it is a photothermal effect and results when energy from a laser beam passing through a sample is absorbed, causing heating of the sample along the beam path. The lens is created through the temperature dependence of the sample refractive index. The lens usually has a negative focal length since most materials expand upon heating and the refractive index is proportional to the density. This negative lens causes beam divergence and the signal is detected as a time dependent decrease in power at the center of the beam.