We can also use the complex mode coefficients to compute the phase (or impedance) of the diffraction orders. This can be used to generate a phase map of the binary grating as a function of its geometric parameters. Phase maps are important for the design of subwavelength phase shifters such as those used in a metasurface lens. When the period of the unit cell is subwavelength, the zeroth-diffraction order is the only propagating wave. In this demonstration, which is adapted from the previous example, we compute the transmittance spectra and phase map of the zeroth-diffraction order (at 0°) for an E_{z}-polarized planewave pulse spanning wavelengths of 0.4 to 0.6 μm which is normally incident on a binary grating with a periodicity of 0.35 μm and height of 0.6 μm. The duty cycle of the grating is varied from 0.1 to 0.9 in separate runs.

In [3]:

```
import meep as mp
import numpy as np
import numpy.matlib
import matplotlib.pyplot as plt
resolution = 50 # pixels/μm
dpml = 1.0 # PML thickness
dsub = 3.0 # substrate thickness
dpad = 3.0 # padding between grating and PML
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
nfreq = 21 # number of frequency bins
k_point = mp.Vector3(0,0,0)
glass = mp.Medium(index=1.5)
def grating(gp,gh,gdc,oddz):
sx = dpml+dsub+gh+dpad+dpml
sy = gp
cell_size = mp.Vector3(sx,sy,0)
pml_layers = [mp.PML(thickness=dpml,direction=mp.X)]
src_pt = mp.Vector3(-0.5*sx+dpml+0.5*dsub,0,0)
sources = [mp.Source(mp.GaussianSource(fcen, fwidth=df), component=mp.Ez if oddz else mp.Hz, center=src_pt, size=mp.Vector3(0,sy,0))]
symmetries=[mp.Mirror(mp.Y, phase=+1 if oddz else -1)]
sim = mp.Simulation(resolution=resolution,
cell_size=cell_size,
boundary_layers=pml_layers,
k_point=k_point,
default_material=glass,
sources=sources,
symmetries=symmetries)
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=100)
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=300)
freqs = mp.get_eigenmode_freqs(mode_mon)
res = sim.get_eigenmode_coefficients(mode_mon, [1], eig_parity=mp.ODD_Z+mp.EVEN_Y if oddz else mp.EVEN_Z+mp.ODD_Y)
coeffs = res.alpha
mode_wvl = [1/freqs[nf] for nf in range(nfreq)]
mode_tran = [abs(coeffs[0,nf,0])**2/input_flux[nf] for nf in range(nfreq)]
mode_phase = [np.angle(coeffs[0,nf,0]) for nf in range(nfreq)]
return mode_wvl, mode_tran, mode_phase
gp = 0.35
gh = 0.6
gdc = np.linspace(0.1,1.0,10)
mode_tran = np.empty((gdc.size,nfreq))
mode_phase = np.empty((gdc.size,nfreq))
for n in range(gdc.size):
mode_wvl, mode_tran[n,:], mode_phase[n,:] = grating(gp,gh,gdc[n],True)
```

The phase of the zeroth-diffraction order is simply the angle of its complex mode coefficient. Note that it is generally only the relative phase (the phase difference) between different structures that is useful. The overall mode coefficient α is multiplied by a complex number given by the source amplitude, as well as an arbitrary (but deterministic) phase choice by the mode solver MPB (i.e., which maximizes the energy in the real part of the fields via `ModeSolver.fix_field_phase`

) — but as long as you keep the current source fixed as you vary the parameters of the structure, the relative phases are meaningful.

The figure below shows the transmittance spectra (left) and phase map (right). The transmittance is nearly unity over most of the parameter space mainly because of the subwavelength dimensions of the grating. The phase variation spans the full range of -π to +π at each wavelength but varies weakly with the duty cycle due to the relatively low index of the glass grating. Higher-index materials such as titanium dioxide (TiO_{2}) generally provide more control over the phase

In [4]:

```
plt.figure(dpi=150)
plt.subplot(1,2,1)
plt.pcolormesh(mode_wvl, gdc, mode_tran, cmap='hot_r', shading='gouraud', vmin=0, vmax=mode_tran.max())
plt.axis([wvl_min, wvl_max, gdc[0], gdc[-1]])
plt.xlabel("wavelength (μm)")
plt.xticks([t for t in np.linspace(wvl_min,wvl_max,3)])
plt.ylabel("grating duty cycle")
plt.yticks([t for t in np.arange(gdc[0],gdc[-1]+0.1,0.1)])
plt.title("transmittance")
cbar = plt.colorbar()
cbar.set_ticks([t for t in np.arange(0,1.2,0.2)])
cbar.set_ticklabels(["{:.1f}".format(t) for t in np.linspace(0,1,6)])
plt.subplot(1,2,2)
plt.pcolormesh(mode_wvl, gdc, mode_phase, cmap='RdBu', shading='gouraud', vmin=mode_phase.min(), vmax=mode_phase.max())
plt.axis([wvl_min, wvl_max, gdc[0], gdc[-1]])
plt.xlabel("wavelength (μm)")
plt.xticks([t for t in np.linspace(wvl_min,wvl_max,3)])
plt.ylabel("grating duty cycle")
plt.yticks([t for t in np.arange(gdc[0],gdc[-1]+0.1,0.1)])
plt.title("phase (radians)")
cbar = plt.colorbar()
cbar.set_ticks([t for t in range(-3,4)])
cbar.set_ticklabels(["{:.1f}".format(t) for t in range(-3,4)])
plt.subplots_adjust(wspace=0.5)
```

See Tutorials/Near to Far Field Spectra/Focusing Properties of a Metasurface Lens for a related example.