Nonlinear Optics of Skin: Enhancement of Autofluorescence and Second Harmonic Generation Signals by Immersion Optical Clearing

Multiphoton microscopy and especially two-photon microscopy are actively used in dermatology for the diagnosis and analysis of skin diseases. The typical skin probing depth for these methods is limited to 150–200 µm due to the strong scattering properties of skin. The application of the optical clearing method to biological tissues makes it possible to control their optical properties, namely light scattering, by matching the refractive indices of the structural components of tissues. Reducing the scattering of the upper layers of tissue increases the depth of probing for any method of optical imaging by increasing the intensity of the optical signal recorded from the depth up to 6.6 times. This paper presents a brief review of the methods of nonlinear optics used to assess the condition of the skin, discusses the possibility of improving the efficiency of diagnosing skin diseases through the use of the optical clearing method. In particular, this paper discusses the enhancement of the intensity of autofluorescence and second harmonic generation signals during two-photon microscopy of the skin.