# Questions about the material setting in inverse design model: 'y_branch_base_TE_modematch.lsf'

#1

Hello:

Why the index of the waveguide (input wg, output wg top, output wg bottom) in ‘y_branch_base_TE_modematch.lsf’ (FDTD 2D simulation) is seted as 2.8? Does that mean the refractive index of the silicon slab with a thickness of 220nm is 2.8? Could you give me some reference papers? And what’s the exact meaning of the scalar ‘index’ in material and why the index has a length units?

Fig.1 material setting for input wg
y_branch_base_TE_modematch.lsf (2.2 KB)

If I want to set the source mode as the following higher order mode (TE1), are the settings of the material in this waveguide same as Fig.1?

Thank you,

Ya Han

#2

Material setup

The meaning of the scalar value of “index” is the real part of refractive index. This means that the waveguides and the Y Branch are constructed of lossless dielectric material with refractive index of 2.8.

You can notice we set also scalar value of background index using 1.44 value (shown as blue on your plot). This means that the structure is buried in lower index dielectric.

Setting up higher order modes

To inject higher order modes, you need to change the source settings in the python script in FOM definition:

######## DEFINE FIGURE OF MERIT ########

# The base simulation script defines a field monitor named 'fom' at the point where we want to modematch to the 3rd mode (fundamental TE mode).

fom = ModeMatch(monitor_name = 'fom', mode_number = 4, direction = 'Forward', target_T_fwd = lambda wl: np.ones(wl.size), norm_p = 1)


Specifically, change the “mode_number” attribute to your desired mode.

#3

Thanks a lot!

On 3/14/2019 15:24，Miroslav Support Benes via Lumerical Knowledge Exchangelumerical@discoursemail.com wrote：

closed #4

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