Optical properties of the Bi+ center in KAlCl4
Introduction
Near infrared (NIR) luminescence of Bi-doped media was discovered in 2001 by Fujimoto et al. in silicate glasses [1], and optical amplification was demonstrated later in this material [2]. To date lasing and optical amplification have been demonstrated in ~1.1–1.6 μm range using Bi-doped media [3], [4], [5], [6]. Luminescence properties of various Bi-doped media: glasses [1], [7], [8], [9], [10], crystals [11], [12], [13], [14], [15], [16] as well as Bi-containing crystals [17], [18], [19], [20], [21] were investigated. Nevertheless the nature of Bi-related NIR luminescence is still a subject of intense debates. A lot of various Bi-centers that could be responsible for the observed NIR luminescence were suggested: BiO, Bi+, Bi24+, Bi53+, Bi82+, Bi22−, Bi2−, Bi4, Bi-related defects etc. Often several luminescent species are observed simultaneously in a Bi-doped media [8], [10], [22], [23], [24], [25]. To date only a few Bi-luminescent centers (e.g. Bi+ [11], [26], [27], [28], Bi53+ [17], [18], [19], [20] and Bi82+ [19]) were obtained solely.
Recently it was shown that Bi-doped KAlCl4 possess intense NIR-luminescence near 1 μm [27]. By analysis of spectroscopic data this band was assigned to Bi+ ion that isomorphically substitutes for K+ in this crystal. Probably the luminescence of Bi+ was observed in other crystalline hosts also [11], [12], [14], [16]. The Bi-doped RbPb2Cl5 [11], KMgCl3 [26], CsCdCl3 [28] and several other chlorides [29] are the most prominent and reliable examples in this case. Frequently the NIR luminescence observed in Bi-doped glass media is ascribed to Bi+ ion also [24], [30], [31]. Quite reliably it was demonstrated for ZnCl2/AlCl3/BiCl3 glass [24].
In most cases the observed optical properties and NIR luminescence of Bi+ ascribed to intraconfigurational transitions of an open external 6p2 shell of the monocation, implying strong dependence of its optical properties on a host material. Nevertheless up to now there is no detailed investigation on the Bi+ optical center features and, in particular, its vibronic properties. The investigations in terms of electron–phonon interactions are often performed for optically active centers having rather strong coupling: various impurity centers (Ce3+ [32], Eu2+ [33], Cr3+ [34], [35], Tl+ [36] etc.) as well as color centers [37], [38]. On the one hand it allows to refine features of a center in a specific host medium, on the other hand the previously obtained data on the coupling could be used as auxiliary information for identification of optical centers.
The results of further investigation of the Bi+ center optical properties in KAlCl4 crystal are reported in this paper. The temperature dependences of shapes, widths, positions, and intensities of emission and excitation bands as well as luminescence decay law over the temperature range of 77-410 K are investigated. The obtained experimental data are considered in terms of electron–phonon interaction of the optical center with the host crystal. Estimated parameters of the interaction give the additional evidence that namely Bi+ impurity but not a defect center is responsible for the luminescence.
Section snippets
Experimental
The same as before [27] Bi-doped KAlCl4 polycrystalline sample is used in present investigation. The details of the sample preparation as well as experimental setup description could be found in our previous works [24], [27]. In brief, the Bi-doped KAlCl4 specimens were synthesized by melt crystallization. To be sure that bismuth monocation is formed in melt, AlCl3 was taken in excess and the controlled reduction of BiCl3 by zinc metal was performed. The experimental setup has not been changed,
Luminescence. Experimental results
The luminescence spectrum of Bi+ in KAlCl4 crystal strongly depends on the temperature (Fig. 1). However the luminescence peak shape is not only determined by usual changes like thermal broadening, peak shift etc. with change of temperature only, but also by the structural phase transitions of the host. It is known, that KAlCl4 exists in three polymorphic modifications: I – low temperature (the space group is unknown), II – room temperature (P212121 space group) and III – high temperature (Pnma
Discussion
Further investigation of the Bi+ optical center in KAlCl4 demonstrates that abrupt changes of the main luminescent properties occur during two phase transitions of the crystal. In the case of the luminescence lifetime both of these changes could be observed directly in experiment whereas the additional analysis is needed to detect the luminescence band parameters leaps in the case of II↔III phase transition.
The existence of the phase transitions in KAlCl4 host requires to consider each state of
Summary
The optical properties of the Bi+ substitutional center in KAlCl4 crystal are studied in details. The strong dependence of the Bi+ center features on its local surrounding is demonstrated experimentally. The two independent but closely spaced energy states of the ion are responsible for the observed emission near 1 μm. The observed temperature dependences of the emission lifetime and the band shape are determined mainly by thermalization process, which takes place between these levels. The
Acknowledgments
The reported work was supported by the Russian Foundation for Basic Research (RFBR, Research Project number 13-03-00777).
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