Dr John Scales - Everything you always wanted to know about Terahertz waves, but were afraid to ask, Oct 28, 2012

Dr John Scales

Everything you always wanted to know about Terahertz waves, but were afraid to ask - Oct 28, 2012

View Dr Scales' abstract here.

Prof. John Scales did his undergraduate work in physics and mathematics at the University of Delaware, then studied Mathematical Physics at University of Colorado, where he got his PhD in 1984. Prof. Scales post-doc was unusual; he wanted to get hands on experience with high-performance computing so he went to work for a (now-defunct) oil company and became involved in enormous seismic computing and data analysis (inverse theory). The latter is statistical inference applied to physical experiments: If you show a color picture of, say, the Cosmic Microwave Background made from finite, noisy measurements, how do you know whether a given blob is really required to fit the data. Maybe it's really an artifact of your algorithm. Scientists tend to hate this problem because it raises existential questions. For instance, it turns out that it is impossible to get finite uncertainty without making assumptions that are completely independent of the data - so called a priori knowledge. But then you must quantify the uncertainty in this information. What does it mean to know something? This is important work but it won't make you many friends. (Error bars? Why in his field they call those axes.) Another classic view is summarized as: if he hadn't believed it, he wouldn't have seen it. Around this time he was deeply influenced by a sabbatical at the Institute de Physique du Globe de Paris, with the late Albert Tarantola, a pioneering empirical Bayesian.

In addition to daunting statistical problems, in order to study inverse problems for waves, one must be able to solve the prediction problem for waves in realistically heterogeneous media. This eventually let to a new career in random dynamical systems, wave chaos, or waves in random media. There were so few people at the time doing experiments in this area that John decided to take the plunge. So, following another sabbatical in Paris, this time at the École supéieure de physique et de chimie industrielles de la ville de Paris with Matthis Fink (the inventor of time-reversed acoustics), he came back to Golden and began setting up a laser ultrasound lab. Using lasers his group could excite waves in any solid and measure the atomic-level particle motions with an interferometer. You'll find a number of their movies here: laser ultrasound movies. Some of these bear an uncanny resemblance to simulations.

Since about 2005 (when he moved from the Geophysics Department to the Physics Department at CSM) Prof. Scales has been focusing on quasi-optical measurements. These exploit electromagnetic waves with small enough wavelength that beams can be small and tightly collimated. Standard items such as lenses, isolators and beam splitters exist, but not from the same materials that you would use in optics. (The dielectric constant cannot be constant!) He and his students are currently extending all of these techniques to higher spatial resolution (for instance by moving farther into the THz region). Their applications include such practical things as millimeter wave vibrometry for land-mine detection; thin film characterization for next-gen solar cells, as well as more fundamental studies of the collective behavior of arrays of quantum dots, and coherent multiple scattering effects such as Anderson Localization