artifacts in get_source

[oskooi]

There are artifacts in the get_source slicing routine demonstrated by the following example involving an arbitrary oblique planewave source defined by an amplitude function spanning the entire cell area.

import meep as mp
import math
import cmath
import numpy as np
import matplotlib.pyplot as plt

resolution = 50        # pixels/μm                                                                                                                                          

sxy = 5
cell_size = mp.Vector3(sxy,sxy,0)

# rotation angle of incident planewave; counter clockwise (CCW) about Z axis, 0 degrees along +X axis                                                                                           
theta_in = math.radians(27.3151)

# k (in source medium) with correct length (plane of incidence: XY)                                                                                                         
k = mp.Vector3(math.cos(theta_in),math.sin(theta_in),0)

def pw_amp(k,x0):
  def _pw_amp(x):
    return cmath.exp(1j*2*math.pi*k.dot(x-x0))
  return _pw_amp

src_pt = mp.Vector3(-0.5*sxy+1.3)
sources = [mp.Source(mp.GaussianSource(1,fwidth=0.2),
                     component=mp.Ez,
                     center=src_pt,
                     size=cell_size,
                     amp_func=pw_amp(k,src_pt))]

sim = mp.Simulation(resolution=resolution,
                    cell_size=cell_size,
                    k_point=k,                                                                                                                                            
                    sources=sources)

sim.init_sim()

src_data = sim.get_source(mp.Ez, center=mp.Vector3(), size=cell_size)

plt.figure(dpi=150)
plt.imshow(np.real(src_data.transpose()), interpolation='spline36')
plt.axis('off')
plt.show()

For the case in which the source center is src_pt = mp.Vector3(-0.5*sxy+1.3) (i.e., off center), a line artifact appears in the output at the source location:

If the k_point is removed from the Simulation constructor, the line artifact becomes an empty finite region which extends to the edge of the cell:

Finally, if the source center is at the origin, src_pt = mp.Vector3(), there are no artifacts and the output is as expected (with or without k_point):

Note that these examples do not use symmetries which were fixed in #828.

[oskooi]

Actually, this is not a bug in get_source because the position and size of the source (src_pt and cell_size) were inadvertently chosen such that the source was extending outside of the computational cell. This causes the results from get_source to be unpredictable especially when there are periodic boundaries.

When the source object is defined to lie entirely within the cell volume, get_source works fine.

import meep as mp
import math
import cmath
import numpy as np
import matplotlib
matplotlib.use('agg')
import matplotlib.pyplot as plt

resolution = 50        # pixels/μm                                                                                                      

sxy = 10
cell_size = mp.Vector3(sxy,sxy,0)

# rotation angle of incident planewave; counter clockwise (CCW) about Z axis, 0 degrees along +X axis                                   
theta_in = math.radians(27.3151)

# k (in source medium) with correct length (plane of incidence: XY)                                                                     
k = mp.Vector3(math.cos(theta_in),math.sin(theta_in),0)

def pw_amp(k,x0):
  def _pw_amp(x):
    return cmath.exp(1j*2*math.pi*k.dot(x-x0))
  return _pw_amp

src_pt = mp.Vector3(-1.2,2.3)
sources = [mp.Source(mp.GaussianSource(1,fwidth=0.2),
                     component=mp.Ez,
                     center=src_pt,
                     size=mp.Vector3(3,3),
                     amp_func=pw_amp(k,src_pt))]

sim = mp.Simulation(resolution=resolution,
                    cell_size=cell_size,
                    k_point=k,
                    sources=sources)

sim.init_sim()

src_data = sim.get_source(mp.Ez, center=mp.Vector3(), size=cell_size)

plt.figure(dpi=150)
plt.imshow(np.real(src_data.transpose()), interpolation='spline36')
plt.axis('off')
plt.savefig('test_get_source.png',bbox_inches='tight')