Regensburg 2013 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 50: Exciton Polaritons and their Condensates (jointly with HL)

TT 50.3: Talk

Thursday, March 14, 2013, 10:00–10:15, H2

Coherent Propagation of blue Polaritons in Cu₂O — •Johannes Schmutzler, Dietmar Fröhlich, and Manfred Bayer — Experimentelle Physik 2, Technische Universität Dortmund, D-44221 Dortmund, Germany

The lowest excitons in Cu₂O are intensively studied as they are considered as candidates for Bose-Einstein Condensation. The excitons of the so-called yellow and green series are of even parity and thus only quadrupole allowed. There are, however, two other series of excitons (blue and violet series) which are dipole allowed and should thus exhibit a pronounced polariton structure.

Despite the fact that Cu₂O has inversion symmetry we report sum-frequency generation of blue exciton polaritons. This is possible since in our case the SFG is of quadrupole-dipole type. Typically the observation of non-linear effects requires power levels in the kW to MW range, necessitating pulsed laser sources. Here, we achieve strong sum-frequency (SF) signals far above the bandgap by spectrally narrow excitation of two exciton-polariton resonances in Cu₂O with two continuous wave lasers in the mW to W power range.

In the case of an antiparallel laser beam configuration pronounced oscillations of the SF-signal can be observed. We attribute these oscillations to a phase matching effect. This observation is rather surprising since the absorption length in one-photon absorption is in the range of 150 nm [1]. The occurrence of oscillations clearly indicates the creation of coherent exciton-polaritons, the damping of which is suppressed.

[1] S. Brahms et al., Physics Letters. 21, 31 (1966)