Gradual Evolution is the most capable synthesis tool. It runs needle optimization to local optimality, then deliberately forces a needle insertion even when that briefly raises the merit function, and refines the new state. Forcing an uphill step is what lets the design climb out of a local minimum that plain needle insertion cannot escape. It keeps a running record of the best design seen, so at the end the lowest-merit result is restored and you can review the whole series of designs it passed through.

Each outer cycle runs an inner needle pass to needle-optimality, forces in the best available needle regardless of whether the merit goes up, refines, then prunes sub-threshold layers and merges adjacent same-material layers. This repeats for the number of cycles you set.

Settings

Candidate materials — the pool of materials Gradual Evolution may insert. All / Clear select or empty the pool in one click.

Max layers — an upper limit on how many layers the design may grow to.

Target MF — stop once the merit function reaches this value (0 means run all cycles).

The advanced section exposes the synthesis tuning:

Refine iterations — how many refinement steps run after each forced insertion.

dMin (nm) — the insertion floor and prune threshold. Gradual Evolution can push below the minimum thickness limit during its forced step, which is part of how it escapes a tight minimum.

GE cycles — the number of outer cycles (needle-opt plus a forced step); typically 20–60.

Inner refiner — which method refines the stack between steps. See Optimization Methods for the choices.

Candidate search — how thoroughly each step explores improving candidates (fast, balanced, or thorough).

Seed mode — whether to refine the starting stack first or preserve its bulk before growing, which matters when you begin from a thick seed design.

How to read it

The design series is a sortable table of every accepted state, each with its merit function and layer count. The Pareto chart plots merit against layer count — look for the knee, where adding more layers stops buying much improvement, rather than chasing the absolute minimum merit. Best restores the all-time-best design, and you can Restore any earlier state from the series.

You may sometimes see only needle rows in the series and no forced-step rows. That is expected: Gradual Evolution only forces a step once the inner needle loop is exhausted, so as long as ordinary needle insertions keep improving the design, it stays inside that loop.

A larger candidate pool (three to six materials) often beats simply allowing more layers. In both_independent surface mode it grows both sides as it goes.

References

• Tikhonravov et al., Appl. Opt. 46, 6936 (2007).
• Sullivan & Dobrowolski, Appl. Opt. 35, 5484 (1996).