loss rate calculation is off

[alexysong]

I suspect it is either because of multiple layers, or because cladding is not 1. An example:

import numpy as np
import legume
from legume.phc import PhotCryst, Lattice
from legume.shapes import Square, Circle, Poly
from legume.gme import GuidedModeExp
from legume.utils import plot_reciprocal
import matplotlib.pyplot as plt


# Initialize a lattice (can be 'square', 'hexagonal', or defined by primitive vectors)
lattice = Lattice('square')
# Initialize a PhC (optional permittivity in the lower and upper cladding)
phc = PhotCryst(lattice, eps_l=3.2826**2, eps_u=3.2826**2)

phc.add_layer(d=1.007, eps_b=3.5032**2)  # PhC layer

# A triangle
triangle = Poly(x_edges=[0., 0.5256, 0.], y_edges=[0., 0., 0.5256], eps=1.)
phc.layers[-1].add_shape(triangle)

phc.add_layer(d=0.375, eps_b=3.5032**2)  # sch layer
phc.add_layer(d=0.495, eps_b=3.3955**2)  # active layer

# We can plot an overview of what we've built so far
phc.plot_overview()

# Initialize GME and plot what we're simulating
gmax = 5  # plane wave truncation
gme = legume.GuidedModeExp(phc, gmax=gmax)
gme.plot_overview_ft()

n_k = 21
path = phc.lattice.bz_path(['G', np.array([np.pi/2., 0])], [n_k-1])

# See the legume README to understand the options below
neig = 12
options = {'gmode_inds': np.arange(8), 'gmode_npts': 4000, 'numeig': neig, 'verbose': True}


gme.run(kpoints=path.kpoints, **options)

ax = legume.viz.bands(gme)

ks = np.arange(n_k)
freqs_i = []
for kind in ks:
    # kind = 0  # index of the k point
    minds = np.arange(0, neig)  # mode indexes whose loss rates will be computed
    (freqs_im, coup_l, coup_u) = gme.compute_rad(kind=kind, minds=minds)
    freqs_i.append(freqs_im)
    # Note that states below light line still have zero losses
freqs_i = np.array(freqs_i)
qf = gme.freqs/2./freqs_i
# print("Frequencies of first 4 modes at kx = %f are: " % gme.kpoints[0, kind], gme.freqs[kind, minds])
# print("Loss rates of first 4 modes at kx = %f are : " % gme.kpoints[0, kind], freqs_im)

f1 = plt.figure()
ax = f1.add_subplot(111)
ax.plot(np.log10(qf))

Expect: Q factor at gamma to be in the 10^5 range.

Actual outcome: Q factor in 10^2 range.

[alexysong]

Update: I think it's probably related to the cladding being not air. If I delete these two lines:

phc.add_layer(d=0.375, eps_b=3.5032**2)  # sch layer
phc.add_layer(d=0.495, eps_b=3.3955**2)  # active layer

I am left with a one layer structure, with claddings being not air. The calculated loss rate is still off.

[momchilmm]

So the code above doesn't run at all, because currently loss rates are only implemented for even gmode_inds. So I can't really reproduce what you're seeing.

Maybe when I'm done with the loss rates fully, we can have a look into that again.

P. S. Note that I've never tried GME with such a small contrast between slab and claddings, but I hope that it will still work well - we'll see.

[momchilmm]

Fixed in afc9bc0

The Q of the band starting around f = 0.3 is now > 10^5.