diff --git a/gdsfactory/simulation/effective_permittivity.py b/gdsfactory/simulation/effective_permittivity.py
new file mode 100644
index 0000000000..a11e85dfc4
--- /dev/null
+++ b/gdsfactory/simulation/effective_permittivity.py
@@ -0,0 +1,92 @@
+"""Calculate the effective refractive index for a 1D mode."""
+
+from typing import Literal
+
+import numpy as np
+from scipy.optimize import fsolve
+
+
+def calculate_effective_permittivity(
+    epsilon_film: float,
+    epsilon_substrate: float,
+    epsilon_cladding: float,
+    thickness: float,
+    wavelength: float,
+    polarization: Literal["te", "tm"],
+):
+    """
+    Calculate the effective refractive index for a 1D mode.
+
+    .. code::
+
+        -----------------      |
+        epsilon_cladding      inf
+        -----------------      |
+        epsilon_film        thickness
+        -----------------      |
+        epsilon_substrate     inf
+        -----------------      |
+
+    Args:
+        epsilon_film: Relative permittivity of the film.
+        epsilon_substrate: Relative permittivity of the substrate.
+        epsilon_cladding: Relative permittivity of the cladding.
+        thickness: Thickness of the film.
+        wavelength: Wavelength.
+        polarization: Either "te" or "tm".
+
+    Returns:
+        List of effective permittivity.
+    """
+    if polarization == "te":
+        tm = False
+    elif polarization == "tm":
+        tm = True
+    else:
+        raise ValueError('Polarization must be "te" or "tm"')
+
+    k_0 = 2 * np.pi / wavelength
+
+    def k_f(e_eff):
+        return k_0 * np.sqrt(epsilon_film - e_eff) / (epsilon_film if tm else 1)
+
+    def k_s(e_eff):
+        return (
+            k_0 * np.sqrt(e_eff - epsilon_substrate) / (epsilon_substrate if tm else 1)
+        )
+
+    def k_c(e_eff):
+        return k_0 * np.sqrt(e_eff - epsilon_cladding) / (epsilon_cladding if tm else 1)
+
+    def objective(e_eff):
+        return 1 / np.tan(k_f(e_eff) * thickness) - (
+            k_f(e_eff) ** 2 - k_s(e_eff) * k_c(e_eff)
+        ) / (k_f(e_eff) * (k_s(e_eff) + k_c(e_eff)))
+
+    # scan roughly for indices
+    # use a by 1e-10 smaller search area to avoid division by zero
+    x = np.linspace(
+        min(epsilon_substrate, epsilon_cladding) + 1e-10, epsilon_film - 1e-10, 1000
+    )
+    indices_temp = x[np.abs(objective(x)) < 0.1]
+    if not len(indices_temp):
+        return []
+
+    # and then use fsolve to get exact indices
+    indices_temp = fsolve(objective, indices_temp)
+
+    # then make the indices unique
+    indices = []
+    for index in indices_temp:
+        if not any(np.isclose(index, i, atol=1e-7) for i in indices):
+            indices.append(index)
+
+    return indices
+
+
+if __name__ == "__main__":
+    print(
+        calculate_effective_permittivity(
+            3.4777**2, 1.444**2, 1.444**2, 0.22e-6, 1.55e-6, "te"
+        )
+    )