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Optical Materials 29 (2007) 1289–1294

Fabrication and laser performance of polycrystal and singlecrystal Nd:YAG by advanced ceramic processing

A. Ikesue a,*, Yan Lin Aung a, T. Yoda b, S. Nakayama c, T. Kamimura d

a World-Lab Co., Ltd., Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japanb Optoquest. Co., Ltd. 1335 Haraichi, Ageo, Saitama 362-0021, Japan

c Department of Applied Chemistry and Biotechnology, Niihama National College of Technology,

7-1 Yagumo-cho, Niihama, Ehime 792-8580, Japand Department of Electronics, Information and Communication Engineering, Osaka Institute of Technology,

5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan

Received 27 October 2005; accepted 12 December 2005Available online 7 November 2006

Abstract

We report the first demonstration of polycrystalline Nd-doped YAG ceramics with almost perfect pore-free structure and Nd-dopedYAG single crystal by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd:YAG ceramicsis extremely high and its optical quality is comparable to that of commercial high quality Nd:YAG single crystal. Moreover, we havesucceeded also in fabrication of Nd:YAG single crystal, which enables laser oscillation, by solid-state reaction method. Laser oscilla-tion efficiency was very low when the pores were remained inside single crystal, however the laser oscillation efficiency of pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having grain boundaries. From thisfact, it was found that the optical scattering inside the Nd:YAG ceramics occurs mainly at the residual pores and the scattering atthe grain boundary is very little. In addition, we confirmed that high concentration Nd:YAG single crystal can be fabricated by sinteringmethod.� 2006 Elsevier B.V. All rights reserved.

1. Introduction

Since high efficiency polycrystalline Nd:YAG ceramiclaser was reported by the present authors in 1995 [1], vari-ous transparent ceramics laser materials such asYb:YSAG, Nd:Y2O3, etc. [2–9] with optical grade havebeen developed in the world. Nowadays, the ceramicsmaterials enabled ultra-short pulse (pico-second, femto-second) laser oscillation by controlling the spectral widthof the fluorescent elements such as Nd, Yb, etc. Moreover,development of ceramics composite laser with complicatedstructure has also been reported [10,11] and solid-statelaser is at the dawn of a new era. Since ceramic laser per-mits design flexibility, it is extremely important that it

0925-3467/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2005.12.013

* Corresponding author.E-mail address: poly-ikesue@s5.dion.ne.jp (A. Ikesue).

can provide the possibility of high power laser, develop-ment of high output laser using microchip and the lasertechnologies, which cannot be realized in conventional sin-gle crystal materials. Although the optical quality of theconventional transparent ceramics was inferior to that ofsingle crystal materials, optical grade transparent ceramicshave no optically heterogeneous regions such as facets andcore in Nd:YAG single crystal materials. In general, cera-mic materials include residual pores and grain boundaries.For laser materials, the technology to remove those scatter-ing sources becomes highly important. To decrease opticalloss, it is extremely important to fabricate (i) super fulldense ceramics with pore-free structure and (ii) ceramicswith no grain boundaries (single crystal laser media) bynon-melting (sintering) method because both pores andgrain boundaries in ceramic materials act as optical scatter-ing centers [12,13]. In this paper, we discuss on the fabrica-

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tion and laser properties of both (i) super full dense ceram-ics and (ii) no grain boundary ceramics.

2. Experimental

Fig. 1 shows the fabrication process of polycrystallineNd:YAG ceramics and Nd:YAG single crystal by solid-state reaction method. Nd:YAG ceramics (pore-free andextremely low optical scattering) with uniform microstruc-ture were fabricated by a simple solid-state reactionmethod [1]. For the fabrication of polycrystalline Nd:YAGceramics, we controlled pore size (ca. several tens of nano-meter) and packing density (ca. 55%) of powder compactsin order to decrease the porosity in sintered ceramics. Weobtained perfect fully dense polycrystal Nd:YAG ceramicsby sintering the powder compacts at 1780 �C for 10–20 hunder vacuum.

In the of case of single crystal fabrication, after pre-sin-tering the powder compacts at 1550 �C–3 h under vacuumthe surface of specimen was mirror polished. The polishedsurface of the polycrystal Nd:YAG ceramics was bondedwith YAG single crystal (seed crystal of h11 1i, h110iand h100i) by the Czochralski (Cz) method. Then thebonded sample was heated at 1700–1840 �C. The continu-ous grain growth occurred from the seed crystal towards

Fig. 1. Fabrication process of polycrystal

Fig. 2. Experimental setu

polycrystal region and finally the polycrystal changed toNd:YAG single crystal.

Fig. 2 shows the experimental setup for laser oscillation.Ti:Sapphire laser (808 nm) was used as pumping source.The coating mirror was put on the both surfaces of ceramicsamples; one side is AR and another is partial-reflection(R = 95%). The optical resonator (transmittance of coatingmirror at 1064 nm) is not optimized in this experiment inorder to compare the relative oscillation characteristics ofeach test sample. The laser power emitted from the ceram-ics was measured with an optical power meter.

3. Results and discussion

Fig. 3 shows the microstructure of Nd:YAG ceramicsheated at 1780 �C for 0, 2, and 5 h. Sudden abnormal graingrowth was observed when the soaking time was over 2 hand the grain was rapidly grown up as seen in the photo-graph. We applied this driving force of abnormal graingrowth in the synthesis of Nd:YAG single crystal.

Fig. 4 shows the relationship between the reciprocal ofthe heated temperature and abnormal grain growth rate.Sudden grain growth occurred at near 1700 �C and thegrain growth rate was drastically increased with an increaseof the heated temperature.

and single crystal Nd:YAG material.

p for laser oscillation.

Fig. 3. Microstructure of Nd:YAG ceramics at high temperature for soaking time of 0 h, 2 h, and 5 h.

Fig. 4. Relationship between the heated temperature and the grain growthrate.

A. Ikesue et al. / Optical Materials 29 (2007) 1289–1294 1291

When the heated temperature reached 1840 �C, theabnormal grain growth rate reached 1.7 mm/h. The graingrowth rate depends not only on the heated temperaturebut also on the grain size of the ceramics. We have also con-firmed several mm/h level grain growth rates, however thedetails of those data will be reported in our future paper.

Fig. 5 shows the image of single crystallization by sinter-ing method. At the first step, polycrystal ceramic is con-tacted with seed crystal. Next, this specimen is heated at

Fig. 5. Image of single crystalli

an adequate temperature. As shown in this figure, whenthe condition that the surface energy (G1) of the seed crys-tal is sufficiently smaller than the surface energy (G2) of thefine grain of polycrystal Nd:YAG is satisfied, the seed crys-tal can easily absorb the fine grains. With the absorption ofthe fine grains, the initial interface between the seed crystaland the fine grains moves to the advanced interface shownin the figure. In order to keep the continuous crystalgrowth, the above condition (G1� G2) must be main-tained constantly because the grain growth easily occursamong the grains of the polycrystal during the heating athigh temperature.

Fig. 6(a) is an appearance of an Nd heavily doped YAGceramics (2.4, 3.6, and 4.8 at% Nd content) with top seedafter heat-treating at high temperature. It was confirmedthat the single crystallization occurred from the seed crystaltowards polycrystalline region about a few mm (3�5 mm).Fig. 6(b) shows the reflection and polarizing image of thecross-section of 3.6 at% Nd:YAG ceramics after thermaletching the polished sample. It was confirmed that the sin-gle crystallization occurred from single crystal side towardspolycrystal side about 4 mm. The growth interface isslightly uneven and it is in irregular structure at the levelof grain size of polycrystal. In the case of a single crystalproduced by the Cz method, many optically heterogeneousphases such as core at central region [14,15], facets aroundthe outer regions, and striations are seen. In addition, Ndsegregation occurs when >1.5 at% Nd ions are doped intoYAG single crystal because the segregation coefficient of

zation by sintering method.

Fig. 6. (a) Appearance of an Nd heavily doped YAG ceramics with top seed after heat-treating at high temperature. (b) Reflection and polarizing image ofthe cross-section of 3.6 at% Nd:YAG ceramics after thermal etching the polished sample.

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Nd ions into YAG single crystal is very low, and it resultsthat the obtained sample is not applicable for laser opera-tion. On the other hand, the Nd heavily doped YAG singlecrystal obtained by sintering method includes no opticallyheterogeneous phases because solid–liquid interface, seenin the conventional melt-growth method, does not existin present fabrication method. Single crystallization by sin-tering is a process in which continuous absorption of finegrains by coarse grains (seed single crystal) occurs duringheat-treatment at high temperature. Therefore, if the opti-cal quality of the fine grains is perfect, the newly grown sin-gle crystal will also become optically perfect. This sinteringmethod permits the fabrication of Nd heavily doped YAGsingle crystal which could not be produced by conventionalmelt growth method.

The sintering theory [16] shows that the crystal orienta-tion of one grain is rearranged in accordance with that ofanother grain (absorber) when the grains with differentcrystal orientations are combined each other at the sinter-ing stage. If the rearrangement obeys the volume diffusionmechanism [17,18], crystal growth rate in solid state can beestimated as <several lm/h with regard to the diffusioncoefficient of each atom. However, the crystal growth rateobtained in the present sintering method reached mm/horder, so this phenomenon cannot be explained by the con-ventional crystal growth mechanism (diffusion mechanism)in solid state. As seen in Fig. 6(b), the microstructure,

Fig. 7. Growth interface between fine grains a

which is in progress of the absorption of fine grains intosingle crystal, was not observed at the crystal growth inter-face. This suggests that the absorption rate is remarkablyfast. The details on the mechanism of single crystallizationby sintering method will be reported in our future paper.

For the fabrication of Nd:YAG single crystal, largercontent (ca. 500 mass ppm) of colloidal silica was addedinto powder compacts. When the temperature of theceramics bodies reached over 1700 �C, sudden grain growth(coarse grains) occurred in the specimen. In the case of seedfree sample, the grain growth rate of Nd:YAG was below1.7 mm/h. To obtain single crystal laser gain media, poly-crystalline Nd:YAG ceramics was contacted and bondedwith h111i or h110i YAG single crystal seed producedby Cz method. When the seeded polycrystal Nd:YAGceramics was heated at high temperature, small grains ofNd:YAG ceramics were absorbed by large seed single crys-tal. After continuous absorption of small grains, polycrys-talline Nd:YAG ceramics finally changed into ‘‘SingleCrystal Materials’’.

Fig. 7 shows the growth interface between fine grainsand single crystal observed by TEM-EDX. In left TEMphotograph, extra grain boundary phase was not observednear growth interface. In this figure, the spots a, b and cindicate growth interface, interior of single crystal and finegrain, respectively. The spectra of spot analysis are shownin the right figure. Characteristic element of Si was detected

nd single crystal observed by TEM-EDX.

Fig. 9. Etch-pit pattern of the prepared Nd:YAG single crystal afteretching in hot phosphoric acid.

Fig. 10. Laser performance of commercial high quality 1 at% Nd:YAGsingle crystal and, 1 at%, 2.4 at% and 3.6 at% Nd:YAG polycrystalceramics.

A. Ikesue et al. / Optical Materials 29 (2007) 1289–1294 1293

at spot a, however it is not clear yet whether Si is related tothe grain growth of ceramics or not.

In Fig. 8, XRD patterns of (a) the conventional poly-crystalline Nd:YAG ceramics without seed, and the samplegrown on (b) h11 1i and (c) h110i YAG single crystal seedare shown. (The Nd content of each sample is 2.4 at%.) TheXRD pattern of polycrystalline ceramic was as same asthat of powder YAG and many diffraction patterns wasconfirmed. Only the diffraction patterns, which are inaccordance with the direction of seed crystal, wereobserved in h111i seeded sample and h110i seeded sample.From the results such as the formation of ceramic withoutgrain boundaries shown in Fig. 6(b) and the diffraction pat-tern at a unique direction, it can be concluded that thepolycrystal ceramics grown on seed crystal finally changedto single crystal.

Fig. 9 shows the etch-pit pattern of the preparedNd:YAG single crystal after etching in hot phosphoricacid. The growth direction of the crystal is near h100i,and the etch-pit pattern, which belongs to near h100i ori-entation, was observed. As shown in this figure, weobserved a region in which the etch-pits mostly appeared.Other etch-pits were not observed on the surface of thesample. The dislocation density of the prepared sample isa few/cm2 and it was very low.

Fig. 10 shows the laser performance of commercial highquality 1 at% Nd:YAG single crystal and, 1 at%, 2.4 at%and 3.6 at% Nd:YAG polycrystal ceramics. The thresholdand the slope efficiency of polycrystalline Nd:YAG ceram-ics generally depend on Nd concentration in ceramics,however we achieved very high laser conversion efficiencyin each ceramics because of pore-free conditions.

In the case of 1 at% Nd:YAG samples, the slope effi-ciency (g) of the polycrystalline ceramics was 65% and thisvalue is almost equivalent to that of single crystal producedby Cz method. The quantum efficiency of laser with Nd asa luminescent element pumped by 808 nm light source is74% at exciting state. From this fact, it can be concludedthat the ceramic laser media, which provide high laser con-version efficiency and very low threshold, have very little

a b

Fig. 8. XRD patterns of three t

optical scattering. Accordingly, it was shown that the opti-cal scattering at the grain boundary of the polycrystal1 at% Nd:YAG ceramics is extremely low.

Fig. 11 shows the laser oscillation characteristics ofNd:YAG samples, which are a polycrystal and a single crys-tal with almost pore-free quality and a single crystal with a

c

ypes of Nd:YAG ceramics.

Fig. 11. Laser oscillation characteristics of a polycrystal and a singlecrystal with almost pore free quality and a single crystal with a smallamount (several tens of vol. ppm) of residual pores.

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small amount (several tens of vol. ppm) of residual pores.The Nd content is 2.4 at% in each of the sample. It was foundthat the slope efficiency of the high quality single crystal pro-duced by sintering method is higher about 6% than that ofthe polycrystalline ceramic with same composition. The dif-ference in microstructure of a polycrystal and a single crystalis the existence of grain boundary. It can be considered thatthe scattering at the grain boundary of ceramics generatesthe difference in the efficiency about 6%. In laser oscillation,light amplification occurs inside the optical resonator (insideof laser gain media) by the numerous reflections of theexcited ray or the laser beam generated. In this result, how-ever, the difference in amplification efficiency of the poly-crystal and the single crystal is about 6% and it can beestimated that the scattering loss in both samples is extre-mely low. The optical slope efficiencies of 3.6 at% Nd:YAGsingle crystal lasers by sintering method was 56% (not shownin this figure). From these data, it was recognized that thelaser efficiency of single crystal was somewhat improved incomparison with those of polycrystal materials.

4. Conclusions

Full dense Nd:YAG polycrystal and single crystal withoptical grade were fabricated successfully by the solid-state

reaction method and vacuum sintering. The laser oscilla-tion with the highest efficiency was performed for the firsttime using both heavily Nd-doped YAG polycrystal andsingle crystal with sufficiently low losses. The importantresults of this work can be summarized as follows:

(1) The laser oscillation efficiency of pore-free polycrys-talline Nd:YAG ceramics fabricated by sinteringmethod was very competitive to that of high qualityNd:YAG single crystal.

(2) Solid-state reaction method enabled the fabricationof Nd heavily doped YAG single crystal, which couldnot be produced by the Cz method.

(3) The laser oscillation efficiency of Nd:YAG singlecrystal is slightly higher than that of Nd:YAGpolycrystal with grain boundaries.

(4) The optical scattering from grain boundaries of poly-crystal Nd:YAG ceramics on the laser oscillation effi-ciency is fairly small, however the effect of residualpores is very large.

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