  When propagating through periodically structured media, i. e. photonic
crystals, optical waves will be modulated with the periodicity. As a result,
the dispersion of waves will no longer behave as in a free space, and so called
frequency band structures appear. Under certain conditions, waves may be
prohibited from propagation in certain or all directions, corresponding to
partial and complete bandgaps respectively. Here we report a new fascinating
phenomenon associated with the partial gaps, that is, deflection of optical
waves. This phenomenon will render novel applications in manipulating light
flows.
