micromode

micromode

An electromagnetic mode solver using the FDFD method on a rectilinear Yee-grid.

pip install micromode

Why Use It?

• Grid-first API: pass already-rasterized material tensors and grid edges directly, with no required CAD or geometry model.
• Readable SciPy implementation: sparse operators are assembled in Python and solved with SciPy/ARPACK, so the numerical path can be inspected by users who do not want to trust a custom native solver.
• Small integration surface: MicroMode is the solver piece you embed after geometry has already been rasterized by an FDTD, FEM, or custom photonics pipeline.
• Tensor-aware: supports scalar, diagonal anisotropic, and full tensor material grids, plus transformed angle and bend solves.
• Practical outputs: coordinate-aware fields, n_eff, k_eff, mode area, polarization fractions, Lorentz overlaps, plotting, dataframe export, and HDF5 save/load.
• Works for both 2D cross sections and 1D slices.

MicroMode is intentionally not a full simulation platform or geometry package. That is the point: you give it a mode-plane material grid, and it returns guided modes with the diagnostics and result helpers needed by downstream simulation workflows.

Micromode is the default mode solver in the BEAMZ FDTD engine.

Quick Start

import micromode as mm

wavelength_um = 1.55
freq = mm.C_0 / wavelength_um

# Arrays from your own rasterizer.
eps_xx, x_edges, y_edges = mode_plane_arrays(...)

materials = mm.Materials.from_diagonal(
    eps_xx=eps_xx,
    x_edges=x_edges,
    y_edges=y_edges,
)

data = mm.solve_modes(
    material_grid=materials,
    freqs=[freq],
    num_modes=2,
    target_neff=2.5,
)

print(data.n_eff.values)
data.plot_field("Ex", mode_index=0)
data.to_hdf5("modes.h5")

Examples

Tidy3D Waveguide

The Tidy3D modal monitor example recreates the strip-waveguide setup from Flexcompute’s modal sources and monitors notebook. It solves the first three x-propagating modes of a silicon waveguide on a silica substrate and plots |Ey| and |Ez| on the same y-z mode plane. (See Tidy3D, “Defining Mode Sources and Monitors”.)

uv run --extra dev python examples/tidy3d_modal_sources_monitors.py

Hybridization Sweep

The SOI hybridization example sweeps the width of a 220 nm silicon ridge and solves several modes at each step. It shows how nearby modes exchange character as the geometry changes by plotting effective index and TE fraction across the sweep in separate figures, then rendering representative field profiles.

uv run --extra dev python examples/soi_hybridization_sweep.py

Physics

MicroMode solves the source-free frequency-domain Maxwell equations on a rasterized Yee mode plane, $\nabla\times\mathbf{E}=-i\omega\mu\mathbf{H}, ; \nabla\times\mathbf{H}=i\omega\epsilon\mathbf{E},$ with modal fields $\mathbf{E},\mathbf{H}\propto e^{i k_0 n_\mathrm{eff} z}.$

On diagonal material grids this becomes a transverse eigenproblem, while full tensor or transformed grids use a first-order tensorial form. The detailed derivation is in docs/physics-model.md, and the public solver controls are summarized in docs/mode-solver-methods.md.

Solver

MicroMode assembles the finite-difference sparse operators in Python and solves the shift-invert eigenproblems with SciPy/ARPACK. That makes the numerical method easier for academic users to inspect and debug, while still using a trusted sparse eigensolver stack.

This means MicroMode is differentiated by workflow and inspectability rather than by a custom solver engine: it is built for users who already have permittivity on a Yee grid and want a focused, auditable mode-solver component that fits cleanly into a larger photonics pipeline.