Formation of luminescence centers in Cr/sup 3+/, Li: Mg/sub 2/SiO/sub 4 crystal for one-micron tunable lasing

Abstract

For the first time, the key role of lithium in the formation of one: micron luminescence centers by Clz' ions in forsterite single crystals has been discovered. This effect can be used for creation of C?forsterite laser. All the investigated crystals of high optical quality were grown by the Czochralski technique in a neutral growth atmosphere (100% AI). The chromium content in the initial charge was 0.06 wt.% for all the crystals. The lithium content in the initial melt was varied in the range from 0 to 0.3 wt.%. The co-doping by lithium ions of Cr3+:Mg2Si04 single crystals leads to drastic changes of the absorption and luminescence spectra (Fig.1, Phnm notation, the lithium content in the initial charges was 0, 0.02, 0.03, 0.3wt.% for curves 1-4, respectively). Figure shows the spectra of polarized luminescence excited by a He-Ne laser (Lx -633 nm) along c and a axes in the same scale of intensity. The most substantial changes in the shape of both luminescence and ahsorption spectra take place in Ellc polarization. The broadband luminescence in this range is usually attributed to Cr3'-ions in the M2-sites of the forsterite structure with mirror symmetry, Cr(M2). An increase of the lithium content in the crystals results in a gradual shift of the position of the peak of the broadband luminescence (fig. Ib) from -890 nm (L, band) toward -950nm (Lz hand). Simultaneously the half-width of the hand increases from AX -200 nm to A2 -240 nm. The new Lz-band is polarized along c-axis much more than the initial L,-band of Cr(M2) ions at h,,, -890 nm 111. From Ellh polarization was observed that Fig. 1. Absorption and luminescence spectra. transfonhagon of lumi'nescence band -from L, to L? is accompanied by decreasing of !he number of the initial luminescence centers of Cr3+(M2) with peak at Lm, -860 nm [I]. The presence of lithium in the crystals results in the formation of an additional luminescence band L, (Fig.1~) centered at &,,-750 nm (AX-150 nm) and mainly polarized along a-axis. It is possible that the L3-hand belongs to Cr-ions in a stronger crystal field and corresponds to the new absorption hand at -420 tun which appears in all polarizations. The Cr ions are responsible for the unwanted absorption in the lasing range. Their content decreased with increasing the lithium content in the crystals as also reported earlier in [2]. Fig.1 shows that the dependence of the intensities of both absorption and luminescence bands on the lithium content in the crystals, which reflects the changes of the concentration of the ,luminescence centers in the samples is nonmonotonic and has a minimum at some non-zero lithium content CLim'-0.03 wt.%. On reaching the CL~ lithium concentration in the crystal, the changes of the shape ofthe luminescence and absorption spectra are completed. If the lithium concentration in the crystal is greater than CLim', the intensity of the luminescence of the Lj-band increases up to the value, which is by a factor of 1.3 greater than that in an undoped crystal. We think that the minimum on the dependence of the Ll/Ll luminescence intensity upon the lithium content results from gradual decomposition of chromium-vacancy optical centers and simultaneous formation of Cr-Li associates along with an increase of the lithium concentration in the crystals. In conclusion, note thavthe red shift of the Cr3+ luminescence peak toward the one-micron spectral range and simultaneous reduction of the Ci' ions content are promising for achieving lasing in lithium co-doped Cr-forsterite crystals.