Learning Forum

[smzia]

Hi,

Greetings. I would like to ask about the "T" attribute in Lumerical FDTD. Does it mean power transmission or electric field transmission? Could anyone please give any idea with explanation and proof? 

 

Regards,

Zia

[Guilin Sun]

Hi Zia,

It is power transmission, the same as the script command "transmission":

https://optics.ansys.com/hc/en-us/articles/360034405354-transmission-Script-command

Please note that it uses the Poyntine vector dot surface normal, and the surface normal is always defined as along positive axis. Therefore the result can be negative. However it does not mean the reflected or transmitted power is negative.Its sign is to be used for other purpose, for example forward and backward propagation.

 

I hope this helps.

[smzia]

Thank you so much for the answer. It's truly helpful.

Regards,

Zia

[smzia]

I have another confusion. There are some scalar operation for example, Re, -Re, Abs, Abs_square and so on. 

If I want to see the power result. Should I select Abs or Abs_square? Could you please give any suggestions?

Attached please find the related image.

[Guilin Sun]

Abs=absolute value

Abs^2 is the square of the abs value.

In the case of transmission, they are not useful.

the Visualizer is designed for visualizing all quantities so there are many operations.

 

[smzia]

Hi Guilin,

So, which will visualize power? Real value of T? I have found that Real value of T and Absolute value of T gives the same value. 

You have already mentioned T is power transmission, So real value of T or absolute value of T should give the power transmission result? Isn't it?

I think, If T is electric field then Abs^2 or square of abs value would represent the power then.

Like if T is power then Abs(T)/Re(T) will visulize power transmission.

if T is electic field then, Abs^2(T) will visulaize power transmission. As , P=E^2.

Please let me know your thoughts.

[Guilin Sun]

T is real valued so Real value of T and Absolute value of T gives the same value. 

T should give the power transmission result.

" If T is electric field then Abs^2 or square of abs value would represent the power then." this is from textbook for single plane wave only.

The transmission is a normalized transmitted power. Please check this https://optics.ansys.com/hc/en-us/articles/360034405354-transmission-Script-command

 

 

[smzia]

I have read the link for transmission-script command. T is normalized transmitted power.

There are 7 scalar operation. None, Real, negative Real, Imaginary, Absolute, Absolute_square & Angle.

I need to use one of them to see the transmitted power. if I choose 'None' then there will be no result. I think 'Imaginary' & 'Angle' will not give transmitted power result.

So, which scalar operation should I choose among Real, Negative Real, Absolute & Absolute_square to visulize the transmitted power?

Could you please give any suggestions?

Regards,

Zia

[Guilin Sun]

So for transmission T, only Real or Abs are good to use. Again, the visualzier is a general tool so its operatons may not be available for different physical quantities.

[smzia]

Thank you so much Guilin Sun!

[gpsar]

Hi smzia, 

I am currently trying to figure something out regarding transmission "T" in FDTD and I came upon your discussion here, and I saw that in one of your replies you mentioned that "if T is electic field then, Abs^2(T) will visulaize power transmission. As , P=E^2." Is there any way you could explain to me, or point me in the right direction, as to how P = E^2? This would be of great help to me. Thank you!

[Guilin Sun]

This relationsip is simplified notiation in many textbooks that omit some constants. For a plane wave, it should reald P is linearly proportional to E^2.

[gpsar]

Hi Guilin, 

Thanks for your reply. In this case though, would P be the Poynting vector? So, does the Poynting vector, P = |E|^2? Would it possible to tell me which book the screenshot is from? Thank you

[Guilin Sun]

Not realy: the textbook oversimplifies this relation for plane wave only, and it does not depend on space. However, in simultion,the Poynting vectors are a funciton of space, so Px, Py and Pz are power density (Poynting vector), not the power. Power is the integration of P dot source normal over the monitor plane, as show in the link of the first reply: the script command "transmission".

You should be able to find a textbook for such relationship. You can also search online for Optical Intensity.

 

Please note that in most cases Lumerical's tranmission is always normalized to the source power.

 

In addition, the forum policy is for "one question one answer" for better reference. So if you have quesitons different from the first post, please write a new post. Thank you!

[Tisha Elizabeth Tomy]

Hi,

I wanted to ask you how to get the phase of a transmitted light through metasurface?

how to separate the LCP and RCP components of a transmitted light from metasurfce for a LCP or RCP incident?

[Guilin Sun]

Phase is associated with field component or S paramters. So you will need to access field-quantity. In addition, usually phase is a single value. However is can be distribute in space. ONce you define a reference point or a plane, you can use field component to get the phase. Pleae refer to the Metamaterial exmaple https://optics.ansys.com/hc/en-us/articles/360042097393-Metamaterial-parameter-extraction-Smith

You will decompose the fields into LCP and RCP.

Those are not the scope of this post. Please try and in case you cannoy find solutions, please write new posts for each.

[Sijing Zhong]

Hi,

I have two input light and I use frequency domain field and power to measure the output. Does the transmission I get equal to the output light intensity over the intensity of two input light?

[Guilin Sun]

It is normalized with source power, not intensity.

Please note that if the two sources do not have the signal, or thebandwidth/center wavelegth, the frequency domai normalization can have peoblem in theory. If you have questions regarding to this please write another post.