Multipolar interband absorption in a semiconductor quantum dot I Electric quadrupole enhancement

We present a theoretical investigation of a semiconductor quantum dot interacting with a strongly localized optical field as encountered in high-resolution near-field optical microscopy. The strong gradients of these localized fields suggest that higher-order multipolar interactions will affect the standard electric dipole transition rates and selection rules. For a semiconductor quantum dot in the strong confinement limit we calculated the interband electric quadrupole absorption rate and the associated selection rules. We found that the electric quadrupole absorption rate is comparable with the absorption rate calculated in the electric dipole approximation. This implies that near-field optical techniques can extend the range of spectroscopic measurements beyond the standard dipole approximation. However, we also show that spatial resolution cannot be improved by the selective excitation of electric quadrupole transitions.