Surface phonon imaging through the photoelastic effect



Surface phonon imaging is very useful for testing thin structures, in particular because it allows film thicknesses or elastic constants to be derived. We present a method for imaging surface phonons based on optical reflectivity measurements combined with excitation using picosecond optical pulses. Surface phonons are generated in a thin aluminium film on a transparent glass substrate. The induced strain modulates the refractive index of the film and substrate through the photoelastic effect. Probe laser pulses incident from the film or substrate side can be used to image surface phonon propagation in real time by simply monitoring the modulation in the reflected probe beam intensity. When we image from the substrate side, surface-skimming longitudinal bulk phonons are clearly imaged in addition to the surface phonons. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)