Fix for single frequency FieldMonitor

CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Polygon vertices cleanup in `ClipOperation.intersections_plane`.
 - Removed sources from `sim_inf_structure` simulation object in `postprocess_adj` to avoid source and background medium validation errors.
 - Revert overly restrictive validation of `freqs` in the `ComponentModeler` and `TerminalComponentModeler`.
+- Fixed `ElectromagneticFieldData.to_zbf()` to support single frequency monitors and apply the correct flattening order.
 
 ## [2.9.0] - 2025-08-04
 

tests/test_data/test_monitor_data.py

@@ -906,6 +906,17 @@ def field_data(self) -> td.FieldData:
         )
         return self.simdata(monitor)["fields"]
 
+    @pytest.fixture(scope="class")
+    def field_data_single_frequency(self) -> td.FieldData:
+        """Make random field data with single frequency from an emulated simulation run."""
+        monitor = td.FieldMonitor(
+            size=(td.inf, td.inf, 0),
+            freqs=self.freqs[0],
+            name="fields",
+            colocate=True,
+        )
+        return self.simdata(monitor)["fields"]
+
     @pytest.fixture(scope="class")
     def mode_data(self) -> td.ModeData:
         """Make random ModeData from an emulated simulation run."""
@@ -919,18 +930,41 @@ def mode_data(self) -> td.ModeData:
         )
         return self.simdata(monitor)["modes"]
 
+    @pytest.fixture(scope="class")
+    def mode_data_single_frequency(self) -> td.ModeData:
+        """Make random ModeData from an emulated simulation run."""
+        monitor = td.ModeMonitor(
+            size=(td.inf, td.inf, 0),
+            freqs=self.freqs[0],
+            name="modes",
+            colocate=True,
+            mode_spec=td.ModeSpec(num_modes=2, target_neff=4.0),
+            store_fields_direction="+",
+        )
+        return self.simdata(monitor)["modes"]
+
+    @pytest.mark.parametrize("field_data_fixture", ["field_data", "field_data_single_frequency"])
     @pytest.mark.parametrize("background_index", [1, 2, 3])
     @pytest.mark.parametrize("freq", [*list(freqs), None])
     @pytest.mark.parametrize("n_x", [2**5, 2**6])
     @pytest.mark.parametrize("n_y", [2**5, 2**6])
     @pytest.mark.parametrize("units", ["mm", "cm", "in", "m"])
     def test_fielddata_tozbf_readzbf(
-        self, tmp_path, field_data, background_index, freq, n_x, n_y, units
+        self,
+        tmp_path,
+        request,
+        field_data_fixture,
+        background_index,
+        freq,
+        n_x,
+        n_y,
+        units,
     ):
         """Test that FieldData.to_zbf() -> ZBFData.read_zbf() works"""
         zbf_filename = tmp_path / "testzbf.zbf"
 
         # write to zbf and then load it back in
+        field_data = request.getfixturevalue(field_data_fixture)
         ex, ey = field_data.to_zbf(
             fname=zbf_filename,
             background_refractive_index=background_index,
@@ -960,13 +994,20 @@ def test_fielddata_tozbf_readzbf(
         assert np.allclose(ex.values, zbfdata.Ex)
         assert np.allclose(ey.values, zbfdata.Ey)
 
+    @pytest.mark.parametrize("mode_data_fixture", ["mode_data", "mode_data_single_frequency"])
     @pytest.mark.parametrize("mode_index", [0, 1])
-    def test_tozbf_modedata(self, tmp_path, mode_data, mode_index):
+    def test_tozbf_modedata(
+        self,
+        tmp_path,
+        request,
+        mode_data_fixture,
+        mode_index,
+    ):
         """Tests ModeData.to_zbf()"""
         zbf_filename = tmp_path / "testzbf_modedata.zbf"
 
         # write to zbf and then load it back in
-        ex, ey = mode_data.to_zbf(
+        ex, ey = request.getfixturevalue(mode_data_fixture).to_zbf(
             fname=zbf_filename,
             background_refractive_index=1,
             freq=self.freq0,

tidy3d/components/data/monitor_data.py

@@ -1163,9 +1163,14 @@ def to_zbf(
         else:
             freq = freq.item()
 
-        mode_area = mode_area.interp(f=freq)
-        e_x = e_x.interp(f=freq)
-        e_y = e_y.interp(f=freq)
+        # If the data has just one frequency, avoid Nans at the interpolation
+        if len(e_x.f) > 1:
+            mode_area = mode_area.interp(f=freq)
+            e_x = e_x.interp(f=freq)
+            e_y = e_y.interp(f=freq)
+        else:
+            e_x = e_x.isel(f=0, drop=True)
+            e_y = e_y.isel(f=0, drop=True)
 
         # If the data is ModeData, choose one of the modes to save
         if "mode_index" in e_x.coords:
@@ -1241,7 +1246,7 @@ def to_zbf(
             )
             fout.write(struct.pack("<8d", 0, 0, 0, 0, 0, 0, 0, 0))  # unused values
             for e in (e_x, e_y):
-                e_flat = e.values.flatten(order="C")
+                e_flat = e.values.flatten(order="F")
                 # Interweave real and imaginary parts
                 e_values = np.ravel(np.column_stack((e_flat.real, e_flat.imag)))
                 fout.write(struct.pack(f"<{2 * n_x * n_y}d", *e_values))

tidy3d/components/data/zbf.py

@@ -121,11 +121,11 @@ def read_zbf(filename: str) -> ZBFData:
             ) from None
 
         # load E field
-        Ex_real = np.asarray(rawx[0::2]).reshape(nx, ny)
-        Ex_imag = np.asarray(rawx[1::2]).reshape(nx, ny)
+        Ex_real = np.asarray(rawx[0::2]).reshape(nx, ny, order="F")
+        Ex_imag = np.asarray(rawx[1::2]).reshape(nx, ny, order="F")
         if ispol:
-            Ey_real = np.asarray(rawy[0::2]).reshape(nx, ny)
-            Ey_imag = np.asarray(rawy[1::2]).reshape(nx, ny)
+            Ey_real = np.asarray(rawy[0::2]).reshape(nx, ny, order="F")
+            Ey_imag = np.asarray(rawy[1::2]).reshape(nx, ny, order="F")
         else:
             Ey_real = np.zeros((nx, ny))
             Ey_imag = np.zeros((nx, ny))