Towards Perfect Vertical Photonic Band Gap Confinement in a Photonic Crystal Slab

A distributed Bragg reflector (DBR) with air rods was studied theoretically as cladding for a photonic crystal slab in an effort to obtain perfect vertical gap confinement. This DBR has photonic band gaps in the perpendicular and parallel directions, but they do not overlap. When an air-rod-free (i.e., conventional) DBR is used for the cladding, the light line decreases profoundly in the photonic band diagram and the photonic band gap also becomes small, indicating that this type of DBR is greatly inferior to a DBR with air rods as the cladding. The light line of the DBR cladding with air rods is only slightly smaller than that for the extensively used air cladding because of the large air rod radius of 0.47a (a is a photonic crystal period). In addition, the photonic band gaps between the lowest and second lowest TE-like modes and the lowest TE- and TM-like modes are still large for this DBR cladding. Thus, this cladding can be used for a photonic crystal slab. Numerical analysis also predicts that a guided mode has a nearly zero group velocity, because the DBR cladding acts as a reflector for a light propagating in the perpendicular direction.