Ultraviolet Pulsed Laser-Induced Fluorescence Nonlinearity in Optically Thick Organic Samples

A simple two-component model is worked out to investigate pulsed laser-induced fluorescence in complex organic samples, like biological tissues, optically thick at the excitation wavelength. Expression for emitted fluorescence signal is obtained. Saturation process is shown to be determined not only by fluorophores excited by the laser, but non-fluorescent chromophores with overlapping absorption band as well. For homogeneous samples the forms of saturation curves are determined by fluorophore's features. Experimental saturation curves of bulk paper and mice tissues ultraviolet pulsed laser-induced fluorescence are discussed considering this model. For the ns and shorter laser pulse durations with wavelength in 200-300 nm region, pulse energy density should be less than 200 μJ/cm² for correct quantitative comparison of fluorescence spectra of biological tissues with primarily tryptophan fluorescence.