Nonlinear optical effects in PDT

Abstract
Use of near infrared instead of visible light would markedly improve tissue penetration, making larger tumors candidates for PDT. Since common photosensitizers exhibit virtually no absorption in this wavelength region, conditions are required where the simultaneous action of two photons is possible. Healthy tissue (rat ears), sensitized by hematoporphyrin derivative, sulfonated chloroaluminium phthalocyanine (CIBA-GEIGY) or pheophorbide a (PORPHYRIN PRODUCTS), was irradiated (1064 nm, 10 nsec) with power densities up to 200 MWcm^-2 and energy densities up to 200 kJcm^-2. No reproducible photodynamic lesions were observed, but there was a strong sensitizer fluorescence which depended quadratically on the excitation intensity. This may be useful to determine the tissue sensitizer concentration to an otherwise inaccessible depth.