  The effect of electromagnetic wave irradiation on the phase dynamics of
intrinsic Josephson junctions in high temperature superconductors is
investigated. We predict three novel effects by variation of the radiation
amplitude and frequency: changing of the longitudinal plasma wavelength at
parametric resonance; double resonance of the Josephson oscillations with
radiation and longitudinal plasma wave; charging of superconducting layers in
the current interval corresponding to the Shapiro step. The "bump" structure in
IVC recently observed experimentally is demonstrated. We also observe ragged
Shapiro steps at double resonance.
