  Searching for pseudo-Nambu--Goldstone bosons (pNGBs) in weak-coupling domains
is crucial for understanding the dark components in the universe. We propose
searching for pNGBs coupled to two photons in the mass range from 0.1~eV to
10~keV. This could provide opportunities to test string-theory-based pNGBs
beyond the GUT scale $M \sim 10^{16}$~GeV included in the weak coupling
proportional to $M^{-1}$. We provide formulae that are applicable to
photon--photon scattering via a pNGB resonance exchange with a stimulation
process in an asymmetric head-on photon--photon collider by mixing three laser
pulses in laboratory experiments. We discuss the quantum electrodynamic effects
on the pNGB exchange in the same mass--coupling domain as a background process
from the standard model. We find that a large unexplored mass--coupling domain
is accessible by combining existing laser facilities, including free-electron
lasers.
