Spectroscopic properties and energy transfers in Cr, Tm, Ho triple-doped Y3Al5O12 transparent ceramics

Binjie Fei; Wang Guo; Jiquan Huang; Qiufeng Huang; Jian Chen; Junting Li; Weidong Chen; Ge Zhang; Yongge Cao

doi:10.1364/OME.3.002037

1. Introduction

Ho³⁺ ions lasers emitting around 2.09 μm are attractive because of extensive applications in remote sensing, military technology [1,2], and especially in medical field [3]. It is believed that Ho³⁺ ions laser is a perfect operation source, since it not only penetrates the tissue shallowly but also has the advantages of high precision and hemostasis effect [4]. However, laser operation in singly doped Ho³⁺ materials is not easy to realize due to the lack of suitable absorption feature in the 780 – 980 nm regions, which leads to the failure of being pumped directly by traditional diode lasers [5,6]. Methods to circumvent the above problem have been carried out by additionally doping Tm³⁺, Cr³⁺, Cr³⁺ and Tm³⁺, Er³⁺ and Tm³⁺, etc, into the Ho³⁺ doped laser materials. Among these materials, Ho³⁺:YAG sensitized by Cr³⁺ and Tm³⁺ simultaneously is considered as an ideal material for gain medium, due to the high energy transfer efficiency among rare earth ions, optional pumping source, and excellent physicochemical property of YAG [7]. To date, Cr, Tm, Ho triple-doped YAG single crystal has been grown [8], even so, it is still a hard work because of the serious lattice defects [9]. Besides, high cost of the equipment for the single crystal growth also restricts its mass production and commercial application. On the other hand, as the development of advanced ceramic technique [10], transparent Cr, Tm, Ho triple-doped YAG ceramics can be fabricated and have the potential to replace the single crystals. The ceramic laser materials could be much cheaper than the single crystals while their optical qualities are almost the same as (and even better than) those of single crystals [11]. Furthermore, high doping concentration and large-size samples can be easily fabricated without the adoption of complicated facilities and critical techniques. However, so far, there have been no efforts reported on the Cr:Tm:Ho:YAG laser ceramic, though other ceramic laser materials, such as Nd:YAG [12,13], Yb:YAG [14,15], Tm:YAG [16,17] and co-doped Tm:Ho:YAG [10], Yb:Er:YAG [18] etc, have been intensively investigated

In this paper, we report on the fabrication of Cr:Tm:Ho:YAG transparent ceramic by advanced ceramic technology, and study the spectroscopic properties of the host with the interest in infrared laser application at 2.09 μm.

2. Experimental details

High-purity Al₂O₃ (99.99%), Y₂O₃ (99.99%), Cr₂O₃ (99.999%), Tm₂O₃ (99.99%) and Ho₂O₃ (99.99%) commercial powders were used as starting materials (Fig. 1) and weighed according to the designed nominal dopant concentrations of 0.56 at% for Cr³⁺, 5.80 at% for Tm³⁺, and 0.36 at% for Ho³⁺. Meanwhile, 0.45 wt% tetraethoxysilane (purity > 99.99%) was used as a sintering aid and 0.25 wt% Oletic Acid (purity > 99%) was chosen as a dispersant. Then the chemicals were mixed thoroughly in ethanol for 24 h using a planetary-milling machine to prepare ceramic slurries. After drying at 65 °C for 48 h, the mixtures were sieved through 300-mesh screen and uniaxially pressed into $φ$ 10 mm disks. The green bodies were pre-sintered in the air at 800 °C for 10 h to remove the organic ingredients before cold isostatic pressed (CIP) under 200 Mpa. The as-obtained plates were sintered at 1703 °C under vacuum condition of 10⁻⁵ Pa for 15 h, accompanied by annealing at 1400 °C in air and polishing to optical grade..

Fig. 1 SEM images of the starting oxide powders. (a) Cr₂O₃; (b) Tm₂O₃; (c) Ho₂O₃; (d) Y₂O₃; (e) Al₂O₃; (f) mixed powders after ball milling for 24 h.

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The structural properties and phase purity were tested by X-ray diffraction (XRD, Dmax2500, Rigaku), while the microstructure was investigated by scanning electron microscopy (SEM, JSM-6700F, JEOL). The optical transmittance and absorption spectra were measured on a Perkin-Elmer UV-visible-near infrared (NIR) spectrometer (Lambda-900). The fluorescence spectra and decay curves were measured by an Edinburgh FSP920 spectrometer equipped with an OPO laser as the excitation source.

3. Results and discussion

Figures 1(a)-1(e) display the SEM images of the starting powders (i.e., Cr₂O₃, Tm₂O₃, Ho₂O₃, Y₂O₃, α-Al₂O₃, respectively). All powders have a high purity level, but different morphologies. The Al₂O₃ powders employed in our experiment have a regular morphology and uniform size, whereas Cr₂O₃, Tm₂O₃, Ho₂O₃ and Y₂O₃ powders are quite coarse, which are generally considered as unsuitable for the fabrication of transparent ceramics. However, a SEM image shown in Fig. 1(f) suggests that the modified morphology and a fine particle size distribution have been obtained by a sufficient ball-milling process. This is beneficial to obtain a pore-free microstructure during sintering, especially for the ceramics doped with several rare earth ions.

Figure 2 shows the XRD pattern of the Cr:Tm:Ho:YAG ceramic, which is well indexable under Ia-3d lattice symmetry and is consistent with the standard JCPDF file [No. 79-1891 for Y₃Al₅O₁₂ (YAG)]. Figure 3(a) and its inset display the transmittance spectrum and the photograph of the Cr:Tm:Ho:YAG ceramic with diameter of 8 mm and thickness of 1.2 mm, respectively. This sample shows good transparency, with the in-line transmittances in both the visible and infrared region are almost 81%. The transmittance reaches 81.4% at 2.09 nm, which is helpful to the 2.09 μm laser emission. The SEM surface morphology of the ceramic is shown in Fig. 3(b). Obviously, the density is quite high, and neither pores nor second phases can be observed. The average grain size is 15 μm, and no abnormal grain growth arises.

Fig. 2 The XRD pattern of the Cr, Tm, Ho triple-doped YAG ceramic.

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Fig. 3 (a) Transmittance spectrum and (b) SEM image of the transparent ceramic. The inset in (a) is the photograph of the sample.

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The room temperature ground state absorption spectrum of the sample was shown in Fig. 4.The spectrum is well resolved with almost all the Stark components corresponding to different manifold of Cr³⁺, Tm³⁺ and Ho³⁺ are observed. The absorption band positions of Tm³⁺ and Ho³⁺ are well matched with the spectral data of Tm³⁺, Ho³⁺ co-doped YAG ceramic [10]. The inset of Fig. 4 shows the detail of absorption positions ranging from 372 nm to 850 nm, from which four typical transitions for Cr³⁺ and Tm³⁺ are indicated. In the range of 372-650 nm, two absorption bands around 430 nm (⁴A₂→⁴T₁) and 600 nm(⁴A₂→⁴T₂)are of the absorption levels of Cr³⁺ ions, and the calculated absorption coefficients are 3.46 cm⁻¹ and 1.82 cm⁻¹, respectively. The full widths at half maximum (FWHM) of these two absorption bands are very broad (69 nm and 80 nm, respectively), which are consistent with the Cr:Tm:Ho:YAG single crystal and beneficial to the flash lamp pumping [19]. Other two intense absorption bands around 680 nm (³H₆→³F_{2, 3}) and 782 nm (³H₆→³H₄) are attributed to the transitions of Tm³⁺. Due to the lack of appropriate pumping source at 680 nm, the absorption band around 782 nm is the main location for discussion. This absorption has a peak value of 3.68 cm⁻¹ with FWHM of 15 nm, which matches well with the emitting wavelength of high-power AlGaAs laser diodes. Obviously, this Cr, Tm, Ho triple-doped YAG ceramic has the potential advantage of optional pumping source, flash lamp or diode.

Fig. 4 Optical absorption spectrum of the mirror polished Cr:Tm:Ho:YAG ceramic. The detail of absorption positions range from 372 nm to 850 nm is shown in the inset.

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The room temperature fluorescence spectrum of the sample in the range of 1700-2200 nm, obtained with excitation under an OPO laser at 430 nm, is shown in Fig. 5. It consists of 6 main lines range from 2000 to 2200 nm, likely arising from the transition ⁵I₇→⁵I₈ and the maximum emission is at 2.088 μm with FWHM of 40 nm. This emission decays exponentially with the decay time of 8.93 ms (standard error of 0.09 ms, inset of Fig. 5), which is in good agreement with the literature data [20]. The acquisition of fluorescence at 2.088 μm indicates that the energy transfer from Cr³⁺ to Ho³⁺ is realized with Tm³⁺ acted as an intermediate media. The energy transfer mechanism is schematized in Fig. 6.Energy can be absorbed by Cr³⁺ in either the ⁴T₁ or ⁴T₂ bands and relaxes to the ²E level with a high quantum efficiency, since Cr is a transition metal atom with overlapping bands [20]. Then, energy transfers from ²E level of Cr³⁺ to ³F₃ and ³H₄ levels of Tm³⁺ (where ³F₃ decays nonradiatively to ³H₄). At the same time, the cross-relaxation interaction between excited and ground states of Tm³⁺ gives rise to two Tm³⁺ ions in the ³F₄ excited state according to the schematic process. Subsequently, the Tm atoms in the ³F₄ level transfer the energy to the upper laser level (⁵I₇) of Ho³⁺. This energy transfer process is consistent with Cr:Tm:Ho:YAG single crystal [7], indicating the feasibility of fabricating Cr:Tm:Ho:YAG materials for infrared laser application by ceramic technology.

Fig. 5 Emission spectrum of Cr:Tm:Ho:YAG ceramic obtained by exciting the Cr³⁺ at 430 nm. The decay profile of this emission is shown in the inset.

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Fig. 6 Scheme of the energy transfer processes.

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The efficiencies of the energy transfer (Tm³⁺) ³F₄→ (Ho³⁺) ⁵I₇ and its back-transfer process are important parameters in this co-doped system [21], which deeply affect the laser performance at 2.09 μm. The values of these transfer coefficients can be calculated from the emission and absorption cross sections by means of the Forster- Dexter theory [22]:

C_{T m \to H o} = \frac{9 χ^{2} c}{16 π^{4} n^{2}} \int σ_{a b s, H o} (λ) σ_{e m, T m} (λ) d λ,

C_{H o \to T m} = C_{T m \to H o} \exp (\frac{E_{z l : H o} - E_{z l : T m}}{k T}),

where

χ^{2}

includes the orientational average and is usually taken to be 2/3, c is the speed of light, n is the refractive index,

σ_{e m, Χ}

and

σ_{a b s, Χ}

are the emission and absorption cross sections of X ion, respectively, and E_zl:X is the zero phonon line of ion X. We define E_zl:X as the energy of the first absorption peak in the absorption spectra and obtain E_zl:Tm = 5875 cm⁻¹ and E_zl:Ho = 5449 cm⁻¹. To calculate the energy transfer parameters, the measured emission spectra of Tm^{3+ 3}F₄ manifold and the absorption spectra of Ho^{3+ 5}I₇ manifold are introduced, as shown in Fig. 7.The values obtained for the Tm - Ho and Ho - Tm transfer coefficients are C_Tm→Ho = 6.61 × 10⁻⁴⁰ cm⁶ s⁻¹ and C_Ho→Tm = 0.84 × 10⁻⁴⁰ cm⁶ s⁻¹. The main uncertainty of the results is from

σ_{e m, T m} (λ)

using the Forster- Dexter theory, which is calculated by the reciprocity method [23]:

σ_{E M} (λ) = \frac{A (J \to J^{'}) λ^{5} I (λ)}{8 π n^{2} c \int I (λ) λ d λ},

where

A (J \to J^{'})

is the averaged spontaneous emission probability,

I (λ)

indicates the fluorescence intensity, n and c are the refractive index and velocity of light. Among these values,

A (J \to J^{'})

is calculated based on the Judd-Ofelt (J-O) theory, which introduces the room-mean-square (RMS) deviation between the calculated and experimental results. Therefore, the uncertainty of the results can be simply interpreted as the RMS deviation. Due to the value of RMS deviation is always very small, the uncertainty of the results is in the acceptable range.

Fig. 7 YAG-based Emission cross section of Tm^{3+ 3}F₄ manifold and Absorption cross section of Ho^{3+ 5}I₇ manifold relevant to energy transfer parameters.

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The values of the transfer coefficients are reported in Table 1 together with the values calculated with the same method for other hosts. The table shows that our values are similar to those obtained from YAG single crystal [24], a garnet that has already shown laser effect in the 2 μm region when co-doped with Tm³⁺ and Ho³⁺ [25]. Furthermore, the ratio between the direct Tm - Ho energy transfer and its back-process for Cr:Tm:Ho:YAG ceramic is 7.87, slightly lower than that of YAG single crystal, but still higher than that of YLF [26]. It is worth noting that a high value of the ratio between the Tm - Ho transfer and its back-process is an important condition for an efficient Ho 2 μm laser [21]. Therefore, this Cr:Tm:Ho:YAG ceramic is a promising material for the realization of efficient diode pumped laser devices.

Table 1. Comparison of the transfer coefficient between Tm³⁺ and Ho³⁺ in our ceramic and in other laser hosts.

View Table

In order to obtain more reliable information about the perspective of application of our material, we have calculated the emission cross section by means of the McCumber theory [27]:

σ_{e} (λ) = σ_{a} (λ) \frac{Z_{l}}{Z_{u}} \exp [\frac{ε - E}{k T}]

where

σ_{a}

is the absorption cross-section which can be calculated by

α / N

(

α

is the absorption coefficient shown in Fig. 4 and N is the ionic concentration); Z_l and Z_u are the partition functions for the lower and the upper levels, respectively, involved in the considered optical transition; T is the temperature (here is the room temperature); k is the Boltzmann constant, and ε is the energy for the transition between the lower Stark sublevels of the emitting multiplets and the lower Stark sublevels of the receiving multiplets (zero-phonon line); E is the energy corresponding to the peak wavelength of the absorption. The result is shown in Fig. 8(a) with the relevant absorption cross section at this region. The peak cross section is 7.4 × 10⁻²¹ cm² at 2088 nm, rather similar to that reported by Fan et al [28]. In addition, the spectral overlap between the absorption and emission cross section indicates that the infrared laser could only be realized within 2000-2200 nm.

Fig. 8 (a) Absorption and emission cross section spectra of Ho³⁺ in YAG ceramic corresponding to the transition from ⁵I₇ to ⁵I₈ level. (b) Simulated optical gain spectra for this transition under various population inversion (P) conditions.

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A meaningful assessment about the potentiality as laser medium can be obtained from the curves of the potential laser gain versus wavelength, which are calculated according to the formula [29]:

G (λ) = N [P σ_{e} (λ) - (1 - P) σ_{a} (λ)]

where P is the population of the emitting level. The gain spectra for P ranging from 0 to 1.0 in the 1800 – 2200 nm spectra regions are shown in Fig. 8(b). The gain increases with the population inversion and positive gain in the system can be achieved for a population inversion of 40% and above. At 100% population inversion, a maximum gain coefficient of 6.66 cm⁻¹ is obtained. According to the laser principle, laser can be realized when

G (λ) \geq 0

. Therefore, this kind of Cr:Tm:Ho:YAG ceramic is suitable for laser gain medium.

Preliminary laser test was carried out to verify the possibility of Cr:Tm:Ho:YAG ceramic used as laser gain medium. Laser operation at 2.09 μm had been acquired with sparks observed when the laser hit a seared wood. It is believed that better laser performance would be realized by the improvement of ceramic fabrication details and optimization laser experiments in the further study.

4. Conclusions

Cr, Tm, Ho triple-doped Y₃Al₅O₁₂ laser ceramic, which was fabricated by the one step vacuum sintering method, displays excellent quality and optical characteristics comparable to the single crystal. The ⁵I₇ → ⁵I₈ fluorescence at 2.09 μm indicates an excellent infrared emission under flash lamp or diode pumping with maximum emission cross section of 7.4 × 10⁻²¹ cm² and optical gain coefficient of 6.66 cm⁻¹. The efficiencies of the energy transfer (Tm³⁺) ³F₄ → (Ho³⁺) ⁵I₇ and its back-transfer process were also calculated. The obtained ratio between the Tm – Ho transfer and its back-process was 7.87, which was comparable to that of YAG and YLF. It is therefore favorable to consider this material as a potential host to make flash lamp or diode pumped solid state laser operating at 2.09 μm.

Acknowledgments

This work was financially supported by the Key Program in Major Research Plan of National Natural Science Foundation of China (91022035).

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Host	C_Tm→Ho (10⁻⁴⁰ cm⁶ s⁻¹)	C_Ho→Tm (10⁻⁴⁰ cm⁶ s⁻¹)	C_Tm→Ho/ C_Ho→Tm	Ref.
YLF	17	2.7	6.3	[26]
YAG(crystal)	10.897	1.144	9.53	[24]
YAG(ceramic)	6.61	0.84	7.87	This work

Spectroscopic properties and energy transfers in Cr, Tm, Ho triple-doped Y₃Al₅O₁₂ transparent ceramics

Abstract

1. Introduction

2. Experimental details

3. Results and discussion

4. Conclusions

Acknowledgments

References and links

Cited By

Figures (8)

Tables (1)

Equations (5)

Optical Materials Express