The figure in the right shows a field snapshot from a simulation with a larger cell along the y direction, in which you can see that the second-order leaky mode decays away, leaving us with the fundamental mode propagating downward. Moreover, since we are operating just barely below the cutoff frequency for single-mode behavior, we are able to excite a second leaky mode after the waveguide bend, whose second-order mode pattern (superimposed with the fundamental mode) is apparent in the animation. It is clear that the transmission around the bend is rather low for this frequency and structure-both large reflection and large radiation loss are clearly visible. This results in the following animationВ : Here, we are using an animated GIF format for the output. Finally, just for variety, we'll specify the (vacuum) wavelengthВ instead of the frequency again, we'll use a wavelength such that the waveguide is half a wavelength wide.

Second, instead of turning the source on suddenly at tВ = 0В (which excites many other frequencies because of the discontinuity), we will ramp it on slowly (technically, Meep uses aВ tanhВ turn-on function) over a time proportional to the widthВ of 20 time units (a little over three periods). First, a point source does not couple very efficiently to the waveguide mode, so we'll expand this into a line source the same width as the waveguide by adding a sizeВ property to the source (Meep also has an eigenmode source feature which can be used here and is covered in a separateВ tutorial). While we're at it, we'll make a couple of other changes. We also need to shift our source to yВ = в€’11.5В so that it is still inside the waveguide. Also shown in green is the source plane at xВ = в€’ 7В (see Figure at side). As illustrated in the figure, the originВ (0,0)В of the coordinate system is at the center of the computational cell, with positive yВ being downwards in h5topng, and thus the block of size 12Г—1 is centered atВ ( в€’ 2, в€’ 11.5). Note that we now have twoВ blocks, both off-center to produce the bent waveguide structure pictured at right. ( set! geometry-lattice ( make lattice ( size 16 16 no-size ))) ( set! geometry ( list ( make block ( center -2 -11.5 ) ( size 12 1 infinity ) ( material ( make dielectric ( epsilon 12 )))) ( make block ( center 3.5 2 ) ( size 1 28 infinity ) ( material ( make dielectric ( epsilon 12 )))))) ( set! pml-layers ( list ( make pml ( thickness 1.0 )))) ( set! resolution 10 )

Fdtd meep