---
title: 300 nm SiN atom-strip first-step seed-width sweep
---

# 300 nm SiN atom-strip first-step seed-width sweep

**Status:** complete · **progress:** 11 / 11 widths complete · **updated:** 2026-09-05 14:30 UTC

This controlled sweep asks only how the first temporal-gradient seed changes
when the immutable atom-access strip is moved from 1.0 to 2.0 µm. Every point
starts independently from density 0.5; no geometry or optimizer state is
carried from one width to the next.

The common stack is a 300 nm SiN film embedded between 300 nm of SiO2 per side,
with SiO2 filling the complete x-y plane (including x/y PML) and air outside
the finite stack in z. The physical design region is 12 × 4 µm and the retained
simulation uses x/y/z symmetry with an Ey atom response. The hard air strip
cuts through the complete SiO2/SiN stack.

The reported objective gain is not inferred from the gradient. It is measured
by a fresh forward replay after the single raw-Adam update.

[Download the numerical sweep table](../_static/generated/atom_strip_seed_width_sweep_sin300_metrics.csv) ·
[Read the exact method](https://github.itap.purdue.edu/hoodlabpurdue/fdtdx-hoodlab/blob/main/benchmarks/cases/atom_strip_seed_width_sweep/METHOD_SIN300.md) ·
{doc}`Compare with the 100 nm SiN sweep <atom_strip_seed_width_sweep_dashboard>`

| air strip (µm) | J before | J after replay | ΔJ | fidelity after | dominant lag (µm) | correlation peak |
|---:|---:|---:|---:|---:|---:|---:|
| 1.0 | -1.13638 | -1.09929 | 0.0370809 | 0.476351 | 0.25 | 0.8342 |
| 1.1 | -1.12419 | -1.08294 | 0.0412439 | 0.48009 | 0.25 | 0.8174 |
| 1.2 | -1.10005 | -1.05808 | 0.0419695 | 0.48589 | 0.25 | 0.8244 |
| 1.3 | -1.06237 | -1.02314 | 0.039229 | 0.494282 | 0.25 | 0.8239 |
| 1.4 | -1.02091 | -0.985904 | 0.0350026 | 0.503549 | 0.25 | 0.829 |
| 1.5 | -0.984511 | -0.954203 | 0.0303079 | 0.511718 | 0.25 | 0.8366 |
| 1.6 | -0.95567 | -0.930118 | 0.0255519 | 0.518103 | 0.25 | 0.8391 |
| 1.7 | -0.932727 | -0.912024 | 0.0207032 | 0.523006 | 0.25 | 0.8491 |
| 1.8 | -0.915083 | -0.898271 | 0.0168115 | 0.526795 | 0.25 | 0.7981 |
| 1.9 | -0.905602 | -0.890362 | 0.0152406 | 0.528999 | 0.25 | 0.7425 |
| 2.0 | -0.907259 | -0.892234 | 0.0150253 | 0.528476 | 0.25 | 0.7542 |


## Objective comparison

```{image} ../_static/generated/atom_strip_seed_width_sweep_sin300_objectives.png
:alt: Temporal objective, objective gain, fidelity, and seed regularity versus air-strip width
:width: 100%
```

## Updated geometries

The density panels use the physical 0-to-1 scale. Because this is exactly one
step from 0.5, the second atlas is the more sensitive view of the emerging
seed.

```{image} ../_static/generated/atom_strip_seed_width_sweep_sin300_geometry.png
:alt: Full physical density after one temporal update for every air-strip width
:width: 100%
```

## Geometry differences

```{image} ../_static/generated/atom_strip_seed_width_sweep_sin300_geometry_delta.png
:alt: One-step density change for every air-strip width
:width: 100%
```

## Longitudinal regularity telemetry

```{image} ../_static/generated/atom_strip_seed_width_sweep_sin300_autocorrelation.png
:alt: Longitudinal autocorrelation of each first-step density change
:width: 100%
```


### Interpretation guardrails

- Larger $J$ (less negative) and positive $\Delta J$ mean a better match to the
  fixed ideal waveform.
- The density-change panels compare the actual first Adam moves. Fixed air and
  feedthrough pixels remain unchanged.
- The autocorrelation peak is descriptive only. It summarizes repetition in
  the complete two-dimensional update and does not rank, select, or modify a
  seed.