Biological Effects of Laser Radiation. Volume IV. Optical Second Harmonic Generation in Biological Tissues

摘要

Radiation which appeared to be due to optical second-harmonic generation (SHG) at 347 nm. was observed from cornea, tendon, sclera, and skin on 694 nm. Q-switched ruby laser irradiation. A possible source of this second-harmonic generation was tissue collagen; because of high tissue content and relatively high order. SHG was also observed from glutathione, but not from the lens; and from frog skeletel muscle. Studies were consequently conducted on purified collagen fibers; SHG was observed from a commercial preparation. Collagen order is reduced near 60 C. A large reduction in conversion efficiency at 347 nm. was observed when cornea and tendon were heated above the collagen phase transformation temperature. Therefore, the collagen component of tissue may be the principal site for the SHG. Experiments on normal corneas showed the 347 nm. scattering pattern to more closely resemble coherent second-harmonic generation from a crystalline material than it did incoherent second-harmonic generation from an amorphous material. Corneal models for SHG were developed. The observed conversion efficiency of the normal cornea was in much better agreement with that calculated for coherent than for incoherent conversion. The reduction in conversion efficiency for corneas treated above the collagen phase transformation temperature was explained as a reduction in coherent second-harmonic generation in corneal stroma lamellae due to a decrease in collagen fibril order. (Author)