The Electric Field window plots the squared field amplitude |E(z)|² against depth through the coating at a chosen wavelength. It shows where light intensity concentrates inside the stack — the key piece of information for laser-damage-threshold analysis, since the layer carrying the highest field is usually the one that fails first under a high-power beam. It also makes it clear why a particular layer’s thickness matters so much to performance.

The field is normalized so that the incident intensity is 1, shown as 100 %. It combines the forward- and backward-travelling waves at every depth, matched across each layer boundary. In a high-reflectance mirror the field in the incident medium can reach 400 %, because the incident and reflected waves add nearly in phase.

Settings

Wavelength — the single wavelength at which the standing-wave profile is computed, in nm. It defaults to the design’s reference wavelength.

AOI — the angle of incidence in degrees.

Polarization — s, p, or their average. At oblique incidence s and p give different profiles, so compare them when working at an angle.

Side — profile the front coating or the back coating. Each side shows that coating’s standing wave on the substrate, evaluated from its own incident medium; the substrate is the exit medium.

How to read it

The horizontal axis is physical depth in nanometres; vertical dotted lines and the colored bands mark the layer boundaries and materials. Peaks of |E(z)|² are field anti-nodes and troughs are nodes. For laser-damage work, the layer containing the highest in-material field is the bottleneck — lowering the field there raises the damage threshold. In a well-designed mirror the anti-nodes sit preferentially in the more robust material, which is part of why mirrors tolerate high power.

The readout reports the peak field, the layer count and the total physical thickness, and the data table lists the field versus depth for the curves on screen. Reading the field against the Refractive Index Profile shows which layer the standing-wave peak lands in.

References

• H. A. Macleod, Thin-Film Optical Filters, 5th ed., Ch. 3 (Eqs. 3.5–3.6) — fields in thin films.