Broadly tunable optical harmonic generation in type I collagen using IR nanosecond laser pulses

Under pulsed nanosecond laser irradiation of type I pure collagen over the 760-1070 nm spectral range, optical upconversion of the incident radiation is observed. More specifically irradiation of collagen at 1064, 901, 892, 828, 785 and 766 nm produces monochromatic second harmonic signals at half the original wavelengths i.e. 532, 451, 446, 414, 393 and 383nm respectively. The dependence of the second harmonic signal on the excitation intensity was found to be quadratic (log[I-532]=1.92*log[I-1064]). A weaker third harmonic signal was also observed from collagen at 355nm when irradiated by 1064nm nanosecond pubes. This signal was found to bear a near cubic dependence upon the irradiation intensity (log[I-356]=2.53*log[I-1064]). The polar distribution of the second harmonic radiation was recorded for both pure fibrous collagen as well as for collagen diluted in acetic acid and forming a semitransparent dry film. In the latter case, significant optical behavior was demonstrated, potentially important for further studies of these nonlinear phenomena and for novel applications.