Non-perturbative measurement of evanescent fields

Evanescent electromagnetic fields that decay exponentially with distance from a substrate surface are challenging to measure accurately because the experimental procedure usually perturbs the distribution. We propose a non-perturbative method of characterizing an evanescent field by scanning a particle that is illuminated by a second wavefront that creates a controlled interference pattern. Because it can always be located in a dark region of the interference fringes, the particle does not scatter light and distort the evanescent field. We demonstrate the measurement principle with Finite-difference time-domain numerical simulations.