Pumped phonons in atmospheric aerosols

摘要

Phonons describe packets of vibrational energy in the same way that photons describe packets of electromagnetic energy. While pumping of electronic levels is a familiar concept, the notion that phonons could be pumped and change the population of vibrational levels did not appear until 1980 when the late Herbert Froehlich foresaw that such a mechanism might be of importance in biosystems. For these, his insight has still to be given experimental vindication but an application for his ideas has been found in explaining the results of atmospheric absorption measurements of millimetre and submillimetre waves by showing how apparent anomalies could be attributed to aerosols. Aerosols in the sea level atmosphere contain less than a millionth of the water that is typically present as vapour, their interaction with visible radiation is inconspicuous and only shows in atmospheric path lengths of the order of kilometres. At first sight, this suggests that as Rayleigh scatterers their effects at longer wavelengths would be negligible, but this is not so since they have surface mode resonances which, when excited by pumping, can produce spectacular effects at millimetre wavelengths. For 30 years or more studies at these wavelengths gave many intriguing observations but little or no understanding of the underlying mechanisms. Froehlich's pumped phonon model was new thinking and the results it explained came from a new technique in measuring. My aim here will be to show how these two new innovations have come together to give an understanding which will lead to new applications. Atmospheric science will be the first to benefit but the expectations for biophysics from both theory and experiment are high. Since I was involved from the beginning in the new experimental technique and there are some useful lessons from its history, I will outline the story of the development.