  Many theories of quantum gravity, as string theory, loop quantum gravity, and
doubly special relativity, predict the existence of a minimal length scale and
outline the need to generalize the uncertainty principle. This generalized
uncertainty principle relies on modified commutation relations that - if
applied to the second quantization - imply an excess energy of the
electromagnetic quanta with respect to $\hbar \omega$. Here we show that this
"dark energy of the photon" is amplified during nonlinear optical process.
Therefore, if one accepts the minimal length scenario, one must expect to
observe specific optical frequencies in optical harmonic generation by intense
laser fields. Other processes as four-wave mixing and supercontinuum generation
may also contain similar spectral features of quantum-gravity. Nonlinear optics
may hence be helpful to falsify some of the most investigated approaches to the
unification of quantum mechanics and general relativity.
