Luminescence Anisotropy and Thermal Effect of Magnetic and Electric

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Luminescence anisotropy and thermal effect of magnetic- and electric-dipole transitions of Cr3+ ions in Yb:YAG transparent ceramic Fei Tang, Honggang Ye, Zhicheng Su, Yitian Bao, Wang Guo, and Shijie Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14061 • Publication Date (Web): 23 Nov 2017 Downloaded from http://pubs.acs.org on November 24, 2017

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ACS Applied Materials & Interfaces

Luminescence anisotropy and thermal effect of magnetic- and electric-dipole transitions of Cr3+ ions in Yb:YAG transparent ceramic Fei Tang,† Honggang Ye,† Zhicheng Su,† Yitian Bao, † Wang Guo,‡ and Shijie Xu†* †

Department of Physics, and Shenzhen Institute of Research and Innovation (HKU-SIRI), The

University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China. ‡

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou

350002, P. R. China. KEYWORDS: Trivalent Cr ion activated transparent ceramic, magnetic/electronic-dipole transitions, ion-phonon coupling, luminescence, thermal effect.

ABSTRACT: In this article, we present an in-depth optical study on luminescence spectral features and thermal effect of the magnetic-dipole (MD) transitions (e.g. the R lines of 2E → 4A2) and the associated electric-dipole (ED) transitions (e.g. phonon induced sidebands of the R lines) of Cr3+ ions in ytterbium-yttrium aluminum garnet (Yb:YAG) polycrystalline transparent ceramic. The doubly split R lines predominately due to the doublet splitting of 2E level of Cr3+ ion in octahedral crystal field are found to show a very large anisotropy in both emission intensity and thermal broadening. The large departure from the intensity equality between them could be interpreted in terms of large difference in coupling strength with phonons for the doubly

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split states of 2E level. For the large anisotropy in thermal broadening, very different effective Debye temperatures for the two spilt states may be responsible for it. Besides the 2E excited state, the higher excited states, e.g. 4T1 and 4T2 of Cr3+ ion, also exhibit a very large inequality in coupling strength with phonons at room temperature. By examining the Stokes phonon sidebands of the MD R lines at low temperatures with the existing ion-phonon coupling theory, we reveal that they indeed carry fundamental information of phonons. For example, their broad background primarily reflects Debye density of states of acoustic phonons. These new results significantly enrich our existing understanding on interesting but challenging luminescence mechanisms of ion-phonon coupling systems.

1. Introduction Inspired by the successful applications of yttrium aluminum garnet (YAG)-based single crystals doped with various active ions in lasers,1-3 their polycrystalline counterparts, namely transparent ceramics (TCs), are alternatively extensively being investigated with the rapid development of high-temperature sintering technology.2-9 This kind of novel material possesses a full-dense microstructure with numerous grains bonded together via grain boundaries.10 Such unique microstructures endow it not only outstanding optical and thermal properties, but also better mechanical properties with respect to single crystals.8 Like single crystals, TCs formed via solid-state reaction sintering process can also be optically activated by doping various active ions, such as rare-earth (RE) and transition-metal ions. Currently, RE-doped YAG single crystals and TCs are the most extensively studied and widely used for high power lasers.9 Nd:YAG and Yb:YAG lasers are the two application examples of RE-doped YAG single crystals and TCs. Unlike RE ions having a stable 4f electronic


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configuration, transition metal ions often show changeable luminescent behaviors in insulating crystals and TCs, depending sturdily on the surrounding lattice environment.11-16 For example, transition-metal Cr3+ ion with a 3d3 electronic configuration,16 which has been widely recognized as one of most significant active ions since 1960s when inventors developed the first solid-state ruby laser,17 may show a +4 chemical state and thus exhibits a 3d2 configuration in YAG crystal.18,19 However, it has been shown that Cr3+ rather than Cr4+ acts as an efficient luminescent center in some host lattices such as ZnAl2O4, MgAl2O4 and MgO crystals.20,21 On the other hand, the coupling of transition-metal ions with lattice vibrations (phonons) in these crystals is relatively stronger than that of RE ions in the same crystals.22 In general, stronger impurityphonon coupling results in more noticeable phonon sidebands in luminescence and absorption spectra. Very recently, there has been a renewed interest in the control and manipulation of magnetic dipole (MD) transitions of transition metal ions in various crystal fields.16,23 For transition metal ions such as Cr ions in crystals, they often simultaneously show a strong MD transition and broadened electric-dipole (ED) transitions.24-26 The former transition leads to socalled zero-phonon line (ZPL), whereas the latter ED transitions contribute to the broadened phonon sidebands.25,26 Clearly, investigating the MD and ED transitions of Cr3+ ions in YAG TC materials is of both fundamental and technological significance. In this article, we present a comprehensive study of luminescent properties of Cr3+ ions in Yb:YAG TC which was prepared with a high-temperature vacuum sintering method. It is found that the double sharp MD lines of Cr3+ ions exhibit a large anisotropy in both luminescence intensity and thermal broadening. Our study reveals that the large fluorescence anisotropy may stems from very dissimilar coupling strengths of the relevant impurity levels with host phonons. 2. Experimental Section


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2.1 Material preparation As mentioned earlier, Cr3+-activated Yb:YAG transparent ceramic was prepared through a solidstate reaction process by means of high-temperature vacuum sintering technology. Commercial available powders of Al2O3, Y2O3, Yb2O3 and Cr2O3 were chosen as reaction precursors. Tetraethyl orthosilicate (TEOS) was used as the sintering additive, when oleic acid (OA) served as the dispersion agent. Based on the formula of (Y0.99Yb0.01)3(Al0.997Cr0.003)5O12, stoichiometric amounts of the precursors were thoroughly mixed in ethanol for 20 h in planetary-milling machine. The obtained slurry was dried, then ground and sieved through 200-mesh screen. Ceramic green body was formed via cold-isostatically pressing the powder into 15 mm disk at ~2 MPa, and followed by de-binding process at 800 oC. Then, high-temperature sintering process was carried out under a vacuum of 5×10-7 Torr at 1750 oC for 10 h. Finally, transparent ceramic was obtained after mirror-polishing on both surfaces. There is no need of additional thermal annealing on the sintered ceramics. 2.2 Structural characterization Crystalline structure of the ceramic was determined via X-ray powder diffraction (XRD, Type D8 Advance ECO, Bruker, UK) with Cu Kα radiation (λ=1.54 Å) as the radiation source. The current and cathode voltage of the radiation source were set to be 40 mA and 40 kV, respectively. Coupled 2θ/θ was selected as the scan type with continuous power spectral density fast as scan mode. Measured scanning range in this study was set from 2θ=10o to 120o with an increment of 0.02o/step and delay time of 1s. Obtained XRD data of the as-sintered transparent ceramic were utilized as an initial base for the Rietveld refinement using the GSAS package. Morphology of the sample was observed with a field emission scanning electron microscope (JOEL JSM6700-F).


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2.3 Variable-temperature PL measurements High-resolution PL spectra of the ceramic were measured at temperatures ranging from 4 K to 300 K by means of a home-assembled variable-temperature laser spectroscopy system, in which a 405 nm laser light was employed as the excitation source. Variable-temperature PL measurements were realized through mounting the ceramic sample on the cold finger of a closed cycle cryogen-free refrigerator (Janis Research), and a temperature controller (Lakeshore 325) was employed to control the sample temperature. Light signal emitted from the sample was collected by a pair of lenses, and then guided into a 1200 g/mm grating monochromator (Spex 750M) with a photomultiplier detector (Hamamatsu R928) for dispersion and detection. In order to improve the signal to noise ratio, a standard optical chopper plus lock-in amplifier was adopted. The major parts of the PL system were controlled with a desktop computer. 3. Results and discussion Figure 1 shows a scanning electron microscope (SEM) image of the surface-polished Cr3+activated Yb:YAG TC sample. Full-dense microstructures without any pores or other second phase can be clearly seen from the image, which indicates good quality of the prepared transparent ceramic. Figure 2(a) presents a photograph of the TC sample taken under the conditions of natural light. Obviously, the prepared ceramic exhibits light-green color under the illumination of natural light. However, it emits intense red light when it was illuminated with a 405 nm laser beam, as illustrated by the photograph in Figure 2(b). Note that strong white background in the photograph is due to the mixture of emitted red light and scattered blue laser by the TC sample. As reported and argued in literature, the intense red light emission can be ascribed to the R lines of 2E→ 4A2 of Cr3+ ion in YAG host crystal.


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Figure 1. SEM image of the surface-polished TC sample prepared with a high-vacuum hightemperature sintering technology. It has been well argued from crystal-chemistry considerations that in garnet crystals including YAG crystal, Cr3+ ions preferentially occupy the octahedral 16(a) sites rather than the tetrahedral 24(d) sites.27,28 To determine the major occupation state of Cr3+ ions in the studied YAG TC sample, a theoretical calculation was performed via applying Rietveld refinement method on the experimental XRD data of the TC sample. As seen in Figure 2(c), theoretical results are in excellent agreement with the measured XRD patterns. Crystalline structure of the TC sample was thus determined and can be uniquely characterized in a cubic space group of Oh10-Ia3d. Determined unit-cell parameters were tabulated in Table 1. It can be concluded that incorporating Yb ions and Cr ions into YAG lattice results in a slightly enlarged unit cell. And some Al3+ ions surrounded by six nearest O2- ions are most likely replaced by Cr3+ ions.


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Figure 2. Photographs of the as-prepared transparent ceramic under the illumination of natural light (a) and a 405 nm laser light (b). Measured and simulated XRD patterns of the sample (c). The inset shows crystal structure of the sample. Table 1. Unit cell parameters obtained from Rietveld structural refinement of XRD data. Samples YAG29 Yb:YAG Cr,Yb:YAG

a=b=c (Å) 12.01159±0.000034 12.015681±0.000775 12.019558±0.000031

α=β=γ (ο) 90 90 90

V (Å3) 1733.01 1734.78 1736.46

Herein, YAG denotes Y3Al5O12.

For Cr3+ ion occupying Al3+-octahedral center site in YAG host crystal, dependence of its energy levels on crystal field strength may be illustrated by using Tanabe-Sugano energy diagram.30 In the crystal-field approximation, the effect of the electric field produced by


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surrounding ions on the energies of the impurity levels may be calculated by use of four parameters: spin-orbit coupling ζ, crystal-field Dq, and two electrostatic interaction parameters, B and C defined by Racah in terms of the Slater integrals.31 Crystal-field parameters for Cr3+ in YAG host crystal adopted by Wood et al.20 were relisted here: ζ = 170 cm-1, Dq =1640 cm-1, B = 650 cm-1, and C = 3250 cm-1. Note that the spin-orbit coupling in the Cr3+ ion is relatively much smaller, and thus it contributes only in a very minor way to the energy level locations of Cr3+ ion. In fact, the Tanabe-Sugano energy matrices of Cr3+ ion are valid provided the spin-orbit interaction is neglected entirely. According to the calculations of Tanabe and Sugano, the 2E level of Cr3+ ion located approximately 9B+3C above the ground state 4A2. The separation between the ground level 4A2 and the first quartet state 4T2 was just 10 Dq, whereas the second quartet 4T1 was higher by ~12B. Figure 3(a) shows the calculated splittings of 2E and 4A2 levels of Cr3+ ion under the combined action of crystal field and spin-orbit coupling.32 From a theoretical view of point, four possible transitions may occur. However, only double R lines were resolved in the PL spectrum, as shown in Figure 3(c). We thus sketched two downward arrows in Figure 3(a) to show R1 and R2 transitions. At 300 K the PL spectrum are dominated by R1 and R2 lines located at 688.75 nm and 687.79 nm, respectively. At 4 K, the R1 and R2 lines peak at 687.31 and 686.38 nm, respectively. The R line splitting still remains as 20 cm-1. These data are in good agreement with ones reported previously.28,33 The experimental R line splitting is smaller by about 9 cm-1 than the theoretical splitting of 2E level of Cr3+ in cubic crystal field. The discrepancy has not yet well understood by now. The complicated crystal structure of YAG and inaccurately known local lattice distortion may be important factors for the obvious discrepancy,20,27 although from a macroscopic viewpoint the overall symmetry is that of


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a cubic nature.34 Nevertheless, it is still concluded that the observed double R lines are predominantly due to the doublet splitting of 2E level under the action of crystal field. In addition to the 2E excited state, there are another two higher excited states, namely 4T2 and 4

T1, for Cr3+. Evidently, an order from a smaller displacement difference is obtained for 2E, 4T2,

and 4T1 with respect to the ground state 4A2, as depicted in the configurational coordinate diagram in Figure 3(b). The parabolas in the figure represent the potential profiles of Cr3+ electronic orbits coupled with a host lattice vibration (phonon). The difference between the equilibrium positions of parabolas may be uniquely characterized by Huang-Rhys factor.35 Larger equilibrium-position difference implies greater lattice relaxation during relevant vibronic transition and larger Huang-Rhys factor which is the most important parameter in theoretical calculations of absorption and emission of electron-phonon coupling system.36-38 For the 4T2, and 4

T1 states, two major photoluminescence excitation (PLE) bands due to the optical transitions

from 4A2 to 4T1 and 4T2, respectively, were observed at room temperature, as shown in Figure 3(c). The monitored wavelength was set at the R1 line for the PLE measurements. A generalized multimode Brownian oscillator (MBO) model, which is developed to calculate nonlinear optical responses of a two electronic-level system with some primary nuclear coordinates coupled linearly to it and to a harmonic phonon bath,39,40 was adopted to compute the two broad PLE bands following the pioneering application of MBO model in the calculation of luminescence spectrum of electron-phonon coupling system in solid.41 The calculated curves with MBO model are illustrated by green solid lines in Figure 3(c). Obviously, they are in good agreement with the experimental spectra. The parameters used in the calculations are EZPL = 1.8931( 2.3846 ) eV,

S = 3.1( 7.6 ) , γ =140 ( 50 ) cm-1 , hωP = 547 cm −1 , and T = 300 K for the PLE band located at


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lower (higher) energy side. Here EZPL represents so-called ZPL location, S the Huang-Rhys factor, γ the coupling constant between the primary phonon mode and the continuous phonon bath, and hωP = 547 cm −1 the characteristic energy of primary phonon mode.42 Interestingly, good agreement between theory and experiment reveals that there is a much larger Huang-Rhys factor for the absorptive transition from 4A2 to 4T1 with respect to the transition from 4A2 to 4T2. As argued earlier, a relatively larger lattice relaxation may occur upon the optical transition from 4

A2 to 4T1. It is thus expected that relatively much larger Huang-Rhys factor is attached to the

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A2 → 4T1 transition because of S ∝ ∆ 2 in the strong electron-phonon coupling cases.43

Figure 3. (a) Energy level splitting diagram induced by the combined effect of both distorted crystal field and spin-orbit coupling. Two downward arrows are sketched to point out R1 and R2


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lines. (b) Configurational coordinate diagram for Cr3+ ions in YAG host and possible optical transitions. (c) Room-temperature PL and PLE spectra of Cr3+-activated Yb:YAG transparent ceramic. The inset shows the ZPL spectrum enlarged in the wavelength range of 686 nm-692 nm where the R1 and R2 lines can be well resolved. Besides the sharp R lines and the two broad PLE bands, actually, both Stokes and anti-Stokes phonon sidebands were also observed in the long- and short-wavelength sides of the split R lines at room temperature, as shown in Figure 3(c). These sidebands of the R lines are outcomes resulting from interactions between Cr3+ ion and surrounding phonons. They carry important information of lattice vibrations, especially low-frequency phonon modes. Figure 4 shows the R lines and the phonon sidebands at 4 K. The inset shows the overall spectrum of the R lines and the phonon sidebands at 4 K, while the main figure depicts a closer spectrum of the phonon sidebands. Being consistent with the spectral data previously reported by Wall et al.,33 at 4 K only highly structured Stokes phonon sidebands due to the simultaneous phonon emission were observed. Only when the temperature is beyond 100 K, the anti-Stokes sidebands become observable due to the thermal excitation and simultaneous absorption of phonons. It has been well established that being as a paramagnetic impurity, Cr3+ ions in various crystals including the studied YAG crystal often show a sharp MD ZPL luminescence and broadened ED phononassisted luminescence.24-26 For the phonon sidebands, they have been extensively theoretically treated in terms of lattice relaxations.35-38, 43-46 For the ith level of an impurity ion in host crystal, the phonon relaxation probability due to one phonon, or “direct” processes may be written as46

Wi d =

π  ∑ ωij ρ (ωij ) j C i M υ 2 h  j i

2

 N (ωij )  , 

(1)


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where M is the mass of host crystal, υ is the average sound velocity in the host crystal, C is the linear coupling operator, ρ (ωij ) is the detailed phonon density of states at frequency ωij , and

N (ωij ) =  exp ( hωij k BT ) -1 is the thermal equilibrium phonon population of the κ (ωij ) mode. -1

When the temperature approaches zero, Eq. (1) becomes

Wi d =

π ∑ωij ρ (ωij ) j C i M υ 2 h j

Luminescence Anisotropy and Thermal Effect of Magnetic and Electric

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