The fluorescence of melanin in human skin tissue reflects the structural changes in the process of skin cancer progression towards malignant melanoma. A selectively excitation of melanin in skin tissue, however, is only possible by stepwise two-photon absorption (TPA) in the NIR spectral range (λ exc ≈ 800 nm). Due to the very short lifetime of the energy level populated by the absorption of the first photon (< 5 ps) an effective TPA process can be only achieved with laser pulses on the femtosecond time scale. To use the two-photon excited fluorescence (TPF) of melanin for early diagnosis of malignant melanoma a small, mobile TPF spectrometer was developed. It consists (i) of a fiber laser the radiation of which was amplified in a short pulse nitrogen laser pumped dye cell, (ii) an articulated arm with special mirrors for nearly lossless direction of the fs laser beam to the skin position of investigation, (iii) a camera objective which include visualization and excitation as well as acquisition of fluorescence and (iv) a monochromator with a multichannel photomultiplier and a PC with special software for spectrometer control and data processing. First investigations of the equipment concerning the early diagnosis of malignant melanoma were carried out.
The intensity-dependent transmission of primary leaves of Triticum aestivum seedlings at lambda = 694 nm was measured with single pulses of a Q-switch ruby laser. At photon flux densities above 2 x 10(17) cm-2s-1 a decrease of transmission was observed. The result is interpreted as a two-step absorption of cooperative units of 10(5)-10(6) chlorophyll molecules.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.