Nano-optics: imaging the resonant nonlinear response of individual localized excitons

Summary form only given. Micro-optical probe techniques have been developed by numerous groups to study and exploit features of excitonic systems since the initial visualization of disorder induced exciton localization in quantum wells by near-field scanning optical microscopy. The most recent work in narrow GaAs quantum wells has demonstrated that these structures have features consistent with quantum dots, including sharp line optical spectra, excited states, and a nonlinear optical response that is distinct from higher dimensional systems. Indeed, many of the optical and electronic features of these systems are similar to simple atomic systems, leading to proposed applications in novel quantum optoelectronic devices. In this paper, we present the first sub-wavelength resolution images of the coherent nonlinear optical response of these strongly localized excitons. Experimental data are collected on a narrow (42 /spl Aring/) single quantum well grown with long growth interruptions. Data is obtained using a 4 K near-field microscope to allow imaging of the system. The measurements are based on homodyne detected differential transmission, which, unlike typical luminescence studies, directly probes all optically accessible excitonic states.