Add waveguide mode converter readme, schematic diagram, design patterns, and script

waveguide_mode_converter/README.md

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+# Waveguide Mode Converter
+
+This is a waveguide mode converter test problem. The design objective is to maximize the conversion of power from the fundamental waveguide mode of the input waveguide to the second-order mode of the output waveguide. Specifically, the worst-cast (maximum) reflection is minimized and the worst case transmission (minimum) is maximized. The geometry of the device is two-dimensional.
+
+## Schematic
+
+![Waveguide mode converter schematic](mode_converter_schematic.png)
+
+## Parameters
+
+| Parameter                            | Value           |
+|--------------------------------------|-----------------|
+| Design region width, dx              | 1.6 um          |
+| Design region height, dy             | 1.6 um          |
+| Waveguide width                      | 400 nm          |
+| Solid material relative permittivity | 12.25 (silicon) |
+| Void material relative permittivity  | 2.25 (oxide)    |
+| Operating wavelength range           | 1260 - 1300 nm  |
+| Simulation resolution                | 10 nm           |
+| Design pixel size                    | 10 nm           |
+| Expected design array shape          | 160 x 160       |
+
+These parameter values correspond to the mode converter demonstrated in
+[Inverse Design of Photonic Devices with Strict Foundry Fabrication Constraints](https://doi.org/10.1021/acsphotonics.2c00313).
+
+## Designs
+
+Mode converter designs are under the `designs/` subfolder.
+
+| File                                               | Description                                                                                             |
+|----------------------------------------------------|---------------------------------------------------------------------------------------------------------|
+| converter_schubert_circle_x33491673_w307_s134.csv  | Circular brush design from [Schubert et al. (2022)](https://doi.org/10.1021/acsphotonics.2c00313)       |
+| converter_schubert_notched_x33491673_w183_s159.csv | Notched-square brush design from [Schubert et al. (2022)](https://doi.org/10.1021/acsphotonics.2c00313) |
+
+## Usage
+
+The mode converter test problem can be run on a list of specified designs via
+the `run.sh` script. For example,
+
+```
+python3 ./run.py designs/converter_schubert_circle_x33491673_w307_s134.csv designs/converter_schubert_notched_x33491673_w183_s159.csv
+```
+
+The script will print out CSV-formatted performance metrics. For example,
+
+```
+# Worst-case reflection (dB), Worst-case transmission (dB)
+-34.11, -0.19
+-30.67, -0.26
+```