Electromagnetically induced grating
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Through Light and Coherence (physics)
Writing, reading and phase-matching conditions for EIG diffraction
Figure 1 shows a possible beam configuration to write and read an EIG. The period of the grating is controlled by the angle . The writing and reading frequencies are not necessarily the same. EB is referred as the "backward" reading beam and ER is the signal obtained by diffraction on the grating.
The phase-matching conditions for the EIG for the plane-wave approximation is given by the simple geometric relation:
,
where the angles are given according to Fig. 2, and are the frequencies of the writing (W, W') and reading beam (R), respectively, and n is the effective index of refraction of the medium.
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Types of EIG
Matter gratings
The writing lasers form a grating by modulating density of matter or by localizing matter (trapping) on the regions of maxima (or minima) of the writing interference fields. A thermal grating is an example. Matter gratings have slow dynamics (milliseconds) compared to population and phase gratings (potentially nanoseconds and faster).
Population gratings
The writing lasers are resonant with optical transitions in the matter and the grating is formed by optical pumping (See Fig. 3). This type of grating can be easily tuned to produce multiple orders of diffraction.
Coherence gratings
A grating where the writing lasers form a coherent matter pattern. An example is a pattern of electromagnetically induced transparency.
Applications
Usually two lasers at an angle are used to build an EIG. The EIG is used to diffract a third laser, to monitor the behavior of the underlying substrate where the EIG was written or to serve as a switch for one of the lasers involved in the process.