s to explore the light emission mode of a microdisc on a nanometre scale.