Focus on: Needle/Tunneling Optimization
When needle optimization finds an "optimal" design, that design may still not have the
desired performance. Needle optimization has merely found a local minimum of the merit
The purpose of the tunneling method is to find another -- better -- local minimum. It does this by growing a layer -- which has the effect of tunneling to another valley of the merit function. See the diagram below for a 2-layer example.
The arrow indicates how this method tunneled from one valley to another.
A benefit of the needle/tunneling method is that the initial design can be very simple -- perhaps only 1 QWOT thick. The method will grow the design to meet the desired performance.
Another benefit is that a sequence of designs is created, giving the designer a choice of designs to manufacture.
Finally, this method excels at finding the thinnest design that meets a given performance specification; it just grows the design until the requirement is attained. For an example of this, see Growing an Antireflection Coating.
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