Adding Sellmeier.from_dispersion()

Author: momchil-flex

CHANGELOG.md

@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- ``Selmeier.from_dispersion()`` method to quickly make a single-pole fit for lossless weakly dispersive materials.
 - Stable dispersive material fits via webservice.
 - Validates simulation based on discretized size.
 

tests/test_components.py

@@ -449,6 +449,25 @@ def test_medium_dispersion_create():
         struct = Structure(geometry=Box(size=(1, 1, 1)), medium=medium)
 
 
+def test_sellmeier_from_dispersion():
+    n = 3.5
+    wvl = 0.5
+    freq = C_0 / wvl
+    dn_dwvl = -0.1
+    with pytest.raises(ValidationError) as e:
+        # Check that postivie dispersion raises an error
+        medium = Sellmeier.from_dispersion(n=n, freq=freq, dn_dwvl=-dn_dwvl)
+
+    # Check that medium properties are as epected
+    medium = Sellmeier.from_dispersion(n=n, freq=freq, dn_dwvl=dn_dwvl)
+    epses = [medium.eps_model(f) for f in [0.99 * freq, freq, 1.01 * freq]]
+    ns = np.sqrt(epses)
+    dn_df = (ns[2] - ns[0]) / 0.02 / freq
+
+    assert np.allclose(ns[1], n)
+    assert np.allclose(-dn_df * C_0 / wvl ** 2, dn_dwvl)
+
+
 def eps_compare(medium: Medium, expected: Dict, tol: float = 1e-5):
 
     for freq, val in expected.items():

tidy3d/components/medium.py

@@ -13,7 +13,7 @@
 from .validators import validate_name_str
 
 from ..constants import C_0, pec_val, EPSILON_0, HERTZ, CONDUCTIVITY, PERMITTIVITY, RADPERSEC
-from ..log import log
+from ..log import log, ValidationError
 
 
 def ensure_freq_in_range(eps_model: Callable[[float], complex]) -> Callable[[float], complex]:
@@ -268,7 +268,7 @@ def from_nk(cls, n: float, k: float, freq: float):
             Real part of refractive index.
         k : float = 0
             Imaginary part of refrative index.
-        frequency : float
+        freq : float
             Frequency to evaluate permittivity at (Hz).
 
         Returns
@@ -507,6 +507,39 @@ def pole_residue(self):
             name=self.name,
         )
 
+    @classmethod
+    def from_dispersion(cls, n: float, freq: float, dn_dwvl: float = 0):
+        """Convert ``n`` and wavelength dispersion ``dn_dwvl`` values at frequency ``freq`` to
+        a single-pole :class:`Sellmeier` medium.
+
+        Parameters
+        ----------
+        n : float
+            Real part of refractive index. Must be larger than or equal to one.
+        dn_dwvl : float = 0
+            Derivative of the refractive index with wavelength (1/um). Must be negative.
+        frequency : float
+            Frequency to evaluate permittivity at (Hz).
+
+        Returns
+        -------
+        :class:`Medium`
+            medium containing the corresponding ``permittivity`` and ``conductivity``.
+        """
+
+        if dn_dwvl >= 0:
+            raise ValidationError("Dispersion ``dn_dwvl`` must be smaller than zero.")
+        if n < 1:
+            raise ValidationError("Refractive index ``n`` cannot be smaller than one.")
+
+        wvl = C_0 / freq
+        nsqm1 = n ** 2 - 1
+        c_coeff = -(wvl ** 3) * n * dn_dwvl / (nsqm1 - wvl * n * dn_dwvl)
+        b_coeff = (wvl ** 2 - c_coeff) / wvl ** 2 * nsqm1
+        coeffs = [(b_coeff, c_coeff)]
+
+        return cls(coeffs=coeffs)
+
 
 class Lorentz(DispersiveMedium):
     """A dispersive medium described by the Lorentz model.

tidy3d/log.py

@@ -43,7 +43,7 @@ class Tidy3dKeyError(Tidy3dError):
 
 
 class ValidationError(Tidy3dError):
-    """eError when constructing Tidy3d components."""
+    """Error when constructing Tidy3d components."""
 
 
 class SetupError(Tidy3dError):