Spectroscopy of Chromium Doped Garnets in a Wide Temperature Range
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1053-EE04-01
Spectroscopy of Chromium Doped Garnets in a Wide Temperature Range Humeyra Orucu1,2, Gonul Ozen3, John Collins4, and Baldassare Di Bartolo1 1 Department of Physics, Boston College, Chestnut Hill, MA, 02467 2 Department of Physics, On leave from Ege University, Izmir, 35100, Turkey 3 Department of Physics, Istanbul Technical University, Istanbul, Turkey 4 Department of Physics, Wheaton College, Norton, MA, 02766 ABSTRACT Chromium doped garnets are tunable solid state laser materials which emit in the red and near infrared. We have studied the emission spectra of Y3Al5O12:Cr3+ (YAG:Cr), Gd3Ga5O12:Cr3+ (GGG:Cr) and Gd3Sc2Ga3O12:Cr3+ (GSGG:Cr) in a wide range of temperatures 25-800 K and have related their spectroscopic properties to the interaction between the 2E and 4 T2 energy levels of the chromium ion. The measurements included absorption, lifetime, luminescence under continuous and pulsed excitations, and thermal effects on sharp lines. INTRODUCTION Garnets have technological importance as materials for laser and optical spectroscopic applications [1, 2]. Garnet crystals are cubic and belong to the space group Ia3d (Oh10 in Schoenflies notation). Their stoichiometric formula is {C3}[A2](D3)O12, where C, A, and D denote different lattice sites with respect to their oxygen coordination (dodecahedral, octahedral, and tetrahedral respectively) and local symmetry. The relevance of this investigation is based on the information it may provide regarding the frequency distribution of the lattice vibrational modes and the dependence of the spectral output on the strength of the crystalline field. The absorption spectrum of the chromium doped garnets presents two prominent bands classified as 4T2 and 4T1 ~ 70 nm wide. The peaks of these bands are located at 600 and 440 nm for YAG:Cr; 634 and 432 nm for GGG:Cr; 641 and 470 nm for GSGG:Cr. An energy level scheme of the Cr3+ ion in solids is presented in figure 1. The luminescence R lines correspond to transitions from the two doublet sublevels of the metastable 2E level to the ground level 4A2.
Figure 1 Energy level diagram of the Cr3+ ion in solids.
The crystals considered were, in order of decreasing crystalline field, YAG (∆E (4T2 - 2E) = 1000 cm-1), GGG (∆E = 380 cm-1), and GSGG (∆E = 50 cm-1). Cr3+ goes into octahedral sites, which are trigonally distorted, presenting inversion symmetry for YAG and GGG but not for GSGG [3]. In YAG Cr3+ replaces Al3+, in GGG Cr3+ replaces Ga3+, and in GSGG Cr3+ replaces Sc3+. Electric dipole transitions within the d3 configuration are parity forbidden. However, when 3+ Cr is in a site with inversion symmetry, the coupling of Cr3+ levels to asymmetric vibrational modes of the crystal may allow electric dipole phonon-assisted transitions, even when the zerophonon transitions maintain their magnetic dipole character. EXPERIMENT Luminescence measurements were performed by using a 1.0 m McPherson Model 2051 monochromator. For continuous luminescence measurements, the source used was an Omnichrome model 532 Argon-ion Laser whic
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