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10/2/2019
1
Advanced Electromagnetics:
21st Century Electromagnetics
Diffraction Gratings
Lecture Outline
• Fourier series• Diffraction from gratings
• The plane wave spectrum• Plane wave spectrum for crossed gratings
• The grating spectrometer
• Littrow gratings
• Patterned fanout gratings• Diffractive optical elements
Slide 2
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Fourier Series
Jean Baptiste Joseph Fourier
Born:
Died:
March 21, 1768in Yonne, France.
May 16, 1830in Paris, France.
Slide 3
1D Complex Fourier Series
Slide 4
If a function f(x) is periodic with period x, it can be expanded into a complex Fourier series.
2
2 2
2
1
mxj
m
mxj
f x a m e
a m f x e dx
Typically, only a finite number of terms are retained in the expansion.
2 mxM j
m M
f x a m e
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2D Complex Fourier Series
Slide 5
For 2D periodic functions, the complex Fourier series generalizes to
2 2 2 2
1, , , ,x y x y
px qy px qyj j
p q A
f x y a p q e a p q f x y e dAA
Diffraction from Gratings
Slide 6
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Fields are Perturbed by Objects
Slide 7
A portion of the wave front is delayed after travelling through the dielectric object.
inck
Fields in Periodic Structures
Slide 8
Waves in periodic structures take on the same periodicity as their host.
k
1,trnk
2,trnk
3,trnk
4,trnk 5,trnk
x
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Diffraction Orders
Slide 9
The field must be continuous so only discrete directions are allowed. The allowed directions are called the diffraction orders.The allowed angles are calculated using the famous grating equation.
Allowed Not Allowed Allowed
Diffraction from Gratings
Slide 10
The field is no longer a pure plane wave. The grating “chops” the wave front and sends the power into multiple discrete directions that are called diffraction orders.
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Grating Lobes
Slide 11
If we were to plot the power exiting a periodic structure as a function of angle, we would get the following. The power peaks are called grating lobes or sometimes side lobes. The power minimums are called nulls.
Grating Lobe Level
Main Beam (Zero‐order mode)
inc incincr,avg
wave 1wave 2 wave 3
2 2
j k K r j k K rjk r A AA e e e
Field in a Periodic Structure
12
r r,avg cosr K r
The dielectric function of a sinusoidal grating can be written as
A wave propagating through this grating takes on the same symmetry.
incjk rAE r er
incr,avg cos jk rA K r e
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Grating Produces New Waves
13
The applied wave splits into three waves.
inc
incinc
inc
jk r
j k K rjk r
j k K r
e
e e
e
Each of those splits into three waves as well.
inc
incinc
inc
jk r
j k K rjk r
j k K r
e
e e
e
inc
inc inc
inc
2
j k K r
j k K r j k K r
jk r
e
e e
e
inc
inc inc
inc 2
j k K r
j k K r jk r
j k K r
e
e e
e
And each of these split, and so on.
inc ,..., 2, 1,0,1,2,...,k m k mK m This equation describes the total
set of allowed diffraction orders.
Wave Incident on a Grating
14
inc
K
Boundary conditions required the tangential component of the wave vector be continuous.
?
,trn ,incx xk kThe wave is entering a grating, so the phase matching condition is
,incx x xk m k mK
The longitudinal vector component is calculated from the dispersion relation.
22 20 avgz xk m k n k m
For large m, kz,m can actually become imaginary. This indicates that the highest diffraction‐orders are evanescent.
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The Grating Equation
15
The Grating Equation
0avg inc incsin sin sinn m n m
Note, this really is just
,incx x xk m k mK
Proof:
,inc
0 avg 0 inc inc
avg inc inc0 0
0avg inc inc
0avg inc inc
2sin sin
2 2 2sin sin
sin sin
sin sin sin
x x x
x
x
x
k m k mK
k n m k n m
n m n m
n m n m
n m n m
inc
K
inck
0
‐1
+1
+2
+10
‐1+2
x
z
incn
avgn
trnn
Diffraction in Two Dimensions
Slide 16
• Everything is known about the direction of diffracted waves just from the grating period, wavelength, and refractive index.
• The grating equation says nothing about how much power is in the diffracted modes.• That information must come from solving Maxwell’s equations.
Diffraction tends to occur primarily along the lattice planes.
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Diffraction Configurations
Slide 17
Planar Diffraction from a Ruled Grating
• Diffraction is confined within a plane• Numerically much simpler than other cases
• E and H modes are independent
transmitted
Conical Diffraction from a Ruled Grating
• Diffraction is no longer confined to a plane
• Almost same analytical complexity as crossed grating case, but simpler numerically
• E and H modes are coupled
transmitted
reflected
Conical Diffraction from a Crossed Grating with Planar Incidence
• Diffraction occurs in all directions
• Almost same numerical complexity as next case
• E and H modes are coupled transmitted
reflected
Conical Diffraction from a Crossed Grating
• Diffraction occurs in all directions• Most complicated case numerically
• E and H modes are coupled• Essentially the same as previous case
transmitted
incidentreflected
Grating Equation in Different Regions
Slide 18
The angles of the diffracted modes are related to the wavelength 0, refractive index, and grating period x through the grating equation.
The grating equation only predicts the directions of the modes, not how much power is in them.
Reflection Region
0trn inc incsin sin
x
n m n m
0ref inc incsin sin
x
n m n m
Transmission Region
ref incn n
trnn
x
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Grating Equation for Planar Diffraction
Slide 19
The angles of the diffracted modes are related to the wavelength and grating period through the grating equation.
The grating equation only predicts the directionsof the modes, not how much power is in them.
Reflection Region
0trn inc incsin sinm
x
n n m
0ref inc incsin sinm
x
n n m
Transmission Region
ref incn n
trnn
x
Effect of Grating Periodicity
Slide 20
0
avgx n
0 0
avg incxn n
0
incx n
0
incx n
SubwavelengthGrating
“Subwavelength”Grating
Low Order Grating High Order Grating
navg navg navg navg
ninc ninc ninc ninc
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Animation of Grating Diffraction (1 of 3)
Slide 21
Animation of Grating Diffraction (2 of 3)
Slide 22
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Animation of Grating Diffraction (3 of 3)
Slide 23
Wood’s Anomalies
24
Type 1 – Rayleigh Singularities Type 2 – Resonance Effects
Robert W. Wood observed rapid variations in the spectrum of light diffracted by gratings which he could not explain. Later, Hessel explained them an identified two different types.
Rapid variation in the amplitudes of the diffracted modes that correspond to the onset or disappearance of other diffracted modes.
A resonance condition arising from leaky waves supported by the grating. Today, we call this guided‐mode resonance.
R. W. Wood, Phil. Mag. 4, 396 (1902)A. Hessel, A. A. Oliner, “A New Theory of Wood’s Anomalies on Optical Gratings,” Appl. Opt., Vol. 4, No. 10, 1275 (1965).
Robert Williams Wood1868 ‐ 1955
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Grating Cutoff Wavelength
Slide 25
0sinx
n m m
When m becomes imaginary, the mth mode is evanescent and cut off.
Assuming normal incidence (i.e. inc = 0°), the grating equation reduces to
The first diffracted modes to appear are m = 1.
The cutoff for the first‐order modes happens when (±1) = 90°.
0
0
1 90
sin 90 1x
x
n
n
To prevent the first‐order modes, we need
0x n
To ensure we have first‐order modes, we need
0x n
Total Number of Diffracted Modes
Slide 26
Given the grating period x and the wavelength 0, we can determine how many diffracted modes exist.
Again, assuming normal incidence, the grating equation becomes
0 0
avg avg
sin sin 1x x
m mm m
n n
avgmax
0
xnm
Therefore, a maximum value for m is
The total number of possible diffracted modes M is then 2mmax+1
avg
0
21xn
M
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Determining Grating Cutoff Conditions
Slide 27
Condition Requirements0‐order mode Always exists unless there is total‐internal reflection
No 1st‐order modes Grating period must be shorter than what causes 1) = 90°Ensure 1st‐order modes Grating period must be larger than what causes 1) = 90°No 2nd‐order modes Grating period must be shorter than what causes 2) = 90°Ensure 2nd‐order modes Grating period must be larger than what causes 2) = 90°No mth‐order modes Grating period must be shorter than what causes m) = 90°
Ensure mth‐order modes Grating period must be larger than what causes m) = 90°
Three Modes of Operation for 1D Gratings
Slide 28
Bragg GratingCouples energy between counter‐propagating waves.
Diffraction GratingCouples energy between waves at different angles.
Long Period GratingCouples energy between co‐propagating waves.
Applications• Thin film optical filters• Fiber optic gratings• Wavelength division multiplexing
• Dielectric mirrors• Photonic crystal waveguides
Applications• Beam splitters• Patterned fanout gratings• Laser locking• Spectrometry• Sensing• Anti‐reflection• Frequency selective surfaces• Grating couplers
Applications• Sensing• Directional coupling
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Analysis of Diffraction Gratings
Slide 29
Direction of the Diffracted Modes Diffraction Efficiency and Polarizationof the Diffracted Modes
0inc incsin sin sinn m n m
0
0
E j H
H j E
E
H
We must obtain a rigorous solution to Maxwell’s equations to determine amplitude and polarization of the diffracted modes.
Applications of Gratings
Slide 30
Subwavelength GratingsOnly the zero‐order modes may exist.
Applications• Polarizers• Artificial birefringence• Form birefringence• Anti‐reflection• Effective index media
Littrow GratingsGratings in the littrowconfiguration are a spectrally selective retroreflector.
Applications• Sensors• Lasers
Patterned Fanout GratingsGratings diffract laser light to form images.
HologramsHolograms are stored as gratings.
SpectrometryGratings separate broadband light into its component colors.
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The Plane Wave Spectrum
Slide 31
Periodic Functions Can Be Expanded into a Fourier Series
Slide 32
2
2
x
x
mxj
m
mxj
A x S m e
S m A x e dx
The envelop A(x) is periodic along x with period x
so it can be expanded into a Fourier series.
Waves in periodic structures obey Bloch’s equation
, j rE x y A x e
x A x
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Rearrange the Fourier Series (1 of 2)
Slide 33
x A x
A periodic field can be expanded into a Fourier series.
, j rE x y A x e
2
2
2
x
x
yx xz
mxj
j r
m
mxj
j r
m
mxj
j yj x j z
m
S m e e
S m e e
S m e e e e
Here the plane wave term 𝑒 · ⃗ is brought inside of the summation.
Rearrange the Fourier Series (2 of 2)
Slide 34
x A x
x can be combined with the last complex exponential.
2
, yx xz
mxj
j yj x j z
m
E x y S m e e e e
2
xy xz
mj x
j y j z
m
S m e e e
Now let and , ,, y m y z m zk k
,
2x m x
x
mk
, jk m r
m
E x y S m e
2ˆ ˆ ˆx x y y z z
x
mk m a a a
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The Plane Wave Spectrum
Slide 35
, jk m r
m
E x y S m e
Terms were rearranged to show that a periodic field can also be thought of as an infinite sum of plane waves at different angles. This is the “plane wave spectrum” of a periodic field.
FieldPlane Wave Spectrum
Longitudinal Wave Vector Components of the Plane Wave Spectrum
The wave incident on a grating can be written as
,inc ,inc
,inc 0 inc inc
,incinc 0
,inc 0 inc inc
sin
0,
cos
x z
xj k x k z
y
z
k k n
kE x y E e
k k n
Phase matching into the grating leads to
,inc
2x x
x
k m k m
, 2, 1,0,1, 2,m
Each wave must satisfy the dispersion relation.
22 20 grat
2 20 grat
x z
z x
k m k m k n
k m k n k m
There are two possible solutions here.1. Purely real kz
2. Purely imaginary kz.
Note: kx is always real.
incx
z
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Visualizing Phase Matching into the Grating
Slide 37
The wave vector expansion for the first 11 modes can be visualized as…
inck
0xk 1xk 2xk 3xk 4xk 5xk 1xk 2xk 3xk 4xk 5xk
2n
1n
kz is real. A wave propagates into material 2.
kz is imaginary. The field in material 2 is evanescent.
kz is imaginary. The field in material 2 is evanescent.
Each of these is phase matched into material 2. The longitudinal component of the wave vector is calculated using the dispersion relation in material 2.
Note: The “evanescent” fields in material 2 are not completely evanescent. They have a purely real kx so power flows in the transverse direction.
x
z
Conclusions About the Plane Wave Spectrum
• Fields in periodic media take on the same periodicity as the media they are in.• Periodic fields can be expanded into a Fourier series.• Each term of the Fourier series represents a spatial harmonic (plane wave).• Since there are in infinite number of terms in the Fourier series, there are an infinite number of spatial harmonics. •Only a few of the spatial harmonics are actually propagating waves. Only these can carry power away from a device. Tunneling is an exception.
Slide 38
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Plane Wave Spectrum from
Crossed Gratings
Slide 39
Grating Terminology
Slide 40
1D gratingRuled grating
Requires a 2D simulation
2D gratingCrossed grating
Requires a 3D simulation
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Diffraction from Crossed Gratings
Slide 41
Doubly‐periodic gratings, also called crossed gratings, can diffract waves into many directions.
They are described by two grating vectors, Kx and Ky.
Two boundary conditions are necessary here.
,inc
,inc
..., 2, 1,0,1, 2,...
..., 2, 1,0,1,2,...
x x x
y y y
k m k mK m
k n k nK n
inck
xK
yK
2ˆ
2ˆ
xx
yy
K x
K y
Transverse Wave Vector Expansion (1 of 2)
Slide 42
Crossed gratings diffraction in two dimensions, x and y.
To quantify diffraction for crossed gratings, we must calculate an expansion for both kx and ky.
% TRANSVERSE WAVE VECTOR EXPANSIONM = [-floor(Nx/2):floor(Nx/2)]';N = [-floor(Ny/2):floor(Ny/2)]';kx = kxinc - 2*pi*M/Lx;ky = kyinc - 2*pi*N/Ly;[ky,kx] = meshgrid(ky,kx);
,inc
,inc
2 , , 2, 1,0,1, 2,
2 , , 2, 1,0,1, 2,
ˆ ˆ,
x xx
y yy
t x y
mk m k m
nk n k n
k m n k m x k n y
We will use this code for 2D PWEM, 3D RCWA, 3D FDTD, 3D FDFD, 3D MoL, and more.
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Transverse Wave Vector Expansion (2 of 2)
Slide 43
The vector expansions can be visualized this way…
xk m yk n
ˆ ˆ,t x yk m n k m x k n y
Longitudinal Wave Vector Expansion (1 of 2)
Slide 44
The longitudinal components of the wave vectors are computed as
2 2 2,ref 0 ref
2 2 2,trn 0 trn
,
,
z x y
z x y
k m n k n k m k m
k m n k n k m k m
The center few modes will have real kz’s. These correspond to propagating waves. The others will have imaginary kz’s and correspond to evanescent waves that do not transport power.
,ref ,zk m n ,tk m n
% Compute Longitudinal Wave Vector Componentsk0 = 2*pi/lam;kzinc = k0*nref;kzref = -sqrt((k0*nref)^2 - kx.^2 - ky.^2);kztrn = +sqrt((k0*ntrn)^2 - kx.^2 - ky.^2);
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Longitudinal Wave Vector Expansion (2 of 2)
Slide 45
The overall wave vector expansion can be visualized this way…
,ref ,zk m n ,tk m n
,ref ,zk m n ,trn ,zk m n
The Grating Spectrometer
Slide 46
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What is a Grating Spectrometer
47
Input Light
Separated Colors
Diffraction Grating
Spectral Sensitivity
48
0avg inc incsin sin
x
n m n m
Start with the grating equation.
Define spectral sensitivity as how much the diffracted angle 𝜃 𝑚 changes with respect to wavelength 𝜆 .
0 avg cosx m
m m
n
This equation shows how to maximize sensitivity.
1. Diffract into higher order modes ( m).2. Use shorter period gratings ( x).3. Diffract into larger angles ( m)).4. Diffract into lower refractive index materials ( navg).
0avg cosx
mm
n m
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Ocean Optics HR4000 Grating Spectrometer
49
http://www.oceanoptics.com/Products/benchoptions_hr.asp
1 SMA Connector
2 Entrance Slit
3 Optional Filter
4 Collimating Mirror
5 Diffraction Grating
6 Focusing Mirror
7 Collection Lenses
8 Detector Array
9 Optional Filter
10 Optional UV Detector
Fiber Optic Input
Littrow Gratings
Slide 50
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Littrow Configuration
51
Incident
0+1
In the littrow configuration, the +1 order reflected mode is parallel to the incident wave vector. This forms a spectrally selective mirror.
Conditions for the Littrow Configuration
52
0incsin sin
x
n m n m
The grating equation is
The littrow configuration occurs when
inc1
0inc2 sin
x
n
The condition for the littrow configuration is found by substituting this into the grating equation.
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Spectral Selectivity
53
Typically only a cone of angles reflected from a grating is detected.
It is desired to find 𝑑𝜆/𝑑𝜃 by differentiating the previous equation.
0 2 cosx
dn
d
Typically this is used to calculate the bandwidth of the reflected light.
0 2 cosxn
2
0
2 cosxn ff
c
Linewidth (optics and photonics)
Bandwidth (RF and microwave)
0inc2 sin
x
n
Example (1 of 2)
54
SolutionRight away, we know that
0
inc
3.00 cm2.12 cm
2 sin 2 1.0 sin 45x n
inc
80
0
1.0
45
3 10 3.00 cm
10 GHz
ms
n
c
f
The grating period is then found to be
Design a metallic grating in air that is to be operated in the littrow configuration at 10 GHz at an angle of 45.
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Example (2 of 2)
55
Solution continuedAssuming a 5 cone of angles is detected upon reflection, the bandwidth is
2
8
2 1.0 2.12 cm 10 GHz cos 455 0.87 GHz
3 10 180ms
f
PatternedFanout Gratings
Slide 56
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Near‐Field to Far‐Field
57
, ,0E x y
, ,E x y L
L
FFT
After propagating a long distance, the field within a plane tends toward the Fourier transform of the initial field.
What is a Patterned Fanout Grating?
58
A patterned fanout is formed whenever a diffraction grating forces the field to take on the profile of the inverse Fourier transform of an image. After propagating very far, the field takes on the profile of the image.
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Gerchberg‐Saxton Algorithm:Initialization
59
Far‐Field
amplitude phase
Step 1 – Start with desired far‐field image.
Near‐Field
amplitude phase
FFT-1
Step 2 – Calculate near‐field
amplitude phase
Step 3 – Replace amplitude
amplitude phase
FFTStep 4 – Calculate far‐field
Gerchberg‐Saxton Algorithm:Iteration
60
Far‐FieldNear‐Field
amplitude phase
FFT-1
amplitude phase
FFT
amplitude phase
Step 6 – Calculate near‐field
Step 8 – Calculate far‐field
Step 5 – Replace amplitude with desired image.
Step 7 – Replace amplitude
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Gerchberg‐Saxton Algorithm:End
61
Far‐FieldNear‐Field
amplitude phase amplitude phase
FFT
This is what the final image will look like.
This is the phase function of the diffractive optical element.
After several dozen iterations…
The Final Fanout Grating
62
A surface relief pattern is etched into glass to induce the phase function onto the beam of light.
This can also be accomplished with an amplitude mask fabricated in a high‐resolution laser printer.
Phase Fu
nctio
n (rad
)
Phase function as calculated by the Gerchbert‐Saxton algorithm.
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Diffractive Optical Elements
Slide 63
What is a Diffractive Optical Element
64
Conventional Lens Diffractive Optical Lens (Fresnel Zone Plate)
If the device is only required to operate over a narrow band, devices can be “flattened.”
The flattened device is called a diffractive optical element (DOE).
Lukas Chrostowski, “Optical gratings: Nano-engineered lenses,” Nature Photonics 4, 413-415 (2010).
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