YAG composite ceramics by tape casting

Journal of Alloys and Compounds 640 (2015) 317–320

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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Fabrication and planar waveguide laser behavior of YAG/Nd:YAG/YAG composite ceramics by tape casting Chaoyang Ma a,1, Fei Tang a,⇑,1, Haifeng Lin b, Weidong Chen b,⇑, Ge Zhang b, Yongge Cao a,⇑, Wenchao Wang a, Xuanyi Yuan a, Zhen Dai a a b

Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China Key Lab of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

a r t i c l e

i n f o

Article history: Received 24 February 2015 Received in revised form 19 March 2015 Accepted 30 March 2015 Available online 9 April 2015 Keywords: Tape casting Planar waveguide structure Laser behavior

a b s t r a c t Planar waveguide laser output was successfully demonstrated on Nd:YAG sandwich composite ceramics. Tape casting method combined with vacuum sintering technology was used to fabricate this sample. The core layer was 0.1 mm thickness with Nd doping concentration of 2 at.%. Clean morphology of microstructure was observed with average grain size of 10 lm. The sample exhibited high optical transmittance of above 80% at the wavelength of 632 nm. With the quasi-monolithic Fabry–Perot cavity configuration, the maximum slope efficiency of 63% was achieved in continuous wave (C.W.) operation mode, with a maximum output power of 840 mW and threshold absorbed pump power of about 0.16 W. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In the past two decades, ceramic laser technology has emerged as a promising candidate for single-crystal lasers because of its equal or more superior properties [1]. Combining it with the waveguide technology is recently considered as a prospective approach to construct a high power laser [2,3]. Actually, planar waveguide structure (PWs) materials have always been an interesting research aspect in the field of solid-state laser because of its architecture of waveguide confining the one-dimension pump [4]. The two dimensional geometry provides a large surface area for water-cooling allowing effective heat dissipation. Besides, such materials also possess other advantages like low threshold absorbed pumped power, high gain, high optical confinement in comparison to the traditional uniform bulk laser materials [5–8]. All of these merits allow it to be applied in the compact thin-disk laser [9]. Nowadays the use of both liquid-phase epitaxy (LPE) and thermal bonding technologies is well known to make the fabrication of composite crystals possible, and some excellent results have been reported on PWs materials. Rogin et al. firstly adopted LPE technology to fabricate Nd:YLF crystal, and realized the linear waveguide laser output [10]. Bonner et al. used the same technology to fabricate YAG/Nd:YAG/YAG composite crystal, and achieved 6.2 W laser

output [11]. Romanyuk et al. fabricated Yb:KY(WO4)2 planar waveguide laser, and obtained slope efficiency of above 80% [12]. Machenzie group and Beach group reported Nd:YAG claddingpumping planar waveguide laser and both gained excellent laser output [13,14]. Although some successful reports are given, the fabrication of composite crystals still faces great challenges, such as high cost, long growth period, thermal bonding of multilayer crystals. In recent years, the established fabrication technology of transparent ceramics provides a new route for fabricating PWs materials. Recently, Ge et al. reported YAG/1 at.% Nd:YAG/YAG transparent ceramics, and attempted to use it for PW laser [15]. However, they just mainly analyzed the diffusion distance of Nd ions by ICP-MS technique. The longer sintering time and higher sintering temperature caused their sample to have a longer diffusion distance, and no laser output was given in that report. In this paper, multilayer YAG/2 at.% Nd:YAG/YAG transparent ceramics was presented with core layer of 0.1 mm thickness. We firstly demonstrated experimental result on waveguide laser behavior in continuous wave (C.W.) operation mode. The excellent laser performance indicated the suitability of tape casting combined with vacuum sintering technology for PWs ceramic materials. 2. Experimental procedure 2.1. Sample fabrication

⇑ Corresponding authors. Tel.: +86 010 62513093. 1

E-mail addresses: [email protected] (C. Ma), [email protected] (F. Tang). First authors.

http://dx.doi.org/10.1016/j.jallcom.2015.03.246 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

The brief fabrication process is similar to our previous report [16], as shown in Fig. 1, which is described as follows: high-purity powders of a-Al2O3, Y2O3 and Nd2O3 were weighted in accordance with the chemical composition of

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C. Ma et al. / Journal of Alloys and Compounds 640 (2015) 317–320

Fig. 1. Brief fabrication process of the Nd:YAG composite ceramic cladded with each side of YAG ceramics.

(NdxY1x)3Al5O12 (x = 0, and 0.02) respectively. Then, they were mixed in solvent with fish oil as the dispersant because of its high deflocculating ability. The used solvent was the mixture of ethanol and xylene. Followed by addition of plasticizers and binder, the slurries were second-milled. The obtained slurries were de-aired, and then cast to form thin tapes. These tapes were cut into pieces that were stacked and laminated into green parts. These green bodies were composed of two parts. One was the core layer of Nd:YAG while another was the clad-layers of YAG. The organics in these parts were burned off in oxygen atmosphere prior to the following cold-isostatic press process. Sintering process was carried out at 1720 °C for 10 h under vacuum condition of 107 torr. The annealing treatment was carried out under oxygen atmosphere at 1450 °C for 10 h. The annealed ceramic was then polished until the surface flatness achieved about k/10 (k is 632.8 nm). Finally, multilayer composite waveguide laser ceramic with the size of about 4  0.5  15 mm3 (the thickness of doping region was 0.1 mm) was obtained. 2.2. Properties characterization To investigate the sample properties, some characterizations were carried out. The microstructures on surface and cross section were observed using scanning electron microscope (SEM, JSM-6700F). Fluorescence microscope was used to observe the surface variation of sandwich structure. Optical transmittance was recorded by using UV/Vis/NIR spectrophotometers (PerkinElmer, Lambda 900) and the samples studied were detected ranging from 230 nm to 1100 nm. Finally, laser performance was measured using a 808 nm laser diode as the pumping source.

3. Results and discussion In order to gain a perfect bonding structure between YAG and Nd:YAG ceramics, sintering processing is required to be refined after choosing appropriate additives and ball-milling process. For both YAG and Nd:YAG ceramics, the optimum sintering temperature is slightly different. Commonly, the doping of Nd ions decreases the temperature of sintering to be transparent ceramics. Therefore, the amount of sintering aids (i.e. the mixture of TEOS and MgO) was experimentally increased for YAG to 0.50 wt.% while the same aids was doped into Nd:YAG with amount of 0.48 wt.%. The obtained PWs sample is shown in Fig. 2. It is obviously transparent from the surface (as shown in Fig. 2(a)), which indicates our fabrication technology suitable for PWs ceramics. Moreover, from the viewpoint of cross section, we also find it much transparent through a light path of 15 mm. This result further evidences that our fabrication process can be used to successfully prepare PWs ceramics. Fig. 3(a) and (b) exhibits the surface and cross section morphologies. Both images show full dense microstructure with average grain size of about 10 lm. No pores or secondary phase can be observed. From the cross section morphology, little black point can be found in ceramic matrix, which is attributed to the

Fig. 2. Photos of Nd:YAG composite planar waveguide ceramics: (a) surface and (b) cross section.

contamination. From Fig. 3(b), it also can be found that each grain grows well, and they are bonded tightly with each other after high temperature processing under vacuum condition. Fig. 4 shows the high resolution transmission electron microscope (HRTEM) image of Nd:YAG ceramics. The sample has a perfect crystal lattice arrangement. By analyzing this image, the interplanar distances of (2 1 1), (4 0 0) and (0 2 2) crystal faces are obtained, as listed in Table 1. The little deviation of interplanar distances between YAG crystal and 2 at.% Nd:YAG ceramics indicates a relative little effect of Nd doping ions on crystal structure by the comparison with pure YAG crystal. After Fourier transformation of HRTEM image, an electron diffraction image of Nd:YAG ceramics in the reciprocal space is obtained. Some diffraction points are obviously found. They are analyzed and labeled to the corresponding indices of crystal faces. The obtained result reflects a high degree crystallization of our sample studied. Fig. 5 shows the composite structure photo of cross section for the ceramics by using fluorescence microscopy. Sandwich structure can be found with core layer of Nd:YAG, which is bonded with each side of YAG. The doping layer is measured to be 100 lm thickness, which is used to restrict the transmission of laser. The straight interfaces between layers are found, which indicates that the diffusion of ions does not deteriorate our composite structure. Perfect and full dense microstructure ensures the high optical quality of the samples, as seen in Fig. 6. The optical transmittance excesses above 80% at the wavelength of 632 nm compared to the theoretical value of about 83.55% [17], which indicates the perfect

C. Ma et al. / Journal of Alloys and Compounds 640 (2015) 317–320

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Fig. 3. SEM images of surface (a) and cross section (b) for Nd:YAG PWs ceramics.

Fig. 5. Photo of cross section composite structure for Nd:YAG PWs ceramics.

Fig. 4. HRTEM image of 2 at.% Nd:YAG ceramics (the inset figure is FET electron diffraction).

Table 1 Comparison of interplanar distances between YAG and 2 at.% Nd:YAG ceramics. Crystal face index

YAG (A)

2 at.% Nd:YAG (A)

Deviation (%)

(2 1 1) (4 0 0) (0 2 2)

4.90 3.00 4.25

4.92 3.05 4.23

0.41 1.6 0.47

structure of multilayer PWs ceramics. Some characteristic absorption peaks can be found from 300 nm to 950 nm, which are corresponding to the energy level transitions from ground state of 4I9/2 to various excited state levels, e.g. 4F5/2, 4F7/2, 4G5/2, 2K13/2 and 4D3/2 et al. [15,18]. In laser experiment, we chose a commercial 808 nm multimode laser diode (LIMO) as the pumping source for Nd:YAG PWs ceramics. As exhibited before, our sample is composed of core and clad layers. They have a slightly difference of refractive index resulting from the Nd ions doping [19,20]. The pump beam was directly focused into the core layer of our sample along the cross section of 4  0.5 mm2 by using an aspheric lens of f = 15 mm, and the pump spot radius was 25 lm. A simple plane-plane quasi-monolithic Fabry–Perot type oscillator was constructed for evaluating the laser behavior of Nd:YAG PWs ceramic materials. The input mirror had a dichroic coating with high transmission at 808 nm and super-high reflectivity at 1064 nm. Additionally, both input and output mirrors were attached to both faces of our sample. The entire schematic diagram of experimental setup was shown in Fig. 7. When the pump beam was focused on the core layer and excited it, the output beam would be guided because of the slightly difference of refractive index between core layer, clad layers, as well as air.

Fig. 6. Optical transmittance spectrum of Nd:YAG PWs ceramics.

Fig. 7. Schematic diagram of experimental setup for laser-diode pumped Nd:YAG PWs composite ceramics.

Two separate output couplers were experimentally utilized for evaluating laser performance of our sample with the transmittance of T1 = 8.5% and T2 = 14.1% respectively at the laser wavelength. Fig. 8(a) displays both C.W. laser output power as a function of absorbed pump power. For the output coupler of T1 = 8.5%, the output power increases with pump power, and the threshold absorbed pump power is about 100 mW with the slope efficiency of g1 = 41%. By comparison, for the output coupler of T2 = 14.1%, we

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4. Conclusions Planar waveguide laser output was successfully demonstrated by using YAG/2 at.% Nd:YAG/YAG sandwich composite ceramics. This sample was fabricated by tape casting and vacuum sintering technology with full dense microstructure and high optical quality. By using the output coupler of T = 14.1%, we obtained waveguide laser behavior in continuous wave operation mode. The slope efficiency attained to 63%, with the maximum output power of 840 mW and the threshold absorbed pump power of 160 mW. Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (No. 2014030192), and the program of National Natural Science Foundation of China (No. 51272282), the program of National Natural Science Foundation of China (No. 51302311), the program of Beijing Municipal Commission of Science and Technology (No. 0080043581300100), and the program of Beijing Municipal Commission of Education (No. 2011010329). References

Fig. 8. (a) C.W. laser output power as a function of absorbed pump power and (b) laser spectrum.

obtained the slope efficiency as high as g2 = 63%. When the absorbed pump power is 1.35 W, the maximum output power achieves 0.84 W corresponding to the optical conversion efficiency of 62%. Besides, the cavity loss was estimated based on the obtained slope efficiencies gi (i = 1, 2), which, as measured before, is tightly related with the transmittance of output couplers. The round cavity loss L is given by [21]

[1] [2] [3] [4] [5] [6] [7] [8] [9]

[10] [11] [12] [13] [14] [15]

L¼

ðT 2 =T 1 ÞT 1  ðg2 =g1 ÞT 2 g2 =g1  T 2 =T 1

ð1Þ

[16] [17]

and it was calculated to be about 0.6. Finally, we used a high resolution fiber-coupled optical spectrometer (Ocean Optics HR4000) to examine the lasing spectrum centered at 1064 nm in continuous wave mode, as shown in Fig. 8(b). The obtained result exhibits no evident variation of output wavelength for PWs ceramics. All the measured results above indicate excellent laser properties to be gained for our sample.

[18] [19] [20] [21]

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