In this example, we demonstrate the use of stackrt command to design an antireflective circular polarizer to reduce the ambient light reflection of an OLED display.
Understand the simulation workflow and key results
The bottom metal electrode of OLED display can be used to enhance the light extraction efficiency of the device. However, it also has the detrimental effect of increasing the reflection of the ambient light, resulting in a reduced contrast ratio when the display is used outdoors. In this example, we demonstrate the use of circular polarizer for minimizing the reflection of light with certain linear polarization . The configuration and working principle of the circular polarizer is illustrated below:
For simplicity, the multi-layer OLED structure is represented by a metallic reflector. The light incident on the linear polarizer become 30 o linearly-polarized after it propagates through the half-wave plate and then is circular-polarized after passing quarter-wave plate. The reflected light would finally become orthogonally-polarized with respect to the polarization of the linear polarizer, hence blocked out by it.
The reflected light can be decomposed into two parts as is shown in the illustration above. R1 represents the reflection at the air/polarizer interface and R2 is associated with the circular polarizer. In this example, we will be focusing on how we can minimize the R2. For minimization of R1, please refer to te "Taking the model further" section.
For decomposing R1 and R2, one approach is to add an artificial layer with refractive index 1.5 as the illustration below.
The refractive index 1.5 is chosen to be close to the refractive index of the linear polarizer so that the overall reflection of the circular polarizer is almost the same with or without the artificial layer. We will then convert the reflectance from STACK solver (brown arrow) to R2 (blue arrow) by script commands. Details could be found in "Taking the model further" section.
The polarizer and wave plates are made of anisotropic materials, meaning their refractive indices can be different in different directions. Their rotations of the polarization/slow axis are fully considered in the Ansys STACK solver by rotating the corresponding permittivity tensors.
The workflow includes the following steps:
Step 1 Initial test
The main purpose of this step is to ensure the simulation is set up correctly and to validate the anti-reflection behavior of the circular polarizer at normal incidence.
Step 2 Sweep angles
In this step, the reflection properties of the circular polarizer is characterized by sweeping the incident angles (theta and phi). This metric can be useful when further assessing the behavior of a display in terms of viewing angles in ray optics tools such as Ansys SPEOS.
Run and results
Instructions for running the model and discussion of key results
Step 1 Initial test
- Open and run the script file stackrt_antireflection.lsf . The script plots the reflection spectrum of the circular polarizer at normal incidence.
The thicknesses of the wave plates were chosen for a minimal reflection at the target wavelength of 0.55 um, which is confirmed in the above plot. The small ripples in the reflection spectrum can be attributed to the Fabry-Perot resonance by the multlayer films.
Step 2 Sweep angles
- Open and run the script file stackrt_antireflection_angular_sweep.lsf . The script will sweep over incident angles (phi) by rotating the permittivity tensors and then give the reflectance as a function of wavelength and angles (theta and phi).
- Set the visualizer to check the polar image of R_ave, which is the average of Rs and Rp.
- Change the Nz value ( Nz=(nx-nz)/(nx-ny) ) in the script file from 1.5 to 0.5 and compare the results at 550 nm.
We could find that the higher reflection for larger incident angle theta, which implies the antireflection breaks down at large incident angles. User may find there are some negative reflection. This is due to the interpolation from (theta, phi) system to (u1, u2) system since we plot polar image in (u1, u2) system. This could be further improved by increasing the number of interpolation points or setting negative points to be zero directly.
Next, referring to paper , we study two different anisotropic films:
Nz is one of the key parameters of an anisotropic materials film, which is defined as (nx-nz)/(nx-ny). From the figure above, we could find Nz=0.5 could achieve a better antireflection performance for all incident angles, which agrees with paper .
Important model settings
Description of important objects and settings used in this model
The STACK solver always assumes the plane of incidence to be the xz plane (phi=0). To obtain the response of the anisotropic layers to an incident light with a certain polar angle (phi), we need to rotate the optic axis (equivalently, the permittivity tensor) of the corresponding materials by -phi.
We utilize an artificial layer to decompose R1 and R2 in this example. However, it is needed to convert the result of STACK solver to R2.
From the figure above, we utilized stackrt command to obtain R pol . T ag stands for the transmission of the air-glass interface. (ag: from air to glass, ga: glass to air) R2 is then obtained using R2=T ag *R pol *T ga .
Updating the model
Instructions for updating the model based on your device parameters
The refractive indices in this example are non-dispersive. To update the model with dispersive materials, please refer to this page . Note that the material database allows only a diagonalized permittivity. To obtain a broadband response, the permittivity rotation should be applied to the diagonal permittivity matrix for each frequency.
Taking the model further
Information and tips for users that want to further customize the model
Antireflection coating for minimizing reflection at air-polarizer interface
In this example, we ignore the reflectance R1 at the air-1.5 interface. A multilayer reflection coating could be further attached on the top of the polarizer to reduce the interface reflection .
Additional documentation, examples and training material
- Bong Choon Kim, Young Jin Lim, Je Hoon Song, Jun Hee Lee, Kwang-Un Jeong, Joong Hee Lee, Gi-Dong Lee, and Seung Hee Lee, "Wideband antireflective circular polarizer exhibiting a perfect dark state in organic light-emitting-diode display," Opt. Express 22, A1725-A1730 (2014)
- Qi Hong, Thomas X. Wu, Ruibo Lu, and Shin-Tson Wu, "Wide-view circular polarizer consisting of a linear polarizer and two biaxial films," Opt. Express 13, 10777-10783 (2005)