Three-Photon Magnetoabsorption of Excitons in Alkali Halides

Abstract

Three-photon magnetoabsorption measurements in fields up to 9 T are presented for the first time. A narrow exciton line of Ff symmetry is resolved in Csl at 5.8076 eV. A Rvalue of 1.17 is deduced from the Zeeman splitting of this exciton. At high magnetic fields an exciton of Fj" symmetry is detected. The analysis of the field-dependent mixing of the Ff" and Ff excitons yields a zero-field shift of the Ty exciton to lower energies of 0.54 meV and an effective g value for the admixture of 1.20. 42.65.Ft, 71.70.Ej Three-photon spectroscopy was recently introduced by Beerwerth and Frohlich^ as a powerful technique to study exciton polaritons in crystals with inversion symmetry. The authors measured resonances on the upper polariton branch and even the longitudinal exciton of the one-photon-allowed exciton of Ff symmetry in several alkali halides. Besides this one-and three-photon-allowed F^ exciton there are excitons of Ff, Ff, TJ, and Ff symmetry, which are only allowed in three-photon absorption, as can be easily derived with the use of the tables of Koster et al? In this Letter we present the first demonstration of such a one-photon-forbidden transition. Again the alkali halides are chosen, since they show strong odd-parity exciton transitions. Among the alkali halides the cesium halides have attracted much interest recently, since application of hydrostatic pressure causes a drastic red shift of the band gap.^ There are even predictions of metallization of these ionic crystals at superhigh pressures.-^' "^ Kuznetsov et al? showed that in Csl, excitons derived from the lowest ^conduction band (Fg" symmetry) shift to lower energy by 0.25 eV under hydrostatic pressure of 120 MPa. We were able to resolve a new, very narrow exciton line of Ff symmetry in Csl, which is assigned to the same t/conduction band. The width of this line is only 0.2 meV at 4.2 K. This small linewidth makes possible, for the first time, the direct observation of the Zeeman splitting of an exciton transition in alkali halides. In onephoton absorption, ^values of the F^ exciton can only be determined by magnetoreflectance^ or magnetoabsorption dichroism,^ because the linewidth of about 30-50 meV is much larger than the expected Zeeman splitting (about 1 meV at 9 T).For our measurements we used ultrapure single crystals of Csl which were cut for measurements in three orientations ([001], [110], and [111]). The crystals were oriented by use of stress-induced birefringence as described by Maier.^ The magnetic fields for the measurements in the Faraday configuration were produced by a superconducting magnet at 4.2 K; fields up to 9 T were available. As a light source we used a frequency-doubled Nd-doped yttriumaluminum-garnet laser (Quanta Ray model DCR-2 A) to pump a tunable dye laser (Lambda Physik model FL 2002). Three-photon absorption was detected via the luminescence of the self-trapped exciton at 290 nm.In Fig. 1 we present a three-photon spectrum of Csl and the one-photon data of Teegarden and Baldini.^ The three rather broad lines corr...

Linewidth Collapse in Three-Photon Exciton-Polariton Spectra of CsI under Pressure

“…The first measurement confirming the prediction of such a state in CsI was made by Beerwerth and Fröhlich [25] using TPS. In their work, the paraexciton state was assigned tentatively as G 2 5 belonging to the d-like conduction band of the Cs ion.…”

Magnetic Field Splitting and Dispersion of the Lower 1S‐Exciton Polariton in KI and RbI

“…The linewidth of the resonances of r, symmetry (upper polariton branch and longitudinal exciton) ranged from 10 to 40 meV. In addition to the r; transitions, which are one-photon allowed, a very narrow line of r; symmetry was excited by TPS in CsI [5]. In high magnetic fields transitions to paraexcitons were seen in KI [6] and RbI [4].…”

Band-structure parameters and Fermi resonances of exciton-polaritons in CsI and CsBr under hydrostatic pressure

“…An alternative to the OP experiments on the alkali halides mentioned above offers the technique of three-photon excitation spectroscopy (TPS) [20]. Not only can this technique access the same excitonic states as OPS but additionally can probe states of different symmetry.…”