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 grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
. 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 that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance...
s", as they do not remain guided, and have been observed in one and two-dimensional photonic crystal
Photonic crystals are periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons...
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
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
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 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 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 perpendicular to the first...
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 of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
), 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.
- David Rosenblatt, Avener Sharon, Asher A. Friesem
Asher A. Friesem is a professor at the Weizmann Institute of Science in Israel.Friesem received B.Sc. and Ph.D. degrees from the University of Michigan in 1958 and 1968, respectively. From 1958 to 1963 he was employed by Bell Aero Systems Company and Bendix Research Laboratories...
, "Resonant Grating Waveguide Structures", IEEE Journal of Quantum Electronics
The IEEE Journal of Quantum Electronics is a scientific journal covering optical, and electrical, and electronics engineering, and some applied aspects of lasers and physical optics. It is published by the Institute of Electrical and Electronics Engineers since 1965...
, Vol. 33, No.11 (1997).