  The invention of the laser immediately enabled the detection of nonlinear
photon-matter interactions, as manifested for example by Franken et al.'s
detection of second-harmonic generation. With the recent advancement in
high-power, high-energy lasers and the examples of nonlinearity studies of the
laser-matter interaction by virtue of properly arranging lasers and detectors,
we envision the possibility of probing nonlinearities of the photon interaction
in vacuum over substantial space-time scales, compared to the microscopic scale
provided by high-energy accelerators. Specifically, we introduce the
photon-photon interaction in a quasi-parallel colliding system and the
detection of higher harmonics in that system. The method proposed should
realize a far greater sensitivity of probing possible low-mass and weakly
coupling fields that have been postulated. With the availability of a large
number of coherent photons, we suggest a scheme for the detection of higher
harmonics via the averaged resonant production and decay of these postulated
fields within the uncertainty of the center-of-mass energy between incoming
laser photons. The method carves out a substantial swath of new experimental
parameter regimes on the coupling of these fields to photons, under appropriate
laser technologies, even weaker than that of gravity in the mass range well
below 1 eV.
