Photonic band structure of periodically etched high index waveguide layer:mode suppression in multimode waveguides

There is considerable current interest in using photonic band gap structures to control spontaneous emission and mode density in miniature laser resonators. In this paper, the photonic band structure and possible applications of resonant modes that occur within strongly modulated periodic dielectric films are reported. The structure analysed consists of a planar waveguide formed between planar sheets of perfect metal. Within the waveguide are alternating lines of high index dielectric and air. The guided modes in this structure (solid lines on figure) can be compared with those that would occur in a multimode waveguide with a homogeneous guiding layer of the same average index (dashed lines). At the Bragg condition a bandgap appears and this can be used to reduce the number of guided modes. From the figure it can be seen that at a normalised frequency of 4.2 there is a 13:1 mode reduction. In this way a fairly thick multi-mode waveguide can be made in which all but the highest order mode are suppressed, rendering the structure single-mode. The field microstructure of this mode shows that most of its intensity is a concentrated in the high index regions. This mode could be excited by rare-earth doping in the high index regions alone. This could have applications in single-mode waveguide lasers where an increased mode volume could be very beneficial