Phonons are quantized vibrations. They can range from low frequencies, such as sound, to very high frequencies, such as heat. The data shown above is at the high frequency end—about 600 GHz. These phonons (heat pulses) are emanating from a small laser-heated region on one surface of a sample cooled to 2 Kelvin and are detected on the opposite surface. The pictures show how the heat arrives at the detection surface; each is essentially a snap-shot of the temperature distribution on the surface at a given instant. The set of images shows how the heat propagates through time.
This work was done by UIUC graduate student, Matt Hauser (Ph.D., 1995, "Acoustic Waves in Crystals: I. Ultrasonic Flux Imaging and Internal Diffraction. II. Imaging Phonons in Superconducting Niobium") under the direction of Professor James P. Wolfe.