1.1. Layer Editor

The layer editor serves for assembling a grating stack. It is activated whenever either the button “Add” or the button “Edit” is hit in the Grating Stack Tab. For the latter, a layer has to be selected before. The layer editor is context sensitive depending on what kind of grating is selected.

Unigit offers four different basic types of stacks where each has it’s own genuine layer types. These basic types distinguish between dimension (1D and 2D) and method (RCWA and C-method) – see also Grating Stack Tab in section 2.4. The specific layer types of these different gratings are discussed in the following sub-sections.

1.1.1. Layer Editor for 1D RCWA

Currently, there are 9 layer types available for 1D RCWA gratings (see section 2.4.2). They fall in

five basic and four composite types. The layer type is selected via the arrow-down button in the drop list “Layer Type”. All layer types are presented briefly in what follows.

1.1.1.1. Thin Film

A thin film is the most elementary layer type. It consists of a flat homogeneous layer on top of a plane underlay. Thus, it requires only two parameters for complete description – thickness and material (see Fig. 28). In addition, an arbitrary name can be assigned to each layer.

Fig. 28: Thin film layer

1.1.1.2. Rayleigh-Fourier Polygon

The editor look for the selection of a Rayleigh-Fourier (RF) polygon is shown in Fig. 29. The Rayleigh-Fourier method is an additional solver method beside the RCWA. It is very fast but fails if the so-called Rayleigh hypothesis (for a sine profile with y = h*cos(kx) & k = 2pi/l holds that the method only converges if k*h < 0.448) is violated (in many cases it works also beyond this limit).

Fig. 29: Rayleigh-Fourier polygon

Actually, the Rayleigh-Fourier polygon represents an interface rather than a “layer”. The interface itself follows the polygon points in the list box. Moreover, the materials above and below the interface (medium 1 and 2) have to be specified. The absolute PV-value of the interface is given by the entrance in the “Thickness” field. The z-points in the polygon list are renormalized correspondingly. Polygon points can be added (“Insert”), removed or replaced by means of the buttons below the list. Furthermore a polygonal interface can also be load from a file (“Load” button). As a default, all x-values (lateral coordinate) are normalized to 1. The absolute values are shown when the box “abs” is checked. In addition, it is possible to check the box “trap” when the total number of polygon points equals 6 or to check the box “blaze” when the number of polygon points is 3. In either case, the appearance of the editor changes from polygon point input to parametric input. The new parameters are CD and side wall angles (SWA) for the trapezoid and only SWA for the blaze grating (see Fig. 30).

Fig. 30: Parametric input modus for RF-polygon layer (left trapezoid, right blaze)

In both cases, the profile can be shifted laterally by means of entering a phase shift value for x0 (an example for the trap case is shown in Fig. 31).

Fig. 31: Lateral shift of the RF profile by modifying x0

1.1.1.3. Rayleigh-Fourier Polygon

The RF Sine is similar to the polygon (same method, similar parameters) however the polygonal profile is replaced by a sine profile (see Fig. 32).

Fig. 32: RF Sine Layer

1.1.1.4. RCWA Slice

Beside the thin film, the RCWA slice is the most elementary layer type of the 1D RCWA grating.

One of the strengths of RCWA is that quite arbitrary and complicated grating profiles can be put together with these slices (so-called stair case approximation).

Fig. 33: RCWA slice (discrete)

The appearance of the layer editor for the RCWA slice is depicted in Fig. 33. Each slice is a slab of constant thickness consisting of an arbitrary number of partitions (as it is called in Unigit) meaning areas of homogeneous material that are separated by vertical borders in the default case, i.e. skew value = 0. The partitions (and thus the grating lines) can be skewed by entering a value different from zero (skew angle in degrees). The location of the border is indicated by the first column (x) in the list. The third column contains the material descriptor. Partitions can be inserted, replaced or deleted by means of the buttons on the bottom. In order to insert a new partition, an x-value has to be entered and the refractive index has to be edited.