Near-field optical spectroscopy using an incoherent light source

We present a method to perform near-field optical spectroscopy. It consists of an apertureless near-field optical microscope combined with an incoherent light source and a monochromator. We show that the optical response of the tip depends both on the wavelength and the tip apex shape. The experimental optical response of the tips is in good agreement with the predicted theoretical one. The near-field optical spectrum obtained on a gold island in the visible spectrum range (lambda=400-600 nm) is presented and shows that the method is reliable to perform spectroscopic measurements on submicrometer-sized objects. (C) 2000 American Institute of Physics. [S0003-6951(00)02204-X].