Nonlinear optical behaviour of fish scales

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Abstract
Observations of second harmonic generation and ferroelectric behaviour in in vitro biological structures are reported. The scales of the Indian freshwater fish (Catla catla, Cytrinidae) have been found to have nonlinear optical properties, exhibiting frequency doubling (second harmonic generation, SHG). In contrast to this behaviour it has not proved possible to obtain SHG signals from the scales of British freshwater fish (Roach, Rutilus rutilus L). Experiments were also conducted on scales treated with various organic solvents and on dry collagen. Optical transmission and infra-red spectra have also been measured and the latter indicate that the scales obtained from both species have nearly the same composition and are collagen rich. The dry collagen itself gives rise to an SHG signal which is about eight times as intense as that from the Indian fish scales. The fish scales and collagen are found to be ferroelectric at room temperature.