Post on 19-Dec-2015
ULTRASHORT LASER PULSE PROPAGATION IN HIGHLY
SCATTERING MEDIA
A.P. Popov
What is the task?• Pulse structure
• Output pulse duration τ(ti)
• Photon scattering multiplicity• Absorption, scattering,
transmission• Change of medium parameters• Effect of stranger particles on skin
properties
2
Modelled setup3
3
4
2 1
5
6 7
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
Scattering particle
Detector
Absorbing particle
DetectorIncident laser beam
Scheme of the modelled experiment
4
0 1 2 3 4 5 6 7 8 9 10
0,1
1
10
100
g = 0.98
s = 85 mm-1
a = 0.6 mm-1
l* = 0.43 mm
L = 0.1 mm L = 0.2 mm L = 0.4 mm
Rel
. n
um
ber
of
ph
oto
ns,
%
Registration time, ps
Forward detected photons vs registration time
5
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
Rel
. n
um
ber
of
ph
oto
ns,
%
Registration time, ps
g = 0.98
s = 85 mm-1
a = 0.6 mm-1
l* = 0.43 mm
L = 0.6 mm L = 0.8 mm L = 1.0 mm
Forward detected photons vs registration time6
0 100 200 300 400 500 600 700 800 900 10001E-5
1E-4
1E-3
0,01
0,1
1
Thickness, m
Unscattered part Tc
Scattered part Ts
Beer's law
g = 0.9
s = 10 mm-1
a = 0.01 mm-1
n = 1.37
Tra
nsm
issio
n, re
l. u
nit
s
Behavior of scattered and unscattered parts of radiation at various sample thickness
7
0 10 20 30 40 50 60
0
2
4
6
8
10
12
g = 0.98
s = 85 mm-1
a = 0.6 mm-1
L = 0.1 mm L = 0.2 mm L = 0.4 mm
Rel
. n
um
ber
of
ph
oto
ns,
%
Number of scattering acts
Forward detected photons vs their scattering multiplicity
8
0 50 100 150 200 250 300 350
0,0
0,2
0,4
0,6
0,8
g = 0.98
s = 85 mm-1
a = 0.6 mm-1
L = 0.6 mm L = 0.8 mm L = 1.0 mm
Rel. n
um
ber
of
ph
oto
ns, %
Number of scattering acts
Forward detected photons vs their scattering multiplicity
9
0,0 0,2 0,4 0,6 0,8 1,00
20
40
60
80
100
Y = A + B * XA=-1,61096+/-0,73594
B=95,69863+/-1,21262 mm-1
g = 0.98
s = 85 mm-1
a = 0.6 mm-1
Max
imum
of
scat
teri
ng m
ultip
licity
Medium thickness, mm
Maximum of scattering multiplicity vs medium thickness10
0,0 0,2 0,4 0,6 0,8 1,0
0
5
10
15
20
25
30
Depth, mm
g = 0.95
a = 1 mm-1
L = 1 mm
s = 95 mm-1
s = 75 mm-1
Rel. n
um
ber
of
scatt
eri
ng
, %
Radiation scattering within medium11
0 10 20 30 40 50
0,00
0,02
0,04
0,06
0,08
g = 0.85
s = 35 mm-1
L = 1 mm
a = 0.1 mm-1
a = 0.4 mm-1
a = 0.8 mm-1
Rel. n
um
ber
of
ph
oto
ns, %
Registration time, ps
Forward detected photons vs registration time12
Forward detected photons vs registration time
0 10 20 30 40 50
0
50
100
150
200
250
300
350
g = 0.76
s = 20 mm-1
à = 0.076 mm-1
n = 1.39
Nu
mb
er
of
ph
oto
ns
Registration time, ps
ti = 1 ps - 20 times
ti = 5 ps - 4 times
ti = 10 ps - 2 times
13
0 5 10 15 20 25 30 35 40 45 50
0
50
100
150
200
250
300
Nu
mb
er
of
ph
oto
ns
g << 1
s = 35 mm-1
a = 0.6 mm-1
L = 1 mm
ti = 0.1 ps - 100 times
ti = 1.0 ps - 10 times
ti = 5.0 ps - 2 times
ti = 10.0 ps - no
Registration time, ps
Forward detected photons vs registration time14
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
4 56
15
Skin structure Stratum corneum
Epidermis
Dermis - 1
Dermis - 2
Dermis
Dermis - 3
Subcutaneous fat
16
Parameters of epidermis with substancesSkin type μa,
mm-1
μs,
mm-1
g Diffuse reflectance R,
%
TransmissionT, %
AbsorptionA, %
Skin 0.15 20 0.8 20.4 75.2 4.4
Skin + 0.25хFluo
1.00 20 0.8 14.3 64.1 21.6
Skin + 1хTiO2
0.15 30 0.8 26.5 68.7 4.8
Skin + 2хTiO2
0.15 40 0.8 31.2 63.6 5.2
Skin + 0.25хFluo + 1хTiO2
1.00 30 0.8 18.6 57.1 24.3
Skin + 0.25хFluo + 2хTiO2
1.00 40 0.8 22.3 51.7 26.0
17
Ratio of radiation components for epidermis with various substances
1 2 3 40
10
20
30
40
50
60
70
80
90
100
Inte
nsit
y, %
Skin type
Dif. reflectance Transmission Absorption
18
Layer withTiO2
Rest of stratum corneum
Incident radiation
Transmitted radiation
Stratum corneum with particles of TiO2 19
0 1 2 3 4 5 6 7 80,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7R
el. n
um
ber
of
ph
oto
ns, %
Thickness, m
Whole stratum corneum Rest stratum corneum layer with particles of TiO
2
Absorption in different parts of stratum corneum20
0,08 0,10 0,12 0,14 0,16 0,18 0,20 0,22 0,24
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
g = 0.8
1
s = 150 mm-1
2
s = 100 mm-1
a = 0.1 mm-1
n = 1.5
ti = 0.01 ps
ti = 0.05 ps
ti = 0.10 ps
Rel. n
um
ber
of
ph
oto
ns, %
Registration time, ps
Pulses of various durations passed through stratum corneum with scattering substance (TiO2)
21
Summary• Ballistic, snake- and diffusion
components registration• Scattering multiplicity, time detection of
pulse maximum depends linear on μs, L
• Decreasing of μa makes detected pulses longer and increase their amplitude
• There exists maximum of scattered and absorbed radiation under the medium surface
22
Summary• Multiple scattering makes short
pulses longer• Scattering substance injection
causes absorption and reflectance increase in upper skin layers, transmission decrease
23