  Since the forward and backward waves are coupled with each other and a
standing wave with no net propagation of energy is formed in the photonic
bandgap, it is a commonsense of basic physics that, any kinds of effects
associated with wave propagation including four-wave-mixing (FWM) are thought
to be impossible. However, we lay great emphasis here on explaining that this
commonsense could be broken under specific circumstances. In this article, we
report with the first experimental observation of the energy conversion in the
photonic bandgap into other channel via FWM. Owing to the phase manipulation by
fast light effect in the photonic bandgap, we manage to achieve the phase-match
condition and thus occurred FWM transfer energy into other channels outside the
photonic bandgap efficiently. As one-dimensional photonic crystal, simulations
on fiber Bragg grating (FBG) with and without fast light were conducted
respectively, and an enhanced FWM in photonic bandgap of FBG was observed. The
experimental result shows great agreement with the analysis.
