Positive nonlinearity

A positive nonlinearity of the defect leads to the effect observed in Fig. A.8.

Such unexpected patterns can probably be explained in terms of instability phase modulation.

We didn’t investigate deeply this effect because we were in condition not related to (super)fluidity of photons, where it is required to be in a Kerr defocusing regime.

I report it briefly anyway, hoping that it will be useful for someone.

Here the defect is focused and all the parameter and the setup, except the frequency, are the same of section A.2. In this situation the observed effect depends on the relative angle between the two beams.

Here the observed effect is presented with the defect atf_def = 384,2330T Hz, where it is in focusing regime and the other beam has a frequency lower than the higher energy transitions of the D₂ line.

The defect and the photon fluid are both in Kerr self-focusing regime.

When the defect beams overcome a frequency of fdef = 384,2324 T Hz, that is '220 M Hz higher than the higher energy D2 transition 5S_1/2(F = 2) −→ 5P_3/2 (f₀= 384,23218T Hz), it is possible to observe this effect and there is an immediate (not smooth) switch between a bright spot (without fringes) and the image observed.

Figure A.8. On the left the relative ∆k_⊥=0, on the right there is a relative angle ∆k_⊥6=0 between fluid and the defect. Herefdef = 384,2324T Hz.

105

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Dedicato alle mie migliori amiche Eugenia e Valeria, con le quali ho condiviso gli anni di università e che si sono rivelate persone speciali.