Abstract: An exact representation of dispersion relation for one dimensional periodic system composed of dielectric layers with a defect is derived by means of transfer matrix, in order to calculate the group velocity of the defect modes in the photonic band gap. It is found that slow group velocities can be tuned by modifying size of the defect layer,as moving towards the band edge the frequency values get lower shifts.Effect of the different defect dielectric constants on the group velocity is also presented.

Keywords: photonic crystal, defect mode, dispersion relation, group velocity, slow light.