Nonlinear scattering in hard tissue studied with ultrashort laser pulses

Area of Science:

• Biophotonics
• Laser-tissue interactions
• Non-linear optics

Background:

• Ultrashort laser pulses can induce non-linear optical phenomena in biological tissues.
• Understanding these interactions is crucial for developing advanced imaging and diagnostic tools.
• Human dental tissues exhibit unique optical properties that warrant investigation.

Purpose of the Study:

• To investigate the back-scattered non-linear optical spectrum of ultrashort laser pulses in human dental tissues.
• To characterize the generation of second harmonic generation (SHG), frequency tripled radiation, and two-photon fluorescence.
• To assess the potential of these optical signals for dental diagnostics.

Main Methods:

• In vitro study of human dental and other hard tissues using ultrashort laser pulses (800 nm, 0.2 ps) below the ablation threshold.
• Detection and relative yield measurement of SHG, frequency tripled radiation, and two-photon fluorescence.
• Analysis of SHG signal dependence on probe thickness and polarization.

Main Results:

• Frequency doubled radiation (SHG), frequency tripled radiation, and two-photon fluorescence were detected in back-scattered spectra.
• SHG signals were measured in dentin, cementum, and enamel, with dentin showing larger signals.
• No SHG signal was detected in carious dental tissues.
• SHG signal intensity demonstrated dependence on probe thickness and incident laser polarization.

Conclusions:

• Non-linear optical processes, particularly SHG, can be generated in human dental tissues using ultrashort laser pulses.
• The distinct SHG responses in different dental tissues and absence in carious areas highlight potential for diagnostic applications.
• Further research into higher harmonic radiation could lead to novel microscopy and diagnostic techniques for dentistry.