Abstract: A method and apparatus for detecting optical radiation by optically monitoring temperature changes in a microvolume caused by absorption of optical radiation to be detected. More specifically, a thermal lens-forming material is provided which has first and second opposite, substantially parallel surfaces. A reflective coating is formed on the first surface, and a radiation-absorbing coating is formed on the reflective coating. Chopped, incoming optical radiation to be detected is directed to irradiate a small portion of the radiation-absorbing coating. Heat generated in this small area is conducted to the lens-forming material through the reflective coating, thereby raising the temperature of a small portion of the lens-forming material and causing a thermal lens to be formed therein. The thus-formed thermal lens is optically detected by irradiating the thermal lens by a collimated light beam.

Abstract: A method and apparatus for measuring thermal diffusivity and molecular relaxation processes in a sample material utilizing two light beams, one being a pulsed laser light beam for forming a thermal lens in the sample material, and the other being a relatively low power probe light beam for measuring changes in the refractive index of the sample material during formation and dissipation of the thermal lens. More specifically, a sample material is irradiated by relatively high power, short pulses from a dye laser. Energy from the pulses is absorbed by the sample material, thereby forming a thermal lens in the area of absorption. The pulse repetition rate is chosen so that the thermal lens is substantially dissipated by the time the next pulse reaches the sample material. A probe light beam, which in a specific embodiment is a relatively low power, continuous wave (cw) laser beam, irradiates the thermal lens formed in the sample material.