---
title: 100 nm SiN atom-strip first-step seed-width sweep
---

# 100 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 100 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_metrics.csv) ·
[Read the exact method](https://github.itap.purdue.edu/hoodlabpurdue/fdtdx-hoodlab/blob/main/benchmarks/cases/atom_strip_seed_width_sweep/METHOD.md) ·
{doc}`Compare with the 300 nm SiN sweep <atom_strip_seed_width_sweep_sin300_dashboard>`

| air strip (µm) | J before | J after replay | ΔJ | fidelity after | dominant lag (µm) | correlation peak |
|---:|---:|---:|---:|---:|---:|---:|
| 1.0 | -0.896661 | -0.886168 | 0.010493 | 0.530176 | 0.25 | 0.75 |
| 1.1 | -0.890217 | -0.879247 | 0.0109698 | 0.532128 | 0.25 | 0.7785 |
| 1.2 | -0.893688 | -0.881725 | 0.0119627 | 0.531427 | 0.25 | 0.78 |
| 1.3 | -0.906972 | -0.893989 | 0.0129833 | 0.527986 | 0.25 | 0.8025 |
| 1.4 | -0.925854 | -0.911901 | 0.0139533 | 0.52304 | 0.25 | 0.8005 |
| 1.5 | -0.947361 | -0.932506 | 0.0148556 | 0.517463 | 0.25 | 0.8118 |
| 1.6 | -0.97036 | -0.954752 | 0.0156081 | 0.511574 | 0.25 | 0.8033 |
| 1.7 | -0.993566 | -0.977438 | 0.0161285 | 0.505705 | 0.25 | 0.8078 |
| 1.8 | -1.01487 | -0.998465 | 0.016406 | 0.500384 | 0.25 | 0.7986 |
| 1.9 | -1.03203 | -1.01575 | 0.016282 | 0.496094 | 0.25 | 0.7919 |
| 2.0 | -1.0445 | -1.02908 | 0.0154252 | 0.492835 | 0.3 | 0.783 |


## Objective comparison

```{image} ../_static/generated/atom_strip_seed_width_sweep_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_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_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_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.