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

Area of Science:

• Optics and Photonics
• Biophysics
• Spectroscopy

Background:

• Pulsed laser-induced fluorescence (LIF) is vital for analyzing complex organic samples, including biological tissues.
• Understanding fluorescence saturation is critical for accurate quantitative analysis.
• Optical properties of biological tissues can complicate LIF measurements.

Purpose of the Study:

• To develop a two-component model for pulsed laser-induced fluorescence in optically thick organic samples.
• To investigate the factors influencing fluorescence signal saturation.
• To provide guidelines for accurate quantitative fluorescence measurements in biological tissues.

Main Methods:

• Development of a theoretical two-component model for LIF.
• Derivation of an expression for the emitted fluorescence signal.
• Analysis of experimental saturation curves for bulk paper and mouse tissues.

Main Results:

• The fluorescence saturation process is influenced by both excited fluorophores and non-fluorescent chromophores with overlapping absorption.
• For homogeneous samples, saturation curves are primarily determined by fluorophore characteristics.
• Experimental data from paper and mouse tissues align with the model's predictions.

Conclusions:

• The proposed model accurately describes pulsed LIF in complex organic samples.
• Accurate quantitative comparison of biological tissue fluorescence, especially tryptophan fluorescence, requires pulse energy density below 200 μJ/cm² for ns and shorter pulses (200-300 nm).
• The model highlights the importance of considering non-fluorescent components in LIF saturation analysis.