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Photonic Solutions Enewsletter
October 2014
Importing Custom Refractive Index Data into the RSoft CAD
While the Material Editor in the RSoft CAD contains many common materials, you can import custom materials using refractive index data from experiments or published references. Materials can be conveniently imported using refractive index data files and, if needed, can fit to Lorentzian dispersion models. A Group Library is also included to allow materials to be shared easily among RSoft design files as well as different users on a computer.
Importing Custom Refractive Index Data
It is simple to create a material from a data file that contains real and/or imaginary refractive index data. First, open the RSoft CAD and go to the Material Editor. Then, click New Material in the top toolbar to create a new Project Material, click the Import NK Data button highlighted in Fig 1a, and then select the material data file. To illustrate the procedure, we use Molybdenum (Mo) data from the CRC Handbook of Chemistry and Physics. The refractive index data must be in the RSoft file format: three columns of data for wavelength (μm), real and imaginary parts of the refractive index. For more information about this format, please refer to the October 2013 RSoft Enews. If needed, click Display to show the contents of the data file as in Fig.1b.
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Figure 1) a) The RSoft Material Editor, and b) displayed index vs. wavelength for Mo. The dashed green line represents the imaginary component of the refractive index.
Fitting NK Data to a Lorentzian-Based Dispersion Model
The previously described method is valid for all RSoft device tool simulations with the exception of non-CW FullWAVE simulations that require a Lorentzian dispersion model. If used in this case, the result is valid at the central wavelength only. The Material Editor can be used to fit a material data file to the required Lorentzian coefficients. We will illustrate this with the previously used Mo data file. Create a Lorentzian-based material by selecting the Dispersive option and click the Fit NK Data button highlighted in Fig 2a. This will open the Fit NK Data Options dialog shown in Fig 2b.
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Figure 2: a) RSoft Material Editor, b) the Fit NK Data Options dialog,
c) fitted data with default parameters (top),
d) fitted data with 10 Max # Lorentzians and a 1e-10 Fitting Tolerance (bottom).
Select the Mo data file and leave the other fitting options unchanged for the moment. Click the Try Fit button to view the current fit: Fig 2c (top) shows that it is a decent fit for such wide spectral range. To achieve the better fit shown in Fig 2c (bottom), use more Lorentzians and a smaller fitting tolerance.
Click the Accept Fit button once an acceptable fit is found; the calculated coefficients will be automatically placed into the material editor as shown in Figure 3.
Figure 3: Material editor with linear dispersion coefficients imported from the curve
Note: A better fit can usually be found by using the smallest wavelength range necessary for your application. For example, the default fitting settings for this example give a good fit if the wavelength range is restricted to 0.6 – 0.8 μm.
Using the Group Library to Build Your Own Customized Libraries
The Group Library allows the creation of a user-defined library of materials that can be easily shared between different RSoft .ind files and among users on the computer. To use this feature, the Group Library Path must be set. Go to Options > Preference > Import/Export. After the path is set, you can use the Material editor to create and store your own libraries and materials. These materials can be copied from existing .ind files, or new materials can be created as described above.
Figure 4: Setting the path for the Group Library feature.