  We propose a method for ultra-sensitive displacement and phase metrology
based on the interferometric evanescent wave excitation of nano-antennas. We
show that with a proper choice of nano-antenna, tiny displacements or relative
phase variations can be converted into sensitive scattering direction changes
in the Fourier $k$-space. These changes stem from the strong position
dependence of the imaginary Poynting vector orientation within interfering
evanescent waves. Using strongly-evanescent standing waves, high sensitivity is
achieved in the nano-antenna's zero scattering direction, which varies linearly
with displacement over a long range. With weakly-evanescent wave interference,
even higher sensitivity to tiny displacement or phase changes can be reached
around chosen location. The high sensitivity of the proposed method can form
the basis for many applications.
