Dear MPB users,
I found a weird output of the epsilon tensor in MPB while trying to simulate an
anisotropic nanofiber. Here, the epsilon.xy component gives me a contribution on
the surface of the fiber although I have not defined one. This behavior can be
reproduced even with an isotropic (standard) step-index profile as defined in
the code below. Just copy the code to a file, run it, and then compare the
outputs of the following commands:
h5topng epsilon.h5:epsilon_inverse.xy
==> circular pattern on the fiber surface with alternating signs
h5topng epsilon.h5:epsilon_inverse.yz
h5topng epsilon.h5:epsilon_inverse.xz
==> 0 in the whole region (black)
I have cross-checked the h5-file with Mathematica and find matching results: 4
pairs of two spikes with a magnitude of 0.1 in the diagonals with alternating
signs.
Has anyone an idea whether this is real or just in the output function? Every
help is appreciated.
Regards, Christian
Version: MPB 1.4.2, using libctl 3.1 and Guile 1.8.8 (=current Ubuntu)
Code:
=====
;Fiber parameters
(define-param mu 0.5); Diameter (micrometer)
(define (eps-func p) (make dielectric (epsilon (* 1.45258 1.45258)) ) )
(define clad (make material-function (material-func eps-func)))
(define-param radclad 0.5)
(define-param wl .850); wavelength (um)
(define-param omega (/ mu wl)) ; conversion to mpb-units
(define-param sc-x 5) ; supercell width (mu)
(define-param sc-y 5) ; supercell height (mu)
(set! geometry-lattice (make lattice (size sc-x sc-y no-size)))
; define the fiber geometry
(set! geometry
(list
(make cylinder (center 0 0 0) (height infinity) (axis 0 0 1) (material
clad) (radius radclad))
)
)
(set-param! num-bands 2)
(set-param! resolution 100) ; pixels/um
(define-param bestguess 1.14504)
; Output the x component of the Poynting vector for num-bands bands at omega
(find-k NO-PARITY omega 1 num-bands (vector3 0 0 1) 1e-5
(* omega bestguess) (* omega .5) (* omega 2)
fix-efield-phase output-efield display-group-velocities
)
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