| Abstract: | We demonstrate the direct 1064 nm two-photon excitation of hematoporphyrin derivative (HPD), a complex mixture of photosensitizing porphyrins which is selectively retained in tumor tissue and used in cancer photochemotherapy. Although 1064 nm is outside of the one-photon HPD absorption spectrum, two-photon induced fluorescence from HPD was observed following excitation by the 20 ns output of an amplified, Q-switched Nd-YAG laser at peak power levels of 0.1 to 3 GW/cm2. Evidence for the successful two-photon excitation to vibrational levels of the S1 state consists of the observation of the known HPD fluorescence spectrum exhibiting peaks at approximately 615 and 675 nm, with the observed two-photon induced fluorescence intensity exhibiting a quadratic dependence on the excitation laser intensity as required for a direct two-photon process. More generally, these results suggest the possibility for the achievement of photosensitized oxidations utilizing photons of lower energy than that required for single photon excitation, offering the potential for both greater selectivity and a reduction in competing photochemical processes. |
