# Mode Decomposition

This tutorial demonstrates the mode-decomposition feature which is used to decompose a given mode profile into a superposition of harmonic basis modes. There are examples for two kinds of modes in lossless, dielectric media: (1) localized (i.e., guided) and (2) non-localized (i.e., planewave).

## Reflectance of a Waveguide Taper

This example involves computing the reflectance — the fraction of the incident power which is reflected — of the fundamental mode of a linear waveguide taper. The structure and the simulation parameters are shown in the schematic below. We will verify that computing the reflectance using two different methods produces nearly identical results: (1) mode decomposition and (2) directly from the fields via its Poynting flux. Also, we will demonstrate that the scaling of the reflectance with the taper length is quadratic, consistent with analytical results from Optics Express, Vol. 16, pp. 11376-92, 2008.

The structure, which can be viewed as a two-port network, consists of a single-mode waveguide of width `w1`

(1 μm) coupled to a second waveguide of width `w2`

(2 μm) via a linearly-sloped taper of length `Lt`

. The structure is homogeneous ε=12 in vacuum. PML absorbing boundaries surround the computational cell. An eigenmode source with E_{z} polarization is used to launch the fundamental mode with a wavelength of 6.67 μm. There is an eigenmode-expansion monitor placed at the midpoint of the first waveguide. This is a line monitor which extends beyond the waveguide in order to span the entire mode profile including its evanescent tails. The Fourier-transformed fields along this line monitor are used to compute the basis coefficients of the harmonic modes. These are computed separately via the eigenmode solver MPB. Technical details are described in Mode Decomposition where it is shown that the squared magnitude of the mode coefficient is equivalent to the power (i.e., Poynting flux) in the given eigenmode. The ratio of the complex mode coefficients can be used to compute the S parameters. In this example, we are computing |S_{11}|^{2} which is the reflectance. Another line monitor could have been placed in the second waveguide to compute the transmittance or |S_{21}|^{2}. Also not considered is the scattering into radiative modes.

The structure has mirror symmetry in the direction which can be exploited to reduce the computation size by a factor of two. This requires that we use `add_flux`

and specify `eig_parity=mp.ODD_Z+mp.EVEN_Y`

in the call to `get_eigenmode_coefficients`

. This is because `add_mode_monitor`

, which is an alias for `add_flux`

, is not optimized for symmetry.

At the end of the simulation, the reflectance of the fundamental mode computed using the two methods are displayed. The simulation script is in examples/mode-decomposition.py.

```
import meep as mp
resolution = 61 # pixels/μm
w1 = 1 # width of waveguide 1
w2 = 2 # width of waveguide 2
Lw = 10 # length of waveguide 1/2
dair = 3.0 # length of air region
dpml_x = 6.0 # length of PML in x direction
dpml_y = 2.0 # length of PML in y direction
sy = dpml_y+dair+w2+dair+dpml_y
Si = mp.Medium(epsilon=12.0)
boundary_layers = [mp.PML(dpml_x,direction=mp.X),
mp.PML(dpml_y,direction=mp.Y)]
# mode wavelength
lcen = 6.67
# mode frequency
fcen = 1/lcen
symmetries = [mp.Mirror(mp.Y)]
for m in range(5):
Lt = 2**m
sx = dpml_x+Lw+Lt+Lw+dpml_x
cell_size = mp.Vector3(sx,sy,0)
src_pt = mp.Vector3(-0.5*sx+dpml_x+0.2*Lw,0,0)
sources = [mp.EigenModeSource(src=mp.GaussianSource(fcen,fwidth=0.2*fcen),
component=mp.Ez,
center=src_pt,
size=mp.Vector3(0,sy-2*dpml_y,0),
eig_match_freq=True,
eig_parity=mp.ODD_Z+mp.EVEN_Y)]
# straight waveguide
vertices = [mp.Vector3(-0.5*sx-1,0.5*w1),
mp.Vector3(0.5*sx+1,0.5*w1),
mp.Vector3(0.5*sx+1,-0.5*w1),
mp.Vector3(-0.5*sx-1,-0.5*w1)]
sim = mp.Simulation(resolution=resolution,
cell_size=cell_size,
boundary_layers=boundary_layers,
geometry=[mp.Prism(vertices,height=mp.inf,material=Si)],
sources=sources,
symmetries=symmetries)
mon_pt = mp.Vector3(-0.5*sx+dpml_x+0.7*Lw,0,0)
flux = sim.add_flux(fcen,0,1,mp.FluxRegion(center=mon_pt,size=mp.Vector3(0,sy-2*dpml_y,0)))
sim.run(until_after_sources=mp.stop_when_fields_decayed(50,mp.Ez,mon_pt,1e-9))
res = sim.get_eigenmode_coefficients(flux,[1],eig_parity=mp.ODD_Z+mp.EVEN_Y)
incident_coeffs = res.alpha
incident_flux = mp.get_fluxes(flux)
incident_flux_data = sim.get_flux_data(flux)
sim.reset_meep()
# linear taper
vertices = [mp.Vector3(-0.5*sx-1,0.5*w1),
mp.Vector3(-0.5*Lt,0.5*w1),
mp.Vector3(0.5*Lt,0.5*w2),
mp.Vector3(0.5*sx+1,0.5*w2),
mp.Vector3(0.5*sx+1,-0.5*w2),
mp.Vector3(0.5*Lt,-0.5*w2),
mp.Vector3(-0.5*Lt,-0.5*w1),
mp.Vector3(-0.5*sx-1,-0.5*w1)]
sim = mp.Simulation(resolution=resolution,
cell_size=cell_size,
boundary_layers=boundary_layers,
geometry=[mp.Prism(vertices,height=mp.inf,material=Si)],
sources=sources,
symmetries=symmetries)
refl_flux = sim.add_flux(fcen,0,1,mp.FluxRegion(center=mon_pt,size=mp.Vector3(0,sy-2*dpml_y,0)))
sim.load_minus_flux_data(refl_flux,incident_flux_data)
sim.run(until_after_sources=mp.stop_when_fields_decayed(50,mp.Ez,mon_pt,1e-9))
res = sim.get_eigenmode_coefficients(refl_flux,[1],eig_parity=mp.ODD_Z+mp.EVEN_Y)
coeffs = res.alpha
taper_flux = mp.get_fluxes(refl_flux)
print("refl:, {}, {:.8f}, {:.8f}".format(Lt,abs(coeffs[0,0,1])**2/abs(incident_coeffs[0,0,0])**2,-taper_flux[0]/incident_flux[0]))
```

We compute the reflectance for five different taper lengths: 1, 2, 4, 8, and 16 μm. A quadratic scaling of the reflectance with the taper length appears as a straight line on a log-log plot. The Bash commands to run the simulation and extract the plotting data from the output are:

```
mpirun -np 3 python -u mode-decomposition.py |tee taper_data.out;
grep refl: taper_data.out |cut -d , -f2- > taper_data.dat
```

The results are plotted using the Python script below. The plot is shown in the accompanying figure. The reflectance values computed using the two methods of mode decomposition and flux are nearly identical. For reference, a line with quadratic scaling is shown in black. The reflectance of the linear waveguide taper decreases quadratically with the taper length consistent with analytic theory.

```
import numpy as np
import matplotlib.pyplot as plt
f = np.genfromtxt("taper_data.dat", delimiter=",")
Lt = f[:,0]
Rmode = f[:,1]
Rflux = f[:,2]
plt.figure(dpi=150)
plt.loglog(Lt,Rmode,'bo-',label='mode decomposition')
plt.loglog(Lt,Rflux,'ro-',label='flux')
plt.loglog(Lt,0.005/Lt**2,'k-',label=r'quadratic reference (1/Lt$^2$)')
plt.legend(loc='upper right')
plt.xlabel('taper length Lt (μm)')
plt.ylabel('reflectance')
plt.show()
```

## Diffraction Spectrum of a Binary Grating

The mode-decomposition feature can also be applied to planewaves in homogeneous media with scalar permittivity/permeability (i.e., no anisotropy). This will be demonstrated in this example to compute the diffraction spectrum of a binary phase grating. The unit cell geometry of the grating is shown in the schematic below. The grating is periodic in the direction with periodicity `gp`

and has a rectangular profile of height `gh`

and duty cycle `gdc`

. The grating parameters are `gh`

=0.5 μm, `gdc`

=0.5, and `gp`

=10 μm. There is a semi-infinite substrate of thickness `dsub`

adjacent to the grating. The substrate and grating are glass with a refractive index of 1.5. The surrounding is air/vacuum. Perfectly matched layers (PML) of thickness `dpml`

are used in the boundaries.

### Transmittance Spectra for Planewave at Normal Incidence

A pulsed planewave with E_{z} polarization spanning wavelengths of 0.4 to 0.6 μm is normally incident on the grating from the glass substrate. The eigenmode monitor is placed in the air region. We will use mode decomposition to compute the transmittance — the ratio of the power in the direction of the diffracted mode relative to that of the incident planewave — for the first ten diffraction orders. Two simulations are required: (1) an empty cell of homogeneous glass to obtain the incident power of the source, and (2) the grating structure to obtain the diffraction orders. At the end of the simulation, the wavelength, angle, and transmittance for each diffraction order are computed.

The simulation script is in examples/binary_grating.py.

```
import meep as mp
import math
resolution = 60 # pixels/μm
dpml = 1.0 # PML thickness
dsub = 3.0 # substrate thickness
dpad = 3.0 # padding between grating and PML
gp = 10.0 # grating period
gh = 0.5 # grating height
gdc = 0.5 # grating duty cycle
sx = dpml+dsub+gh+dpad+dpml
sy = gp
cell_size = mp.Vector3(sx,sy,0)
pml_layers = [mp.PML(thickness=dpml,direction=mp.X)]
wvl_min = 0.4 # min wavelength
wvl_max = 0.6 # max wavelength
fmin = 1/wvl_max # min frequency
fmax = 1/wvl_min # max frequency
fcen = 0.5*(fmin+fmax) # center frequency
df = fmax-fmin # frequency width
src_pt = mp.Vector3(-0.5*sx+dpml+0.5*dsub,0,0)
sources = [mp.Source(mp.GaussianSource(fcen, fwidth=df), component=mp.Ez, center=src_pt, size=mp.Vector3(0,sy,0))]
k_point = mp.Vector3(0,0,0)
glass = mp.Medium(index=1.5)
symmetries=[mp.Mirror(mp.Y)]
sim = mp.Simulation(resolution=resolution,
cell_size=cell_size,
boundary_layers=pml_layers,
k_point=k_point,
default_material=glass,
sources=sources,
symmetries=symmetries)
nfreq = 21
mon_pt = mp.Vector3(0.5*sx-dpml-0.5*dpad,0,0)
flux_mon = sim.add_flux(fcen, df, nfreq, mp.FluxRegion(center=mon_pt, size=mp.Vector3(0,sy,0)))
sim.run(until_after_sources=mp.stop_when_fields_decayed(50, mp.Ez, mon_pt, 1e-9))
input_flux = mp.get_fluxes(flux_mon)
sim.reset_meep()
geometry = [mp.Block(material=glass, size=mp.Vector3(dpml+dsub,mp.inf,mp.inf), center=mp.Vector3(-0.5*sx+0.5*(dpml+dsub),0,0)),
mp.Block(material=glass, size=mp.Vector3(gh,gdc*gp,mp.inf), center=mp.Vector3(-0.5*sx+dpml+dsub+0.5*gh,0,0))]
sim = mp.Simulation(resolution=resolution,
cell_size=cell_size,
boundary_layers=pml_layers,
geometry=geometry,
k_point=k_point,
sources=sources,
symmetries=symmetries)
mode_mon = sim.add_flux(fcen, df, nfreq, mp.FluxRegion(center=mon_pt, size=mp.Vector3(0,sy,0)))
sim.run(until_after_sources=mp.stop_when_fields_decayed(50, mp.Ez, mon_pt, 1e-9))
freqs = mp.get_eigenmode_freqs(mode_mon)
nmode = 10
res = sim.get_eigenmode_coefficients(mode_mon, range(1,nmode+1), eig_parity=mp.ODD_Z+mp.EVEN_Y)
coeffs = res.alpha
kdom = res.kdom
for nm in range(nmode):
for nf in range(nfreq):
mode_wvl = 1/freqs[nf]
mode_angle = math.degrees(math.acos(kdom[nm*nfreq+nf].x/freqs[nf]))
mode_tran = abs(coeffs[nm,nf,0])**2/input_flux[nf]
if nm != 0:
mode_tran = 0.5*mode_tran
print("grating{}:, {:.5f}, {:.2f}, {:.8f}".format(nm,mode_wvl,mode_angle,mode_tran))
```

Note the use of the keyword parameter argument `eig_parity=mp.ODD_Z+mp.EVEN_Y`

in the call to `get_eigenmode_coefficients`

. This is important for specifying **non-degenerate** modes in MPB since the `k_point`

is (0,0,0). `ODD_Z`

is for modes with E_{z} polarization. `EVEN_Y`

is necessary since each diffraction order which is based on a given k_{x} consists of *two* modes: one going in the +y direction and the other in the -y direction. `EVEN_Y`

forces MPB to compute only the +k_{y} + -k_{y} (cosine) mode. As a result, the total transmittance must be halved in this case to obtain the transmittance for either the +k_{y} or -k_{y} mode. For `ODD_Y`

, MPB will compute the sine mode but this will have zero power because the source is even. If the parity is left out, MPB will return a random superposition of the cosine and sine modes. Specifying the `eig_parity`

parameter this way ensures that the ordering of the modes corresponds to only the non-degenerate diffraction orders. Finally, note the use of `add_flux`

instead of `add_mode_monitor`

when using symmetries.

The simulation is run and the results piped to a file (the grating data is extracted to a separate file for plotting) using the following shell script:

```
#!/bin/bash
python -u binary_grating.py |tee grating.out
grep grating grating.out |cut -d , -f2- > grating.dat
```

The diffraction spectrum is plotted using the following script and shown in the figure below.

```
import matplotlib.pyplot as plt
import numpy as np
d = np.genfromtxt("grating.dat",delimiter=",")
nmode = 10
nfreq = 21
thetas = np.empty((nmode,nfreq))
wvls = np.empty((nmode,nfreq))
tran = np.empty((nmode,nfreq))
for j in range(nfreq):
tran[:,j] = d[j::nfreq,2]
thetas[:,j] = d[j::nfreq,1]
wvls[:,j] = d[j,0]
tran_max = round(tran.max(),1)
plt.figure(dpi=150)
plt.pcolormesh(wvls, thetas, tran, cmap='Blues', shading='flat', vmin=0, vmax=tran_max)
plt.axis([wvls.min(), wvls.max(), thetas.min(), thetas.max()])
plt.xlabel("wavelength (μm)")
plt.ylabel("diffraction angle (degrees)")
plt.xticks([t for t in np.arange(0.4,0.7,0.1)])
plt.yticks([t for t in range(0,35,5)])
plt.title("transmittance of diffraction orders")
cbar = plt.colorbar()
cbar.set_ticks([t for t in np.arange(0,tran_max+0.1,0.1)])
cbar.set_ticklabels(["{:.1f}".format(t) for t in np.arange(0,tran_max+0.1,0.1)])
plt.show()
```

Each diffraction order corresponds to a single angle. In the figure below, this angle is represented by the *lower* boundary of each labeled region. For example, the m=0 order has a diffraction angle of 0° at all wavelengths. The representation of the diffraction orders as finite angular regions is an artifact of matplotlib's pcolormesh routine. Note that only the positive diffraction orders are shown as these are equivalent to the negative orders due to the symmetry of the source and the structure. The transmittance of each diffraction order should ideally be a constant for all wavelengths. The slight wavelength dependence shown in the figure is due to numerical discretization which can be mitigated by increasing the resolution.

The diffraction orders/modes are a finite set of propagating planewaves. The wavevector k_{x} of these modes can be computed analytically: for a frequency of ω (in c=1 units), these propagating modes are the **real** solutions of sqrt(ω²n²-(k_{y}+2πm/Λ)²) where m is the diffraction order (an integer), Λ is the periodicity of the grating, and n is the refractive index of the propagating medium. In this example, n=1, k_{y}=0, and Λ=10 μm. Thus, at a wavelength of 0.5 μm there are a total of 20 diffraction orders of which we only computed the first 10. The wavevector k_{x} is used to compute the angle of the diffraction order as cos^{-1}(k_{x}/ω). Evanescent modes, those with an imaginary k_{x}, exist for |m|>20 but these modes carry no power. Note that currently Meep does not compute the number of propagating modes for you. If the mode number passed to `get_eigenmode_coefficients`

is larger than the number of propagating modes at a given frequency/wavelength, MPB's Newton solver will fail to converge and will return zero for the mode coefficient. It is therefore a good idea to know beforehand the number of propagating modes.

In the limit where the grating periodicity is much larger than the wavelength and the size of the diffracting element (i.e., more than 10 times), as it is in this example, the diffraction efficiency can be computed analytically using scalar theory. This is described in the OpenCourseWare Optics course in the Lecture 16 (Gratings: Amplitude and Phase, Sinusoidal and Binary) notes and video. For a review of scalar diffraction theory, see Chapter 3 ("Analysis of Two-Dimensional Signals and Systems") of Introduction to Fourier Optics (fourth edition) by J.W. Goodman. From the scalar theory, the diffraction efficiency of the binary grating is 4/(mπ)^{2} when the phase difference between the propagating distance in the glass relative to the same distance in air is π. The phase difference/contrast is (2π/λ)(n-1)s where λ is the wavelength, n is the refractive index of the grating, and s is the propagation distance in the grating (`gh`

in the script). A special feature of the binary grating is that the diffraction efficiency is 0 for all *even* orders. This is verified by the diffraction spectrum shown above.

To convert the diffraction efficiency into transmittance in the *x* direction (in order to be able to compare the diffraction-theory results with those from Meep), the diffraction efficiency must be multiplied by the Fresnel transmittance from air to glass and by the cosine of the diffraction angle. We compare the analytic and simulated results at a wavelength of 0.5 μm (for which the scalar theory is valid) for diffraction orders 1, 3, 5, and 7. The analytic results are 0.3886, 0.0427, 0.0151, and 0.0074. The Meep results are 0.3891, 0.04287, 0.0152, and 0.0076. This corresponds to relative errors of approximately 1.3%, 0.4%, 0.8%, and 2.1% which indicates good agreement.

### Reflectance and Transmittance Spectra for Planewave at Oblique Incidence

As an additional demonstration of the mode-decomposition feature, the reflectance and transmittance of all diffracted orders for any grating with no material absorption and a planewave source incident at any arbitrary angle and wavelength must necessarily sum to unity. Also, the total reflectance and transmittance must be equivalent to values computed using the Poynting flux. This demonstration is somewhat similar to the single-mode waveguide example.

The following script is adapted from the previous binary-grating example involving a normally-incident planewave. The total reflectance, transmittance, and their sum are displayed at the end of the simulation on two separate lines prefixed by `mode-coeff:`

and `poynting-flux:`

.

Results are computed for a single wavelength of 0.5 μm. The pulsed planewave is incident at an angle of 10.7°. Its spatial profile is defined using the source amplitude function `pw_amp`

. This anonymous function takes two arguments, the wavevector and a point in space (both `mp.Vector3`

s), and returns a function of one argument which defines the planewave amplitude at that point. A narrow bandwidth pulse is used in order to mitigate the intrinsic discretization effects of the Yee grid for oblique planewaves. Also, the `stop_when_fields_decayed`

termination criteria is replaced with `until_after_sources`

. As a general rule of thumb, the more oblique the planewave source, the longer the run time required to ensure accurate results. There is an additional line monitor between the source and the grating for computing the reflectance. The angle of each reflected/transmitted mode, which can be positive or negative, is computed using its dominant planewave vector. Since the oblique source breaks the symmetry in the direction, each diffracted order must be computed separately. In total, there are 59 reflected and 39 transmitted orders.

The simulation script is in examples/binary_grating_oblique.py.

```
import meep as mp
import math
import cmath
import numpy as np
resolution = 50 # pixels/μm
dpml = 1.0 # PML thickness
dsub = 3.0 # substrate thickness
dpad = 3.0 # length of padding between grating and PML
gp = 10.0 # grating period
gh = 0.5 # grating height
gdc = 0.5 # grating duty cycle
sx = dpml+dsub+gh+dpad+dpml
sy = gp
cell_size = mp.Vector3(sx,sy,0)
pml_layers = [mp.PML(thickness=dpml,direction=mp.X)]
wvl = 0.5 # center wavelength
fcen = 1/wvl # center frequency
df = 0.05*fcen # frequency width
ng = 1.5
glass = mp.Medium(index=ng)
use_cw_solver = False # CW solver or time stepping?
tol = 1e-6 # CW solver tolerance
max_iters = 2000 # CW solver max iterations
L = 10 # CW solver L
# rotation angle of incident planewave; CCW about Z axis, 0 degrees along +X axis
theta_in = math.radians(10.7)
# k (in source medium) with correct length (plane of incidence: XY)
k = mp.Vector3(math.cos(theta_in),math.sin(theta_in),0).scale(fcen*ng)
symmetries = []
eig_parity = mp.ODD_Z
if theta_in == 0:
k = mp.Vector3(0,0,0)
symmetries = [mp.Mirror(mp.Y)]
eig_parity += mp.EVEN_Y
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*sx+dpml+0.3*dsub,0,0)
sources = [mp.Source(mp.ContinuousSource(fcen,fwidth=df) if use_cw_solver else mp.GaussianSource(fcen,fwidth=df),
component=mp.Ez,
center=src_pt,
size=mp.Vector3(0,sy,0),
amp_func=pw_amp(k,src_pt))]
sim = mp.Simulation(resolution=resolution,
cell_size=cell_size,
boundary_layers=abs_layers,
k_point=k,
default_material=glass,
sources=sources,
symmetries=symmetries)
refl_pt = mp.Vector3(-0.5*sx+dpml+0.5*dsub,0,0)
refl_flux = sim.add_flux(fcen, 0, 1, mp.FluxRegion(center=refl_pt, size=mp.Vector3(0,sy,0)))
if use_cw_solver:
sim.init_sim()
sim.solve_cw(tol, max_iters, L)
else:
sim.run(until_after_sources=100)
input_flux = mp.get_fluxes(refl_flux)
input_flux_data = sim.get_flux_data(refl_flux)
sim.reset_meep()
geometry = [mp.Block(material=glass, size=mp.Vector3(dpml+dsub,mp.inf,mp.inf), center=mp.Vector3(-0.5*sx+0.5*(dpml+dsub),0,0)),
mp.Block(material=glass, size=mp.Vector3(gh,gdc*gp,mp.inf), center=mp.Vector3(-0.5*sx+dpml+dsub+0.5*gh,0,0))]
sim = mp.Simulation(resolution=resolution,
cell_size=cell_size,
boundary_layers=abs_layers,
geometry=geometry,
k_point=k,
sources=sources,
symmetries=symmetries)
refl_flux = sim.add_flux(fcen, 0, 1, mp.FluxRegion(center=refl_pt, size=mp.Vector3(0,sy,0)))
sim.load_minus_flux_data(refl_flux,input_flux_data)
tran_pt = mp.Vector3(0.5*sx-dpml-0.5*dpad,0,0)
tran_flux = sim.add_flux(fcen, 0, 1, mp.FluxRegion(center=tran_pt, size=mp.Vector3(0,sy,0)))
if use_cw_solver:
sim.init_sim()
sim.solve_cw(tol, max_iters, L)
else:
sim.run(until_after_sources=200)
nm_r = np.floor((fcen*ng-k.y)*gp)-np.ceil((-fcen*ng-k.y)*gp) # number of reflected orders
if theta_in == 0:
nm_r = nm_r/2 # since eig_parity removes degeneracy in y-direction
nm_r = int(nm_r)
res = sim.get_eigenmode_coefficients(refl_flux, range(1,nm_r+1), eig_parity=eig_parity)
r_coeffs = res.alpha
Rsum = 0
for nm in range(nm_r):
r_kdom = res.kdom[nm]
Rmode = abs(r_coeffs[nm,0,1])**2/input_flux[0]
r_angle = np.sign(r_kdom.y)*math.acos(r_kdom.x/(ng*fcen))
print("refl:, {}, {:.2f}, {:.8f}".format(nm,math.degrees(r_angle),Rmode))
Rsum += Rmode
nm_t = np.floor((fcen-k.y)*gp)-np.ceil((-fcen-k.y)*gp) # number of transmitted orders
if theta_in == 0:
nm_t = nm_t/2 # since eig_parity removes degeneracy in y-direction
nm_t = int(nm_t)
res = sim.get_eigenmode_coefficients(tran_flux, range(1,nm_t+1), eig_parity=eig_parity)
t_coeffs = res.alpha
Tsum = 0
for nm in range(nm_t):
t_kdom = res.kdom[nm]
Tmode = abs(t_coeffs[nm,0,0])**2/input_flux[0]
t_angle = np.sign(t_kdom.y)*math.acos(t_kdom.x/fcen)
print("tran:, {}, {:.2f}, {:.8f}".format(nm,math.degrees(t_angle),Tmode))
Tsum += Tmode
print("mode-coeff:, {:.6f}, {:.6f}, {:.6f}".format(Rsum,Tsum,Rsum+Tsum))
r_flux = mp.get_fluxes(refl_flux)
t_flux = mp.get_fluxes(tran_flux)
Rflux = -r_flux[0]/input_flux[0]
Tflux = t_flux[0]/input_flux[0]
print("poynting-flux:, {:.6f}, {:.6f}, {:.6f}".format(Rflux,Tflux,Rflux+Tflux))
```

Since this is a single-wavelength calculation, the frequency-domain solver can be used instead of time stepping for a possible performance enhancement. The only changes necessary to the original script are to replace two objects: (1) `GaussianSource`

with `ContinuousSource`

and (2) `run`

with `solve_cw`

. Choosing which approach to use is determined by the `use_cw_solver`

boolean variable. In this example, mainly because of the oblique source, the frequency-domain solver converges slowly and is less efficient than the time-stepping simulation. The results from both approaches are nearly identical. Time stepping is therefore the default.

The following are several of the lines from the output for eight of the reflected and transmitted orders. The first numerical column is the mode number, the second is the mode angle (in degrees), and the third is the fraction of the input power that is concentrated in the mode. Note that the thirteenth transmitted order at 19.18° contains nearly 38% of the input power.

```
...
refl:, 7, 6.83, 0.00006655
refl:, 8, -8.49, 0.00005703
refl:, 9, 8.76, 0.00015782
refl:, 10, -10.43, 0.00001277
refl:, 11, 10.70, 0.04414104
refl:, 12, -12.38, 0.00005981
refl:, 13, 12.65, 0.00041466
refl:, 14, -14.34, 0.00001991
...
```

```
...
tran:, 12, -18.75, 0.00095295
tran:, 13, 19.18, 0.38261656
tran:, 14, -21.81, 0.00198510
tran:, 15, 22.24, 0.00107184
tran:, 16, -24.93, 0.00098452
tran:, 17, 25.37, 0.04148787
tran:, 18, -28.13, 0.00137329
tran:, 19, 28.59, 0.00113850
...
```

The mode number is equivalent to the band index from the MPB calculation. The ordering of the modes is according to *decreasing* values of k_{x}. The first mode has the largest k_{x} and thus angle closest to 0°. As a corollary, the first mode has the smallest |k_{y}+2πm/Λ|. For a non-zero k_{y} (as in the case of an obliquely incident source), this expression will not necessarily be zero. The first seven reflected modes have m values of -3, -4, -2, -5, -1, -6, and 0. These m values are not monotonic. This is because k_{x} is a nonlinear function of m as shown earlier. The ordering of the transmitted modes is different since these modes are in vacuum and not glass (recall that the medium's refractive index is also a part of this nonlinear function). In the first example involving a normally incident source with k_{y}=0, the ordering of the modes is monotonic: m = 0, ±1, ±2, ...

The two main lines of the output are:

```
mode-coeff:, 0.061007, 0.937897, 0.998904
poynting-flux:, 0.061063, 0.938384, 0.999447
```

The first numerical column is the total reflectance, the second is the total transmittance, and the third is their sum. Results from the mode coefficients agree with the Poynting flux values to three decimal places. Also, the total reflectance and transmittance sum to unity. These results indicate that approximately 6% of the input power is reflected and the remaining 94% is transmitted.