Hi Steven, Thank you for answering my query.
>(I assume that you have uniform medium, then a sample of periodic bandgap >medium, then uniform medium, with PML in the uniform media.) Right. I have a 20x20 sample, periodic boundary conditions above and below, 8 units PML and 10 units PAD to the left and right of the sample. >An easy way to check whether this has anything to do with the PML is to >increase the distance between the sample and the PML; if the oscillations are >due to PML reflections then this will increase the frequency of the >oscillations. Here are my observations: doubling the PAD does increase the frequency of the oscillations (the ocillations are frequency dependent so it is not very easy to asses the increase in the frequency, but I think is roughly double), doubling the resolution decreases the amplitude (roughly by a factor of two), and doubling the PML does not change much. Interestingly enough, the transmission seems fine, only the reflection is affected. The configuration is as follows: half-way in the PAD region there is a line source (5 units from the sample), half way between the source and the sample (2.5 units) there is the reflection detector. On the other side, half way in the PAD region (5 units from the sample) there is the transmission detector. The simulation ends when the field decays by 5e-4 at the transmission detector position. >The decay coefficient within any PML is well-known to have a cos(theta) >factor, where theta is the incidence angle, which makes the decay become >slower as you approach glancing incidence. However, this is true even at >infinite resolution, so if you are seeing an effect that decreases with >increasing resolution then it is something different. e.g. it may be >transition reflections, which you can also decrease by making the PML thicker >instead of increasing the resolution. >Transition reflections also increase as you approach glancing incidence, >because at glancing incidence the phase-velocity mismatch between incident and >reflected waves goes to zero. See e.g. equation (13) in our >paper: http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-15-11376 I realize this, and that is what I was afraid of. However the effect seems rather strong: at a resolution of 64, the amplitude is .02 and I can not run resolutions higher than 128 (where the oscillations are still clearly visible). Is there anything I can do ? (modify the functional absorption in the PML ?) My guess is that what I see are reflections from the first PML layers (the results seem to do not depend on the PML thickness). >The next question is why you would be getting glancing-angle waves. >Glancing-angle waves will occur right after the onset of each diffracted >order. However, for normal-incident light, the first diffracted order occurs >when the wavelength drops below the period, but since you say you are looking >around the band gap then I'm guessing you are operating at a longer wavelength >than this. Merely oblique waves that are are not approaching glancing >incidence, however, should not normally be a problem. Is there some reason >why you think you may have near glancing waves? Right. What I have is a short-range ordered dielectric material that gives rise to the PBG-like behavior. The material is disordered (two-point correlations fall rapidly to 1 as a function of distance), and, depending on the wavelength, the propagating (and reflected) radiation have a strong diffusive component. --SGJ _______________________________________________ meep-discuss mailing list [email protected] http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss _______________________________________________ meep-discuss mailing list [email protected] http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
