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

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 grating with a periodic structure that diffraction, diffracts light, or another type of electromagnetic radiation, into several beams traveling in different directions (i.e., different diffractio ...

or prism. Such guided modes are also called "

leaky mode A leaky mode or tunneling mode in an optical fiber or other waveguide is a mode having an electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as ...

s", as they do not remain guided, and have been observed in one and two-dimensional photonic crystal 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 Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...

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 In optics, diffraction efficiency is the performance of diffractive optical elements – especially diffraction gratings – in terms of power throughput. It's a measure of how much optical power In optics, optical power (also refer ...

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

A grating 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 and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

), 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