Heads-up display (HUD) is a display which combines the background view and several essential information from other devices. This helps a driver/pilot to keep their viewpoint at the front. To achieve this, a HUD should be a transparent display and be able to reflect/project light from other directions.
This example demonstrates a multi-stack coating that provides simultaneous high reflection at multi-wavelengths . We start from micro-scales optics using STACK and then export data to macro optics tools, e.g. SPEOS.
The dielectric film is composed of repeated periods of two materials with refractive index (n1, n2). We design appropriate phase differences such that all reflected lights at the interface could match the constructive interference condition. As a result of this, reflection is then enhanced.
With this approach, we could design a reflector for a particular wavelength band. However, it is transparent for wavelength outside this band. In order to reflect light at R,G,B three bands simultaneously, we would cascade three dielectric stacks. Each of them reflects a particular band. (R : 632nm, G : 535nm, B : 450nm) Additionally, the order of the sub-stacks (e.g. R,G,B or G,B, R…etc) can affect the overall response in some cases that the reflection at the stopbands is completely suppressed for individual filters.
The script contains three parts. The first part defines the materials and simulation settings. For example, the target wavelength range, incident angle, etc. In the following part, with the constructive interference condition, we apply the STACK simulation. This includes both S & P polarization simulations. The result would contain the reflection and transmittance of S&P polarization over a specified wavelength and incident angle range. Finally, we plot results and export them using a SPEOS file format, i.e. “.coated” file.
Step 1: Stack Analysis
- Open the script file SPEOS-Lumerical-STACK_Reflector_R_filter.lsf and run it.
This plot illustrates the refractive index of different layers. As the above picture shows, the final structure contains three parts. Each part contains ~100 layers and it could respectively reflect specific wavelength. It should be mentioned that we use refractive index apodization in this step. Apodization is a well-known method to reduce transmission spectrum ripples.
The following figure shows the result of STACK which illustrates the angular and the wavelength dependence of the reflectance. (left: RP, right: RS) It is obvious that we achieve high reflectance in three wavelengths at the angle 30 we set in the simulation.
We can also check the spectrum at this angle.
Step 2: Export a SPEOS format file
The script would generate a “.coated” file automatically. The data format should be the following: