Near-field optical spectroscopy of excitons in single quantum dots

InGaAs single quantum dot photoluminescence spectra and images are investigated by using a low-temperature near-field optical microscope. By modifying the commonly used near-field apertured probe, a high spatial resolution and high detection efficiency are achieved simultaneously. Local collection of the emission signal through a 500 nm (λ/2) aperture contributes to the single-dot imaging with a λ/6 resolution, which is a significant improvement over the conventional spatially resolved spectroscopy. Tailoring the tapered structure of the near-field probe enables us to obtain the emission spectra of single dots in the weak excitation region, where the carrier injection rate is ∼10⁷ excitons/s per dot. By employing such a technique, we examine the evolution of single-dot emission spectra with excitation intensity. In addition to the ground-state emission, excited-state and biexciton emissions are observed for higher excitation intensities. By a precise investigation of the excitation power dependences of individual dots, two-dimensional identification of their emission origins is obtained for the first time.