Localized Structures
Introduction
Localized structures ("LS") we call large amplitude peaks that are
localized to a small area of the light field. Stimulated by the theoretically
predicted possibility to excite these solitary structures in compact semiconductor
devices and by their possible application in optical data communication,
they have attracted great interest in recent years.
In our experiment we use sodium vapour in an external magnetic field
B
as a nonlinear medium. The sodium vapor is driven by a circularly polarized
cw dye laser beam tuned several GHz above the D1-line. 99% of
the transmitted power is fed back into the sodium cell by a plane mirror
at distance d behind the vapour. A second beam of smaller width can be
injected into the cell at an arbitrary position in order to ignite or erase
localized structures.
Within a certain parameter range, we observe stable localized structures
with a CCD camera located behind the feedback mirror. It turns out that
the intensity distribution of the transmitted light is rather flat, whereas
the light that is reflected back into the cell has a pronounced maximum
surrounded by circular diffraction fringes.
If two or more LS exist simultaneously they are not isolated but interact
with each other. This interaction leads to stationary states which are
well defined by the distance between the LS.
Numerical and analytical calculations reveal that these localized structures
appear in a region where the homogeneous solution is nearly bistable. This
nascent bistability can be traced back to the nonlinear resonance caused
by a light-shift induced level crossing of the Zeeman ground states of
the sodium atom.
The existence of discrete stable distances between two localized structures
can be reproduced in numerical simulations. By increasing the pump rate
in the simulations, the system goes from the state of single localized
structures to a pattern that fills the whole integration area.
Publications
- Marc Feldmann
Eigenschaften und Wechselwirkung lokalisierter Strukturen in einem nichtlinearen optischen System.
Graduation Thesis, December 1999 (download pdf-version -german only-)
 List of publications
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