Gradual Evolution (GE)

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).