Modeling Focused Beam
Propagation in scattering media
Janaka Ranasinghesagara
Teaching Objectives
► The need for computational models of focused beam propagation in
scattering medium
► Introduction to the principles and mathematical models underlying
focus beam propagation.
► Learn how fundamental concepts are applied to develop an efficient
focused beam propagation model.
Focused Beam Propagation in free space
Amp.
Phase
Focused Beam Propagation in free space
Analytical solution Richards and Wolf. Proc. Royal Soc. Lond. A 253(1274) 1959
Amp.
Phase
Focused Beam Propagation in scattering medium
• Scattering distorts the excitation volume
• Scattering is the main limiting factor for penetration depth in
microscopy
Focused Beam Propagation in scattering medium
• Scattering distorts the excitation volume
• Scattering is the main limiting factor for penetration depth in
microscopy
Spatial scales of Imaging and Models
Ntziachristos, Nat. Methods, 7 (2010)
Leigh, Chen and Liu, Biomed Opt. Exp., 5 (6) (2014)
Existing Focused Beam Propagation Models
Finite Difference Time Domain (FDTD)
Solution for Maxwell’s equations
• Solve Maxwell’s equations rigorously in a
voxelized space
• Size of the voxel has to be small
• Need enormous computational resources
(15m×15m ×50m 12GB, 500hours*)
• “Stair case” errors
*Starosta and Dunn, Opt. Express 17(15), (2009)
Capoglu et al. Opt. Express 21(1), (2013)
Elmaklizi et al. JBO 19(7) (2014) http://www.angorafdtd.org/
Existing Focused Beam Propagation Models
Finite Difference Time Domain (FDTD)
Solution for Maxwell’s equations
• Solve Maxwell’s equations rigorously in a
voxelized space
• Size of the voxel has to be small
• Need enormous computational resources
(15m×15m ×50m 12GB, 500hours*)
• “Stair case” errors
*Starosta and Dunn, Opt. Express 17(15), (2009)
Capoglu et al. Opt. Express 21(1), (2013)
Elmaklizi et al. JBO 19(7) (2014) http://www.angorafdtd.org/
Monte Carlo Simulation
• Propagates photons towards the focal
point
• Ignores the wave nature of light
• Considers far-field phase function
• Provides mean behavior
• Requires large number of photons
Song et al, Appl. Opt.. 38(13) (1999)
Blanca et. Al Appl. Opt. 37(34) (1998)
Dunn et al, Appl. Opt.. 39(7) (2000)
Hayakawa et al, Biomed. Opt. Exp. 2(2) (2011)
Cai et al, Prog. in Electromag. Res. 142 (2013)
Cai et al, JBO, 19(1) (2014)
►Electromagnetic wave
►Maxwell’s equations
►Plane wave solution to Maxwell’s
equations
►Properties of plane wave
Key Concepts, Equations and Properties
Light is an Electromagnetic Wave
Maxwell’s Equations
► Provide exact model for EM wave propagation
► Provide theoretical foundation of optics
► Model wave interference, diffraction and polarization
0t
HE
0t
EH J
0
E
0 H
(Faraday’s Law)
(Gauss’ Law)
(Gauss’ Law
for Magnetism)
(Ampere’s Law)
𝐄: Electric field
𝐇: Magnetic field
𝐉: Current density
𝜌: Charge density
𝜇0: Permeability
𝜀0: Permittivity
Origin of Wave Equation from Maxwell’s Equations
► Provide exact model for EM wave propagation
► Provide theoretical foundation of optics
► Model wave interference, diffraction and polarization
0t
HE
0t
EH J
0
E
0 H
(Faraday’s Law)
(Gauss’ Law)
(Gauss’ Law
for Magnetism)
(Ampere’s Law) 0t
EH
0 E
No free
charges
No flow
of current
22
0 0 2t
EE
Wave equation
in free space
22
2 2
1
c t
EE
where
0 0
1c
Plane Wave
22
2 2
1
c t
EE
Plane wave solution
( ) cos E 0z E k z ct
( ) sin E 0z E k z ct
OR
Wave equation
Plane Wave
( ) cos E 0z E kz t
2
0 0
1 1Re
2 2Intensity cE E H
k: wave number = 2/
𝜔: angular frequency
𝜀0: Permittivity
c: Speed of light in vacuum
Amplitude Phase
Amplitude
Phase
Plane wavefront
22
2 2
1
c t
EE
Plane wave solution
( ) cos E 0z E k z ct
( ) sin E 0z E k z ct
OR
Wave equation
Complex Representation of Waves
( ) cos sin E 0 0z E kz t iE kz t
Amplitude =
Im ( )
Re ( )
zArcTan kz t
z
E
EPhase () =
( ) exp0z E iE
{Euler’s Formula}
2 2
0Re ( ) Im ( ) E Ez z E
real imaginary
► Complex representation of wave enable us to combine the
amplitude and the phase into a single function.
{Original function}
Polarization
► Polarization is described by specifying orientation of the “electric field”.
Considering x-z plane
exp E Ex 0E ikz
0 E E y
Considering y-z plane
0 E E y
exp E Ex 0E ikz
exp0
E ikzx
E
0y E
=
z
y
x
►Mie Solution to Maxwell’s Equations
(commonly known as Mie Theory)
►Mie Simulator GUI
Plane Wave Incident on a Spherical Scatterer
Plane Wave Incident on a Spherical Scatterer
Diameter < λ Diameter > λ
Uniform electric field
throughout sphere
Non-uniform electric
field throughout
sphere
Incident field
Rayleigh Limit of Mie Scattering
Hertzian Dipole
Diameter < λ
Rayleigh Limit of Mie Scattering
Parallel (p) Perpendicular (s)
Polarization
Hertzian Dipole
0
180
0
180
x-z plane y-z plane
Diameter < λ
Mie Simulator GUI Mie Simulator GUI
Mie Solution to Maxwell’s Equations (Mie Theory)
Van de Hulst, H. C., Light scattering by small particles, Dover publications (1981)
Bohren and Huffman, Absorption and Scattering of Light by Small Particles (1983)
http://www.scattport.org/index.php/light-scattering-software
2 2
21
2(2 1)
sca n n
n
Q n a bx
Scattering efficiency (Qsca)
Scattering cross section (𝜎𝑠) 2 s scaQ a
Far-field amplitude scattering matrix components
1
1
1
1
cos2 1cos
1 sin
n
n n n
n
Pn dS a b P
n n d
1
1
2
1
cos2 1cos
1 sin
n
n n n
n
Pn dS b a P
n n d
Phase function 2 2
1 2
2( )
ave
sca
S Sp
Q x
2
1
( ) 0( )
0 ( )
SS
S
Mie solution is an analytic solution to Maxwell’s equation for an incident plane wave
Mie Simulator GUI
s=Nss
g
’
Ns
s = s(1-g)
Poly disperse: Gelebart et al. Pure Appl. Opt., 5 (1996) http://virtualphotonics.codeplex.com/wikipage?title=Getting%20Started%20Mie%20Simulator%20GUI
Plane Wave Incident on a Spherical Scatterer
Detector A
• Mie solution provides 3-D scattering field
What information is necessary to
find the total electric field at point
“A”?
- Incident electric field
- Scattering field (from Mie Theory)
- Distance to point “A”
Plane Wave Incident on a Spherical Scatterer
A
HC Van de Hulst, “Light Scattering by small particles” Dover, (1981)
cos sin
sin cos 0
E E
E
i inc
i
Incident field on scattered plane
Plane Wave Incident on a Spherical Scatterer
Scattered electric field at A is given by
2
1, , exp( ) , E Es ir ikr S r
ikr
A
HC Van de Hulst, “Light Scattering by small particles” Dover, (1981)
cos sin
sin cos 0
E E
E
i inc
i
Incident field on scattered plane
1
1, , exp( ) , E Es ir ikr S r
ikr
Phase Scattering
amplitude 1/r
{Sph. Wave}
Plane Wave Incident on a Spherical Scatterer
Scattered electric field at A is given by
2
1, , exp( ) , E Es ir ikr S r
ikr
A
HC Van de Hulst, “Light Scattering by small particles” Dover, (1981)
cos sin
sin cos 0
E E
E
i inc
i
Incident field on scattered plane
1
1, , exp( ) , E Es ir ikr S r
ikr
Phase Scattering
amplitude
2
1
( , ) ( , ) 01exp( )
( , ) 0 ( , )
E E
E E
s i
s i
S rikr
S rikr
1/r
{Sph. Wave}
For non-spherical scatterers 2 3
4 1
( , , ) ( , , )
( , , ) ( , , )
S r S r
S r S r
Focused beam as a summation of plane waves
Richards and Wolf. Proc. Royal Soc. Lond. A 253(1274) (1959)
Capoglu et al. Opt. Express 16(23), (2008)
Elmaklizi et al. JBO 19(7) (2014)
Huygens-Fresnel (HF) principle: Each point of an advancing wavefront
act as a source of outgoing secondary spherical waves
HF Wavelet: A small section of
a secondary spherical wave
Airy Pattern Formation
Airy disk radius (r) (the distance between the central maximum and the
first minimum) 0.61 /r NA
Constructive interference Destructive interference
http://zeiss-campus.magnet.fsu.edu/tutorials/basics/airydiskformation/index.html
Focused Beam Propagation in Free Space
max 2
0 0
1( , , ) exp( ) ( , ) exp cos sin cos( ) sin
2
E E kf
z ikf i kz k d di
Phase at 𝜌, 𝜑, 𝑧
w.r.t. focal point
Electric field at
lens surface
Phase at the focal
point w.r.t. lens Richards and Wolf. Proc. Royal Soc. Lond. A 253(1274) 1959
Geometrical representation
Analytical solution:
max
Focused Beam Propagation in Free Space
Focused Beam Simulator GUI: Analytical solution
HF Wavelets to Model Focused Beam Propagation
Implementation in a non scattering medium
Generate uniformly distributed points (HF radiating source
locations) on the spherical cap
Project wavelets from each radiating source to a detector point
Phase advances with traveling distance
( , ) cos sin
( , ) sin cos 0
E E
E
inc ( , ) ( , ) exp( )A
jikd E E
( , ) ( , ) exp( )A
jikd E E
A
( , )
( , )
E
E
A
x y z
A
x y z
E i E j E k
E i E j E k
║and ┴ polarization calculation HF wavelet propagation Wave summation (interference)
Verifying results in a non scattering medium with the analytical solution
(A)
Analytical Solution
(B)
HF-WEFS*
Simulation parameters
: 800nm, nm:1.33, f:500m, NA:0.667
(A) – (B)
HF Wavelet based Electric Field Superposition (HF-WEFS)
*Ranasinghesagara et al, JOSA A 31(7) 2014
HF Wavelet based Electric Field Superposition (HF-WEFS)
Verifying results in a non scattering medium with the analytical solution
Detection of Scattering Fields
Primary and secondary scattering detection
HF Wavelets to Model Focus Beam Propagation
Implementation in a medium with spherical scatterers
Generate uniformly distributed points (HF radiating source
locations) in the spherical cap
Project wavelets from each radiating source to a scatterer
Phase advances with traveling distance
Find scattering angle and distance from scatterer to the detector
point
Calculate scattered field contribution at the detector from Mie
solution
( , ) cos sin
( , ) sin cos 0
E E
E
inc( , ) ( , ) exp( )
P
iikd E E
( , ) ( , ) exp( )P
iikd E E
2
1
( , ) ( , ) 01exp( )
( , ) 0 ( , )
Ps i
Ps i
S rikr
S rikr
E E
E E
( , )
( , )
A
x y z
A
x y z
E i E j E k
E i E j E k
E
E
Comparing HF-WEFS results with FDTD
Simulation parameters: : 800nm, nm:1.33, f:500m, NA:0.667
Ranasinghesagara et al, JOSA A 31(7) (2014)
A B C D
HF-WEFS
FDTD
10µm
Amplitude correlation as a function of scatterer location
Ranasinghesagara et al, JOSA A 31(7) (2014)
Axial displacement of the largest amplitude point
Ranasinghesagara et al, JOSA A 31(7) (2014)
HF-WEFS in a medium with spherical scatterers