Excitation Tab

The Excitation Tab (see Fig. 8) is devised for defining the excitation conditions for the grating computation. It offers means to run loops over the excitation parameters.

Fig. 8: Excitation Tab

1. Loop Group

Especially, the following parameters may be defined:

• The wavelength (to be entered in microns),
• the truncation number (i.e., the minimum and maximum Rayleigh order to be kept in the diffraction matrix algorithm),
• the (polar) incidence angle and
• the azimuthal incidence angle (This option is activated only in the conical and 2D case).

A loop option is linked to each of these four parameters. This means that the computation will be performed for several values of this parameter defined by a start, stop and step value.

Optionally, one geometry parameter of the stack may be also integrated in the loop selection. In order to do this, the particular parameter has to be indicated by means of a dollar sign at the end of its line in the stack file. You can do this by opening the file with the notepad button. The example in Fig. 9 shows the selection of the grating thickness as a parameter.

Fig. 9: Activation of a wild card loop (via $ sign)

The mark is recognized automatically and an additional radio button together with input field(s) pops up after saving the change and leaving the notepad editor. This is shown in ???. If the “/d” box is checked, than the loop parameters are related to the pitch, i.e., entering 0.2 at pitch 0.5 microns means 0.1 micron.

Fig. 10: Activated and selected wild card loop

2. Flexi Loop

The looping group permits already quite some versatility in terms of looping. However, it is limited to the variation of just one parameter and to equidistant steps. This can be overcome by the Flexi loop. In order to activate it, the Flexi loop box has to be checked (see Fig. 11) and a loop control file (extension .ulf) has to be loaded.

Fig. 11: Activated Flexi Loop (Excitation Tab)

An example of an .ulf file is shown in Fig. 12. The parameters have to be listed in exactly the order as shown in the figure. Furthermore, the total number of parameter sets has to be entered in the second line (here 6). Beside of these constraints, the parameter values in each line (corresponding to a parameter set) are quite arbitrary within the range of physical values. Of course, single loops can also be specified but as opposed to the single loop selection, a variable step width can be invoked such as shown in the example of Fig. 12. Finally, the output parameter of the set which shall be appear in the output file (parameter to be printed) can also be selected via the radio buttons (theta_i in the example of Fig. 11).

Fig. 12: Example of an Unigit Loop File (.ulf)

3. Mount Selection

The mount selection offers two choices – the classical and conical mount. In the classical mount, the incident beam remains in the grating plane (defined by the grating vector which is perpendicular to the grating grooves and the normal vector for 1D gratings). Whereas in the conical mount it can be outside. Different solvers are required in 1D for the polarization modes do couple for conical mount and don’t couple for classical mount. In this way, the selection determines which solver has to be run (unigit_1D or unigit_1C for classical and unigit_1DC or unigit_1CC for conical mount). It has to be noted, that comparisons can be made for both solvers by selecting on the one hand classical mount and on the other hand conical mount and setting Phi = 0°.

In general, the mount doesn’t matter for 2D gratings. However, if unigit_2DSX shall be run for symmetry acceleration, the corresponding check box “Sym Oblique” in the Run Tab will be only available by choosing conical mount and setting Phi = 90° while the unigit_2DS0 check box “Sym Normal” will only be available when choosing classical mount and setting AOI = 0°.