COMSOL 6.4 - Fresnel Equations

A plane electromagnetic wave propagating through free space is incident at an angle upon an infinite dielectric medium. This model computes the reflection and transmission coefficients and compares the results to the Fresnel equations.

Model Definition

A plane wave propagating through free space (n = 1) as shown in Figure 1 is incident upon an infinite dielectric medium (n = 1.5) and is partially reflected and partially transmitted. If the electric field is p-polarized — that is, if the electric field vector is in the same plane as the Poynting vector and the surface normal — then there are no reflections at an incident angle of roughly 56°, known as the Brewster angle.

Figure 1: A plane wave propagating through free space incident upon an infinite dielectric medium.

Although, by assumption, space extends to infinity in all directions, it is sufficient to model a small unit cell, as shown in Figure 1; a Floquet-periodic boundary condition applies on the top and bottom unit-cell boundaries because the solution is periodic along the interface. This model uses a 3D unit cell and applies also a periodic boundary condition in the out-of-plane direction. However, this periodic boundary condition acts like a continuity condition. The angle of incidence ranges between 0–90° for both polarizations.

To simplify the process of adding the boundary conditions, a Periodic Structure node is used. It automatically adds and configures its subnodes — the ports and the periodic conditions.

For comparison, Ref. 1 and Ref. 2 provide analytic expressions for the reflectance and transmittance¹. Reflection and transmission coefficients for s-polarization and p-polarization are defined respectively as

Reflectance and transmittance are defined as

The Brewster angle at which rp = 0 is defined as

In the first part of the simulation, the Electromagnetic Waves, Frequency Domain interface will be used. The second part instead uses the Electromagnetic Waves, Beam Envelopes interface. Using the Electromagnetic Waves, Beam Envelopes interface, the model is simplified, replacing the glass substrate with an Impedance boundary condition. Furthermore, as the wave vectors for the incident and reflected waves are known, the simulation can be performed with a very coarse mesh.

Results and Discussion

Figure 2 is a combined plot of the y component of the electric-field distribution and the power flow visualized as an arrow plot for the TE case.

Figure 2: Electric field, Ey (slice) and power flow (arrows) for TE incidence at 70° inside the unit cell.

For the TM case, Figure 3 visualizes the y component of the magnetic-field distribution instead, again in combination with the power flow.

Figure 3: Magnetic field, Hy (slice) and power flow (arrows) for TM incidence at 70° inside the unit cell.

Note that the sum of reflectance and transmittance in Figure 4 and Figure 5 equals 1, showing conservation of power. Figure 5 also shows that the reflectance around 56° — the Brewster angle in the TM case — is close to zero.

Figure 4: The reflectance and transmittance for TE incidence agree well with the analytic solutions.

Figure 5: The reflectance and transmittance for TM incidence agree well with the analytic solutions. The Brewster angle is also observed at the expected location.

Figure 6 shows a TE field plot when the Electromagnetic Waves, Beam Envelopes interface is used. Here, the simulation is only performed for the air domain. The bottom glass domain is represented by an Impedance boundary condition, implemented for nonnormal propagation in the exterior (glass) material. This particular implementation of the Impedance boundary condition for the Electromagnetic Waves, Beam Envelopes interface is useful for truncating the simulation domain at an interface between two dielectric materials.

Since the wave vectors for the solution are known, the mesh consists of only one mesh element in the z direction.

Figure 6: Electric field, Ey (slice) and power flow (arrows) for TE incidence at 70° inside the unit cell when the Electromagnetic Waves, Beam Envelopes interface is used.

Figure 7 shows that the TE reflectance results for the Electromagnetic Waves, Beam Envelopes interface agree well with the simulation using the Electromagnetic Waves, Frequency Domain interface and the analytical results.

Figure 7: Same plot as Figure 5, but with the TE reflectance curve from the Electromagnetic Waves, Beam Envelopes interface also included.

References

1. J.D. Jackson, Classical Electrodynamics, 3rd Ed., Wiley, 1999.

2. B.E.A. Saleh and M.C. Teich, Fundamentals of Photonics, Wiley, 1991.

Wave_Optics_Module/Verification_Examples/fresnel_equations

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

In the window, click

In the tree, select > > .

Click .

Click

In the tree, select > .

Click

Global Definitions

Define some parameters that are useful when setting up the geometry and the study.

Parameters 1

In the window, under click .

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
n_air	1	1	Refractive index, air
n_slab	1.5	1.5	Refractive index, slab
lda0	1[um]	1E-6 m	Wavelength
alpha	70[deg]	1.2217 rad	Angle of incidence
beta	asin(n_air*sin(alpha)/n_slab)	0.67701 rad	Refraction angle
alpha_brewster	atan(n_slab/n_air)	0.98279 rad	Brewster angle, TM only
r_s	(n_aircos(alpha)-n_slabcos(beta))/(n_aircos(alpha)+n_slabcos(beta))	-0.54735	Reflection coefficient, TE
r_p	(n_slabcos(alpha)-n_aircos(beta))/(n_aircos(beta)+n_slabcos(alpha))	-0.20613	Reflection coefficient, TM
t_s	(2n_aircos(alpha))/(n_aircos(alpha)+n_slabcos(beta))	0.45265	Transmission coefficient, TE
t_p	(2n_aircos(alpha))/(n_aircos(beta)+n_slabcos(alpha))	0.52925	Transmission coefficient, TM

The angle of incidence is updated while running the parametric sweep. The refraction (transmitted) angle is defined by Snell’s law with the updated angle of incidence. The Brewster angle exists only for TM incidence, p-polarization (polarization is in the plane of incidence).

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

First, create a block composed of two domains. Use layers to split the block.

Block 1 (blk1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.2.

In the text field, type 0.8.

Click to expand the section. In the table, enter the following settings:


Layer name	Thickness (µm)
Layer 1	0.4

Click

Click the

button in the toolbar.

Choose wireframe rendering to get a better view of each boundary.

Click the

button in the toolbar.

Materials

Now set up the material properties based on refractive index. The top half is filled with air.

Air

In the window, under right-click and choose .

In the window for , type Air in the text field.

Select Domain 2 only.

Locate the section. In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Refractive index, real part	n_iso ; nii = n_iso, nij = 0	n_air	1	Refractive index
Refractive index, imaginary part	ki_iso ; kiii = ki_iso, kiij = 0	0	1	Refractive index

Glass

The bottom half is glass.

Right-click and choose .

In the window for , type Glass in the text field.

Select Domain 1 only.

Locate the section. In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Refractive index, real part	n_iso ; nii = n_iso, nij = 0	n_slab	1	Refractive index
Refractive index, imaginary part	ki_iso ; kiii = ki_iso, kiij = 0	0	1	Refractive index

Electromagnetic Waves, Frequency Domain (ewfd, TE)

Set up the physics based on the direction of propagation and the E-field polarization. First, assume a TE-polarized wave which is equivalent to s-polarization or perpendicular polarization. Ex and Ez are zero while Ey is dominant.

In the window, under click .

In the window for , type Electromagnetic Waves, Frequency Domain (ewfd, TE) in the text field.

Periodic Structure 1

In the toolbar, click

and choose .

In the window for , locate the section.

In the α1 text field, type alpha.

The node automatically adds and configures its port and periodic condition subnodes.

Study 1 (ewfd, TE)

In the window, click .

In the window for , type Study 1 (ewfd, TE) in the text field.

Step 1: Wavelength Domain

In the window, under click .

In the window for , locate the section.

In the text field, type lda0.

Click to expand the section. Select the checkbox.

Click

In the table, enter the following settings:


Parameter name	Parameter value list	Parameter unit
alpha (Angle of incidence)	range(0,2[deg],88[deg])	deg

Mesh 1

In the window, under right-click and choose .

Study 1 (ewfd, TE)

In the toolbar, click

Results

Electric Field (ewfd, TE)

In the window for , type Electric Field (ewfd, TE) in the text field.

The default plot is the E-field norm for the last solution, which corresponds to almost tangential incidence. Replace the expression with Ey, add an arrow plot of the power flow (Poynting vector), and choose a more interesting angle of incidence for the plot.

Multislice 1

In the window, expand the node, then click .

In the window for , click in the upper-right corner of the section. From the menu, choose > > > > .

Locate the section. Find the subsection. In the text field, type 0.

Find the subsection. In the text field, type 0.

Locate the section. From the list, choose .

Arrow Volume 1

In the window, right-click and choose .

In the window for , click in the upper-right corner of the section. From the menu, choose > > > .

Locate the section. Find the subsection. In the text field, type 1.

Locate the section. From the list, choose .

Electric Field (ewfd, TE)

In the window, click .

In the window for , locate the section.

From the list, choose .

In the toolbar, click

Click the

button in the toolbar.

The plot should look like that in Figure 2.

Reflectance, Transmittance, and Absorptance (ewfd, TE)

A 1D plot with the reflection and transmission versus the angle of incidence is added by default.

In the window, under click .

In the window for , type Reflectance, Transmittance, and Absorptance (ewfd, TE) in the text field.

Click to expand the section. From the list, choose .

Locate the section. Select the checkbox.

Locate the section. From the list, choose .

Global 1

In the window, expand the node, then click .

In the window for , click to expand the section.

Find the subsection. From the list, choose .

From the list, choose .

Global 2

In the window, right-click and choose .

In the window for , locate the section.

In the table, enter the following settings:


Expression	Unit	Description
abs(r_s)^2		Reflectance, analytic
n_slabcos(beta)/(n_aircos(alpha))*abs(t_s)^2		Transmittance, analytic

Locate the section. From the list, choose .

Click to expand the section. From the list, choose .

In the table, enter the following settings:


Legends
Reflectance, analytic
Transmittance, analytic

In the toolbar, click

Compare the resulting plots with Figure 4.

Polarization Plot (ewfd, TE)

In the window, under click .

In the window for , type Polarization Plot (ewfd, TE) in the text field.

In the toolbar, click

The polarization plot shows that the field is linearly polarized, with a pure out-of-plane polarization. The out-of-plane component of the Jones vector represents the amplitude of a wave polarized in the y direction, whereas the in-plane component represents the amplitude of a wave with a polarization that is orthogonal to both the out-of-plane direction and the propagation direction, resulting in a polarization in the x − z plane.

Electromagnetic Waves, Frequency Domain (ewfd, TE) (ewfd)

The following instructions are for the case of a TM-polarized wave (p-polarization or parallel polarization). In this case, Ey is zero while Ex and Ez characterize the wave. In other words, Hy is dominant while Hx and Hz are zero.

In the window, under right-click and choose .

Electromagnetic Waves, Frequency Domain (ewfd2, TM)

In the window, right-click and choose .

In the dialog, click .

In the window for , type Electromagnetic Waves, Frequency Domain (ewfd2, TM) in the text field.

Periodic Structure 1

In the window, expand the > node, then click .

In the window for , locate the section.

From the list, choose . This will change the port mode fields to be polarized parallel to the plane of incidence.

Add Study

In the toolbar, click

to open the window.

Go to the window.

Find the subsection. In the tree, select .

Click the button in the window toolbar.

In the toolbar, click

to close the window.

Study 2 (ewfd2, TM)

In the window for , type Study 2 (ewfd2, TM) in the text field.

Study 1 (ewfd, TE)

Step 1: Wavelength Domain

In the window, under right-click and choose .

Study 2 (ewfd2, TM)

In the window, right-click and choose .

In the window for , locate the section.

In the column of the table, under , clear the checkbox for .

In the toolbar, click

Results

Magnetic Field (ewfd2, TM)

In the window for , type Magnetic Field (ewfd2, TM) in the text field.

Multislice 1

In the window, expand the node, then click .

In the window for , locate the section.

In the text field, type ewfd2.Hy.

Locate the section. Find the subsection. In the text field, type 0.

Find the subsection. In the text field, type 0.

Locate the section. From the list, choose .

Arrow Volume 1

In the window, right-click and choose .

In the window for , locate the section.

In the text field, type ewfd2.Poavx.

In the text field, type ewfd2.Poavy.

In the text field, type ewfd2.Poavz.

Locate the section. Find the subsection. In the text field, type 1.

Locate the section. From the list, choose .

In the toolbar, click

This reproduces Figure 3.

Reflectance, Transmittance, and Absorptance (ewfd2, TM)

In the window, under click .

In the window for , type Reflectance, Transmittance, and Absorptance (ewfd2, TM) in the text field.

Locate the section. From the list, choose .

Locate the section. Select the checkbox.

Locate the section. From the list, choose .

Global 1

In the window, expand the node, then click .

In the window for , locate the section.

Find the subsection. From the list, choose .

From the list, choose .

Global 2

In the window, right-click and choose .

In the window for , locate the section.

In the table, enter the following settings:


Expression	Unit	Description
abs(r_p)^2		Reflectance, analytic
n_slabcos(beta)/(n_aircos(alpha))*abs(t_p)^2		Transmittance, analytic

Click to expand the section. From the list, choose .

In the table, enter the following settings:


Legends
Reflectance, analytic
Transmittance, analytic

Locate the section. From the list, choose .

In the toolbar, click

The plot should look like Figure 5. The Brewster angle is observed around 56 degrees, which is close to the analytic value.

Polarization Plot (ewfd2, TM)

In the window, under click .

In the window for , type Polarization Plot (ewfd2, TM) in the text field.

In the toolbar, click

Also in this case, the polarization plot shows that the field is linearly polarized, in this case with a pure in-plane polarization.

Component 1 (comp1)

Now, add a simulation using the Electromagnetic Waves, Beam Envelopes interface. In this simulation, the Impedance boundary condition is added to the interface between the air and the glass domain and the simulation is only performed for the air domain. This example demonstrates that the Impedance boundary condition for the Electromagnetic Waves, Beam Envelopes interface can be used for truncating the simulation domain at a boundary between two dielectric material domains.

Add Physics

In the toolbar, click

to open the window.

Go to the window.

In the tree, select > > .

Click the button in the window toolbar.

In the toolbar, click

to close the window.

Electromagnetic Waves, Beam Envelopes (ewbe, TE)

In the window for , type Electromagnetic Waves, Beam Envelopes (ewbe, TE) in the text field.

Select Domain 2 only, to only perform the simulation for the air domain.

Locate the section. Specify the k1 vector as


ewbe.k0*sin(alpha)	x
0	y
-ewbe.k0*cos(alpha)	z

Specify the k2 vector as


ewbe.k1x	x
ewbe.k1y	y
-ewbe.k1z	z

Port 1

In the toolbar, click

and choose .

Select Boundary 7 only.

In the window for , locate the section.

From the list, choose .

For the first port, wave excitation is by default.

Locate the section. From the list, choose , to give a polarization in the y-direction (orthogonal to the plane of incidence).

In the α1 text field, type alpha.

The maximum frequency in the setting window will be used only when button is clicked to generate features representing higher order modes. In this model, no diffraction is expected from the given geometry, so this setting is ineffective.

Periodic Condition 1

In the toolbar, click