Inhomogeneities & Interlayers

Deposition never produces a perfectly abrupt boundary between two materials. There is always a thin region where the index grades from one to the other. Inhomogeneities lets you insert such a graded transition at any interface in your stack and see what it does to the spectrum. Each transition is sliced into many thin sub-layers whose index is mixed linearly from one material to the next along the chosen profile, and the graded stack is evaluated and overlaid against the original step-interface design. The interlayers are a preview only: your design is not changed.

The window configures the interfaces for the surface mode set in the Design Editor: the front stack, the back stack, or both for total.

Settings

λ range / step: the wavelength grid, in nanometres.

AOI: angle of incidence.

T / R / A: which curves to draw, with avg, s and p inside each. Every polarization is computed together, so switching one on adds a curve rather than replacing the averaged one, and costs nothing.

Interfaces

The Interfaces strip below the plot lists every interface in the stack by the two media it joins. Tick one to add a graded interlayer there, then set:

Thickness: the depth of the transition region, in nanometres. The interlayer is added at the interface; the host layers keep their own thicknesses.

Profile: the shape of the index grade across the transition: linear, parabolic, invParabolic, exponential, or sigmoid.

Slices: how many thin sub-layers the transition is divided into for the calculation. Around 10–20 is usually enough; more slices give a smoother grade.

Clear all removes every interlayer.

How to read it

The chart is in percent. Each channel is drawn twice: the original step-interface design as a faint dotted line, and the graded version as a solid line on top. Comparing the two shows what the real, non-abrupt interfaces cost you: typically softened stopband edges and shifted passband ripple. If your design has a Specification, a live verdict in the Results strip tells you whether the graded design still passes. The Interfaces strip header reports how many interlayers are active and their combined thickness.

References

• H. A. Macleod, Thin-Film Optical Filters, 5th ed., §16 (inhomogeneous layers).