Guided-mode resonance or waveguide-mode resonance is a phenomenon wherein the guided

mode Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is ...

s of an

optical waveguide An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid light ...

can be excited and simultaneously extracted by the introduction of a phase-matching element, such as a

diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structur ...

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

. 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 A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic ...

slabs.

Grating coupler

An example of guided-mode resonance is a ''grating coupler'', which is a region on top of or below a waveguide where there is a grating. Off-resonance light incident on the grating behaves almost the same as it would if it was incident in an area where there is no grating. Waveguides are usually made of

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

and are transparent. For specific combinations of incident angles and light frequency, there is resonance, allowing the grating to couple light into a guided mode of the waveguide. Typically, the grating coupler has only a few periods, so light can be coupled into the waveguide, but not back out. In such a case, light will be guided in the waveguide until it reaches the waveguide edge, or an additional coupling element, which will couple the light out. The larger the

diffraction efficiency Diffraction efficiency is the performance of diffractive optical elements – especially diffraction gratings – in terms of power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put ...

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 efficiency, the fewer periods would be needed to couple the light out.

Grating waveguide structures

grating A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicul ...

coupler that is extended over the whole surface of the grating results in a combined structure sometimes called a ''grating waveguide structure.'' In such a structure, light cannot be guided, as any light coupled in is also coupled out. At resonance, a normally transparent structure becomes reflective. If the grating period is sub-wavelength, then the normally-transparent structure becomes a mirror under resonance conditions. These conditions include the angle, frequency (

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

), 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.

Grating coupler

Grating waveguide structures

References

Further reading