Nanometer-scale dielectric constant of Ge quantum dots using apertureless near-field scanning optical microscopy

Tip-enhanced near-field scattering images of Ge quantum dots (QDs) with 20–40 nm height and 220–270 nm diameter grown on a Si substrate have been observed with a spatial resolution of 15 nm. Changing the wavelength of the incident light, the contrast of the images is reversed. It is found that the scattering intensity is caused by the dielectric constants of the materials under the probe. By changing the wavelength of the incident light, we have obtained information about the dielectric constant dispersion of single Ge QDs.