Wavebit

Differentiable electromagnetic simulation: RCWA and FDTD behind one API, on NumPy, JAX and PyTorch.

Wavebit computes how light scatters from periodic nanostructures (RCWA) and how fields evolve in time (FDTD). It differentiates through the RCWA calculation, so the model that predicts a device's response is the model that optimizes it.

• Two solvers, one lifecycle. rcwa and fdtd share check → prepare → run, the same materials, explicit length units, radian angles, and fields in V/m and A/m.
• One implementation of the physics on three array backends. Cross-backend agreement is part of the test suite, not a promise.
• Gradients where you need them. Wavelength, materials, layer thicknesses and raster permittivities differentiate on JAX and PyTorch; shape dimensions on PyTorch. The full table is in the differentiation guide.
• Checked against the outside world. RETICOLO, fmmax and S4 for RCWA; Meep for FDTD; the Li (1997) crossed-grating benchmark. The numbers and how they were taken: validation.

Status: 0.0.3, alpha. RCWA is the mature path. FDTD runs on CPU in double precision; its geometry, material and derivative support is reported by check for each problem. The API may change before 1.0. An experimental RCWA solver for metal contacts is opt-in and has its own, narrower scope; read method selection before enabling it.

Install

Python 3.12 or newer.

pip install wavebit              # NumPy backend
pip install 'wavebit[jax]'       # or 'wavebit[pytorch]', or 'wavebit[jax,pytorch]' — differentiation backends

From a source checkout, for development:

git clone https://github.com/fieldalgo/wavebit.git && cd wavebit
pip install -e '.[jax,docs]'     # editable, with the documentation toolchain

environment.yml builds the full development environment, including what the studies need. The version floors in pyproject.toml are the versions the suite is verified against; each is commented with the reason it sits where it does.

Quickstart

A binary grating on glass under normal incidence, on the NumPy backend:

import numpy as np
from wavebit import Material, Units, rcwa

problem = rcwa.Problem(
    units=Units(length="nm"),
    stack=rcwa.Stack(
        period=(700., 700.),
        bottom=Material(epsilon=2.25),
        layers=(rcwa.Layer(thickness=300., epsilon=np.array([[1., 1., 12.25, 12.25]])),),
    ),
    excitation=rcwa.Excitation(wavelength=900., sources=(rcwa.PlaneWave(polarization="s"),)),
)
numerics = rcwa.Numerics(orders=(10, 0))
result = rcwa.run(problem, numerics=numerics)
print(float(result.R.sum()), float(result.T.sum()))
print(result.transmission.order(0, 0).efficiency)

Diffraction arrays keep (wavelength, order_v, order_u) axes even for a single wavelength. R and T are efficiencies; absolute port power and its integration measure are recorded separately.

To differentiate, prepare once and run with live leaves. JAX double precision is enabled by you, never by importing Wavebit:

import jax, jax.numpy as jnp
jax.config.update("jax_enable_x64", True)

prepared = rcwa.prepare(problem, numerics=numerics, backend="jax")

def transmitted(thickness):
    layer = problem.stack.layers[0].replace(thickness=thickness)
    live = problem.replace(stack=problem.stack.replace(layers=(layer,)))
    return jnp.sum(prepared.run(live).T)

gradient = jax.grad(transmitted)(jnp.asarray(300.))

The FDTD entry point works the same way; examples/fdtd/pulse_slab.py is the first thing to run. The conventions both solvers follow — relative permittivity, exp(-iωt) phasors, bottom-to-top layers, centered cells, explicit units — are listed once in the documentation home and defined normatively in docs/reference/common/conventions.md.

Documentation

The prose is split by who is reading, the way NumPy's manual is:

The same pages build as a site (pip install -e '.[docs]', then sphinx-build -b html docs docs/_build/html); a hosted copy is coming. Every link in docs/ is a relative link to a real file, and a test checks that every path a page cites exists. python -m wavebit.manifest prints every option, refusal and constraint as one JSON document for programs that need to know without reading the source (capability manifest).

Contributing

Bug reports, questions and pull requests are welcome. CONTRIBUTING.md opens with the rule for where a new file belongs, and explains how changes are reviewed and landed.

Citation

@software{wavebit,
    title  = {Wavebit: Differentiable Electromagnetic Simulation Suite},
    author = {Kim, Yongha},
    year   = {2026},
    url    = {https://github.com/fieldalgo/wavebit},
}

License

Apache-2.0. LICENSE also carries the third-party notices this project owes.

Contact

dev@fieldalgo.com