Add aux field monitors

Author: caseyflex

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - New subpixel averaging option `ContourPathAveraging` applied to dielectric material boundaries.
 - A property `interior_angle` in `PolySlab` that stores angles formed inside polygon by two adjacent edges.
 - `eps_component` argument in `td.Simulation.plot_eps()` to optionally select a specific permittivity component to plot (eg. `"xx"`).
+- Monitor `AuxFieldTimeMonitor` for aux fields like the free carrier density in `TwoPhotonAbsorption`.
 
 ### Fixed
 - Compatibility with `xarray>=2025.03`.

tests/test_components/test_medium.py

@@ -797,6 +797,29 @@ def test_nonlinear_medium():
         )
         _ = sim.updated_copy(medium=med, path="structures/0")
 
+    grid_spec = td.GridSpec.auto(min_steps_per_wvl=10, wavelength=1)
+    sim = sim.updated_copy(grid_spec=grid_spec)
+    aux_fields = ("Nfz",)
+    with AssertLogLevel(None):
+        med = td.Medium(
+            nonlinear_spec=td.NonlinearSpec(models=[td.TwoPhotonAbsorption(beta=1, tau=1)])
+        )
+        monitor = td.AuxFieldTimeMonitor(
+            interval=1, size=(0, 0, 0), name="aux_field_time", fields=aux_fields
+        )
+        sim = sim.updated_copy(medium=med, path="structures/0")
+        sim = sim.updated_copy(monitors=[monitor])
+
+    with AssertLogLevel("WARNING", contains_str="stores field"):
+        med = td.Medium(
+            nonlinear_spec=td.NonlinearSpec(models=[td.TwoPhotonAbsorption(beta=1, tau=0)])
+        )
+        _ = sim.updated_copy(medium=med, path="structures/0")
+
+    with AssertLogLevel("WARNING", contains_str="stores field"):
+        med = td.Medium(nonlinear_spec=td.NonlinearSpec(models=[td.KerrNonlinearity(n2=1)]))
+        _ = sim.updated_copy(medium=med, path="structures/0")
+
 
 def test_custom_medium():
     Nx, Ny, Nz, Nf = 4, 3, 1, 1

tests/test_components/test_monitor.py

@@ -376,6 +376,7 @@ def test_monitor():
         name="directivity",
     )
     m10 = td.PermittivityMonitor(size=size, center=center, freqs=FREQS, name="perm")
+    m11 = td.AuxFieldTimeMonitor(size=size, center=center, name="aux_field_time", fields=("Nfx",))
 
     tmesh = np.linspace(0, 1, 10)
 

tests/test_data/test_data_arrays.py

@@ -30,6 +30,7 @@
     ),
 ]
 FIELDS = ("Ex", "Ey", "Ez", "Hx", "Hz")
+AUX_FIELDS = ("Nfz",)
 INTERVAL = 2
 ORDERS_X = list(range(-1, 2))
 ORDERS_Y = list(range(-2, 3))
@@ -51,6 +52,9 @@
 FIELD_TIME_MONITOR = td.FieldTimeMonitor(
     size=SIZE_3D, fields=FIELDS, name="field_time", interval=INTERVAL
 )
+AUX_FIELD_TIME_MONITOR = td.AuxFieldTimeMonitor(
+    size=SIZE_3D, fields=AUX_FIELDS, name="aux_field_time", interval=INTERVAL
+)
 FIELD_MONITOR_2D = td.FieldMonitor(size=SIZE_2D, fields=FIELDS, name="field_2d", freqs=FREQS)
 FIELD_TIME_MONITOR_2D = td.FieldTimeMonitor(
     size=SIZE_2D, fields=FIELDS, name="field_time_2d", interval=INTERVAL
@@ -80,6 +84,7 @@
 MONITORS = [
     FIELD_MONITOR,
     FIELD_TIME_MONITOR,
+    AUX_FIELD_TIME_MONITOR,
     MODE_MONITOR_WITH_FIELDS,
     PERMITTIVITY_MONITOR,
     MODE_MONITOR,

tests/test_data/test_monitor_data.py

@@ -11,6 +11,7 @@
     FreqModeDataArray,
 )
 from tidy3d.components.data.monitor_data import (
+    AuxFieldTimeData,
     DiffractionData,
     DirectivityData,
     FieldData,
@@ -24,6 +25,7 @@
 
 from ..utils import AssertLogLevel
 from .test_data_arrays import (
+    AUX_FIELD_TIME_MONITOR,
     DIFFRACTION_MONITOR,
     DIRECTIVITY_MONITOR,
     FIELD_MONITOR,
@@ -120,6 +122,17 @@ def make_field_time_data_2d(symmetry: bool = True):
     )
 
 
+def make_aux_field_time_data(symmetry: bool = True):
+    sim = SIM_SYM if symmetry else SIM
+    return AuxFieldTimeData(
+        monitor=AUX_FIELD_TIME_MONITOR,
+        Nfz=make_scalar_field_time_data_array("Ez", symmetry),
+        symmetry=sim.symmetry,
+        symmetry_center=sim.center,
+        grid_expanded=sim.discretize_monitor(AUX_FIELD_TIME_MONITOR),
+    )
+
+
 def make_mode_solver_data():
     mode_data = ModeData(
         monitor=MODE_MONITOR_WITH_FIELDS,

tests/test_data/test_sim_data.py

@@ -15,6 +15,7 @@
 from ..utils import get_nested_shape
 from .test_data_arrays import FIELD_MONITOR, SIM, SIM_SYM
 from .test_monitor_data import (
+    make_aux_field_time_data,
     make_diffraction_data,
     make_directivity_data,
     make_field_data,
@@ -32,6 +33,8 @@
 FIELD = make_field_data(symmetry=False)
 FIELD_TIME_SYM = make_field_time_data()
 FIELD_TIME = make_field_time_data(symmetry=False)
+AUX_FIELD_TIME_SYM = make_aux_field_time_data()
+AUX_FIELD_TIME = make_aux_field_time_data(symmetry=False)
 PERMITTIVITY_SYM = make_permittivity_data()
 PERMITTIVITY = make_permittivity_data(symmetry=False)
 MODE = make_mode_data()
@@ -45,6 +48,7 @@
 MONITOR_DATA = (
     FIELD,
     FIELD_TIME,
+    AUX_FIELD_TIME,
     MODE_SOLVER,
     PERMITTIVITY,
     MODE,
@@ -56,6 +60,7 @@
 MONITOR_DATA_SYM = (
     FIELD_SYM,
     FIELD_TIME_SYM,
+    AUX_FIELD_TIME_SYM,
     MODE_SOLVER,
     PERMITTIVITY_SYM,
     MODE,

tests/utils.py

@@ -787,6 +787,9 @@ def make_custom_data(lims, unstructured):
             size=(0, 0, 0), center=(0, 0, 0), fields=["Ex"], freqs=[1.5e14, 2e14], name="field"
         ),
         td.FieldTimeMonitor(size=(0, 0, 0), center=(0, 0, 0), name="field_time", interval=100),
+        td.AuxFieldTimeMonitor(
+            size=(0, 0, 0), center=(0, 0, 0), fields=("Nfx",), name="aux_field_time", interval=100
+        ),
         td.FluxMonitor(size=(1, 1, 0), center=(0, 0, 0), freqs=[2e14, 2.5e14], name="flux"),
         td.FluxTimeMonitor(size=(1, 1, 0), center=(0, 0, 0), name="flux_time"),
         td.PermittivityMonitor(size=(1, 1, 0.1), name="eps", freqs=[1e14]),

tidy3d/__init__.py

@@ -269,6 +269,7 @@
 
 # monitors
 from .components.monitor import (
+    AuxFieldTimeMonitor,
     DiffractionMonitor,
     DirectivityMonitor,
     FieldMonitor,
@@ -467,6 +468,7 @@ def set_logging_level(level: str) -> None:
     "PlaneWaveBeamProfile",
     "FieldMonitor",
     "FieldTimeMonitor",
+    "AuxFieldTimeMonitor",
     "FluxMonitor",
     "FluxTimeMonitor",
     "ModeMonitor",
@@ -503,6 +505,7 @@ def set_logging_level(level: str) -> None:
     "ModeSolverDataset",
     "FieldData",
     "FieldTimeData",
+    "AuxFieldTimeData",
     "PermittivityData",
     "FluxData",
     "FluxTimeData",