feat: Allow nonlinearity with custom medium

Author: caseyflex

CHANGELOG.md

@@ -32,6 +32,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added support for `.lydrc` files for design rule checking in the `klayout` plugin.
 - Added a Gaussian inverse design filter option with autograd gradients and complete padding mode coverage.
 - Added support for argument passing to DRC file when running checks with `DRCRunner.run(..., drc_args={key: value})` in klayout plugin.
+- Added support for `nonlinear_spec` in `CustomMedium` and `CustomDispersiveMedium`.
 
 ### Breaking Changes
 - Edge singularity correction at PEC and lossy metal edges defaults to `True`.

tidy3d/components/nonlinear.py

@@ -27,19 +27,25 @@ class NonlinearModel(ABC, Tidy3dBaseModel):
 
     def _validate_medium_type(self, medium: AbstractMedium) -> None:
         """Check that the model is compatible with the medium."""
-        from .medium import AbstractCustomMedium, DispersiveMedium, Medium
+        from .medium import (
+            AbstractCustomMedium,
+            CustomDispersiveMedium,
+            CustomMedium,
+            DispersiveMedium,
+            Medium,
+        )
 
-        if isinstance(medium, AbstractCustomMedium):
+        if medium.is_time_modulated:
             raise ValidationError(
                 f"'NonlinearModel' of class '{type(self).__name__}' is not currently supported "
-                f"for medium class '{type(medium).__name__}'."
+                f"for time-modulated medium class '{type(medium).__name__}'."
             )
-        if medium.is_time_modulated:
+        if isinstance(medium, AbstractCustomMedium) and not medium.is_isotropic:
             raise ValidationError(
                 f"'NonlinearModel' of class '{type(self).__name__}' is not currently supported "
-                f"for time-modulated medium class '{type(medium).__name__}'."
+                f"for anisotropic medium class '{type(medium).__name__}'."
             )
-        if not isinstance(medium, (Medium, DispersiveMedium)):
+        if not isinstance(medium, (Medium, DispersiveMedium, CustomMedium, CustomDispersiveMedium)):
             raise ValidationError(
                 f"'NonlinearModel' of class '{type(self).__name__}' is not currently supported "
                 f"for medium class '{type(medium).__name__}'."