  The observed accelerating universe indicates the presence of Dark Energy
which is probably interpreted in terms of an extremely light gravitational
scalar field. We suggest a way to probe this scalar field which contributes to
optical light-by-light scattering through the resonance in the quasi-parallel
collision geometry. As we find, the frequency-shifted photons with the
specifically chosen polarization state can be a distinct signature of the
scalar-field-exchange process in spite of the extremely narrow width due to the
gravitationally weak coupling to photons. Main emphasis will be placed in
formulating a prototype theoretical approach, then showing how the weak signals
from the gravitational coupling are enhanced by other non-gravitational effects
at work in laser experiments.
