On Tue, 27 Mar 2007, ? ? wrote:
I¡¯m sorry to trouble you again. In MEEP we could not specify the power of the source. Rather we could specify the amplitude only. If I want to set a source with a power 4w, what is the amplitude of the source? Is it simply obtained by 4^0.5 = 2? However, it seems that the expected nonlinear effect could not be observed. What¡¯s wrong with it? Many thanks. Jack
You are thinking of this in the wrong way.A source in Meep is a current in Maxwell's equations. A given current source does *not* generate a given power. The radiated power depends on the time-dependence and on the surrounding geometry --- exactly the same current, with exactly the same time-dependence, radiates different amounts of power depending on what its surroundings are. This is a consequence of the physical laws, independent of Meep's computational method.
So, in a nonlinear calculation where the absolute amount of power is important, you need to run two calculations. Once to calculate the amount of power radiated by your source with a given amplitude in the linear system, and then again to calculate your nonlinear effect (with either the amplitude adjusted to give your desired power, or your chi3 adjusted to give the desired index shift).
See also: http://ab-initio.mit.edu/wiki/index.php/Units_and_nonlinearity_in_Meep Cordially, Steven G. Johnson
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