Dear Yanbo Pei, In this essentially classical treatment of the interactions between a saturable gain/absorbing medium and the electromagnetic fields, there are three different variables that can all be tuned to control the effective gain/loss that the field sees -- the two that you mentioned, N0 and the various decay and pumping rates (including Rp), as well as the coupling element sigma (or theta, depending on which set of conventions you are using). If you wish to use physical units for N0 and the pumping/decay rates, you'll also need to be careful to pick the coupling element to also use these units (it should be quite small in this case). Note. that N0 should be interpreted as a density, number of atoms per unit pixel of your system.
In the tutorial, the values chosen do not correspond to any particular physical system, they're just a convenient set of values to confirm the stable multimode lasing regime of meep. This choice was made for two reasons: (1) because it is a bit difficult to find physical values for sigma/theta for realistic systems in the literature alongside pumping / decay rates. (at least, at the time when I was focused on these problems, ~6 years ago, maybe that's changed.) and (2) only the combination of these parameters discussed in the tutorial, D0, actually matters for calculating the lasing behavior. (i.e., you have 3 knobs to turn to choose a single parameter.) If you cared about the noise of the system, this is no longer true, and all 3 parameters play independent roles, but the quantum noise of a laser is beyond the scope of what meep currently calculates. alex On Thu, Jul 4, 2019 at 9:59 AM 裴延波 <peiya...@163.com> wrote: > Dear Meep users and developers > I am using multilevel-Atomic Susceptibility of MEEP to do some > simulation. I wonder what is the unit of N0 - total number of gain atoms in > the system and Rp - pumping rate. > In the code of the tutorial documents(Multilevel_atomic Susceptibility), > N0 is equal to 37 and Rp is equal to 0.0051. However the reference [Opt. > Express 2011, 20,474] says that N0 is in in the order of magnitude of 1e23 > /m3 and Rp is in the order of magnitude of 1e8 /s. According to above data, > N0 seems to be normalized. But I cannot find the document that explains how > to normalize. As for Rp, if we normalize Rp with (c/a=3e8/1e-6=3e14), the > result is about 1e-6 which deviates a lot from the value in the tutorial > document 0.0051. > Thanks a lot in advance for any help and also meep developers for this > excellent tool > > > > Yanbo Pei > > > > > > > _______________________________________________ > meep-discuss mailing list > meep-discuss@ab-initio.mit.edu > http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
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