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2D Photonic crystals (PC) with single site defects behave as standing-wave resonators and offer exceptional mode selectivity. This high mode selectivity offers many advantages for higher-order mode suppression and applications at frequencies higher than X-band. In this paper we focus on the RF excitation by electron bunches and RF coupling in a PC cavity resonator. We first compare the RF excitation with that seen in a traditional pill-box cavity operating at the same frequency. We find that in a single defect PC cavity only the TM 010 mode can be excited and confined in the central defect region. We then expand this analysis to include application towards pre-buncher and power extraction cavities for particle acceleration. Power extraction cavity is designed in the form of a two-coupled-cavity structure, in which the excited field is mutually coupled between two defects sites. The pre-buncher cavity is constructed by introducing a stub coupler to one of the cavities in a two-coupled-cavity structure, which detunes the cavity to exclude the fundamental mode from the cavity. In this case the excited field propagates into the adjacent cavity which confines the mode. We cold test and analyse the transmission curve of a stub-coupled structure against the full-band-gap plot. We also discuss the sensitivity of the observed field due to the perturbation of the detecting probe.