Second and third optical harmonic generation in type I collagen, by nanosecond laser irradiation, over a broad spectral region

We report optical up-conversion of pulsed (ns) laser radiation in type I pure collagen, tuneable over a broad excitation spectrum covering the 760-1070 nm wavelength range. We investigated second harmonic generation (SHG) in collagen using lambda(p) = 1064, 901, 892, 828, 785 and 766 nm nanosecond pulsed laser excitation and recorded monochromatic signals at lambda(p)/2, i.e. 532, 451, 446, 414, 393 and 383 nm, respectively, corresponding to the SHG optical process. The SHG signal intensity exhibited a quadratic dependence on the excitation radiation (log[I-532] = 1.92*log[I-1064]). Furthermore, a weaker third harmonic generation (THG) signal from collagen was also observed at lambda(p)/3 (355 nm) using 1064 nm nanosecond pulsed laser excitation. The THG signal was found to have near-cubic dependence upon the irradiation laser intensity (log[I-356] = 2.53*log[I-1064]). The significance of collagen ability to exhibit broadly tuneable second harmonic generation is discussed. (C) 2000 Published by Elsevier Science B.V. All rights reserved.