Periodic modulation of the refractive index by vortex-lattices

Dragomir Neshev
Laser Centre, Vrije Universiteit Amsterdam
De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands

Alexander Dreischuh and Sotir Chervenkov
Sofia University, Department of Quantum Electronics
5 James Bourchier Blvd., 1164 Sofia, Bulgaria

Gerhard G. Paulus and Herbert Walter
Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1,
D-85748 Garching, Germany

We demonstrate experimentally the generation of square and hexagonal lattices of optical vortices and reveal their propagation in a saturable nonlinear medium. If the topological charges (TCs) of the vortices are of the same sign the lattice exhibit rotation, while if alternative we observe stable propagation of the structures. In the nonlinear medium (NLM) the lattices induce a periodic modulation of the refractive index. We observed diffraction of a probe beam by the nonlinearity-induced periodic structure of the refractive index.

In Fig. 1 we show the experimental images for hexagonal honey-comb lattice with equal and opposite TCs and two different powers at the end of the nonlinear medium (ethylene-glycol dyed with DODCI). The images show clearly the rotation of the lattice with opposite TCs and the steady propagation of the other. Due to the self-defocusing the beam spreads out at higher power.

The observed periodic structure of the intensity distribution also induces periodic modulation of the refractive index of the medium. This modulation is sufficient to cause a perpendicularly propagating He-Ne laser beam to diffract. We belive that idea gives a new way for dynamic creation of two-dimensional photonic band-gap structures.

Experimental images of the vortex lattices at 10cm propagation in the NLM. (a) - hexagonal lattice with alternative charges for powers 10mW and 50mW. (b) - hexagonal lattice with equal charges for the same powers. The insets in each image represents the size and the orientation of the elementary cell of each lattice.