Basics of EM wave propagation

[sumeet kumar]

I have to setup a demonstration session on FDTD for a group of students. So I started with the source (10um) in air and set it up along horizontal and vertical. Why did it seem like the wave (Ey with z) was propagating from left to right for horizontal source? Also I wanted to show that conservation of energy (R + T = 1). For that I set up two observation planes. But the sum of power is very smaller than 1. Could someone suggest me what I am missing.

Attachments:

1. Basics32bit.zip

[Scott Newman]

Well you have a number of issues in how you setup your simulation that is causing the problems.

Vertical source appearing to go horizontal

1. The horizontal axis is from 0 to 5 not -2.5 to 2.5 which means when you set the center of the source to be located at 0 um you are setting it to be right on top of the PML and even with 20 PML layers it is not enough to absorb that so what you are seeing is reflections off the edge of your domain. Shift your center over to 2.5 um and you will see what you are expecting.

Power not equaling zero

1. You are using a pulsed input which means many frequency components, the power will be spread across these. To show what you want to show you would need to add up the power for the observation lines for each frequency component. Try a CW simulation instead.

2. You are using PML as your boundary conditions. PML is an absorbing boundary condition, therefore introducing loss that you are not considering.

3. Be cautious, you are specifying the amplitude of your input plane to be 1 but want to compare power. Try specifying power so that you are comparing like quantities.

4. The initialization of the input plane is based on the paraxial approximation, i.e. field profile is much larger than wavelength. With a wavelength of 10 um and a Gaussian Full width at 1/e^2 of 1 um you are very far from this.

[sumeet kumar]

Thanks Mr Newman.

Based upon your reply, I split the work into two parts. Firstly I want to correct the source and TFSF. Later I will work on power and reflectance.

1. Basically we add a point source using input plane. We can control the injection point, beam width and strength corresponding to various positions in layout using the options center, width and rectangular/ gaussian respectively. It worked pretty well. I tried with tilting option and the wave was propagating as it was expected.

2. Now the problem was updating of field in reverse direction. So I added TFSF. But it changed everything and the wave started propagating from left to right again. Also just for curiosity, I want to know that why do we need to update wavelength in TFSF tab? Should not it be considered from the same wavelength which we already entered in source tab?

3. I attached two files. (i) without TFSF (ii) with TFSF.

Please take a look over the files and help me in figuring out how to properly incorporate TFSF into the model.

Thanks

Attachments:

1. TFSF32bit.zip

[Scott Newman]

Just to be clear regarding point 1, you are not creating a point source with the input plane. We have a point source, you are injecting a Gaussian beam along the input plane.

As for your questions regarding the specifics of the 32-bit software, the 32-bit free evaluation is an older version of the OptiFDTD product that we make available for the community. Along with other members of the community we work to help you understanding the use of the product or aspects of design. That being said detailed technical support and work on any bug reports is only available for users of the paid 64-bit software with a valid maintenance license.

[sumeet kumar]

Thanks Mr Newman.

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