Scattering theory of Bardeen's formalism for tunneling: New approach to near-field microscopy

We propose a new theoretical approach to near-field microscopy, which allow s one to deal with scanning tunneling microscopy and scanning near-field op tical microscopy with a unified formalism. Under the approximation of weak tip-sample coupling, we show that Bardeen's perturbation formula, originall y derived from a scattering formalism which extends its validity to electro magnetic vector fields. This result should find broad applications in near- field imaging and spectroscopy.