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Diffraction Grating
Introduction
This tutorial uses the Wave Optics Module and the Ray Optics Module to simulate the propagation of rays through a diffraction grating at different angles of incidence. It uses the S-parameters computed by the Electromagnetic Waves, Frequency Domain interface on a unit cell of the grating to specify the reflectance and transmittance of each diffraction order in the Geometrical Optics interface, allowing ray propagation through the grating to be modeled over length scales much larger than the width of the unit cell.
The Geometrical Optics interface includes a Grating feature that can be used to simulate propagation of electromagnetic waves on fully scaled optical devices without the need to spatially resolve the wavelength, which would be impractical in many cases due to the large number of mesh elements required.
Although the directions of propagation for the diffraction orders can be derived from the wavelength of radiation, the angle of incidence, and the width of a unit cell in the grating, reinitialization of the ray intensity requires prior calculation of the transmittance and reflectance for all diffraction orders as a function of angle of incidence. These quantities can be obtained by computing the S-parameters of each diffraction order for a single unit cell as a function of the angle of incidence using the Port and Diffraction Order features for the Electromagnetic Waves, Frequency Domain interface.
This 2D model is separated in two parts.
Model Definition
This model simulates the interaction of light of free-space wavelength λ0 = 441 nm with a 5 mm wide dielectric grating of grating constant (the distance between the grooves) d = 340 nm.
Notes on Diffraction orders
For a plane wave incident on a diffraction grating at angle of incidence α (SI unit: rad) as in Figure 1, the diffraction orders correspond to the angles at which the difference in optical path length for wavefronts from adjacent unit cells is an integer multiple of the wavelength. A valid angle for a transmitted diffraction order βm (SI unit: rad) must follow the relation
where the diffraction order m (dimensionless) is an integer.
Figure 1: The geometric path lengths of two transmitted parallel rays.The shaded area represents a unit cell of the diffraction grating (SiO2). For this model the grating constant is = 340 nm and the monochromatic TE polarized light has a wavelength of λ0 = 441 nm.
For = 0, the angle of refraction is described by Snell’s law,
For reflected rays, nα = nβ. For = 0, the equation for specular reflection is recovered,
Because the sine functions can only vary between -1 and 1, the existence of higher diffraction orders requires that
In this example only the diffraction orders 0, 1, and -1 can be released, which means that
As mentioned in the introduction, the model consists of two parts: the S-parameter calculation using a single unit cell and the ray trajectory computation in an optically large modeling domain.
S-parameter Calculation
The transmittance and reflectance for the refraction, specular reflection, and first order diffraction of plane TE waves (electric field component in the z-direction, out of the xy-plane) are computed for a single unit cell.
The Electromagnetic Waves, Frequency Domain interface is used to model wave propagation in a single unit cell of the grating, as outlined in Figure 1. On either side of the unit cell, the Periodic Condition boundary condition with Floquet periodicity is used. This condition states that the solution on one side of the unit cell equals the solution on the other side multiplied by a complex-valued phase factor. The phase shift between the boundaries is evaluated from the perpendicular component of the wave vector. Note that due to the continuity of the field, the phase factor is the same for the refracted and reflected waves as for the incident wave.
Port boundary conditions are used to release the incident wave and to absorb the reflected and transmitted waves of order 0. To ensure that no non-physical reflections occur, Diffraction Order subnodes must be added to the Port nodes to absorb outgoing waves of each nonzero diffraction order.
The input to each periodic port is an electric field amplitude vector and an angle of incidence. In this example the angle of incidence is swept from 0° to 90° at 1° intervals.
Ray Tracing
The Geometrical Optics interface computes the intensity of rays of each diffraction order using the transmittance and reflectance computed in the previous study. In order to see the effect of the angle of incidence on the ray trajectories and intensity, 901 rays are released from a point in a 90° cone with a source power density of 901 W/m, or 1 W/m per ray. For each diffraction order, two rays may be released, one transmitted ray and one reflected ray. Because the transmitted ray of order 0 uses the same degrees of freedom as the incident ray, five extra degrees of freedom should be allocated per incident ray: one for the reflected ray of order 0 and two each for the reflected and transmitted rays of order = 1 and = -1. A total of 4505 secondary rays are allocated.
Results and Discussion
The electric field norm for a TE wave with an angle of incidence of 45° is shown in Figure 2. In order to get reliable results one has to use a very fine mesh to resolve the wavelength. To resolve a wave properly, it is necessary to use about 10 mesh elements per wavelength when using linear shape functions, or 5 elements per wavelength when using the default quadratic shape functions.
Figure 2: Norm of the electric field for a TE wave with an angle of incidence of 45 degrees.
The transmittance and reflectance of each diffraction order as functions of the angle of incidence are shown in Figure 3. Most of the radiation is transmitted at diffraction order 0, except at very large angles of incidence for which most of the radiation is reflected.
Figure 3: Reflectance and transmittance of diffraction orders 0, 1, and -1 as functions of the angle of incidence.
The raw data from Figure 3 was used to define a series of six interpolation functions, each corresponding to the reflectance or transmittance of a diffraction order. These interpolation functions were used in the Geometrical Optics interface to define the reinitialized intensity of the transmitted and reflected rays.
In Figure 4 the total intensity of the reflected and transmitted rays, indicated by points at discrete angle intervals, is compared to the sum of the reflectance and transmittance functions defined with the solution data from full wave solution.
The curves for the Electromagnetic Waves, Frequency Domain interface and the Geometrical Optics interface agree closely, which is to be expected because the transmittance and reflectance of the grating in the latter are defined explicitly in terms of the solution to the former.
Figure 4: The transmittance and reflectance computed by both the wave optics and ray optics models.
Application Library path: Ray_Optics_Module/Tutorials/diffraction_grating
Modeling Instructions
From the File menu, choose New.
New
In the New window, click  Model Wizard.
Model Wizard
1
In the Model Wizard window, click  2D.
2
In the Select Physics tree, select Optics>Wave Optics>Electromagnetic Waves, Frequency Domain (ewfd).
3
Click Add.
4
Click  Study.
5
In the Select Study tree, select General Studies>Frequency Domain.
6
Global Definitions
Parameters 1
1
In the Model Builder window, under Global Definitions click Parameters 1.
2
In the Settings window for Parameters, locate the Parameters section.
3
Because this model uses two model Components with different geometries but the same material properties, it is convenient to define global materials before setting up the individual physics interfaces.
Air
1
In the Model Builder window, right-click Materials and choose Blank Material.
2
In the Settings window for Material, type Air in the Label text field.
3
Click to expand the Material Properties section. In the Material properties tree, select Electromagnetic Models>Refractive index>Refractive index, real part (n).
4
Click  Add to Material.
5
Locate the Material Contents section. In the table, enter the following settings:
SiO2
1
Right-click Materials and choose Blank Material.
2
In the Settings window for Material, type SiO2 in the Label text field.
3
Click to expand the Material Properties section. In the Material properties tree, select Electromagnetic Models>Refractive index>Refractive index, real part (n).
4
Click  Add to Material.
5
Locate the Material Contents section. In the table, enter the following settings:
Geometry 1
Create the geometry of a single unit cell in the grating.
Rectangle 1 (r1)
1
In the Geometry toolbar, click  Rectangle.
2
In the Settings window for Rectangle, locate the Size and Shape section.
3
In the Width text field, type d.
4
In the Height text field, type 6*d.
5
Locate the Position section. In the y text field, type -3*d.
Rectangle 2 (r2)
1
In the Geometry toolbar, click  Rectangle.
2
In the Settings window for Rectangle, locate the Size and Shape section.
3
In the Width text field, type d.
4
In the Height text field, type 3*d.
5
Locate the Position section. In the y text field, type -3*d.
Rectangle 3 (r3)
1
In the Geometry toolbar, click  Rectangle.
2
In the Settings window for Rectangle, locate the Size and Shape section.
3
In the Width text field, type d/2.
4
In the Height text field, type d/4.
5
Locate the Position section. In the x text field, type d/4.
Union 1 (uni1)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
Select the objects r2 and r3 only.
3
In the Settings window for Union, locate the Union section.
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Clear the Keep interior boundaries check box.
5
Click  Build All Objects. The geometry should look like the unit cell in Figure 1.
Materials
Material Link 1 (matlnk1)
1
In the Model Builder window, under Component 1 (comp1) right-click Materials and choose More>Material Link.
2
Material Link 2 (matlnk2)
1
Right-click Materials and choose More>Material Link.
2
3
In the Settings window for Material Link, locate the Link Settings section.
4
From the Material list, choose SiO2 (mat2).
Study 1
Step 1: Frequency Domain
It is convenient to specify the frequency in the sweep before setting up the physics, since it can then be used to automatically compute the diffraction orders for the Port boundary conditions.
1
In the Model Builder window, under Study 1 click Step 1: Frequency Domain.
2
In the Settings window for Frequency Domain, locate the Study Settings section.
3
In the Frequencies text field, type f0.
In this model the S-parameters of a TE wave are computed. Select Out-of-plane vector as the component of the electric field to be solved for.
Electromagnetic Waves, Frequency Domain (ewfd)
1
In the Model Builder window, under Component 1 (comp1) click Electromagnetic Waves, Frequency Domain (ewfd).
2
In the Settings window for Electromagnetic Waves, Frequency Domain, locate the Components section.
3
From the Electric field components solved for list, choose Out-of-plane vector.
Create a periodic input port. To model a TE wave, keep the Electric field as the Input quantity and enter the value 1 in the z-component field.
Port 1
1
In the Physics toolbar, click  Boundaries and choose Port.
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3
In the Settings window for Port, locate the Port Properties section.
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From the Type of port list, choose Periodic.
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Locate the Port Mode Settings section. Specify the E0 vector as
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In the α text field, type alpha.
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Locate the Automatic Diffraction Order Calculation section. Clear the Include in automatic diffraction order calculation check box, as the Diffraction Order nodes need to be manually added for this port for normal incidence.
8
In the n text field, type n_air.
Use Diffraction Order nodes to absorb the reflected waves of nonzero diffraction order.
Diffraction Order 1
1
In the Physics toolbar, click  Attributes and choose Diffraction Order.
2
In the Settings window for Diffraction Order, locate the Port Mode Settings section.
3
From the Components list, choose Out-of-plane vector.
4
In the m text field, type -1.
Port 1
In the Model Builder window, click Port 1.
Diffraction Order 2
1
In the Physics toolbar, click  Attributes and choose Diffraction Order.
2
In the Settings window for Diffraction Order, locate the Port Mode Settings section.
3
From the Components list, choose Out-of-plane vector.
4
In the m text field, type 1.
Add the output port. In this case the excitation is set to Off.
Port 2
1
In the Physics toolbar, click  Boundaries and choose Port.
2
3
In the Settings window for Port, locate the Port Properties section.
4
From the Type of port list, choose Periodic.
5
Locate the Port Mode Settings section. Specify the E0 vector as
6
Locate the Automatic Diffraction Order Calculation section. In the n text field, type n_sio2.
Add the Diffraction Order nodes for the second periodic port by clicking the Add Diffraction Orders button on the first periodic port.
Port 1
1
In the Model Builder window, click Port 1.
2
In the Settings window for Port, locate the Automatic Diffraction Order Calculation section.
3
Click Add Diffraction Orders.
Add the periodic boundary condition to the sides of the unit cell.
Periodic Condition 1
1
In the Physics toolbar, click  Boundaries and choose Periodic Condition.
2
3
In the Settings window for Periodic Condition, locate the Periodicity Settings section.
4
From the Type of periodicity list, choose Floquet periodicity.
5
From the k-vector for Floquet periodicity list, choose From periodic port.
Study 1
Parametric Sweep
1
In the Study toolbar, click  Parametric Sweep.
2
In the Settings window for Parametric Sweep, locate the Study Settings section.
3
4
5
In the Study toolbar, click  Compute.
Results
Electric Field (ewfd)
1
In the Settings window for 2D Plot Group, locate the Data section.
2
From the Parameter value (alpha (deg)) list, choose 45.
3
In the Electric Field (ewfd) toolbar, click  Plot.
4
Click the  Zoom Extents button in the Graphics toolbar. Compare the resulting plot to Figure 2.
Global Evaluation 1
1
In the Results toolbar, click  Global Evaluation.
2
In the Settings window for Global Evaluation, locate the Expressions section.
3
4
Click  Evaluate. The resulting table shows the reflectance and transmittance values as functions of the angle of incidence.
Reflectance, Transmittance, and Absorptance (ewfd)
A line plot for the reflectances, transmittances, and absorptance is also generated by default. Move the legend panel to not cover the line plots.
1
In the Model Builder window, click Reflectance, Transmittance, and Absorptance (ewfd).
2
In the Settings window for 1D Plot Group, locate the Legend section.
3
From the Position list, choose Middle left. Compare the resulting plot to Figure 3.
Now add a second model Component to compute the ray trajectories.
Add Component
In the Model Builder window, right-click the root node and choose Add Component>2D.
Geometry 2
Rectangle 1 (r1)
1
In the Geometry toolbar, click  Rectangle.
2
In the Settings window for Rectangle, locate the Position section.
3
From the Base list, choose Center.
4
Locate the Size and Shape section. In the Width text field, type 5[mm].
5
In the Height text field, type 1.35[mm].
Rectangle 2 (r2)
1
In the Geometry toolbar, click  Rectangle.
2
In the Settings window for Rectangle, locate the Size and Shape section.
3
In the Width text field, type 5[mm].
4
Locate the Position section. From the Base list, choose Center.
5
Locate the Size and Shape section. In the Height text field, type 0.675[mm].
6
Locate the Position section. In the y text field, type -0.675[mm]/2.
7
Click  Build All Objects.
Definitions (comp2)
Use the reflectance and transmittance data from the previous study to define a series of interpolation functions for the large-scale geometrical optics analysis.
Interpolation 1 (int1)
1
In the Home toolbar, click  Functions and choose Local>Interpolation.
2
In the Settings window for Interpolation, locate the Definition section.
3
From the Data source list, choose Result table.
4
Find the Functions subsection. In the table, enter the following settings:
5
Locate the Units section. In the Arguments text field, type deg.
6
In the Function text field, type 1.
Now set up the Geometrical Optics interface.
Add Physics
1
In the Home toolbar, click  Add Physics to open the Add Physics window.
2
Go to the Add Physics window.
3
In the tree, select Optics>Ray Optics>Geometrical Optics (gop).
4
Click Add to Component 2 in the window toolbar.
5
In the Home toolbar, click  Add Physics to close the Add Physics window.
Materials
Material Link 3 (matlnk3)
1
In the Model Builder window, under Component 2 (comp2) right-click Materials and choose More>Material Link.
2
Material Link 4 (matlnk4)
1
Right-click Materials and choose More>Material Link.
2
3
In the Settings window for Material Link, locate the Link Settings section.
4
From the Material list, choose SiO2 (mat2).
Geometrical Optics (gop)
1
In the Model Builder window, under Component 2 (comp2) click Geometrical Optics (gop).
2
In the Settings window for Geometrical Optics, locate the Intensity Computation section.
3
From the Intensity computation list, choose Compute intensity and power.
4
Locate the Ray Release and Propagation section. In the Maximum number of secondary rays text field, type 4505.
Ray Properties 1
1
In the Model Builder window, under Component 2 (comp2)>Geometrical Optics (gop) click Ray Properties 1.
2
In the Settings window for Ray Properties, locate the Ray Properties section.
3
From the Ray property specification list, choose Specify frequency.
4
In the ν text field, type f0.
Define the angle of incidence as a function of the wave vector.
Definitions (comp2)
Variables 1
1
In the Model Builder window, under Component 2 (comp2) right-click Definitions and choose Variables.
2
In the Settings window for Variables, locate the Variables section.
3
Geometrical Optics (gop)
Grating 1
1
In the Physics toolbar, click  Boundaries and choose Grating.
2
3
In the Settings window for Grating, locate the Device Properties section.
4
In the d text field, type d.
5
Select the Store total transmitted power check box.
6
Select the Store total reflected power check box.
These check boxes create variables that can be used to compute the total power of all transmitted rays and all reflected rays, respectively, for each angle of incidence.
By default a Diffraction Order subnode is created to release rays of order 0. Modify this default subnode, then create additional Diffraction Order subnodes to release rays of diffraction orders +1 and -1.
Diffraction Order (m = 0)
1
In the Model Builder window, expand the Grating 1 node, then click Diffraction Order (m = 0).
2
In the Settings window for Diffraction Order, locate the Device Properties section.
3
In the R text field, type R0(alpha_ro).
4
In the T text field, type T0(alpha_ro).
Grating 1
In the Model Builder window, click Grating 1.
Diffraction Order (m = 1)
1
In the Physics toolbar, click  Attributes and choose Diffraction Order.
2
In the Settings window for Diffraction Order, locate the Device Properties section.
3
In the m text field, type -1.
4
In the R text field, type Rm1(alpha_ro).
5
In the T text field, type Tm1(alpha_ro).
Grating 1
In the Model Builder window, click Grating 1.
Diffraction Order (m = 1)
1
In the Physics toolbar, click  Attributes and choose Diffraction Order.
2
In the Settings window for Diffraction Order, locate the Device Properties section.
3
In the R text field, type R1(alpha_ro).
4
In the T text field, type T1(alpha_ro).
Release from Grid 1
1
In the Physics toolbar, click  Global and choose Release from Grid.
2
In the Settings window for Release from Grid, locate the Initial Coordinates section.
3
In the qy,0 text field, type 1e-6. The ray will be released an extremely short distance above the grating so that even rays at very large angles of incidence will reach the boundary fairly quickly.
4
Locate the Ray Direction Vector section. From the Ray direction vector list, choose Conical.
5
In the Nw text field, type 901.
6
In the α text field, type pi/4.
7
Specify the r vector as
Define a power density of 901 W/m so that each ray has a power density of 1 W/m.
8
Locate the Total Source Power section. In the Psrc text field, type 901[W/m].
9
Locate the Initial Polarization section. From the Initial polarization type list, choose Fully polarized.
10
In the axy,0 text field, type 0.
11
In the az,0 text field, type 1. The released ray is S-polarized. This is consistent with the use of TE waves in the previous study.
Add Study
1
In the Home toolbar, click  Add Study to open the Add Study window.
2
Go to the Add Study window.
3
Find the Physics interfaces in study subsection. In the table, clear the Solve check box for Electromagnetic Waves, Frequency Domain (ewfd).
4
Find the Studies subsection. In the Select Study tree, select Preset Studies for Selected Physics Interfaces>Ray Tracing.
5
Click Add Study in the window toolbar.
6
In the Model Builder window, click the root node.
7
In the Home toolbar, click  Add Study to close the Add Study window.
Study 2
Step 1: Ray Tracing
1
In the Settings window for Ray Tracing, locate the Study Settings section.
2
From the Time unit list, choose ps.
3
In the Output times text field, type 0 1.
4
In the Home toolbar, click  Compute.
Results
Ray Trajectories (gop)
The default plot shows the paths of the rays as they interact with the grating.
Transmittance and Reflectance (ewfd and gop)
1
In the Home toolbar, click  Add Plot Group and choose 1D Plot Group.
2
In the Settings window for 1D Plot Group, type Transmittance and Reflectance (ewfd and gop) in the Label text field.
3
Click to expand the Title section. From the Title type list, choose None.
4
Locate the Plot Settings section. Select the x-axis label check box.
5
In the associated text field, type Angle of incidence (deg).
6
Select the y-axis label check box.
7
In the associated text field, type Transmittance and Reflectance.
8
Locate the Legend section. From the Position list, choose Middle left.
Global 1
1
Right-click Transmittance and Reflectance (ewfd and gop) and choose Global.
2
In the Settings window for Global, locate the y-Axis Data section.
3
These expressions give the total reflectance and transmittance, respectively, for all diffraction orders.
4
Locate the x-Axis Data section. From the Parameter list, choose Expression.
5
In the Expression text field, type alpha.
6
From the Unit list, choose °.
7
Click to expand the Legends section. From the Legends list, choose Manual.
8
Transmittance and Reflectance (ewfd and gop)
In the Model Builder window, click Transmittance and Reflectance (ewfd and gop).
Ray 1
1
In the Transmittance and Reflectance (ewfd and gop) toolbar, click  More Plots and choose Ray.
2
In the Settings window for Ray, locate the Data section.
3
From the Dataset list, choose Ray 1.
4
From the Time selection list, choose Last.
5
Locate the y-Axis Data section. In the Expression text field, type gop.Qgr.
6
Locate the x-Axis Data section. From the Parameter list, choose Expression.
7
In the Expression text field, type at(0,alpha_ro).
8
From the Unit list, choose °.
9
Click to expand the Coloring and Style section. Find the Line style subsection. From the Line list, choose None.
10
Find the Line markers subsection. From the Marker list, choose Point.
11
In the Number text field, type 40.
12
Click to expand the Legends section. Select the Show legends check box.
13
From the Legends list, choose Manual.
14
Ray 2
1
Right-click Ray 1 and choose Duplicate.
2
In the Settings window for Ray, locate the y-Axis Data section.
3
In the Expression text field, type gop.Qgt.
4
Locate the Legends section. In the table, enter the following settings:
5
In the Transmittance and Reflectance (ewfd and gop) toolbar, click  Plot.
6
Click the  Zoom Extents button in the Graphics toolbar. Compare the resulting plot to Figure 4.