Temperature dependence and quantum efficiency of ultrabroad NIR photoluminescence from Ni^2+centers in nanocrystalline Ba-Al titanate glass ceramics

Abstract

Ultrabroad near-infrared (NIR) photoluminescence from Ni 2 -centers in nanocrystalline Ba-Al titanate glass ceramics was studied by temperature-dependent static and dynamic photoluminescence spectroscopy in the regime of 10 to 300 K. Photoluminescence covers the spectral range of about 1100 nm to >1600 nm with a typical bandwidth (FWHM) greater than 300 nm. For UV-LED excitation at 352 nm, an internal quantum efficiency of 65% is obtained. The excited state lifetime τ at room temperature is 39 μs. The stimulated emission cross section σ em is 8.5 × 10 −20 cm 2 , resulting in a practical figure of merit, σ em τ, of 3.3 × 10 −24 cm 2 s at room temperature. These properties suggest suitability as a broadband gain medium for tunable lasers and optical amplifiers. © 2012 Optical Society of America OCIS codes: 160.2540, 160.2750 3d transition metal ions such as Ni 2 , Co 2 , and Cr 4 doped into inorganic matrices have been a subject of interest for many years due to their broadband nearinfrared (NIR) photoluminescence (PL). In such materials, NIR PL arises from d-d transitions and is hence strongly dependent on ligand field and coordination [1][2][3][4][5]. While various efforts have been undertaken to make use of this property in next-generation broadband optical amplifiers for telecommunication and other applications, especially with Co 2 and Cr 4 , any breakthrough has so far been prevented by difficulties in stabilizing the specific valence and coordination state in a suitable matrix material [1,2]. From a practical point of view, Ni 2 species seem the most promising choice to approach this problem. In an inorganic matrix, they may be present in three different coordination states, tetrahedral (fourfold, VI Ni 2 ), trigonal (fivefold, V Ni 2 ) and octahedral (sixfold, VI Ni 2 ). Only VI Ni 2 has been known to provide efficient NIR emission [3][4][5]. Consequently, VI Ni 2 -containing glass ceramics and single crystalline materials have drawn continuous attention over the last decade [3][4][5]. Of these two materials classes, glass ceramics, produced by controlled nucleation and crystallization of a suitable precursor glass, combine the advantages of glasses and crystalline materials: depending on viscosity and crystallization temperature of the precursor glass, they can be processed into optical fiber, and depending on the type of precipitated crystal species, high quantum efficiency (QE) can be obtained [6][7][8]. As a prerequisite, however, crystal precipitation must occur in sufficiently high number density to ensure very low crystallite size and, hence, high optical transparency. In this setting, the number of available matrix candidates has been limited to only a few systems, which typically rely on rare raw materials or exhibit major process limitations (such as high liquidus temperature and high dynamic fragility).Recently, we reported on glasses of the type 30TiO 2 -30BaO-30SiO 2 -10Al 2 O 3 (TBSA, mol%) as a new matrix candidate [9]. In this system, nanocrystalline hollandite-type (BaAl 2 Ti 6 O 16 , sec...

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