; Calculating 2d ring-resonator modes, from the Meep tutorial.(define-param n 3.4) ; index of waveguide(define-param w 1) ; width of waveguide(define-param r 1) ; inner radius of ring(define-param pad 4) ; padding between waveguide and edge of PML(define-param dpml 2) ; thickness of PML(define sxy (* 2 (+ r w pad dpml))) ; cell size(set! geometry-lattice (make lattice (size sxy sxy no-size))); Create a ring waveguide by two overlapping cylinders - later objects; take precedence over earlier objects, so we put the outer cylinder first.; and the inner (air) cylinder second.(set! geometry (list		(make cylinder (center 0 0) (height infinity)		      (radius (+ r w)) (material (make dielectric (index n))))		(make cylinder (center 0 0) (height infinity)		      (radius r) (material air))))(set! pml-layers (list (make pml (thickness dpml))))(set-param! resolution 10); If we don't want to excite a specific mode symmetry, we can just; put a single point source at some arbitrary place, pointing in some; arbitrary direction.  We will only look for TM modes (E out of the plane).(define-param fcen 0.15) ; pulse center frequency                            (define-param df 0.1)  ; pulse width (in frequency) (set! sources (list               (make source                 (src (make gaussian-src (frequency fcen) (fwidth df)))                 (component Ez) (center (+ r 0.1) 0)))); exploit the mirror symmetry in structure+source:(set! symmetries (list (make mirror-sym (direction Y))))(run-sources+ 200 	      (at-beginning output-epsilon)	      (after-sources (harminv Ez (vector3 (+ r 0.1)) fcen df))); Output fields for one period at the end.  (If we output; at a single time, we might accidentally catch the Ez field when it is; almost zero and get a distorted view.)(run-until (/ 1 fcen) (at-every (/ 1 fcen 20) output-efield-z))