Near-field spectroscopy of Si-doped GaAs

We have measured surface photoluminescence properties of Si-doped bulk GaAs using a near-field scanning optical microscope (NSOM). An apertured fiber probe tip is used as an emitter of excitation laser as well as a collector of luminescence from GaAs. Due to the Fabry-Perot etalon effect, the excitation laser is reflected or transmitted with an oscillation period of λHe-Ne/2 as the gap between the tip and the GaAs surface varies. The luminescence from GaAs also varies with an oscillation period of λGaAs/2 due to the same etalon effect. Therefore, the intensity of luminescence light collected by the probe tip shows a beating between two oscillations of different periods. When the probe approaches the GaAs surface, the collected luminescence intensity increase due to tunneling of evanescent wave. On the other hand, when we collect the luminescence using a lens, the intensity also increases due to similar coupling of evanescent wave into propagating wave in spite of a shadowing effect of the wide metal coating.