Properties of Extraordinary Polaritons in LiNbO3

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Abstract
On the basis of corrected assignments of the optical phonons in LiNbO3 the dispersion branches of extraordinary polaritons have been calculated for different wave vector directions with respect to the optic axis of the material. A fundamental feature of extraordinary polaritons is the lifting of crossing points between dispersion branches for wave vector directions different from the principal directions. A first experimental observation of this effect is presented and has been compared with theoretical data. Furthermore the experiments allow a simultaneous observation of the lowest frequency ordinary and extraordinary polariton branches.