Two-level model of melanin absorption in picosecond laser skin treatment

Abstract

Picosecond laser treatment effect for pigmented skin lesions is triggered by selective absorption of melanin particles packaged in cutaneous melanosomes. The treatment effect has been evaluated based on linear absorption of laser fluence in light distribution calculation. However, nonlinear absorption usually occurs when melanin particles are irradiated with a picosecond laser pulse with a high-power density. This paper proposes a two-level model that simulates absorption and nonradiative decay of melanin particles in picosecond skin laser treatment. The proposed two-level model supposes that the picosecond laser pulse width is comparable to the nonradiative relaxation time of melanin particles, and essentially all the absorbed energy in visible and near-infrared wavelengths is released nonradiatively. The numerical simulation result shows that nonlinear absorption occurred at pulse widths ranging from picoseconds to nanoseconds. The amounts of energy deposition within a single melanosome were reduced by 57% and 21% at pulse widths of 100 ps and 1 ns, respectively, compared with linear absorption, for a laser fluence of 0.05 J/cm² . The numerical results indicate that picosecond laser pulses are less absorbed by pigmented skin lesions than nanosecond laser pulses.