Search

The photonic surface modes supported by multilayer dielectric stacks are highly dispersive. They can therefore be used to design mode-selective taps and narrow-band notch transmission filters for waveguide applications.

The surface-guided Bloch modes guided by the interface between a multilayer dielectric stack and an external medium are highly dispersive. This makes attractive their use in mode-selective coupling of light into and out of side-polished multimode fibers. The particular case of a dual-mode fiber is modeled. Mode selection by means of surface-guided Bloch modes is found to offer advantages in comparison with selection by means of a single-mode waveguiding film of the same average refractive index as the stack. Good modal selectivity is achievable together with high tunability, i.e., high sensitivity to the refractive index of the intervening layer between stack and side-polished fiber.

Photonic band gap materials are dielectrics with a synthetic, three dimensional, multiply periodic microstructure (lattice constant of order the optical wavelength) whose distinguishing feature is a very large modulation depth of refractive index. When appropriately designed, these “photonic crystals” exhibit ranges of optical frequency where light cannot exist — the photonic band gaps 7. The current interest in these materials 1–22 has led us to re-appraise propagation in structures that, while not exhibiting a complete photonic band gap (PBG), nevertheless display anomalous and intriguing propagation effects in the vicinity of their Bragg conditions54–58. In most cases, around each Bragg condition appear incomplete momentum and energy gaps (i.e., ranges of, respectively, wavevector and frequency where propagation is forbidden) with widths that are given approximately by the product of the index difference with, respectively, the vacuum wavevector and h times the optical frequency. With the exception of the multi-layer dielectric stack, most conventional electromagnetic gratings, such as those encountered in holography 27, waveguides 45, distributed feedback lasers 35,37,38, acousto-optic 47 and x-ray 61 diffraction, consist of weak periodic perturbations about a mean refractive index. In these gratings, while strong spatial and temporal dispersion are present around each Bragg condition, the ranges of angles and frequencies over which this occurs are very narrow; and although PBG’s do appear, they are incomplete and mostly very weak.

Photonic band gap materials are dielectrics with a synthetic, three dimensional, multiply periodic microstructure (lattice constant of order the optical wavelength) whose distinguishing feature is a very large modulation depth of refractive index. When appropriately designed, these "photonic crystals" exhibit ranges of optical frequency where light cannot exist - the photonic band gaps. The current interest in these materials has led us to re-appraise propagation in structures that, while not exhibiting a complete photonic band gap (PBG), nevertheless display anomalous and intriguing propagation effects in the vicinity of their Bragg condition. In most cases, around each Bragg condition appear incomplete momentum and energy gaps (i.e., ranges of, respectively, wavevector and frequency where propagation is forbidden) with widths that are given approximately by the product of the index difference with, respectively, the vacuum wavevector and h times the optical frequency. With the exception of the multi-layer dielectric stack, most conventional electromagnetic gratings, such as those encountered in holography, waveguides, distributed feedback lasers, acousto-optic and x-ray diffraction, consist of weak periodic perturbations about a mean refractive index. In these gratings, while strong spatial and temporal dispersion are present around each Bragg condition, the ranges of angles and frequencies over which this occurs are very narrow; and although PBG's do appear, they are incomplete and mostly very weak. ...

A parallel interface between a periodic multilayer structure and a uniform external medium can support surface electromagnetic waves. The energy in such a wave is localised about the boundary, and is excluded from the bulk of the multilayer structure by a Bragg condition. The possibility of quasi-single-mode behaviour, together with the freedom to make k as low as the plane-wave value in the external medium, and the great sensitivity of k to the detailed properties of the edge layer, gives these surface waves many potential applications in optical sensors, modulators, and other devices.