One-photon and two-photon excited autofluorescence and second harmonic generation from ex vivo human skin by femtosecond laser and visible laser irradiations

The skin tissue contains abundantly intrinsic fluorophores. When the laser light with certain wavelength irradiates the skin specimen, native fluorescence species will emit fluorescence spectroscopy [1-2]. In this paper, the spectral properties of one-photon, two-photon excited autofluorescence and second harmonic generation signal in backscattering geometry from ex vivo human skin are systematically investigated by a femtosecond laser and three visible laser irradiations. In particular, the dependences of the two-photon excited fluorescence spectra in the dermis and epidermis and SHG signal in collagen on the laser intensity and excitation wavelength are mainly recorded and analyzed. Figure 1 shows the two-photon excited image of the dermis. Figure 2 gives two-photon excited aufluorescence spectra and SHG signal from the dermis of ex vivo human skin at various excitation wavelengths. Our experimental results indicate that peak positions of fluorescent spectra from the dermis and epidermis shift towards long wavelengths and the fluorescent intensity decreases when the excitation wavelength increases. But the intensity of SHG signal in collagen has the maximum value at 800nm excitation wavelength. The amplitudes of two-photon excited fluorescence and SHG signals all obey the quadratically dependent law on the incident intensity. Comparing with the two-photon excited fluorescence and second harmonic generation signal, the one-photon excited fluorescence in the dermis and epidermis all exhibits different spectral characteristics.