1. ULTRASHORT LASER PULSE PROPAGATION IN HIGHLY SCATTERING MEDIA A.P. Popov

2. What is the task? Pulse structure Output pulse duration τ(t i ) Photon scattering multiplicity Absorption, scattering, transmission Change of medium parameters Effect of stranger particles on skin properties 2

3. Modelled setup 3 3 4 21 5 67 8 1 – Cr: forsterite laser, 2 – attenuator, 3 – “blind” mirrors, 4 – beam splitter, 5 –power meter, 6 – vessel with scattering medium, 7 – streak-camera, 8 – CCD-camera 3

4. Scattering particle Detector Absorbing particle Detector Incident laser beam Scheme of the modelled experiment 4

5. Forward detected photons vs registration time 5

6. 6

7. Behavior of scattered and unscattered parts of radiation at various sample thickness 7

8. Forward detected photons vs their scattering multiplicity 8

9. 9

10. Maximum of scattering multiplicity vs medium thickness 10

11. Radiation scattering within medium 11

12. Forward detected photons vs registration time 12

13. Forward detected photons vs registration time 13

14. Forward detected photons vs registration time 14

15. Determination of optical parameters of skin 1 – incident beam, 2, 3 – integrating spheres, dealing with reflectance and transmission respectively, 4, 5 – photodetectors, 6 – sample 1 2 3 45 6 15

16. Skin structure Stratum corneum Epidermis Dermis - 1 Dermis - 2 Dermis Dermis - 3 Subcutaneous fat 16

17. Parameters of epidermis with substances Skin typeμ a, mm -1 μ s, mm -1 g Diffuse reflectance R, % Transmission T, % Absorption A, % Skin0.15200.820.475.24.4 Skin + 0.25хFluo 1.00200.814.364.121.6 Skin + 1хTiO 2 0.15300.826.568.74.8 Skin + 2хTiO 2 0.15400.831.263.65.2 Skin + 0.25хFluo + 1хTiO 2 1.00300.818.657.124.3 Skin + 0.25хFluo + 2хTiO 2 1.00400.822.351.726.0 17

18. Ratio of radiation components for epidermis with various substances 18

19. Layer with TiO 2 Rest of stratum corneum Incident radiation Transmitted radiation Stratum corneum with particles of TiO 2 19

20. Absorption in different parts of stratum corneum 20

21. Pulses of various durations passed through stratum corneum with scattering substance (TiO 2 ) 21

22. Summary Ballistic, snake- and diffusion components registration Scattering multiplicity, time detection of pulse maximum depends linear on μ s, L Decreasing of μ a makes detected pulses longer and increase their amplitude There exists maximum of scattered and absorbed radiation under the medium surface 22

23. Summary Multiple scattering makes short pulses longer Scattering substance injection causes absorption and reflectance increase in upper skin layers, transmission decrease 23