  We present a novel method for active continuous-tuning of a band gap which
has a great potential to revolutionize current photonic technologies. We study
a periodic structure of x and y-aligned nanorod dimers. Refractive index of a
y-polarized probe pulse can be continuously-tuned by the intensity of an
x-polarized auxiliary (pump) pulse. Order of magnitude index-tuning can be
achieved with a vanishing loss using the plasmon-analog of refractive index
enhancement [Phys. Rev. B 100, 075427 (2019)]. Thus, a large band gap can be
created from a non-existing gap via the auxiliary pulse. We also present a
"proof of principle" demonstration of the phenomenon using numerical solutions
of Maxwell equations. The new method, working for any crystal dimensions, can
also be utilized as a linear photonic switch operating at tens of femtoseconds.
