Abstract
Melanin dominates the endogenous contrasts of in vivo third harmonic generation (THG) imaging of human skin. A recent study investigated the THG in melanin solution and a linear relationship between melanin concentration and THG intensity was reported, in contrast to the expected nonlinear relationship. Since melanin hydrocolloid solution is very different from the skin tissue, here we report our study on the origin of the melanin-enhanced THG by using a live cell model. Different from the previous conclusion, our live cell study has indicated an initial nonlinear process where the THG intensity was enhanced according to the 3.5th power of melanin mass density (MMD). When the MMD is higher than 11 mg/ml, a transition from the resonance-enhanced THG to the high-order hyper-Rayleigh scattering process occurs. This saturation phenomenon of the virtual-transition-based THG nonlinear process is attributed to the multi-melanosome-induced scattering within the sub-femtoliter focal volume.
Highlights
Melanin is the most important pigment in human and is the determinant of our skin color
Since melanin hydrocolloid solution is very different from the skin tissue observed clinically, here we report our study on the origin of the melanin-enhanced third harmonic generation by using a live cell model
Since the goal of this study is to explore the origin of melanin-enhanced third harmonic generation (THG) in human keratinocytes in vivo, natural environment such as a melanin-containing tissue or a melanin-generating cell line could be a better choice than artificial conditions such as melanin dispersed in agar or the melanin/glass interface
Summary
Melanin is the most important pigment in human and is the determinant of our skin color. A linear relationship between melanin concentration and THG intensity was reported, in sharp contrast to the expected nonlinear relationship. This previous study had concluded a three orders larger third-order hyperpolarizability of melanin hydrocolloids than that of water, and indicated that the observed signal might not be originated from coherent resonance-enhanced THG [13,14,15,16,17], but from the incoherent high-order hyper-Rayleigh scattering process [18]
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