Surprising as is sounds, the principle behind LIMM is the same that makes your basement a perfect storage place for your wine. At the surface, there are large temperature fluctuations between day and night, yet due to the limited thermal conductivity of the soil the daily temperature cycle almost vanishes as soon as we look a few meters below ground.
In more precise terms, we have a thermal wave which penetrates into the ground, but with decreasing amplitude as we move away from the surface. How far this wave penetrates into the ground depends on the frequency of the outside temperature oscillation: the faster the oscillation, the smaller the penetration depth. In the example of the wine cellar, the day/night fluctuation is hardly noticable, but the much slower summer/winter cycle still makes the temperature change by a few degrees.
For LIMM, the same physical principle applies: If the modulation frequency of the laser is small, the whole sample is heated homogeneously (watch how the thermometers at the front and back surface oscillate synchronously).
At high frequencies, the picture changes: the temperature variation at the rear surface becomes smaller, and the oscillation at different points inside the sample no longer happens synchronously (note how T2 lags behind T1).