Properties of Cr 4+ :YAG crystal

Garnets are important gain hosts in the infrared emission bands. Some of the synthetic garnets are: yttrium aluminum garnet Y3Al5O12 (YAG), gadolinium gallium garnet Gd3Ga5O12 (GGG), and gadolinium scandium aluminum garnet Gd3Sc2Al3O12

(GSAG), and so on [2.1-2.3]. Transition metal doped garnet crystals are attractive gain media because of their ultra-broadband emission spectra. The active electrons of transition metal ions are not completely shelled by the electron cloud. The subtle interplay between the Coulomb fields of the host matrix ions and the electron-phonon coupling permit ultra-broad absorption and emission spectra. Among the transition-metal-doped crystals, Cr⁴⁺-ion doped garnet is an attractive gain medium due to its broad emission band from 1.1 μm to 1.7 μm that just covers the optical communication bands. Besides, the optical pumping absorption range for Cr⁴⁺-doped garnets cover the near-infrared region that broad-stripe diode laser with generous wavelength tolerance are available. The optical characteristics of Cr⁴⁺ ion doped in various hosts are listed in Table 2.1.The widest 3-dB emission bandwidth of is 277 nm for Cr⁴⁺:YAG covering the wavelength range from 1.2 to 1.6 μm. In addition, Cr⁴⁺:YAG has large ∆

λσ

e

τ

f product, the symbols in sequence mean the 3-dB emission bandwidth, emission cross section, and fluorescence lifetime. Therefore, Cr⁴⁺:YAG has a potential for a broadband light source in optical communication. Such broad band characteristics offer unprecedented one-for-all, convenience, flexibility, and simplicity to multi-band component manufactures in optical communication. The fiber configuration was fabricated to confine the pump and signal lights in a small-core area with a high intensity for enhancing gain. At present, the Cr⁴⁺:YAG

crystal has also been widely used as the gain medium for tunable solid-state lasers in the near infrared (NIR) region and as the saturable absorber medium for Nd:YAG lasers due to its large pump absorption cross-section [2.4-2.5]. In addition, some physical and optical properties of Cr⁴⁺:YAG crystal are listed in Table 2.2 [2.5-2.6].

YAG belongs to the garnet family with a cubic space group Ia3d. The stoichiometric formula is {A3}[B2](C3)O12, where A, B, and C denote different lattice sites with respect to their oxygen coordination that are dodecahedral, octahedral, and tetrahedral, respectively [2.3]. Figure 2.1 shows the YAG structure, the site symmetry is dodecahedral for the Y³⁺ ions, octahedral for 40% for the Al³⁺ ions, and tetrahedral for 60% of the Al³⁺ ions. Compound with the garnet structure have interesting properties. For example, YAG is a good laser host material for the rare-earth ions and Cr³⁺. The dodecahedron sites are ideal for the rare-earth ions and the octahedral sites are of appropriate size for Cr³⁺ ion.

Table 2.1. List of the Cr

⁴⁺

-ion doped hosts with the 3-dB emission bandwidth ∆ λ , peak wavelength, emission cross section σ

e

, fluorescence lifetime τ

f

, and ∆ λσ

e

τ

f

product at room temperature [2.1-2.3].

Host

Table 2.2. Physical and optical properties of Cr

⁴⁺

:YAG crystal [2.5-2.6].

Crystal YAG

Melting point 1970 ^oC

Hardness (Mohr) 8.5

Density 4.56 g/cm³

Refractive index 1.82 @ 1 μm

Thermal conductivity 11-13 W/m⋅K

Thermal change in refractive index 7.3×10^-6 K^-1 Thermal expansion coefficient 8.0×10^-6 K^-1

Lattice constant 12.01 Å

3-dB emission range 1253-1530 nm

Absorption cross section 22×10^-19 cm^-2 Emission cross section 7×10^-19 cm^-2 Excited-state absorption cross section of pump 4.18×10^-19 cm^-2 Excited-state absorption cross section of signal 1.4×10^-19 cm^-2

Fluorescence lifetime at 25 ^oC 4.5 μs

Al Al³⁺

Y³⁺

3+

Fig. 2.1. The cubic structure of garnet crystal.

The NIR emission spectrum from 1.2 μm to 1.6 μm is attributed to the transition between the ³A2 and ³T2 energy state of Cr⁴⁺ ions in the tetrahedral site of YAG.

Because most Cr ions in the YAG structure have the smallest lattice mismatch of 11.9% in the Al³⁺ octahedral site, they tend to become octahedrally coordinated Cr³⁺. In order to incorporate the Cr in the quadrivalent, divalent ions need to be co-doped as the charge compensator to change the Cr³⁺ ions to Cr⁴⁺ ions. In 1997, Markgraf, et al.

had examined several divalent ions, including Ca²⁺, Mg²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mn²⁺, Zn²⁺, Ni²⁺, and Sr²⁺ [2.7]. Other divalent ions were excluded, such as Be²⁺, Pb²⁺, and Cd²⁺, due to safety concern. The results found the only successful co-dopants were Ca²⁺ and Mg²⁺ that radiated the NIR fluorescence from Cr⁴⁺ tetrahedral sites. Ca²⁺

ions are typically adopted as the co-dopants for good lattice match. Table 2.3 summaries the lattice mismatch between dopant ions and YAG ions. Ca²⁺ ions enters Y³⁺ dodecahedral site with a lattice mismatch of 8.7% which is better than Mg²⁺ ions’

replacing Y³⁺ dodecahedral site with a lattice mismatch of -11.1%.

Crt4+ charge compensation depended on Ca²⁺ ions concentration was investigated in [2.8]. The relation between normalized Crt4+/total Cr and Ca²⁺/total Cr is shown in Fig. 2.2. There is no Crt4+ in the YAG host without charge compensator, and the ratio of Crt4+/total Cr is increased with increasing Ca²⁺/total Cr. In literature, the ratio of Crt4+/total Cr is below 6% that saturated in 5-fold atomic concentration of Cr to Ca. In our research, there are only less than 1% of the Ca²⁺ ions become active in charge compensation when Ca and total Cr ions are in the same quantity [2.9]. The low compensation efficiency may be explained by large lattice mismatch (i.e. +8.7%) with Ca²⁺ ions incorporated into the dodecahedrally coordinated Y³⁺ sites. The large lattice mismatch is also accompanied by the production of oxygen vacancies, which result in the de-activation of Ca²⁺ charge compensation. By annealing in oxygen atmosphere at 1350 ^oC for 4 hours, the ratio of Crt4+/total Cr can be improved to 5.5% [2.10]. It is the combined effects of oxygen vacancy re-fill under oxygen atmosphere and the Crt4+

ions migrated ion from the octahedral to tetrahedral sites.

Table 2.3. Comparison of ionic radius mismatch between dopants and YAG host cations [2.7].

Host ion Y³⁺ (D) Al³⁺ (O) Al³⁺ (T)

Dopant Ionic radius (Å) 1.159 0.675 0.53

Cr³⁺ (O) 0.755 +11.9%

Cr⁴⁺ (O) 0.69 +2.2%

Cr⁴⁺ (T) 0.55 +3.8%

Cr⁶⁺ (T) 0.44 -17.0%

Ca²⁺ (D) 1.26 +8.7%

Ca²⁺ (O) 1.14 +68.9%

Mg²⁺ (D) 1.03 -11.1%

Mg²⁺ (O) 0.86 +27.4%

Mg²⁺ (T) 0.71 +34.0%

Cu²⁺ (O) 0.87 +28.9%

Cu²⁺ (T) 0.71 +34.0%

Co²⁺ (O) 0.79 +17.0%

Co²⁺ (T) 0.72 +35.8%

Fe²⁺ (O) 0.75 +11.1%

Fe²⁺ (T) 0.77 +45.3%

Mn²⁺ (O) 0.81 +20.0%

Mn²⁺ (T) 0.80 +50.9%

Ni²⁺ (O) 0.83 +23.0%

Ni²⁺ (T) 0.69 +30.2%

Zn²⁺ (O) 0.88 +30.4%

Zn²⁺ (T) 0.74 +39.6%

Sr²⁺ (D) 1.40 +20.8%

Sr²⁺ (O) 1.32 +95.6%

D, O, and T denote ions in dodecahedral site, octahedral site, and tetrahedral site, respectively.

7

Fig. 2.2. Relation between Cr⁴⁺ and Ca²⁺ concentrations in literature [2.8].

Fig. 2.3. The relation between normalized Cr⁴⁺ and Ca²⁺ concentrations [2.9].

Fig. 2.4. The dependence between normalized and Ca²⁺ concentration several samples with or without oxygen annealing treatment [2.10]. mean the Cr⁴⁺ ions in tetrahedral sites.

A3 (w/o annealing) A4 (w/ annealing) A5 (w/ annealing) A5 (w/o annealing)

Crt4+ /total Cr ions (%)

Ca^{2 +}/total Cr ions

A3 (w/ annealing)

Ca

²⁺

/total Cr ions

在文檔中 摻鉻釔鋁石榴石晶體光纖之生長系統改良與特性研究 (頁 18-24)