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Lecture N. Naka


06 maart 2007 FWN-Building 5116.0116, Nijenborgh 4, 9747 AG, Groningen
Speaker:Dr. N. Naka
Affiliation:Department of Applied Physics, The University of Tokyo, Japan
Title:High-resolution spectroscopy of excitons in cuprous oxide under magnetic field
Date:Tue Mar 6, 2007
Location:FWN-Building 5116.0116
Host:Paul van Loosdrecht
Telephone:+31 50 363 8149


Cuprous oxide (Cu2O) is one of the best potential candidates to reach excitonic Bose-Einstein condensation (BEC). Extensive work has been done to obtain a cold and high-density of excitons in order to eventually realize BEC. However, excess energy shed by the laser light usually prevents excitons from relaxing down to the lattice temperature. To avoid this thermal effect, we use resonant excitation with single-frequency light from a ring laser cavity. The 1s excitonic state in Cu2O consists of orthoexcitons and paraexcitons, separated by 12 meV in energy. Since paraexcitons are a pure spin-triplet state in the absence of an external field, their direct optical transition is spin-forbidden. We applied a magnetic field of up to 10 T to make this transition allowed by mixing of orthoexcitons, which contain spin-singlet components. High-resolution spectroscopy revealed that the resonant width of the excitonic transitions is extremely narrow [1]. For paraexcitons, a resonant width of less than 0. ueV has been measured [2]. Owing to the flat dispersion of phonons in Cu2O, the spectral shape of a phonon-assisted recombination line reflects the thermal distribution of excitons in momentum space. By monitoring the linewidth in the cw excitation regime, we find that the paraexcitons reach equilibrium with the lattice even under high-density excitation. We also studied the dynamics of paraexcitons in a pulsed excitation regime. Transient effects slow down with decreasing excitation density. We obtained decay times of paraexcitons up to 1. usec. *Experiments were done at Dortmund University with support from the Japan-Germany Research Cooperative Program (JSPS). [1] G. Dasbach et al. Phys. Rev. Lett. 91 107401 (2003). [2] D. Froehlich et al. Solid State Commun. 134 139 (2005).
Laatst gewijzigd:22 oktober 2012 14:31