Dear MPB users and developers.
I tried to check MPB group velocity calculation correctness. At
photonic crystal with negligible contrast group velocity should be
roughly equal light speed at material with changing sings because of
folding. But calculation shows that absolute value of group velocity
depends of band number.
MBP code:
(set! num-bands 7)
(set! geometry-lattice (make lattice (size 1 1 no-size)
(basis1 (/ (sqrt 3) 2) 0.5)
(basis2 (/ (sqrt 3) 2) -0.5)))
(set! default-material air)
(set! geometry (list (make cylinder
(center 0 0 0) (radius 0.2) (height infinity)
(material (make dielectric (epsilon 1.00000001))))))
(set! k-points (list (vector3 0 0 0) ; Gamma
(vector3 (/ -3) (/ 3) 0) ; K
))
(set! k-points (interpolate 64 k-points))
(set! resolution 64)
(run-te display-group-velocities )
; END OF MPB CODE
Example velocity output string:
tevelocity:, 22, #(7.78615212026757e-25 -0.999999999272597 0.0),
#(3.40122036260571e-7 0.805441667563366 0.0), #(-3.40122036718879e-7
0.805441669075089 0.0), #(1.18935285286298e-8 -0.183365906857283 0.0),
#(-1.18935288080422e-8 -0.183365906656533 0.0), #(1.4273767716188e-7
-0.90326523923235 0.0), #(-1.42737677309561e-7 -0.90326524059479 0.0)
First band is ok, but absolute value of second band group velocity
near 0.8 not 1.0.
What is wrong?
Alex Friman.
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