Unreasonable output while setting a large number timesteps.
I was simulating a grating based coupler having 4 ports (Endings of both waveguides). I am feeding a broadband pulse centered at 1550nm as input source (modal TE profile) to one port and observing output power spectrum at other 3 ports. As per the principle if we use PML boundaries after a long enough time there should not be any energy left in the observation area since it will be absorbed by PML layers. But in my case when I use timesteps of about 500000 and I see one of the electric fields(in time domain) at any observation points, its magnitude goes above 10^9 which is unreasonable. Electric field doesn’t die off with time, instead it increases exponentially to huge numbers. My simulation properties are as follows.
Dimension of simulation domain = 20um x 2um
cell width = 0.005um (smaller than (lambda/10))
delta_t = 1e-17s (set less than the auto value)
timesteps = 500000 (larger than the auto value)
DFT for 200 points
What could be the reason for this unreasonable output. Image is attached. Can somebody help?
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