Bulletin Spring‧Summer 1996

The Why's and Wherefore's of Refraction Laser　Interactions Light vis-à-vis Water Droplets How does strong ligh t interact with transparent refracting material such as water droplets? A research project by Prof. H.M. Lai and Prof. K. Young of the Department of Physics to study this phenomenon won an 'excellent' rating by the Research Grants Council, an honour it shared w i t h seven other CUHK research projects. A total of only 13 projects have been so rated in Hong Kong. Events That Raised Interest in the Subject Although funding from the Research Grants Council (RGC) was only applied for in the late 1980s, the two researchers became interested in the subject about 20 years ago, when both as new lecturers in the department became aware of a Bell Labs experiment which showed that a strong beam of laser shone perpendicularly onto a liquid surface did not push the surface in, but instead drew it upwards. This contradicted all conventional notions of radiation pressure, and was all the more strange since electromagnetism was supposed to have been completely understood for over a century. A chance visit by Prof. Sir Rudolph Peierls, head of the physics department at Oxford, pointed the way to a new approach. The critical issue, which the investigators recognized, is that the molecules re-arrange themselve s in the presence o f light energy, and a complete solution can be found for moderately long pulses of light passing through transparent refracting material. 1 W.M. Suen worked on this problem for his M.Phil, thesis, and has since obtained his Ph.D. fro m Caltech i n general relativity, become tenured in Washington University, and in 1995 won the Outstanding Young Researcher Award from the Overseas Chinese Physicists Association in the United States. He is currently an associate professor in the University's Department of Physics for one term. The early work on laser interaction with bulk liquids foreshadowed later investigations into interaction with microdroplets. The next major development came about through yet another chance visit, this time by Prof. Richard Chang of Yale University, i n the mid-1980s. There was then much interest in what happens when an intense laser pulse runs into fog — a collection of transparent microdroplets. Since this problem involves precisely the force exerted by light on a transparent liquid, the group at CUHK was one of the few that had the knowledge to solve it, and research results agreed precisely with experiments using single microdroplets. 2 30