So I'm pretty new to FDTD modelling, and I'm having trouble interpreting the LDOS output that I am recieving from MEEP. To simplify the problem I started with a simple silver grating in a Bloch wave surface plasmon polariton (BW-SPP) configuration:
supers = mp.Block(mp.Vector3(sub+sep,mp.inf,mp.inf), center=mp.Vector3(-0.5*sx+0.5*(sub+sep),0,0), material=diel1) grating = mp.Block(mp.Vector3(grating_t, grating_w,mp.inf), center=mp.Vector3(-0.5*sx+sub-0.5*grating_t,0,0), material=Ag) metal = mp.Block(mp.Vector3(mthick,mp.inf,mp.inf), center=mp.Vector3(-0.5*sx+sub+sep+0.5*mthick,0,0), material=Ag) geometry=[supers,grating,metal]
then I added my Gaussian source:
f_cen = 1/0.570 # source frequency center df = 0.03*f_cen # source frequency width pt = mp.Vector3(-0.5*sx+dpml+sub+sep+mthick+0.010,-0.25*sy,0) sources = [mp.Source(mp.GaussianSource(f_cen, fwidth=df),component=mp.Hz,center=pt)]
and ran the simulation for various amounts of time between 1000 and 10000:
sim.run(mp.dft_ldos(f_cen,df,100), until_after_sources=3000)
What I expect to see is a curve with peak LDOS centred around 570nm wavelength, resulting in the excitation of a surface plasmon as seen below in the enhancement field of the Ex and Ey components: Enhancement field of Ex and Ey compnents around the given structure.
Instead, I get the plot below: Normalised LDOS for the specific position given above The original value of the LDOS is also around 0.58 with minimal differences between frequencies. Additionally, the larger I set the gaussian source width, the same pattern I obtain. I.e. there is no 'peak'. This is not what I would expect to see 10nm from the surface of the substrate, and if I change the y position of the dipole source (moving it across the substrate) it does change, but also results in "nan" over where I would expect the most intense spots to be. I've tried running the same code with resolution varying from 50-200 pixels/um but the results remain similar. I admit that I may not have a full understanding of how to utilise the LDOS in MEEP, but am hoping to find some clarity here.
Thank you