When pulses of light energy absorbed by a sample are converted rapidly into heat, a sound wave can be produced through periodic thermal expansion. The intensity of the sound wave directly relates to the amount of absorber in the sample. Since the signal is produced directly from the absorber, as opposed to inferring it from the absence of signal (as in absorbance spectroscopy), it is well-suited for measuring trace contaminants.

Typically, this sound wave is recorded with a sensitive microphone. Impressive detection limits have been obtained, making photoacoustic techniques popular in trace gas detection. In our lab, we use a fiber Fabry-Perot (FFP) cavity in conjunction with the Pound-Drever-Hall (PDH) technique as an optical microphone to record the sound wave.

We have previously used this method to detect analytes in microfluidic devices and in capillary electrophoresis. Recently we use acoustic resonators such as a wineglass or cantilever beams to further amplify the signal.