The assignments of fundamental mode frequencies in LiNbO3 determined by IR-and Raman scattering-experiments show some characteristic differences. Comparative frustrated total reflection (FTR) -and Raman scattering-studies were carried out in order to find the origin of these discrepancies. The most serious deviation was due to the fact that in one case the ordinary phonon is observed by Raman scattering whereas the extraordinary mode is recorded only by FTR. It has been shown, on the other hand, that the oblique phonon dispersion technique did not provide arguments for the correct assignments. The main limit of the method is due to the fact that the influence of exciton states on polaritons hitherto is taken into account only by a constant high frequency dielectric 'function' (ε∞). The uncertainty in the choice of a suitable ε∞-value allows fittings of quite diverging sets of experimentally determined fundamental frequencies.

No abstract

The theory of surface modes on general cuts of anisotropic crystals yields excitations of mixed polarization. This fact results in the existence of additional dispersion branches and pseudosurface modes. The dispersion of the pseudosurface phonon-polaritons may be determined by means of the minima of the calculated reflectivity response. The theoretical results have been examined by attenuated total reflection and show a very good agreement with the experiment. Die Theorie iiber Oberfllichenwellen an allgemeinen Kristallschnitten von anisotropen Kristallen fiihrt zu Moden mit gemischter Polarisation. Dieses Ergebnis hat zuslitzlicheDispersionszweige und Pseudo-Oberfllichenwellen zur Folge. Die Dispersion der Pseudo-Oberflachen-Phononpolaritonen kann durch die Minima in berechneten Reflexionsspektren bestimmt werden. Die theoretischen Ergebnisse wurden durch verhinderte Totalreflexion iiberpriift und zeigen sehr gute tfbereinstimmung n i t dem Experiment.

The dispersion of surface phonon polaritons is calculated taking into account the experimental set-up and complex dielectric functions. Under these conditions the results show the dispersion of surface waves in as well as outside of the frequency range of reststrahlenbands. Furthermore the dispersion branches exhibit backbending. The calculated curves are compared with reflection measurements at a-quartz and discussed in terms of virtual modes. The connection between frequency and angular scans is shown by three-dimensional representations of the reflectivity as a function of the angle of incidence and frequency.

When pure spatial damping is taken into account, i.e. when the wave vector is treated as complex and the frequency as a real quantity, dispersion curves of bulk phonon polaritons exhibit turnabouts caused by limiting momenta for the wave vectors which lead to branches within the frequency range of reststrahlen bands. The evaluation of the dispersion curves from reflection experiments for fixed frequency and swept angle of incidence confirms these features. Depending on the experimental arrangement dispersion curves inside of reststrahlen bands are investigated for a-quartz. The measurements turn out to be a sensitive aid for the determination of damping parameters.Wenn rein raumliche Danipfung in Rechnung gestellt wird. d. h. wenn man den Wellenvektor als komplexe und die Frequenz als reelle GroDe behandelt, weisen Dispersionskurven von Volumen-Phononpolaritonen Umkehrungen auf, die durch Grenzwerte fur die Wellenvektoren hervorgerufen werden und zu Zweigen innerhalb des Frequenzbereichs von Reststrahlenbanden fiihren. Die Ermittlung der Dispersionskurven aus Reflexionsexperimenten mit konstanter Frequenz und variiertem Einfallswinkel bestatigt diese Eigenschaften. I n Abhangigkeit von der experimentellen Anordnung werden Dispersionskurven innerhalb von Reststrahlenbanden an a-Quarz untersucht. Die Messungen erweisen sich als empfindliches Hilfsniittel bei der Ermittlung von Dampfungsparametern.