fix adjoint if refractive index of 0

Author: tylerflex

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - `ModeSolver.plot_field` correctly returning the plot axes.
+- Avoid error if non-positive refractive index used for integration resolution in adjoint.
 
 ## [2.7.0rc2] - 2024-05-14
 

tidy3d/plugins/adjoint/components/structure.py

@@ -101,6 +101,7 @@ def _get_medium_params(
         freq_max = float(max(grad_data_eps.eps_xx.f))
         eps_in = self.medium.eps_model(frequency=freq_max)
         ref_ind = np.sqrt(np.max(np.real(eps_in)))
+        ref_ind = max([1.0, abs(ref_ind)])
         wvl_free_space = C_0 / freq_max
         wvl_mat = wvl_free_space / ref_ind
         return dict(wvl_mat=wvl_mat, eps_in=eps_in)