Dear Steven and MPB's users,

I am attempting to find the mode structure of ridged SiN (n ~ 2) waveguides 
which live on a layer of SiO2 ( ~ 2 um and n ~ 1.45) and with a layer of Si (n 
~ 3.5) underneath. The surrounding environment is air.

I already know the frequency at which I want to solve the modes therefore I am 
using the find-k function.

My problem is that the algorithm always seem to find modes which are localised 
in the layer of Si and I can not find those which are localised in the SiN 
waveguide itself.

Is there any way I can modify my control file (see bellow) to find only the 
modes of interest? Is this an issue because of the enforced periodicity of the 

Thank you for your held and time.


; Define the simulation space (a = 1um)

(define s_y 10)

(define s_z 10)

(set! geometry-lattice (make lattice (size no-size s_y s_z)))

(set! resolution 32)

; Not a periodic structure

(set! ensure-periodicity #f)

; Define dielectric parameters

(define n_env 1)

(set! default-material (make dielectric (index n_env)))

(define n_wg 2)

(define n_sub1 1.45)

(define n_sub2 3.5)

; Define structure parameters

(define h_sub1 2) ; Thickness of SiO2 substrate

(define h_wg 0.12) ; Heigth of waveguide

(define w_wg 0.60) ; Width of waveguide

; Build structures

(set! geometry (list

(make block

(center 0 0 (- (/ s_z 4)))

(size infinity infinity (/ s_z 2))

(material (make dielectric (index n_sub2))))

(make block

(center 0 0 (- (/ h_sub1 2)))

(size infinity infinity h_sub1)

(material (make dielectric (index n_sub1))))

(make block

(center 0 0 (/ h_wg 2))

(size infinity w_wg h_wg)

(material (make dielectric (index n_wg))))))

; Find wavevector corresponding to frequency

(define wcen 1.2739)

; Run

(find-k NO-PARITY wcen 1 100 (vector3 1 0 0) 1e-4 1.87 1 10 fix-hfield-phase 
output-hfield fix-efield-phase output-efield  output-poynting output-epsilon)
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