Field projection for a zone plate¶
This tutorial will show you how to solve for electromagnetic fields far away from your structure using field information stored on a nearby surface.
This field projection technique is very useful for reducing the simulation size needed for structures involving lots of empty space.
As an example, we will simulate a simple zone plate lens with a very thin domain size, and measure the transmitted fields just above the structure. Then, we'll show how to use field projections to compute the fields at the focal plane above the lens.
# standard python imports import numpy as np import matplotlib.pyplot as plt # tidy3d imports import tidy3d as td import tidy3d.web as web
Below is a rough sketch of the setup of the field projection.
The transmitted near fields are measured just above the metalens on the blue line, and are projected to the focal plane denoted by the red line.
Define Simulation Parameters¶
As always, we first need to define our simulation parameters. As a reminder, all length units in
tidy3D are specified in microns.
# 1 nanometer in units of microns (for conversion) nm = 1e-3 # free space central wavelength wavelength = 1.0 # numerical aperture NA = 0.8 # height of lens features height_lens = 200 * nm # space between bottom PML and substrate (-z) # and the space between lens structure and top pml (+z) space_below_sub = 1.5 * wavelength # height of substrate (um) height_sub = wavelength / 2 # side length (xy plane) of entire metalens (um) length_xy = 20 * wavelength # Lens and substrate refractive index n_TiO2 = 2.40 n_SiO2 = 1.46 # define material properties air = td.Medium(permittivity=1.0) SiO2 = td.Medium(permittivity=n_SiO2**2) TiO2 = td.Medium(permittivity=n_TiO2**2)
Next we perform some conversions based on these parameters to define the simulation.
# because the wavelength is in microns, use builtin td.C_0 (um/s) to get frequency in Hz f0 = td.C_0 / wavelength # Define PML layers, for this application we surround the whole structure in PML to isolate the fields boundary_spec = td.BoundarySpec.all_sides(boundary=td.PML()) # domain size in z, note, we're just simulating a thin slice: (space -> substrate -> lens height -> space) length_z = space_below_sub + height_sub + height_lens + space_below_sub # construct simulation size array sim_size = (length_xy, length_xy, length_z)
# define substrate substrate = td.Structure( geometry=td.Box( center=[0, 0, -length_z / 2 + space_below_sub + height_sub / 2.0], size=[td.inf, td.inf, height_sub], ), medium=SiO2, ) # focal length focal_length = length_xy / 2 / NA * np.sqrt(1 - NA**2) # location from center for edge of the n-th inner ring, see https://en.wikipedia.org/wiki/Zone_plate def edge(n): return np.sqrt(n * wavelength * focal_length + n**2 * wavelength**2 / 4) # loop through the ring indeces until it's too big and add each to geometry list n = 1 r = edge(n) rings =  while r < 2 * length_xy: # progressively wider cylinders, material alternating between air and TiO2 cylinder = td.Structure( geometry=td.Cylinder( center=[ 0, 0, -length_z / 2 + space_below_sub + height_sub + height_lens / 2, ], axis=2, radius=r, length=height_lens, ), medium=TiO2 if n % 2 == 0 else air, ) rings.append(cylinder) n += 1 r = edge(n) # reverse geometry list so that inner, smaller rings are added last and therefore override larger rings. rings.reverse() geometry = [substrate] + rings
# Bandwidth in Hz fwidth = f0 / 10.0 # Gaussian source offset; the source peak is at time t = offset/fwidth offset = 4.0 # time dependence of source gaussian = td.GaussianPulse(freq0=f0, fwidth=fwidth) source = td.PlaneWave( center=(0, 0, -length_z / 2 + space_below_sub / 2), size=(td.inf, td.inf, 0), source_time=gaussian, direction="+", pol_angle=0.0, ) # Simulation run time run_time = 40 / fwidth
We'll create a FieldProjectionCartesianMonitor monitor which measures the fields just above the structure, and projects them to a Cartesian plane in the far field a given distance away. We'll also make a dedicated near-field monitor just to see what the near fields look like.
# place the monitors halfway between top of lens and PML pos_monitor_z = ( -length_z / 2 + space_below_sub + height_sub + height_lens + space_below_sub / 2 ) # set the points on the observation grid at which fields should be projected num_far = 40 xs_far = 4 * wavelength * np.linspace(-0.5, 0.5, num_far) ys_far = 4 * wavelength * np.linspace(-0.5, 0.5, num_far) monitor_far = td.FieldProjectionCartesianMonitor( center=[ 0.0, 0.0, pos_monitor_z, ], # center of the near field surface on which fields are recorded size=[ td.inf, td.inf, 0, ], # size of the near field surface on which fields are recorded normal_dir="+", # normal vector direction of the near field surface on which fields are recorded freqs=[f0], name="farfield", x=xs_far, y=ys_far, proj_axis=2, # direction along which fields are projected proj_distance=focal_length, # signed distance along the projection axis at which the observations grid resides far_field_approx=False, # turn off the geometric far field approximations, which in this case would lead to # inaccurate fields because the distance to the focal plane is of the same order as # the size of the structure and the near field monitor ) monitor_near = td.FieldMonitor( center=[0.0, 0.0, pos_monitor_z], size=[td.inf, td.inf, 0], freqs=[f0], name="nearfield", )
Put everything together and define a simulation object. A nonuniform simulation grid is generated automatically based on a given number of cells per wavelength in each material (10 by default), using the frequencies defined in the sources.
simulation = td.Simulation( size=sim_size, grid_spec=td.GridSpec.auto(min_steps_per_wvl=20), structures=geometry, sources=[source], monitors=[monitor_far, monitor_near], run_time=run_time, boundary_spec=boundary_spec, )
[08:10:16] WARNING: Structure at structures has bounds that extend exactly to simulation edges. This log.py:50 can cause unexpected behavior. If intending to extend the structure to infinity along one dimension, use td.inf as a size variable instead to make this explicit. Skipping check for structure indexes > 57.
Note: Tidy3D warns us that one of the rings is touching the PML boundary. This warning is intended to prevent people from placing, for example, a substrate next to the PML without extending into it. In our case, the ring is not supposed to be extending into the PML so we can safely ignore the warning.
fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(20, 8)) simulation.plot_eps(x=0, ax=ax1) simulation.plot_eps( z=-length_z / 2 + space_below_sub + height_sub + height_lens / 2, ax=ax2 ) plt.show()
import tidy3d.web as web sim_data = web.run( simulation, task_name="zone_plate", path="data/simulation.hdf5", verbose=True )
[08:11:39] WARNING: Structure at structures has bounds that extend exactly to simulation edges. This log.py:50 can cause unexpected behavior. If intending to extend the structure to infinity along one dimension, use td.inf as a size variable instead to make this explicit. Skipping check for structure indexes > 57.
Let's inspect the near field using the Tidy3D builtin field visualization methods.
For more details see the documentation of tidy3d.SimulationData.
near_field_data = sim_data["nearfield"] fig, (ax1, ax2, ax3) = plt.subplots(1, 3, tight_layout=True, figsize=(15, 3.5)) near_field_data.Ex.real.plot(ax=ax1) near_field_data.Ey.real.plot(ax=ax2) near_field_data.Ez.real.plot(ax=ax3) plt.show()