Second-harmonic genention (SHG) at normal incidence in transmission geometry was studied in Bi-containing magnetic gamet thin films on gadolinium-gallium garnet (GGG) substrates oF(l1I) and (210) types. A Q-switched Nd-YAG h e r was used as a radiation source.The intensity of the SHG w35 lower than that in crjstalline quam but it was nevertheless reliably detectable. The study of the SHG intensity as a function of the rotation angle of the films around their normals showed that the signal is well described by phenomenoiog?cal expressions derived under the assumption that the point symmetry of the films is Cu, ((210) substrate) or C2, ((111) subsmte). The SHG intensity was found to be independent of temperature in the range 290-405 K. thus excluding a 'magnetic' origin of the SHG. These experiments as well as those previously published on the linear magnetoelectric effect prove that there is no inversion centre in magnetic gamet thin films

Magnetic-dipole two-photon absorption (MD-TPA) is introduced as a new spectroscopic technique for the study of odd-parity states. Contrary to the classical electric-dipole two-photon absorption (ED-TPA), where only spin-allowed even-parity transitions can be excited, MD-TPA allows us to excite spin-forbidden odd-parity states. This new technique is therefore very well suited to study paraexcitons in semiconductors and insulators. As examples MD-TPA measurements of RbI, NaI, and NaBr are presented.PACS numbers: 42.65.k, 71.35.+z Two-photon absorption (TPA) was first treated theoretically more than sixty years ago by Goppert-Mayer [1] using second-order perturbation theory. With respect to selection rules two-photon processes can be interpreted as two successive one-photon transitions. From Laporte's rule which states that one-photon dipole transitions are only allowed between states of diferent parity, one immediately derives that two-photon transitions are allowed only between states of the same parity. About thirty years ago Hopfield and Worlock [2] were the first to do two-photon spectroscopy on excitons. In agreement with the above-mentioned selection rule they found that in alkali halides two-photon transitions are indeed allowed to even-parity P excitons but strictly forbidden to oddparity S excitons.In order to excite odd-parity states by nonlinear spectroscopy one has to step onto three-photon spectroscopy (TPS) as was again demonstrated in alkali halides [3]. By the use of di8erent three-photon techniques one can observe the polariton dispersion and the longitudinal exciton with high accuracy [4].In this Letter we introduce a new spectroscopic technique, which allows us to excite odd-parity states by two-photon absorption. Contrary to classical two-photon absorption where both photons induce electric-dipole transitions (ED-TPA), we consider two-photon processes where one of the photons induces a magnetic-dipole transition and the other an electric-dipole transition.In the following we will briefly outline the theoretical background of magnetic-dipole two-photon absorption (MD-TPA).As in the case of ED-TPA [5] one can derive the twophoton transition probability Wg f for MD-TPA from second-order perturbation theory: -&(fl(MD)2]i)(il(ED) ilg) Ws f (x E, -Eghindi (fI (I&)i I ') (& I (s&)2 I g) ) E, -Eg -~g x 6(Ef -Egh(u)i + (d2)), where E~a nd Ef refer to the energy of the ground and final states, respectively. As intermediate states~i ) (energy E, ) one has to consider all states which are allowed for electric-dipole transitions from the ground state~g ).(ED)"and (MD) refer to the electricand magneticdipole operator, respectively. v = 1, 2 labels the incoming photons with energies hu and polarization directions e . In Eq.(1) we have assumed that the first transition (g~i) is of electric-dipole type, which is expected to be the case for many solids, where the lowest energy transitions (e.g. , 1S excitons) are dipole allowed.As first shown by Inoue and Toyozawa [5), the twophoton polarization selection rules c...

DiNerent nonlinear techniques are applied to investigate excitonic transitions from the upper valence bands (symmetry I-,, T9, r,) to the lowest conduction band (symmetry r7) in ZnO. Coherent techniques like three-photon difference frequency generation are used for resonances on the lower polariton branch, whereas incoherent techniques are well suited for the investigation on the upper polariton branch. Paraexcitons are detected without external perturbation. The angular dependence of the longitudinal transverse splitting is investigated by excitation of mixed-mode polaritons and analyzed with use of a suitable ansatz for the dielectric tensor in the classical wave equation. The isotropic point at 3.157 CV is studied by non-resonant second harmonic generation. Verschiedene nichtlineare Techniken werden angewandt, urn exzitonische Anregungen von den oberen Valenzbandern (Syrnmetrie r,, T9, r,) in das unterste Leitungsband (Symmetrie r,) in ZnO zu untersuchen. Koharente Techniken wie die Drei-Photonen-Differenzfrequenzerzeugung werden benutzt, urn Resonanzen auf dem unteren Polaritonast anzuregen, wahrend inkohirente Techniken fur Untersuchungen auf den oberen Polaritonasten geeignet sind. Paraexzitonen konnen ohne HuDere Storungen nachgewiesen wcrden. AuDerordentliche Polaritonen werden angeregt. Die Winkelabhlngigkeit der Longitudinal-Transversal LAufspaltung wird bestimmt und anhand eines geeigneten Ansatzes fur den dielektrischen Tensor beschrieben. Der isotrope Punkt bei 3,157 eV wird mit nichtresonanter Erzeugung der zweiten Harmonischen vermessen. ') Otto-Hahn-Str./PSF 500500, W-4600 Dortmund, Federal Republic of Germany.

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