Single-and Two-photon Excitation Of Red Blood Cells Under The Fluorescence By Tissue Penetration Of Contrast Studies And Laser Internal Light Distribution: Fdtd Calculated

The penetrations of 405 nm continuous wave(CW) laser and 800 nm femto-second(fs) laser were found to decay monoexponentically in porcine tissues. The scattering loss of 405 nm CW laser are much bigger than that of 800 nm fs laser, indicating that the tissue scattering also obeys the Rayleigh's law.However,the penetration depth of 405 nm single-photon-excitation(SPE) fluorescence is similar to that of 800 nm two-photon-excitation(TPE) fluorescence,because the reduced laser power density after tissue scattering would lower the TPE fluorescence more seriously than the SPE fluorescence.On the other hand,the pulse width of 800 nm fs laser was measured to be linearly broadened with the tissue depth,because of the tissue dispersion effect.The widening factor of the laser pulse was found to be about 1.004~x,where x is the tissue depth in mm unit,and therefore this effect would not seriously influence the TPE fluorescence in the shallow parts of tissues.A thin blood layer can block the transmission of 405 nm but not affect that of 800 nm.Considering all of these related factors,the penetration depth of TPE is certainly deeper than that of SPE in blood contained in vivo systems and living bodies. In medical applications of low power laser irradiations,safety is one of the most conceming problems since the light focused by the biological object itself may cause damage of living organisms.The light distributions in an erythrocyte with the shape of native biconcave,oblate spheroid,or disk sphere under the irradiation of a plane light of 632.8nm were studied with a numerical calculation method of finite difference time domain.The focusing effect by anyone of the biconcave erythrocyte, oblate spheroid,and disk sphere erythrocyte was found to be so remarkable that the light intensities at the focused areas close to the erythrocyte membrane were about 10 times higher than that of the incident light when the light irradiated along the erythrocyte plane.This focusing effect became weak and even disappeared when the irradiation direction deviated from the erythrocyte plane for more than an angle of 15°. Because the highest light intensity in the erythrocyte can be about one order of magnitude higher than that of the incident light,this factor should be taken into account for laser safety in medical applications.