Left panel illustrates the (simplified) electron-hole relaxation dynamics in dielectric crystals (e.g., CdWO4 tungstate scintillator, the densities of states associated to the W and O sites are indicated [Y. Abraham et al., Phys. Rev. B 62, 1733 (2000)]), after excitation of valence band (VB) electrons into the conduction band (CB) by femtosecond XUV pulses. Electrons in the CB relax their energy on short time scale by e-e interaction (e.g., electron impact ionization) and e-phonon interaction. Excitons, i.e., energy correlated electron-hole pair, can get trapped in lattice distortions, forming the so-called Self-Trapped Excitons (STE) in the gap. STE usually decay radiatively on ns to µs timescale via luminescence in the visible. In the presence of XUV-induced high density of excitation, non radiative decay channels, such as STE-STE interaction, can compete with the radiative relaxation.
Right panel : the quenching of the visible luminescence as a function of the excitation density (XUV fluence) allows tracing the non radiative relaxation dynamics; the luminescence decay is well reproduced in the theoretical modelling of the STE-STE interaction (solid curves).
Collaborations : S. Guizard et al., CEA-IRAMIS-Laboratoire des Solides Irradiés (Palaiseau, France)
A. Belski, Laboratoire de Physico-Chimie des Matériaux Luminescents, Université Lyon 1, (Villeurbanne, France)
P. Martin et al., Centre d’Etudes des Lasers Intenses et Applications (Bordeaux, France)
M. Kirm et al., Institute of Physics, University of Tartu (Tartu, Estonia)
A. Vasil’ev, Department of Optics and Spectroscopy, Moscow Lomonosov University (Moscow, Russia)
L. Juha et al., Institute of Physics, Academy of Sciences of the Czek Republic (Prague, Czek Republic)
As compared with IR-visible laser excitation, intense XUV pulses induce specific excitation in solids. The main feature is that single-photon excitation is localized at the surface (absorption length ~ nm), where high density of electronic excitation can be obtained (~1020 /cm3). No further multi-photon heating of the electrons takes place so that the energy is deposited in a controlled manner, in space, time and amount. This type of excitation is particularly relevant for studying relaxation dynamics at femtosecond scale.
As an example, we have investigated the electronic relaxation after XUV excitation in dielectric tungstate crystals, e.g., CdWO4 and CaWO4, which are used as fast scintillators. Long term relaxation – at ns to µs scale – is monitored through the time-resolved luminescence of self-trapped excitons (STE). We evidence that non radiative decay channels can efficiently quench the excitons at short time (< ns) and high excitation density; they involve STE-STE (dipole-dipole) interaction [M. Kirm et al. Physica Status Solidi (c) 4, 870 (2007) ; M. De Grazia et al., Proc. SPIE Vol. 6586, 65860I (2007)].