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Inelastic neutron scattering measurements on single-domain orthorhombic KNbO3 at room temperature show no evidence of overdamped scattering, in contrast with previous neutron results in the cubic phase. As in the case of KTaO3, the observed diffuse x-ray planes are shown to correspond to a highly anisotropic transverse acoustic branch with an unusually flat dispersion in the 'soft' plane. In addition, a well defined optic branch with a polarization related to the orthorhombic-rhombohedral phase transition is seen to become soft as q to 0 in the soft plane. A mode-eigenvector determination confirms the x-ray result that essentially only one atom (niobium) moves in the flat transverse acoustic mode. The connection with other experimental results and with the independent chain model is discussed.

Phonon spectra for the principal directions of orthorhombic KNbO3 have been recorded by Raman scattering using right angle and back scattering techniques. Assignments of all A1 and A2 modes are given and those of B1 and B2 modes discussed. Polariton spectra concerning the A1 and B1 species recorded by near forward scattering made the assignment of longitudinal B1 modes possible which were not observed in the phonon spectra. The upper photon-like B1 polariton branch has been observed using a photographical arrangement. A comparison is made of the dielectric constants obtained from the phonon frequencies using the Lyddane-Sachs-Teller relation with data from the literature. Ramanspektren der Phononen in den Hauptrichtungen des orthorhombischen KNbO3 wurden in Rechtwinkel- und Ruckstreuanordnung registriert. Fur alle A1- und A2-Schwingungen werden Zuordnungen gegeben, die fur die B1- und B2-Moden werden diskutiert. Die Untersuchung von Polaritonenspektren der Typen A1 und B1 bei nahezu Vorwartsstreuung erlaubt die Zuordnung der longitudinalen B1,-Schwingungen, die in den Phononenspektren nicht beobachtet wurden. Der obere photonartige Polaritonast der Symmetrie B1 wurde mit einer photographischen Methode registriert. Die mit der Lyddane-Sachs-Teller- Relation aus den Phononenfrequenzen berechneten statisrhen Dielektrizitatskonstanten werden mit den Literaturwerten verglichen.

Experimental data on polaritons of type A1(TO), B1(TO), and B2(TO) in orthorhombic KNbO3 with wave vectors propagating in the xy-, yz- and xz-planes, respectively, have been compared with dispersion curves calculated on the basis of the phonon data. Data on mixed mode polaritons to type B1 + B2, A1, + B2, and A1 + B1 in the xy-, yz-and xz-planes are shown to fit numerically calculated corresponding dispersion curves. Finally all directional dispersion branches of polaritons in the phonon limit (k ∞) are calculated for the three symmetry planes. Experimental data on the 45°-phonons are presented in order to check the theoretical curves. Experimentelle Daten fur die A1(TO)-, B2(TO)- und B2(TO)-Polaritonen in KNbO3 werden mit Dispersionskurven verglichen, die aufgrund von Phononendaten berechnet wurden. Polaritonen gemischter Symmetrie vom Typ B1 + B2, A1 + B2 und A1 + B1 in der xy-, yz- bzw. xz-Ebene werden quantitativ in sehr guter Weise durch numerisch berechnete Dispersionszweige beschrieben. Schlieslich werden alle Richtungsdispersionszweige der Polaritonen im Phononengrenzfall (k ∞) fur die drei Symmetrieebenen berechnet. Experimentelle Daten fur die 45°-Phononen bestatigen die theoretischen Kurven.

In this survey we have tried to outline the aspects of current laser research and laser technology that are of relevance to heterodyne detection. We have seen that powerful, stable and narrowline sources are available to perform further heterodyne experiments at fixed frequencies. Taking a realistic view, one has to admit, however, that —at least in the far infrared — no really satisfyingtunable source is today available. In the near and the middle infrared, we are confident that further improvements of some of the devices discussed before will allow for true tunable heterodyne observations in the near future.

Thin-film metal-oxide-metal (MOM) diodes were fabricated by a positivephotoresist process using iron oxide and chromium masks. Reproducible junctions were obtained with areas of 2 μm2. The diodes exhibit stable, highly nonlinear I–V characteristics. Several hundred diodes were printed on the same substrate with nonlinearity parameters varying less than 15%. Infrared rectification was tested with a stabilized 10.6 μm CO2 laser.

The reflectivity of single domain crystals of LiNbO 3 and KNbO 3 has been measured using linearly polarized light up to 10 eV. The Kramers-Kronig analysis reveals a large anisotropy of the optical constants in the interband transition region. It is shown that the large electro-optic effects observed in these compounds are due to transitions in a relatively narrow spectral region above the band edge only.

Measurements of birefringence and refractive indices in KNbO3 were performed in the orthorhombic, tetragonal and cubic phases. KNbO3 is phase matchable and collinear phase matching is possible for a Nd-laser. Absorption measurements reveal an exponential law for the absorption coefficient (Urbach tail). The dispersion of the refractive indices is discussed in terms of a single-oscillator model. The parameters obtained are in good agreement with those of other perovskite materials. Relations for the electrooptic effect in an orthorhombic oxygen-octahedron ferroelectric are derived and it is found that this effect can explain the observed temperature dependence of refractive indices. Finally, the coefficients for the linear electrooptic effect are calculated and discussed.

Domain configurations in large KNbO3 single crystals grown by a top seeded flux growth technique are studied by optical methods and etching techniques. The orientation of observed domain walls is consistent with the principles of mechanical compatibility and electrical neutrality. Experimental evidence for the existence of a new type of ferroelectric domain wall (S-wall) is presented. Miller indices of such walls are irrational and depend on spontaneous strain tensor coefficients. The preparation of large single domain crystals (up to 1/3 cm3) is described. Assessment of crystal perfection by x-ray topographic and interference methods reveals a very small mosaic spread (one minute of arc) and a good optical homogeneity.

From the general polariton dispersion theory it can be shown that in an uniaxial crystal the frequencies of optical phonons * which are identical to those of the short wavelength polaritons depend on the angle Θ between the optic axis and the phonon wave vector. For Θ=0 and Θ=Π/2 the phonons are exactly transversal or longitudinal so that they can be assigned to be of totally symmetric or twofold degenerate type. Careful measurements of the directional dispersion of all phonon modes of LiNbO3 form a firm basis for a new complete assignment. 6 of the total number of 13 dispersion branches previously given in the literature had to be reassigned.

Raman scattering experiments in the orthorhombic phase of KNbO3 yield evidence of an anomalous spectrum which is compared to that obtained in the cubic phase.

In the orthorhombic phase of KnbO 3 an EPR spectrum of Fe 3+ substitutional for Nb 5+ has been observed with the main axis parallel to the pseudocubic one of the orthorhombic phase. The cubic parameter a is very close to the one in BaTiO 3 . A second spectrum probably due to local charge compensation is also reported.

Inelastic neutron scattering measurements on single-domain orthorhombic KNbO3 at room temperature show no evidence of overdamped scattering, in contrast with previous neutron results in the cubic phase. As in the case of KTaO3, the observed diffuse x-ray planes are shown to correspond to a highly anisotropic transverse acoustic branch with an unusually flat dispersion in the 'soft' plane. In addition, a well defined optic branch with a polarization related to the orthorhombic-rhombohedral phase transition is seen to become soft as q to 0 in the soft plane. A mode-eigenvector determination confirms the x-ray result that essentially only one atom (niobium) moves in the flat transverse acoustic mode. The connection with other experimental results and with the independent chain model is discussed.