Transfer functions in collection scanning near-field optical microscopy

It is generally accepted that, if in collection near-field optical microscopy the probe-sample coupling can be disregarded, a fiber probe can be considered as a detector of the near-field intensity whose size can be accounted for via an intensity transfer function. We show that, in general, this perception is wrong and it is impossible to introduce such a transfer function for the detected signal. Instead, we introduce an amplitude coupling function that relates the near-field amplitude and the amplitude of a mode guided in a probe fiber toward a detector. Different experimental configurations are considered with respect to the relation between near-field optical images and the corresponding intensity distributions. Our conclusions are supported with numerical simulations and experimental results obtained by using a photon scanning tunneling microscope with an uncoated fiber tip.