{"id":313248,"date":"2023-10-25T10:03:24","date_gmt":"2023-10-25T10:03:24","guid":{"rendered":"\/forum\/?post_type=topic&p=313248"},"modified":"2023-10-25T10:03:24","modified_gmt":"2023-10-25T10:03:24","slug":"plane-wave-and-bloch-boundary-conditions-in-slab-waveguide","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/plane-wave-and-bloch-boundary-conditions-in-slab-waveguide\/","title":{"rendered":"Plane wave and Bloch boundary conditions in slab waveguide"},"content":{"rendered":"

I would like to compute the transmission of a wave travelling though an infinetly wide waveguide slab. The geometry consists of a 220 nm thick waveguide slab in which the wave is injected, followed by a 130 nm deep step. The angle is varied from 0 to 85 degrees. The source wavelength is fixed to 1550 nm.<\/p>\n

To simulate an infinitely wide slab, I use Bloch periodic boundary conditions (in y) in combination with a plane wave source (PlaneSource, injected in x). In my simulation, I find that the Autoshutoff criterium is not reached, because some light is reflected back and forth between the periodic boundaries. As a result, the energy in the system decays only slowly (see image below). In addition, the pulse seems to become wider as it propagates through the slab. These problems are bigger for larger incidence angles.<\/p>\n


y-component of field with incidence angle = 60 degrees shortly after injection of the pulse. No height step, waveguide thickness is 220 nm.<\/em><\/p>\n


y-component of field with incidence angle = 60 degrees when the pulse has passed through the simulation domain. No height step, waveguide thickness is 220 nm.<\/em><\/p>\n

Is this an artefact of the plane wave source? And if yes, what can I do minimise this effect and\/or correct for it in a transmission measurement?<\/p>\n

Many thanks,
Erik<\/p>\n

Below is sample code (lsf) that can be used to recreate my problem. Use d2_z = 220e-9; for the simulation without the height step and d2_z = 90e-9; for a simulation with a step. The angle of incidence can be varied via deg_angle.<\/p>\n

<\/p>\n

“””<\/p>\n

switchtolayout;<\/p>\n

selectall;<\/p>\n

delete;<\/p>\n

 <\/p>\n

### Variables<\/p>\n

size_x = 8000e-9;<\/p>\n

size_y = 10000e-9;<\/p>\n

size_fdtd_x = 4000e-9;<\/p>\n

size_fdtd_y = 2000e-9;<\/p>\n

size_fdtd_z = 1500e-9;<\/p>\n

x_offset = 0;<\/p>\n

deg_angle = -45;<\/p>\n

z_min = 0;<\/p>\n

d1_z = 220e-9; # height<\/p>\n

d2_z = 220e-9;<\/p>\n

center_wavelength = 1550e-9;<\/p>\n

mesh_accuracy = 5;<\/p>\n

 <\/p>\n

### Main<\/p>\n

 <\/p>\n

poly_name = “rectangle”;<\/p>\n

addrect();<\/p>\n

set(‘name’, poly_name + “_1”);<\/p>\n

set(‘x min’,-size_x\/2+x_offset);<\/p>\n

set(‘x max’,0+x_offset);<\/p>\n

set(‘y min’,-size_y\/2);<\/p>\n

set(‘y max’,size_y\/2);<\/p>\n

set(‘z min’,0);<\/p>\n

set(‘z max’,d1_z);<\/p>\n

set(‘material’, ‘Si (Silicon) – Palik’);<\/p>\n

 <\/p>\n

addrect();<\/p>\n

set(‘name’, poly_name + “_2”);<\/p>\n

set(‘x min’,0+x_offset);<\/p>\n

set(‘x max’,size_x\/2+x_offset);<\/p>\n

set(‘y min’,-size_y\/2);<\/p>\n

set(‘y max’,size_y\/2);<\/p>\n

set(‘z min’,0);<\/p>\n

set(‘z max’,d2_z);<\/p>\n

set(‘material’, ‘Si (Silicon) – Palik’);<\/p>\n

 <\/p>\n

## FDTD<\/p>\n

addfdtd();<\/p>\n

set(‘mesh accuracy’,mesh_accuracy);<\/p>\n

set(‘dimension’,’3D’);<\/p>\n

set(‘x min’,-size_fdtd_x\/2);<\/p>\n

set(‘x max’,size_fdtd_x\/2);<\/p>\n

set(‘y min’,-size_fdtd_y\/2);<\/p>\n

set(‘y max’,size_fdtd_y\/2);<\/p>\n

set(‘z min’,-size_fdtd_z\/2.0+200e-9);<\/p>\n

set(‘z max’,size_fdtd_z\/2.0+200e-9);<\/p>\n

set(‘y min bc’,’Bloch’);<\/p>\n

set(‘index’,1.44);<\/p>\n

set(‘pml profile’,3); # steep angle PML profile<\/p>\n

 <\/p>\n

# SOURCE<\/p>\n

 <\/p>\n

addplane();<\/p>\n

set(‘direction’,’Forward’);<\/p>\n

set(‘injection axis’,’x-axis’);<\/p>\n

set(‘y min’,-size_y\/2);<\/p>\n

set(‘y max’,size_y\/2);<\/p>\n

set(‘z min’,0);<\/p>\n

set(‘z max’,d1_z);<\/p>\n

set(‘x’,-size_fdtd_x\/2+0.5e-6);<\/p>\n

set(‘center wavelength’,center_wavelength);<\/p>\n

set(‘wavelength span’,0);<\/p>\n

set(‘angle theta’,deg_angle);<\/p>\n

 <\/p>\n

# MOVIE MONITORS<\/p>\n

 <\/p>\n

addmovie();<\/p>\n

set(‘name’,’Movie_top_intensity’);<\/p>\n

set(‘monitor type’,’2D Z-normal’);<\/p>\n

set(‘x min’,-size_fdtd_x\/2);<\/p>\n

set(‘x max’,size_fdtd_x\/2);<\/p>\n

set(‘y min’,-size_fdtd_y\/2);<\/p>\n

set(‘y max’,size_fdtd_y\/2);<\/p>\n

set(‘z’,d1_z\/2);<\/p>\n

 <\/p>\n

addmovie();<\/p>\n

set(‘name’,’Movie_top_Ey’);<\/p>\n

set(‘monitor type’,’2D Z-normal’);<\/p>\n

set(‘x min’,-size_fdtd_x\/2);<\/p>\n

set(‘x max’,size_fdtd_x\/2);<\/p>\n

set(‘y min’,-size_fdtd_y\/2);<\/p>\n

set(‘y max’,size_fdtd_y\/2);<\/p>\n

set(‘z’,d1_z\/2);<\/p>\n

set(‘field component’,’Ey’);<\/p>\n

set(‘TM field component’,’Hy’);<\/p>\n

set(‘TE field component’,’Ey’);<\/p>\n

 <\/p>\n

addmovie();<\/p>\n

set(‘name’,’Movie_side_intensity’);<\/p>\n

set(‘monitor type’,’2D Y-normal’);<\/p>\n

set(‘x min’,-size_fdtd_x\/2);<\/p>\n

set(‘x max’,size_fdtd_x\/2);<\/p>\n

set(‘y’,0);<\/p>\n

set(‘z min’,-size_fdtd_z\/2.0+200e-9);<\/p>\n

set(‘z max’,size_fdtd_z\/2.0+200e-9);<\/p>\n

 <\/p>\n

addmovie();<\/p>\n

set(‘name’,’Movie_side_Ey’);<\/p>\n

set(‘monitor type’,’2D Y-normal’);<\/p>\n

set(‘x min’,-size_fdtd_x\/2);<\/p>\n

set(‘x max’,size_fdtd_x\/2);<\/p>\n

set(‘y’,0);<\/p>\n

set(‘z min’,-size_fdtd_z\/2.0+200e-9);<\/p>\n

set(‘z max’,size_fdtd_z\/2.0+200e-9);<\/p>\n

set(‘field component’,’Ey’);<\/p>\n

set(‘TM field component’,’Hy’);<\/p>\n

set(‘TE field component’,’Ey’);<\/p>\n

“””<\/p>\n","protected":false},"template":"","class_list":["post-313248","topic","type-topic","status-closed","hentry","topic-tag-3DFDTD-1","topic-tag-Bloch-1","topic-tag-convergence","topic-tag-plane-wave"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t