# COMSOL - 2D axial symmetry for Light Diffraction

1. Nov 3, 2009

### AndrewSzelc

Hello,

I would be grateful if someone could comment on my problem.

I am trying to simulate diffraction of optical wavelength on a sub-wavelength aperture (aperture diameter/wavelength < 0.1).

What I want to achieve is:
- simulate a geometrical model with an aperture of 15um diameter
- wavelength will be varied between 10um to 200um (Parametric analysis).
- estimate what the aperture-transmitted power is as a function of the wavelength (for a fixed aperture diameter value).
- observe the diffracted wavefront (phi component and Electric field).
- understand how the polarization of the wavelength influences the transmission across the aperture cross section (can explain that further if needed).
- This means I will be increasing the wavelength and observing/reading the decreasing transmitted power.

What I did was:
- I am using RF module set for TE waves (harmonic propagation);
- 2D axial symmetry to simplify the calculations
- I drew a model of my setup in 2D axial symmetry. The geometrical model will be revolved around its axis of symmetry that is exactly in the centre of the aperture (centre of the aperture to the aperture edge = 7.5um).
- the axis of symmetry (the boundary of my geometrical model that is on the axis of symmetry) is defined as "axial symmetry" in Boundary settings.
- the remaining boundaries (apart from port) are all set for perfectly matched boundary (no reflections).
- I use "cylindrical" mode settings for emission port (I have also tried TM waves with coaxial settings, but the result was basically the same, see below).

What I get is:
- at first sight, the results seem ok as I get diffraction for "a" < lambda, but...
- the phi component of the electric field (given in V/m) is zero on the axis of revolution (axis is symmetry)
- it is as if the electric field was "disappearing" on the axis of revolution.
- That suggests that it is not a real 2D axial symmetry, since...
- I would expect the Z component to be continuous over the axis of revolution (2D axial symmetry...)

Is there anyone out there that could comment on that?
It is a very simply simulation but I cannot crack it.
All I want to do is to simulate a 2D axially symmetric model of an aperture that diffracts wavelengths.

Thanks,
Andrew

2. Oct 28, 2014

### kripri1991

hai andrew

if you dont mind can your share your program , i am also stuck in same situation

3. Oct 28, 2014

### Staff: Mentor

This person hasn't been here in 5 years, please pay attention to the date.