What is the "Gouy phase shift"?

You can read an article about it by A. Ruffin et al., in Phys. Rev. Lett. 83, 3410 (1999). The following movie shows it, for a plane wave pulse, focused by a parabolic reflector.
Movie
What it shows is:
1) The wave is created by exciting a central plane.
2) The part on the left is absorbed, the part on the right reaches the parabolic mirror.
3) At the parabolic mirror, the reflection changes the polarity of the field (from yellow to blue).
4) When the wave front passes through the focus, its polarity changes again (from blue to yellow). This is called the Gouy phase shift.

One can see that, as is explained in the PRL article, the phase shift is not a complete polarity reversal (the lowest frequencies are not reversed.)

The field shown is the E-field component normal to the viewing plane, in a central cross-section of a 3D simulation. The mirror (on the right) is simply the discretized approximation of a rotationally symmetric parabolic shape. Its shape can clearly be seen at the moment the signal is reflected.

The simulation was done on an FDTD grid with size: space*time = 80*80*80 * 200. Absorbing boundaries are present (Berenger's technique) consisting of the outermost 10 cells. The code that computed it is in the following files:
gouy.f90
bin_io.f90
gif_util.f90

After that, the freeware tool gifsicle was used, with the command:
gifsicle -l -d 25 ?.gif ??.gif ???.gif > gouy.gif
 to combine the separate pictures into a movie.

For more FDTD results see Half-wave dipole

Jos Bergervoet,

bergervo@[this_site].nl

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