Optical coherent transient phenomena come from a physical process known as the photon echo. This process is defined as an echo because the first echoes produced looked exactly like the input pulses. In other words, two pulses of light went into the material, spaced temporally by T21 and out comes a third pulse similair but weaker than the first two exactly T21 after the second pulse.
The photon echo process can only come from certain types of materials. These materials must possess a laser transition at some frequency, with a small homogeneous broadening. This transition must be spread out, usually because of imperfections in the material's structure, over a broad range, known as the inhomogeneous broadening. Assuming these two cases are met, and the dephasing time is short (why we cool our crystals to 4K) a photon echo can be created.
Here is a schematic of the homogeneous and inhomogeneous transitions.
For most of the work we do, we use a third pulse that controls the photon echo process and gives us a stimulated photon echo. This is shown below, with the first two pulses being those shown above and the last big pulse the third pulse.
Here is a zoom of the last pulse with the stimulated photon echo.
The stimulated photon echo process is actually governed by a special integral that has unique and exciting features. This integral is shown on the next page allong with applications.
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