When the two laser beams cross, they interfere with each other to make a light pattern. This pattern acts like a temporary grating because it changes the local properties of the sample. For example, the bright stripes excite the molecules, which changes how the material bends light.
The dark stripes leave the molecules in their normal state. When we shine a third probe beam at the sample, it bounces off this light pattern. As time passes, the excited molecules relax or move into the dark areas.
This movement heats up the sample and makes the temporary pattern disappear. By measuring how fast the pattern fades, scientists can calculate how quickly heat or molecules travel. Many students think a grating must be a solid physical object.
In this technique, the grating is just a temporary pattern made of light inside the liquid or solid. This method is very useful because it does not require touching the sample with physical wires. It lets us study materials that are too fragile or too hot for regular thermometers.