Near-field optical spectroscopy and imaging of single InGaAs/AlGaAs quantum dots

We use near-field optical probing at low temperatures (T=5 K) to image and examine the linear and nonlinear luminescence properties of single InGaAs/AlGaAs quantum dots grown on (311)B oriented GaAs substrates. The high spatial resolution of near-field “nanoprobing,” which is typically 200 nm or less, makes the observation of single dots at different locations on the sample possible, even though the spatial density of quantum dots is on the order of 100/μm2. We observe narrow excitonic emission lines at low excitation powers and, with increasing excitation, we observe biexcitonic emission strongly shifted (3 meV) to the low-energy side of the exciton emission.