Needle Variation

Needle Variation improves a design by inserting infinitesimally thin “needle” layers at the spot where they help most, then refining the result. To find that spot, it scans every position in the stack with every candidate material and computes how the merit function would change if a needle of a given material were inserted there, a quantity known as the P-function. The most promising position and material win, the needle goes in at a finite thickness, and a refinement pass settles the new stack. The cycle repeats until no insertion improves the design any further.

There are two versions in the ribbon. Both share the same candidate-material pool and the same underlying scan.

Needle Automatic

Runs the whole scan-insert-refine loop on its own until it reaches needle-optimality. Inside each layer the scan tests 16 positions, and every point where the needle function has a minimum along the layer becomes a candidate. The wavelength grid the band targets are sampled on is sized from the design and grows with it during the run (see Operand Reference).

Needle Manual

Shows you the same P-function scan as a plot and lets you click the position and material to insert a single needle by hand. This is useful for testing a topology idea, seeding a layer where you know one belongs, or stepping through synthesis one insertion at a time. With Refine after insert on, each insertion is refined with SQP, the Refinement window’s default method, for the number of Refine iterations set.

Settings

Candidate materials: the pool of materials Needle is allowed to insert. All / Clear select or empty the pool in one click. This pool is shared by Needle Automatic, Needle Manual, and Gradual Evolution.

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 to convergence).

The advanced section exposes the synthesis tuning:

Needle scale (nm): the thickness step used when probing candidate positions.

dMin (nm): the minimum inserted thickness and the floor used during refinement (1 nm by default). Keep it at the synthesis default while synthesizing; raise it to your manufacturing minimum only in a later Refinement and Design Cleaner pass, so you don’t hold synthesis back with a manufacturable floor too early. Needle Automatic offers a needle inside a layer only where both parts of the split layer stay at or above this floor; the Needle Manual profile draws the needle function everywhere and leaves the choice to you. A layer that refinement drives down onto the floor stays only while it helps: after refining each candidate, Needle Automatic also refines the same design without its layers on the floor, with half the iterations, and keeps that version when its merit is no higher.

Refine iterations: how many refinement steps run after each insertion.

Inner refiner: which method refines the stack after each insert. The default is Conjugate Gradient, which keeps the design “loose” so the next scan can still find improving needles. See Optimization Methods for the alternatives.

Candidate search: how thoroughly each step explores the improving candidates (fast, balanced, or thorough), trading speed against quality.

Minimum and maximum thickness limits are ignored during synthesis; re-enable them by running Refinement afterwards. The surface mode set in the Design Editor is honoured for every mode, not just the front.

How to read it

The window shows a live preview of the design as needles go in, a scan plot of the P-function versus position for the top candidate materials (the deepest dip marks the best place to insert), and a per-cycle merit history. When you click Done the final design is committed to the active design.

Needle can stall when the topology change it needs is a thick spacer rather than a thin needle; Gradual Evolution escapes those cases with its forced-insertion step, so a common pattern is a few Needle cycles, then Gradual Evolution, then Needle again.

References

• Sullivan & Dobrowolski, Appl. Opt. 35, 5484 (1996).
• Tikhonravov et al., Appl. Opt. 35, 5493 (1996).
• Tikhonravov et al., SPIE 4829 (2003).