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

Abstract

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 λ p=1064, 901, 892, 828, 785 and 766 nm nanosecond pulsed laser excitation and recorded monochromatic signals at λ 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 λ 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.