Observing frequency shifts using mp.fluxregion

[kalaiar]

I have kick-started in pymeep using this transmittance spectra example.

To compare influence of geometry, simulation was carried out with different permittivity for the same material and geometry for Gaussian frequency of 200 KHz.

Question : In the wave propagated geometry's transmittance spectrum, frequency shift is expected. But power is reduced as permittivity increases (Figure 1). Is it possible to observe frequency shift?

Please note that geometry and flux region aligned to collect the wave, can be found in the figure 1.

Code for the above results as follows
`
import matplotlib.pyplot as plt
import numpy as np
import meep as mp

resolution = 20 # pixels/um

sx = 16 # size of cell in X direction
sy = 16 # size of cell in Y direction
cell = mp.Vector3(sx, sy, 0)

dpml = 2.0
pml_layers = [mp.PML(dpml)]

pad = 4 # padding distance between waveguide and cell edge
w = 5 # width of waveguide

wvg_xcen = 0.5 * (sx - w - 2 * pad) # x center of vert. wvg
wvg_ycen = -0.5 * (sy - w - 2 * pad) # y center of horiz. wvg

epsilon_values = [2, 3, 4, 5, 6]

fcen = 200e3 # pulse center frequency
df = 0.1 # pulse width (in frequency)

src = mp.GaussianSource(fcen, fwidth=df)
src_center = mp.Vector3(0, -5, 0)
src_size = mp.Vector3(0, 0, 0)
sources = [mp.Source(src, component=mp.Ex, center=src_center, size=src_size)]

nfreq = 51

all_refl_flux = []
all_tran_flux = []
refl_freqs_all = []
trans_freqs_all = []

def run_simulation(epsilon):
global sim
geometry = [
mp.Block(
size=mp.Vector3(mp.inf, w, mp.inf),
center=mp.Vector3(0, wvg_ycen, 0),
material=mp.Medium(epsilon=epsilon),
)
]

sim = mp.Simulation(
    cell_size=cell,
    boundary_layers=pml_layers,
    geometry=geometry,
    sources=sources,
    resolution=resolution,
)

refl_fr = mp.FluxRegion(center=mp.Vector3(0, wvg_ycen-3, 0), size=mp.Vector3(2, 0, 0))
refl = sim.add_flux(fcen, df, nfreq, refl_fr)

tran_fr = mp.FluxRegion(center=mp.Vector3(0, wvg_ycen+3, 0), size=mp.Vector3(2, 0, 0))
tran = sim.add_flux(fcen, df, nfreq, tran_fr)

pt = mp.Vector3(0.5 * sx - dpml, wvg_ycen)

sim.run(until_after_sources=mp.stop_when_fields_decayed(50, mp.Ez, pt, 1e-3))

straight_refl_flux = mp.get_fluxes(refl)
straight_tran_flux = mp.get_fluxes(tran)

all_refl_flux.append(straight_refl_flux)
all_tran_flux.append(straight_tran_flux)

refl_freqs = mp.get_flux_freqs(refl)
trans_freqs = mp.get_flux_freqs(tran)

refl_freqs_all.append(refl_freqs)
trans_freqs_all.append(trans_freqs)

for epsilon in epsilon_values:
run_simulation(epsilon)

plt.figure(figsize=(12, 8), dpi=200)

plt.subplot(1, 3, 1)
for i, epsilon in enumerate(epsilon_values):
plt.plot(refl_freqs_all[i], all_refl_flux[i], label=f"Epsilon = {epsilon}")

plt.xlabel("Frequency (Hz)")
plt.ylabel("Normalized Flux")
plt.legend(loc="upper right")
plt.title("Reflectance")

plt.subplot(1, 3, 2)
for i, epsilon in enumerate(epsilon_values):
plt.plot(trans_freqs_all[i], all_tran_flux[i], label=f"Epsilon = {epsilon}")

plt.xlabel("Frequency (Hz)")
plt.ylabel("Normalized Flux")
plt.legend(loc="upper right")
plt.title("Transmittance")

plt.subplot(1,3,3)
sim.plot2D(fields=mp.Ez)
plt.title("Simulation Geometry")
plt.xlabel("X")
plt.ylabel("Y")
plt.tight_layout()
plt.show()`