Guided-mode resonance

Guided-mode resonance

Guided-mode resonance or waveguide-mode resonance is a phenomenon wherein the guided-modes of an optical waveguide can be excited and simultaneously extracted by the introduction of a phase-matching element such as a diffraction gratingor . Such guided modes are also called 'leaky modes' as they do not remain guided and have been observed in one and two-dimensional photonic crystal slabs.

Grating coupler

An important exapmle of guided mode resonance is a grating coupler. A grating coupler is a region on top (or below) a waveguide where there is a grating. Off resonance light incident on the grating would behave almost the same as it would if it was incident in an area where there is no grating. Waveguides are usually made of dielectric and are transparent. For specific combinations of incident angles and light frequency there is resonance. At resonance the grating allows to couple light into a guided mode of the waveguide.

Typically the grating coupler has only a few periods so light can be coupled in to the waveguide, but not back out. In such a case light will be guided in the waveguide unill it reaches the waveguide edge or an additional coupling element which will couple the light out. The larger the diffraction efficiency of the grating the larger percent of light that would be coupled in. If the grating is used as a coupling out element the larger the diffraction efficieny is the less periods would be needed to couple the light out.

Grating waveguide structures

A grating coupler which is extended over the whole surface of the grating results in a combined structure sometimes called grating waveguide structure. In such a structure light can not be guided as any light couple in is also coupled out. At resonance a normally transparent structure, becomes reflective. If the grating period is sub-wavelength than the normally transparent structure becomes a miror under resonance condition. These conditions include the angle, frequency, and [polarization] of the incident light. At resonance there is also a much higher intensity in the waveguide region. Such intensities are called evanescent as they decay exponentially outside of the waveguide region. The guided mode resonance can be used to design filters and sensors.


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