## Photonics

#### Problems with Azimuthal Source (Imported Source)

• fdtdisgreat
Subscriber

Hi there,

I am trying to use the azimuthal source as setup in this Ansys post ( https://optics.ansys.com/hc/en-us/articles/360034383054-Using-an-equation-to-define-the-spatial-field-profile-of-a-source-in-FDTD ). However I am encountering many problems whilst trying to adapt it to my setup.

Defining the azimuthal field profile at a wavelength of 1.6um, and then running a broadband simulation from 1.57um to 1.64um with the imported source inside a glass substrate (imported source at z = -1.6um, glass substrate surface at z = 0, above is just air), I observe 2 problems:

1) The chirality of the azimuthal polarization changes in space as the beam propagates. The below screenshot is of a power monitor placed along the propagation of the source (z direction). The alternating red and blue patches indicate that at a given xy plane, the chirality of the polarization is e.g. RH, but a few 100nm above it, it then flips to LH.

2) The field profile changes slightly as I change wavelength. The red and blue lobes shift slightly up in the z direction as the wavelength increases (the first screenshot is at wavelength 1570nm, the second at 1640nm). This means that for a given xy-plane, the azimuthal polarization can be RH polarized for 1570nm, but LH polarized for 1640nm.

Ideally, I want 1) The chirality of the azimuthal polarization to be constant across the propagation direction. 2) The propagation profile of the source to be constant across the wavelength range. Solving 1) would essentially solve 2), since if the chirality of the azimuthal polarization is consistent along the z direction, then it wouldn't matter if there is a small change in the profile with wavelength.

More details of my FDTD setup. My import source span = 7.5um (in x and y), NA = 0.3. Source is 1.6um below the substrate surface. FDTD width = 9.5um in x and y, and 6.3 um in z. BC are 50 PML layers on all 6 boundaries. I've placed an override mesh over the central part of the source, of 20 nm steps in x and y. Autoshutoff is 1e-5, which is typically reached very quickly. The example lsf script from Ansys for generating Azimuthal polarization has 2 options: I am using the 2nd option i.e. defining the field profile in k space, then fourier transforming it.

• Guilin Sun
Ansys Employee

A1: not sure how did you quantify the LH or RH?

A2: currently the import source is only for monochromatic light. Other wavelength will use that imported profile, which might not be correct. You could import several different wavelengths at the same location.

You could also file a feature request, when you have your source as a function of wavelength: ix.lumerical.com

"import source span = 7.5um (in x and y), NA = 0.3"

The source span (and FDTD span) might be a little small, as from the monitor result the fields are cut off, which will cause diffraction.

I strongly suggest to test the single wavelength at this moment.

• jmidkiff
Subscriber

The concept of chirality is not applicable to an azimuthal source. Unlike circular polarization, azimuthal polarization always contains orthogonal electric field components at every point in time. The oscillation you see in the direction of propagation is simply a snapshot of the phase.