<title>Excited-state absorption in the Cr<formula>^{<roman>6+</roman>}</formula>O<formula>^{<roman>2</roman>}</formula>- center in Li<formula><inf><roman>2</roman></inf></formula>B<formula><inf><roman>4</roman></inf></formula>O<formula><inf><roman>7</roman></inf></formula> glass</title>

Abstract

We present excited state absorption (ESA) measurements of the Cr:Li 2 B 4 0 7 glass (Cr:LBO-glass) along with preliminary interpretation. The most distinct feature in the ESA characteristics is connected with the presence of chromium in its hexe-(do) valence states. The Cr 6 ÷ ions appear to contribute in the deexcitation processes and their evidence can be seen in the ESA spectra under excitation wavelength at 308 rnm. We interpret the ESA spectra detected with UV excitation in terms of transitions in the framework of Cr+O" center which forms after charge-transfer -type absorption in [CrO 4 ] 2 -group. We assume the presence of the double-electron state of the 3d 2 2p 4 electronic configuration together with crystal field-split states of the 3d'2p 5 configuration, both allow to reproduce the obtained ESA spectra.Keywords: absorption (ESA), Cr:Li 2 B 4 0 7 glass 1. INTRODUCTION Lithium tetraborate (Li 2 9 4 O 7 ) is a well known material, which found numerous optoelectronic applications 1 3 but so far it has not yet been used as a primary laser host or gain medium. This is because of the tight packing of LBO single crystal lattice and the relatively large sizes of the dopant ions that are of interest, for example Cr+ ions. In glass matrix situation is different because the glasses have more loose and relaxed structures and are more "dopant friendly" allowing much higher concentrations doping than in crystals. The optical spectroscopy of the Cr? ions in the LBO glass has been described in detail in several papers 4-6. In this contribution, after making sure that Cr3+ in the LBO glass is rather poor candidate for lasing medium (because of the high strength of the ESA transitions) 7 , we focus our attention on other chromium ions: Cr