Fix a bug in capacitance when symmetry is present (#2469)

* Fix a bug in capacitance when symmetry is present

* Add test

Author: marc-flex

tests/test_components/test_heat_charge.py

@@ -1559,3 +1559,43 @@ def test_fossum():
     """Check that fossum model can be defined."""
 
     _ = td.FossumCarrierLifetime(tau_300=3.3e-6, alpha_T=-0.5, N0=7.1e15, A=1, B=0, C=1, alpha=1)
+
+
+@pytest.mark.parametrize("symmetry", [(0, 0, 0), (0, 1, 0), (1, 0, 0), (1, 1, 0)])
+def test_symmetry_capacitance(symmetry):
+    """Check that symmetry_expanded_copy works as expected"""
+
+    data = [1, 2, 3]
+    voltages = [0, 1, 2]
+
+    hole_capacitance = td.SteadyVoltageDataArray(
+        data=data,
+        coords={"v": voltages},
+    )
+
+    electron_capacitance = td.SteadyVoltageDataArray(
+        data=data,
+        coords={"v": voltages},
+    )
+
+    monitor = td.SteadyCapacitanceMonitor(
+        center=(0, 0, 0),
+        size=(1, 1, 1),
+        name="test_monitor",
+    )
+
+    mnt_data = td.SteadyCapacitanceData(
+        monitor=monitor,
+        hole_capacitance=hole_capacitance,
+        electron_capacitance=electron_capacitance,
+        symmetry=symmetry,
+    )
+
+    num_symmetries = np.sum(np.array([1 if d > 0 else 0 for d in symmetry]))
+    scaling_factor = np.power(2, num_symmetries)
+
+    for n in range(len(data)):
+        assert mnt_data.symmetry_expanded_copy.hole_capacitance.data[n] == data[n] * scaling_factor
+        assert (
+            mnt_data.symmetry_expanded_copy.electron_capacitance.data[n] == data[n] * scaling_factor
+        )

tidy3d/components/tcad/data/monitor_data/charge.py

@@ -4,6 +4,7 @@
 
 from typing import Dict, Union
 
+import numpy as np
 import pydantic.v1 as pd
 
 from tidy3d.components.base import skip_if_fields_missing
@@ -435,4 +436,27 @@ def field_name(self, val: str) -> str:
     @property
     def symmetry_expanded_copy(self) -> SteadyCapacitanceData:
         """Return copy of self with symmetry applied."""
-        return self
+        num_symmetries = np.sum(np.array([1 if d > 0 else 0 for d in self.symmetry]))
+        scaling_factor = np.power(2, num_symmetries)
+
+        if self.hole_capacitance is None:
+            new_hole_capacitance = None
+        else:
+            new_values = self.hole_capacitance.values * scaling_factor
+            new_hole_capacitance = SteadyVoltageDataArray(
+                data=new_values, coords=self.hole_capacitance.coords
+            )
+
+        if self.electron_capacitance is None:
+            new_electron_capacitance = None
+        else:
+            new_values = self.electron_capacitance.values * scaling_factor
+            new_electron_capacitance = SteadyVoltageDataArray(
+                data=new_values, coords=self.electron_capacitance.coords
+            )
+
+        return self.updated_copy(
+            hole_capacitance=new_hole_capacitance,
+            electron_capacitance=new_electron_capacitance,
+            symmetry=(0, 0, 0),
+        )