Dispersion, tunability and applications of defect modes in photonic band-gap structures

Abstract

Electronic surface states, whose energies lie within the semiconductor band-gap, are normally regarded as a nuisance to be avoided in semiconductor lasers. This is particularly true of micro-pillar laser arrays where the surface-area-to-volume ratio is large. Similarly, intra-photonic-band-gap interface or surface states will reduce the effectiveness of a photonic bandgap material by introducing intra-band-gap states into which unwanted spontaneous emission and lasing can occur. These are not, however, always undesirable. Examples include: i) the DFB laser mode supported by a structural defect at the centre of a uniform Bragg mirror - the resonant frequency of this mode lies within the photonic bandgap of the Bragg mirror; ii) the surface-guided Bloch modes (SGBM) confined at the surface of multilayer stacks; and iii) Bragg waveguide modes (BWGM) in which total internal reflection is replaced by Bragg reflection between two multilayer stacks. A general feature of defect modes is a phase velocity that is highly sensitive both to optical frequency and to the "strength" of the local aperiodicity that defines the defect. Small compositional and structural changes can radically alter the position of the mode within the stop-band, providing an effective tuning mechanism. Owing to these and other unique properties, defect modes may provide the basis for the development of a versatile new family of optoelectronic devices. In this paper we discuss some properties of SGBM's within the context of photonic bandgaps and show how they can be used to realize a tunable narrow-band fibre tap and a mode-selective fibre coupler.

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