Photonic Optical Output Coupling Efficiency Using FDTD : Photonic Optical Output Coupling Efficiency Using FDTD

Requires: Blaze/LED Minimum Versions: Atlas 5.28.1.R

This example demonstrates how to extract the optical output coupling efficiency of an LED/OLED device with photonic crystaline enhancements by using finite-difference time-domain (FDTD).

The finite-difference time-domain method works by direct solution of Maxwell's equations in the time domain and thus explicitly accounts for interference and diffraction in 1, 2 or 3 dimensions.

FDTD uses the same method as reverse ray tracing in that the emission from a set of dipoles located at user specified locations is integrated/averaged over the device.

The device in this case is very similar to the ones discussed in the other examples so far. We have broadened it out in the X direction so we can analyze the effects of guided modes. Guided modes are light trapped in the plane of the device by total internal reflection. We make the device very long the make sure we can capture all the light except that which is definitely captured in the axial modes.

In the previous example we analyzed this same device without the photonic enhancements.

In this example we analyze the effect of introduction of a photonic coupling structure on the top surface to extract light from the axial modes. This demonstrates the advantage of FDTD analysis for photonic coupling. This kind of analysis cannot be done with reverse ray trace or source term methods.

The main difference from what we have seen so far in the other examples in the introduction of the BEAM statement. In example we performed FDTD analysis without the use of the BEAM statement. In this case the BEAM statement is made automatically based on certain assumptions.

Many of the functionalities available on the BEAM statement (including lenses) for FDTD analysis are not yet available on the SAVE or LED statements so the BEAM statement is used here as a convenience and to illustrate the full variety of control parameters available to FDTD analysis.

In the BEAM statement you will first notice the specification of beam origin, direction of propogation and wavelength. These specifications are largely supurfulous for LED analysis but need be specified in accordance with the syntactical rules set forth for photo detection in Luminous. In future LED versions we hope to hide much of this.

The important parameters for LED analysis are FDTD which enables FDTD analysis, TD.SRATE which specifies the number of samples per wavelength PROP.LENG which specifies the duration of the simulation in terms of optical propogation length, BIG.INDEX which enables examination of the largest index of refraction in each region to calculate the local mesh spacing relative to the local wavelength and FD.AUTO which enables automatic meshing.

PMLs are absorbing boundary conditions for FDTD. They are needed at the TOP to characterize the amount of light exiting the top of the device (i.e. the light coupled out of the device). PMLs are placed at the ends of the device to capture and characterize the axial modes.

The PML at the bottom is optional and if left out the boundary is treated as a prefect mirror.

The photonic coupling is introduced using lenslets. The lenslets are defined by LENS statements.

The first LENS statement specifies a flat "lens" extending from Y=-1 to the surface of the device. This lens acts as the fill in between the cylinders to come. The index of the fill pattern is specified by the INDEX parameter.

The second LENS statement specifies a set of cylinders with radius 0.175 microns starting at Y=0 to a height of 0.5 microns in the -Y direction. with an index of 1.9.

The periodicity in the X direction is specified by the PC.X0 parameter that specifies the center of the first duplicate and PC.DX which specifies the spacing in the X direction between duplicates.

And that is it for the photonic crystal. In 3D just 2 more parameters are introduced.

Toward the end of the simulation the SAVE statement is used to initiate FDTD coupling analysis at the operating bias.

The REF.BEAM parameter is used to specify the index of the BEAM in order to tie the LED analysis to the proper set of analysis conditions set up by the BEAM and PML statements.

The SPECTRUM parameter specifies a file prefix for capturing the coupled output spectrum.

The other parameters of the SAVE statement specify the user selected sampling in space and emission wavelength.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.