Lossless interface modes at the boundary between two periodic dielectric structures

Tamm states of light are lossless interface modes decaying exponentially in the surrounding media. We show that they can be formed at the boundary between two periodical dielectric structures, one having a period close to the wavelength of light and another one having a period close to the double of the wavelength. The order of layers at the interface has a crucial effect on the Tamm states. The in-plane dispersion of these states is parabolic with effective masses slightly different for TE and TM polarizations, both of the order of 10^-5 of the free electron mass.