micromode 0.1.0a4

Rust-backed photonics mode solver with a small Python API.

micromode

An electromagnetic mode solver using the FDFD method on a rectilinear Yee-grid, written in native Rust.

pip install micromode

Why Use It?

• Grid-first API: pass arrays directly, with no required geometry model.
• Fast, portable Rust sparse backend: one production solve path.
• Practical outputs: fields, n_eff, k_eff, mode area, polarization fractions, Lorentz overlaps, plotting, dataframe export, and HDF5 save/load.
• Tensor-aware: supports scalar, diagonal anisotropic, and full tensor material grids.
• Works for both 2D cross sections and 1D slices.

You give it a material grid. It returns guided modes: effective indices, six-component fields, polarization metrics, mode area, overlaps, diagnostics, plots, and HDF5 output. MicroMode is intentionally not a CAD or geometry package. It is the solver piece you use after geometry has already been rasterized onto a mode-plane grid.

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, 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 is designed to make high-performance mode solving available without requiring users to install external solver stacks. The production backend is a portable Rust sparse shift-invert eigensolver, so source installs and wheels do not depend on ARPACK, UMFPACK, SuiteSparse, BLAS/ LAPACK, or a Fortran compiler. That matters for simulation workflows that need to run in CI, notebooks, container images, FDTD plugins, and cross-platform design tools.

The native solver is not a dense fallback. It uses sparse finite-difference operators throughout, applies AMD fill-reducing ordering before sparse LU factorization, stores LU factors in a packed format for repeated triangular solves, and runs an Arnoldi iteration targeted around the requested effective index. The Arnoldi stage uses shift-invert, adaptive Ritz-pair checkpointing, early stopping once requested modes are stable, and selective Ritz vector reconstruction so work is spent on the modes that will actually be returned.