On Mon, 3 Jan 2011 10:53:57 +0100, Georg Wachter <[email protected]> wrote:
> > gpipc wrote: >> what resolution you need? >> > > I am not yet sure I am finished with the convergence study. Right now it > looks like this: > My nanostructure has a characteristic radius of curvature of ~ 50 nm, but > is > much larger than that (um range). Since I'm only interested in an estimate > of the effects of beam propagation near the structure, I obtain reasonable > looking results for quite coarse resolutions of ~ 12.5 nm. The maximum I > can > afford is ~ 4 nm, calculating on a cluster, with a simulation box of ~ > 5*5*3 > um and dispersive materials. > > I get decent results for a 20 nm diameter nanosphere in two >> dimensions (so it is actually a cylinder) with 0.5 nm resolution, but >> they >> do not seem to be enough for 3d (I am trying a calculation with 0.25 nm >> resolution now). I compare the FDTD scattering efficiencies with Mie >> theory. >> I need to make a big, big amend to my message above: there is no pressing need for 0.25 nm resolution in 3d for a realistic gold model for a sphere 20 nm diameter at optical frequencies. In fact even a 1 nm resolution is enoguh to get decent results (correct position of the plasmon peak, very nice correspondence of the width, smooth scattering spectrum which follows very nciely the trend of the Mie result - for leaving a precise record of the situation I need to mention that I have more than 10% absolute error on the scattering cross section yet, but that may perhaps be due to some errors I am still making with the incident plane wave, I am not yet sure). The problem I was having, and that was pushing me to increase the resolution, was that the scattering spectrum had spurious oscillations which could be confusd with additional resonances: and I figured out that the reason why they were there is that the box I was using to calculate scattering was too tight around the nanosphere (it had 30 nm side); I have used now a box with 120 nm diameter and the spurious oscillations have disappeared). The level of detail of this message may be a little too much, but I did not want to leave on the Internet a false statement on the resolution that is needed to do this FDTD calculation. > > I personally would be extremely careful with such high resolutions. At 0.25 > nm you are in the range where you have a single atom per pixel/voxel. > Maxwell's Equations (in matter) are a macroscopic theory formulated for > fields which should be considered averaged over many atom distances. I > would > not expect any classical electromagnetic theory to accurately describe > experiments on this length scale. > Ok, thanks. On the other hand the only result I am picking up from the calculation is an integrated flux, so increasing the resolution cannot lead to any inconsistency IMHO (otherwise analytical results, where the resolution is infinite, would never make any sense). Giovanni -- ================================================ Giovanni Piredda Postdoc - AK Hartschuh Phone: ++49 - (0) 89/2180-77601 Fax.: ++49 – (0) 89/2180-77188 Room: E2.062 ---------------------------------------- Message sent by Cup Webmail (Roundcube) _______________________________________________ meep-discuss mailing list [email protected] http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
