Formation mechanism of coherent rainbows

Focusing white laser into samples, many colorful rings (coherent rainbows) come out. Such phenomena have been observed in many materials like water, acetone, absolute ethyl alcohol, soft drink and other liquids, and ice, colored glass, plastics, wax and other solids. From the center of the coherent rainbows to the outer side, the distance between neighboring rings becomes larger and larger. The coherent rainbow is an interference effect, whose optical path difference is induced by locally heating the material with the laser beam. Especially, the coherent rainbows from colored glass in reflection mode can be described with a simple formula, with which simulated results fit the observed interference pattern very well. Several possible mechanisms like nonlinear optical effect, thermal lens effect and self-phase modulation effect are excluded.