  We demonstrate that tunable attractive (bonding) and repulsive (anti-bonding)
forces can arise in highly asymmetric structures coupled to external radiation,
a consequence of the bonding/anti-bonding level repulsion of guided-wave
resonances that was first predicted in symmetric systems. Our focus is a
geometry consisting of a photonic-crystal (holey) membrane suspended above an
unpatterned layered substrate, supporting planar waveguide modes that can
couple via the periodic modulation of the holey membrane. Asymmetric geometries
have a clear advantage in ease of fabrication and experimental characterization
compared to symmetric double-membrane structures. We show that the asymmetry
can also lead to unusual behavior in the force magnitudes of a
bonding/antibonding pair as the membrane separation changes, including
nonmonotonic dependences on the separation. We propose a computational method
that obtains the entire force spectrum via a single time-domain simulation, by
Fourier-transforming the response to a short pulse and thereby obtaining the
frequency-dependent stress tensor. We point out that by operating with two,
instead of a single frequency, these evanescent forces can be exploited to tune
the spring constant of the membrane without changing its equilibrium
separation.
