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 pu re collagen, tuneable over a broad excitation spectrum covering the 760-107 0 nm wavelength range. We investigated second harmonic generation (SHG) in collagen using lambda(p) = 1064, 901, 892, 828, 785 and 766 nm nanosecond p ulsed 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 depende nce on the excitation radiation (log[I-532] = 1.92*log[I-1064]). Furthermor e, a weaker third harmonic generation (THG) signal from collagen was also o bserved at lambda(p)/3 (355 nm) using 1064 nm nanosecond pulsed laser excit ation. The THG signal was found to have near-cubic dependence upon the irra diation laser intensity (log[I-356] = 2.53*log[I-1064]). The significance o f collagen ability to exhibit broadly tuneable second harmonic generation i s discussed. (C) 2000 Published by Elsevier Science B.V. All rights reserve d.