lpor201300140

LASER &PHOTONICS REVIEWS

Laser Photonics Rev.8,No.1,158–164(2014)/DOI 10.1002/lpor.201300140

O R I G I N A L P A P E R

Abstract Currently,the major commercial white light-emitting diode (WLED)is the phosphor-converted LED made of the In-GaN blue-emitting chip and the Ce 3+:Y 3Al 5O 12(Ce:YAG)yellow phosphor dispersed in organic epoxy resin or silicone.However,the organic binder in high-power WLED may age easily and turn yellow due to the accumulated heat emitted from the chip,which adversely affects the WLED properties such as luminous ef?cacy and color coordination,and therefore reduces its long-term reliability as well as lifetime.Herein,an innovative lumi-nescent material:transparent Ce:YAG phosphor-in-glass (PiG)inorganic color converter,is developed to replace the conven-tional resin/silicone-based phosphor converter for the construc-tion of high-power WLED.The PiG-based WLED exhibits not only excellent heat-resistance and humidity-resistance charac-teristics,but also superior optical performances with a luminous ef?cacy of 124lm/W,a correlated color temperature of 6674K and a color rendering index of 70.This easy fabrication,low-

cost and long-lifetime WLED is expected to be a new-generation indoor/outdoor high-power lighting source.

A new-generation color converter for high-power white LED:transparent Ce 3+:YAG phosphor-in-glass

Rui Zhang 1,Hang Lin 2,Yunlong Yu 2,Daqin Chen 2,?,Ju Xu 2,and Yuansheng Wang 1,?

1.Introduction

Nowadays,the white light-emitting diode,as a new type of luminescent source,has played a crucial role in applications of indicator,backlight,automobile headlight and general illumination owing to its excellent performances,such as high luminous ef?cacy (LE),energy saving,environment friendliness,and long lifetime [1–4].The current leading commercial WLED combines an InGaN blue-emitting chip with a Ce:Y AG yellow-emitting phosphor packed on the chip surface using epoxy resin or silicone [5–8].However,for high-power WLED the organic resin or silicone with low thermal conductivity and poor thermal stability may age easily and turn yellow due to the accumulated heat emitted from the chip [9,10].This raises several issues in WLED practical application,including degradation of LE and shift of chromaticity,and ?nally reduction of long-term reliability [11,12].

1State Key Laboratory of Structural Chemistry,Fujian Institute of Research on the Structure of Matter,CAS,Fuzhou,Fujian 350002,P .R.China 2

Key Laboratory of Design and Assembly of Functional Nanostructures,Chinese Academy of Sciences,Fuzhou,Fujian 350002,P .R.China ?

Corresponding authors:e-mail:dqchen@https://www.wendangku.net/doc/4c14606232.html,;yswang@https://www.wendangku.net/doc/4c14606232.html,

To solve this problem,inorganic materials,such as transparent ceramics and glass ceramics,have been investi-gated recently as practical alternatives to the organic poly-mer binders.A maximum LE reaching 93lm/W at a low correlated color temperature (CCT)of 4600K was realized in a thin transparent Ce:Y AG ceramic-based WLED [9].However,the high fabrication cost is an unavoidable chal-lenge to the mass production of the transparent Ce:Y AG ceramics [9,13,14].Ce:Y AG glass ceramic,which is a kind of composite containing Ce:Y AG microcrystals pre-cipitated from precursor glass via well-controlled crystal-lization,has interesting advantages of excellent heat resis-tance and easy formability,etc.[15–17].However,the so far reported maximum quantum yield (QY)for the Ce:Y AG glass ceramic was about 30%and the optimal LE reached merely 20lm/W [18].The reasons causing poor optical features of the Ce:Y AG glass ceramic are that it is

dif?cult to partition all the Ce 3+activators into Y AG host during

C 2013by WILEY-VCH Verlag GmbH &Co.KGaA,Weinheim

Y AG crystallization and to achieve high transparency in such material.

On the other hand,phosphor-in-glass(PiG),where a certain amount of phosphor is dispersed in an inorganic glass matrix,has been considered as a promising alterna-tive for the color converter.PiG is prepared by cosintering of a simple mixture of phosphor and glass powder in a temperature lower than1000?C[19,20].Importantly,var-ious commercial phosphor powders could be adopted to mix with glasses to adjust the emission colors of PiGs.For the PiG material,there are two key factors determining its ef?cient luminescence:one is the excellent heat resistance of phosphor particles against glass melting to keep their original properties;the other is the refractive-index match-ing between phosphor and glass matrix to reduce adverse light scattering and keep PiG transparent.In fact,increasing attention has been paid to explore the low melting tempera-ture glasses(such as borate glasses,phosphate glasses and tellurite glasses)for the dispersion of phosphors[21,22]. However,to the best of our knowledge,there have been no reports on the highly optical transparent and ef?cient PiG color converters usable in WLED to date.

In this paper,we report an innovative Ce:Y AG PiG material that can be used to replace the traditional resin/silicone-based phosphor converter in WLED.This composite was fabricated by introducing Ce:Y AG com-mercial phosphor into the speci?cally selected inorganic glass powders.The mixture was sintered at an optimal temperature at which the glass components were melted while the phosphor powders remained solid as much as possible.By carefully designing the glass composition,ad-justing the phosphor to glass ratio,as well as controlling the sintering temperature/time,the highly transparent PiG sample is achieved and its luminescent QY reaches as high as92%upon460-nm excitation.Furthermore,compared to those of the conventional Ce:Y AG phosphor-in-silicone (PiS),thermal-quenching and thermal-resistance perfor-mances of PiG are greatly improved.Impressively,the LE of the PiG-based WLED reaches as high as124lm/W at an operating current of350mA,superior to that of the conven-tional PiS-based WLED(98lm/W),revealing the promi-nent feasibility of the PiG material in high-power WLED applications.

2.Experimental section

Precursor glasses with following compositions(mol%)of 10–30Sb2O3,10–30B2O3,5–30TeO2,10–25ZnO,5–20 Na2O,0–10La2O3,and0–10BaO were prepared by a con-ventional melting–quenching method.The reagent grade chemicals were mixed thoroughly and melted in a platinum crucible at750–850?C for0.5–1.5h in ambient atmo-sphere.The melt was poured into a cold copper mold and then cooled to room temperature.The prepared glass was milled to powders using a ball grinder,and then mixed with1–9wt%commercial Ce:Y AG phosphors(purchased from XinLi Illuminant Co.Ltd)thoroughly and sintered in a platinum crucible at540–690?C for10–80min in ambient atmosphere.The melt was poured into a220?C preheated copper mold and then cooled to room tempera-ture.The obtained PiG was annealed at260?C for5h in a muf?e furnace to relinquish inner stress,polished and cut into?12mm disks with various thicknesses(0.2–1.2mm).

To study the thermal behaviors of the PiG samples, differential scanning calorimety(DSC)experiments were carried out at a heating rate of10K/min.The refractive index of the sample was measured by a digital refrac-tometer(GI-RDB).The density was measured following the Archimedes’principle using distilled water as medium. The hardness and tenacity were determined using a Vick-ers microindenter(DHV-1000),with a charge of100g. The thermal expansion coef?cient and thermal conductiv-ity were measured by an electronic dilatometer(DIL402PC, Netzsch)and a laser?ash apparatus(LFA457,Netzsch), respectively.Microstructures of the PiG samples were studied using a scanning electron microscope(SEM,JSM-6700F)equipped with an energy-dispersive X-ray spec-troscopy(EDS)system.Emission,excitation spectra and decay curves of PiGs were recorded on a spectro?uoreme-ter(FLS920,Edinburgh Instruments)equipped with both continuous(450W)and pulsed xenon lamps.All the above measurements were carried out at room temperature.The temperature-dependent emission spectra were recorded by a spectro?uoremeter(FLS920),and the sample temperature was controlled by a heating stage(THMS600E,Linkam Scienti?c Instruments).

Internal QY is de?ned as the ratio of the emitted photons to the absorbed photons,and was measured by a spectro?u-oremeter(FLS920).An integrating sphere was mounted on the spectro?uoremeter with the entrance and exit ports located in90o geometry.The PiG sample was located in the center of the integrating sphere.All the recorded spec-troscopic data were corrected for the spectral responses of both the spectro?uoremeter and the integrating sphere. The responses of the detecting systems(integrating sphere, monochromators and detectors)in photon?ux were de-termined using a calibrated tungsten lamp.Based on this setup,internal QY is calculated by the following equitation [23,24]:

η=number of photons emitted

number of photons absorbed

=L sample

E reference?E sample

,(1)

whereηrepresents QY,L sample the emission intensity, E reference and E sample the intensities of the excitation light not absorbed by the reference and the sample,respectively. The precursor glass was used as the standard reference.The difference in integrated areas between the sample and the reference represents the number of the absorbed photons. The photons emitted were determined by integrating the area of the emission band.The error associated with the QY measurement is±3%.

The external QY of PiG,de?ned as the ratio of the emitted photons to the incident photons on the sample,

https://www.wendangku.net/doc/4c14606232.html, C 2013by WILEY-VCH Verlag GmbH&Co.KGaA,Weinheim

LASER&PHOTONICS

REVIEWS

160R.Zhang et al.:Ce:YAG phosphor-in-glass for high-power white LED

was also determined with the procedure similar to that for measuring internal QY,except that the excitation source was not a xenon lamp but a blue-emitting chip.Both the blue chip and the PiG-based WLED were located in the integrating sphere,and their emission spectra were recorded respectively at an operating current of350mA.The number of incident photons(E in)was evaluated by integrating the emission band of the blue chip in the wavelength range of410–520nm,while the number of the emitted photons (E em)was calculated by integrating the emission band of the WLED in the range of490–750nm.The external QY of PiG was determined by E em/E in.

As a proof-of-concept experiment,the WLED device was constructed by encapsulating a PiG or PiS disk on the blue chip,as schematically illustrated in Fig.S1.The sample holder is groove-shaped with the blue chip?xed at the bottom(purchased from Sichuan Baishi Optoelectronic Technology Co.Ltd).The PiG or PiS color converter was horizontally fastened on the blue chip,with opaque silica gel(SD-6020,Shenzhen Saide Electronic Material Co.Ltd) coated around the edge to prevent leakage of blue light.LE, chromaticity coordinate,CCT and color rendering index (CRI)of the PiG-and PiS-based WLEDs were measured in an integrating sphere of50cm diameter,which was con-nected to a CCD detector with an optical?ber(HAAS-2000, Ever?ne Photo-E-Info Co.Ltd).The current for exciting the blue chip was?xed at350mA.

Two kinds of reliability tests were carried out.One was a thermal-resistance test,where the PiG sample and the PiG-based WLED were heat treated in an electric furnace at150?C for0–25days,respectively.The other was a humidity-resistance test,where the PiG-based WLED was heated at85?C in an environment of85%humidity for20 days or immerged in boiling water for24h.In both tests, the measurements were performed after the samples and WLED devices were cooled to room temperature.

3.Results and Discussion

Photographs of the PiG samples with1.0mm in thickness, prepared under various experimental conditions,are shown in Figs.1a–c.All the samples exhibit good transparency and bright yellow color.On increasing the Ce:Y AG phosphor content,the PiG color darkens monotonously(Fig.1a);on increasing of the sintering temperature/time,the apparent yellow color fades gradually(Figs.1b and c).The opti-cal transmission spectrum of the PiG containing5wt% Ce:Y AG phosphor,as presented in Fig.1d,shows that the transmittance reaches80%in the wavelength range of550–800nm,verifying the good transparency of PiG.The ab-sorption at～460nm comes from the4f→5d transition of Ce3+.

The photoluminescence(PL)and PL excitation(PLE) spectra of the Ce:Y AG powder and the corresponding PiG composite are shown in Fig.2a.The PL spectrum of PiG ex-hibits a typical Ce3+:5d→4f broadband emission centered at545nm under460-nm excitation,similar to the case of the Ce:Y AG powder.The PLE spectrum of

the PiG sample Figure1Photographs of PiGs prepared under various experi-mental conditions:(a)containing various Ce:YAG contents(sin-tered at570?C for20min);(b)sintered at various temperatures for20min(containing5wt%Ce:YAG);(c)sintered at570?C for various durations(containing5wt%Ce:YAG).(d)Optical transmission spectrum of PiG with1mm in thickness(containing 5wt%Ce:YAG,and sintered at570?C for20min).

shows two excitation bands centered at340and460nm originating from the4f→5d transition of Ce3+.Notably, the340nm excitation intensity of PiG is weaker than that of Ce:Y AG powder,ascribing to the absorption of glass matrix in the short-wavelength range.

Figures2b–d and Figs.S2and S3exhibit the phosphor content and sintering temperature/time dependence of the PL intensity,internal QY and decay lifetime for the PiG sample.On increasing of the phosphor content,the lumi-nescence of PiG intensi?es correspondingly,while the in-ternal QY(92%)and lifetime(64ns)are not obviously affected(Fig.S2).On increasing of the sintering temper-ature/time,both the PL intensity and internal QY of PiG decrease monotonously(Figs.2b–d,Fig.S3),while the decay lifetime remains unchanged since it is an intrinsic feature of the Ce:Y AG phosphor in PiG.Remarkably,the sintering temperature has a more signi?cant impact on the PL intensity and internal QY than the sintering time.

Scanning electron microscopy(SEM)observations on the PiG samples containing5wt%Ce:Y AG phosphors were carried out to investigate the microstructure variation with increasing of the sintering temperature,as shown in Fig.3. Evidently,the Ce:Y AG particles sized1–10um are ho-mogeneously dispersed in the glass matrix for all the PiG samples.However,the number of Ce:Y AG particles de-creases with increasing sintering temperature,ascribed to the serious corrosion of phosphors by the melting glass at high temperature.This result is consistent with the fading of apparent color,the decreasing of PL intensity and internal QY of the PiG samples on increasing sintering temperature stated above.Figure3d presents the SEM-EDS mapping of the PiG sample that distinguishes an individual Ce:Y AG particle from the glass matrix.The Sb-rich region represents the glass matrix,while the Al-or Y-rich portion exhibits the phosphor particle.

Obviously,low sintering temperature and short sinter-ing time are bene?cial to realizing highly ef?cient lumi-nescence of the PiG sample.However,when the sintering temperature is too low,or the sintering time is too short, the transparency of the prepared PiG sample is impaired,as revealed in Fig.1,probably owing to the existence of large

C 2013by WILEY-VCH Verlag GmbH&Co.KGaA,Weinheim https://www.wendangku.net/doc/4c14606232.html,

ORIGINAL

PAPER

Laser Photonics Rev.8,No.1(2014)

161

Figure 2(a)PLE and PL spectra of the Ce:YAG powder and the corresponding PiG sample;(b)PL intensity,(c)internal QY ,and (d)decay lifetime of PiG versus sintering temperature (sintering time:20min).The measured internal QY and lifetime values of the Ce:YAG powder are also provided in (c)and (d),respectively.

amount of tiny gas bubbles in https://www.wendangku.net/doc/4c14606232.html,prehensively evalu-ating the measured transparency,PL and QY of the samples,PiG with 5wt%Ce:Y AG phosphor sintered at 570?C for 20min is regarded as the most appropriate material for the color converter,and thus is systematically studied in the following section.Table 1lists some of the measured physical parameters of this PiG material.As a comparison,the related parameters of the silicone used for dispersing phosphor powder (QLE 1101,Shenzhen Topgun Technol-ogy Co.Ltd)and those of the Ce:Y AG are also provided.The high refractive index of PiG (1.80)approaching that of Ce:Y AG (1.84)results in a low light scattering loss and therefore high optical transparency of PiG.In addition,the excellent mechanical properties of PiG make it applicable as the outer package for WLED.Furthermore,compared to the silicone,PiG exhibits a much higher thermal conductiv-ity and lower thermal expansion coef?cient,bene?ting the rapid heat-release in the high-power WLED.In the further experiments,the thermal-quenching and thermal-resistance behaviors of the PiG luminescence were investigated and compared to those of the PiS one,as demonstrated in Fig.4.

Table 1Some physical parameters of PiG,silicone and Ce:YAG materials.

PiG

Silicone Ce:Y AG Glass transition temperature T g [?C]463150–Refractive index n 1.80 1.40 1.84Density ρ[g cm ?3] 4.28– 4.57Hardness HV [MPa]340––Tenacity K IC [Mpa m 1/2]

0.31––Thermal expansion coef?cient a [10?6K ?1]16295–Thermal conductivity λ[W m ?1K ?1]

0.71

0.18

–

When the temperature increases from 25to 200?C,the PL intensity of PiG weakens by 5.5%,while that of PiS decreases up to 9.8%(Fig.4a).The improved thermal-quenching feature of PiG originates from its higher thermal

https://www.wendangku.net/doc/4c14606232.html,

C 2013by WILEY-VCH Verlag GmbH &Co.KGaA,Weinheim

LASER &PHOTONICS REVIEWS

162

R.Zhang et al.:Ce:YAG phosphor-in-glass for high-power

white LED

Figure 3SEM images of PiGs prepared at various sintering temperatures:(a)570?C,(b)630?C,and (c)690?C;(d)EDS mapping of an individual Ce:YAG particle embedded

in glass matrix.

Figure 4Variations of the relative PL intensities in (a)temperature-dependent test,and (b)heat-resistance test for the Ce:YAG PiS and Ce:YAG PiG samples.

conductivity (0.71W m ?1K ?1)than that of the silicone (0.18W m ?1K ?1),which bene?ts to the release of the heat emitted from the chip and subsequently reduces the proba-bility of the Ce 3+nonradiative transition.On the other hand,after heating at 150?C for 20days,only 3.9%PL degra-dation is observed in PiG,which is also much smaller than that in PiS (9.0%,Fig.4b).The enhanced thermal resistance of the inorganic PiG is due to its higher thermal stability than that of PiS with organic matrix.These results reveal that the investigated PiG is superior to the conventional PiS as the color converter for the high-power WLED.

Figure 5a shows a transparent PiG color-converter-based WLED.Under an operating current of 350mA,the lamp yields bright white light.The external QY of PiG with 0.4mm in thickness is determined to be 71%(Fig.S4),be-ing lower than the internal QY .This is reasonable since part of the incident light from the blue-emitting chip is not absorbed by but passes through the Ce:Y AG phosphor particles in PiG,and thus does not contribute to the Ce 3+

Table 2Measured optical parameters of the PiG-and PiS-based WLEDs.

PiG

thickness (mm)Chromaticity coordinate LE (lm/W)CCT (K)CRI 0.2(0.273,0.249)7516603690.4(0.312,0.333)1246674700.6(0.339,0.391)1325220640.8(0.364,0.433)1354728631.0(0.381,0.448)1424485611.2(0.393,0.482)138426159PiS

(0.311,0.333)

98

6782

71

emission.The thickness-dependent PL spectra of PiGs and the CIE color coordinates of the WLEDs are shown in Figs.5b and c.To clearly explore the spectral variation,all the PL spectra are normalized to the blue chip emission band (Fig.5b).Obviously,increasing the PiG thickness induces monotonous intensi?cation in the yellow lumines-cence,and the color coordinate of WLED shifts from white to yellow (Fig.5c).For all the PiG–based LED devices,the yielded luminescence is very bright,as demonstrated in the inset of Fig.5c.The measured optical parameters of WLED encapsulated with PiG of various thicknesses are listed in Table 2.On increasing the PiG thickness,LE intensi?es and CCT decreases,owing to more blue light ab-sorbed and more yellow light emitted by PiG.The optimal thickness of PiG is found to be 0.4mm,and the correspond-ing WLED has a LE of 124lm/W,a CCT of 6674K and a CRI of 70.As a comparison,the optical parameters of the PiS-based WLED are also provided in Table 2.The PiG (0.4mm thickness)-and PiS-based WLEDs exhibit similar CCT and CRI,however,the former has much higher LE (124lm/W)than the latter (98lm/W).The 26.5%enhanced LE for the PiG-based WLED is attributed to the remarkable reduction in light-scattering loss in the transparent PiG and thus high light-extraction ef?ciency.

Furthermore,a comparison of the LE loss between PiG (0.4mm thickness)-and PiS-based WLEDs in the heat-resistance test is performed,as shown in Fig.6a.Notably,LE decreases monotonously with prolonging of aging at 150?C for both devices (Fig.6a).However,after aging for 600h,LE loss for the PiG-base WLED (7.6%)is much smaller than that for the PiS-based one (16.5%).Besides,no considerable changes in CCT,CRI and chromaticity co-ordinate are detected for the PiG-based WLED,as exhibited in Figs.6b and c.These results clearly demonstrate that the PiG-based WLED exhibits more excellent heat-resistance performance than the conventional PiS-based one.

Finally,the excellent humidity-resistance feature of the PiG-based WLED was experimentally evidenced.In the humidity-resistance test under the industry standard condi-tion (85%humidity at 85?C),the LE loss for the PiG-based WLED is not obvious (within 1%)after a testing duration of 20days.To further characterize the humidity-resistance behavior in a relative short time,an accelerated experiment,

C 2013by WILEY-VCH Verlag GmbH &Co.KGaA,Weinheim

https://www.wendangku.net/doc/4c14606232.html,

ORIGINAL

PAPER Laser Photonics Rev.8,No.1(2014)163

Figure5(a)Photographs of a WLED lamp encapsulated by a transparent PiG disk(left)and the lamp in operation(right).

(b)Normalized PL spectra,and(c)CIE color coordinate of the PiG-based WLEDs with various PiG thicknesses(in mm);insets of(c)show luminescent photographs of the PiG-based WLEDs at an operating current of350mA.

Figure6(a)LE losses of the PiG-and PiS-based WLEDs,(b)variation of CCT and CRI,and(d)CIE color coordinate of the PiG-based WLED during heat-resistance test at150?C;inset of(c)shows chromaticity coordinate of the PiG-based WLED versus aging time. https://www.wendangku.net/doc/4c14606232.html, C 2013by WILEY-VCH Verlag GmbH&Co.KGaA,Weinheim

LASER&PHOTONICS

REVIEWS

164R.Zhang et al.:Ce:YAG phosphor-in-glass for high-power white LED Table3Humidity-resistance test results of the PiG-based WLED.

Treating condition Chromaticity coordinate LE(lm/W)LE loss(%)CCT(K)CRI Untreated(0.312,0.333)124–667470 After immerging in boiling water for24h(0.311,0.333)117 5.6675971 After reheating at150?C for1h(0.312,0.332)120 3.2669671

i.e.immerging the PiG sample in boiling water for24h,was performed.As exhibited in Table3,only5.6%LE degra-dation is observed after such test,and no marked changes are found for the chromaticity coordinate,CCT and CRI. It is worth noting that the LE recovers to96.8%(with LE loss of3.2%)after reheating PiG at150?C for1h,ascribed to partial release of the adsorbed H2O molecules acting as quenching centers.

4.Conclusion

In summary,we have developed an innovative transparent Ce:Y AG PiG color converter,which is proved to be an ex-cellent alternative to the conventional epoxy resin/silicone-based phosphor converter for high-power WLED.The PiG-based WLED yields a LE of124lm/W,a CCT of 6674K and a CRI of70,under an operating current of350mA.Impressively,this WLED device exhibits admirable heat-resistance and humidity-resistance perfor-mances:only7.6%LE loss is observed after aging at150?C for600h,much superior to that of the conventional PiS-based WLED(16.5%);and only5.6%LE degradation is detected after immersing PiG in the boiling water for24h. Bene?ting from its easy fabrication,low cost,long lifetime, as well as superior optical properties,the PiG-based WLED is expected to be a new-generation indoor/outdoor lighting source.

Acknowledgements.This work was supported by National Natural Science Foundation of China(51172231,21271170, 11204301and51202244),the key innovation project of Haixi Institute of CAS(SZD13001),and Natural Science Foundation of Fujian for Distinguished Y oung Scholars(2012J06014).

Supporting information for this article is available free of charge under https://www.wendangku.net/doc/4c14606232.html,/10.1002/lpor.201300140or from the author.

Received:4September2013,Revised:24October2013, Accepted:18November2013

Published online:10December2013

Key words:optical materials,WLED,Ce3+:YAG,phosphor-in-glass,luminescence.

References

[1]G.Blasse and A.Bril,Appl.Phys.Lett.11,53–55

(1967).

[2]X.Q.Piao,T.Horikawa,H.Hanzawa,and K.Machida,

Appl.Phys.Lett.88,161908(2006).

[3]D.Y.Wang,C.H.Huang,Y.C.Wu,and T.M.Chen,

J.Mater.Chem.21,10818–10822(2011).

[4]D.L.Geng,G.G.Li,M.M.Shang,D.M.Yang,Y.Zhang,

Z.Y.Cheng,and J.Lin,J.Mater.Chem.22,14262–14271 (2012).

[5]S.Nakamura and G.Fasol,The Blue Laser Diode:GaN

Based Light Emitters and Lasers(Springer,Berlin,1997).

[6]H.S.Jang and D.Y.Jeon,Opt.Lett.32,3444–3446(2007).

[7]S.Ye,F.Xiao,Y.X.Pan,Y.Y.Ma,and Q.Y.Zhang,Mater.

Sci.Eng.R71,1–34(2010).

[8]Y.C.Chiu,C.H.Huang,T.J.Lee,W.R.Liu,Y.T.Yeh,

S.M.Jang,and R.S.Liu,Opt.Exp.19,331–339(2011).

[9]N.Wei,T.C.Lu,F.Li,W.Zhang,B.Y.Ma,Z.W.Lu,and

J.Q.Qi,Appl.Phys.Lett.101,061902(2012).

[10]J.Wang,C.C.Tsai,W.C.Cheng,M.H.Chen,C.H.Chung,

and W.H.Cheng,IEEE J.Sel.Top.Quantum Electron.17, 741–746(2011).

[11]J.J.Li,J.D.Chen,R.F.Wei,and H.Guo,J.Am.Ceram.

Soc.95,1208–1211(2012).

[12]L.Yang,M.X.Chen,Z.C.Lv,S.M.Wang,X.G.Liu,and

S.Liu,Opt.Lett.38,2240–2243(2013).

[13]https://www.wendangku.net/doc/4c14606232.html,kshmanan,Sintering of Ceramics-New Emerging

Techniques,(InTech,Rijeka,2012),chap.23.

[14]S.Nishiura,S.Tanabe,K.Fujioka,and Y.Fujimoto,

Opt.Mater.33,688–691(2011).

[15]S.Fujita,S.Yoshihara,A.Sakamoto,S.Yamamoto,and S.

Tanabe,Proc.SPIE5941,594111(2005).

[16]L.Wang,L.Mei,G.He,J.Li,and L.Xu,J.Am.Ceram.

Soc.94,3800–3803(2011).

[17]A.Keshavarzi,W.Wisniewski,and C.Russel,CrystEng-

Comm14,6904–6909(2012).

[18]S.Tanabe,S.Fujita,S.Yoshihara, A.Sakamoto,and

S.Yamamoto,Proc.SPIE5941,594112(2005).

[19]Y.K.Lee,J.S.Lee,J.Heo,W.B.Im,and W.J.Chung,Opt.

Lett.37,3276–3278(2012).

[20]J.T.Fan,X.Q.Yuan,R.H.Li,H.X.Dong,J.Wang,and

L.Zhang,Opt.Lett.36,4347–4349(2011).

[21]H.Segawa,S.Ogata,N.Hirosaki,S.Inoue,T.Shimizu,

M.Tansho,S.Ohki,and K.Deguchi,Opt.Mater.33,170–175(2010).

[22]Z.Y.Mao,Y.C.Zhu,L.Gan,Y.Zeng,F.F.Xu,Y.Wang,

and D.J.Wang,Mater.Lett.80,63–65(2012).

[23]J.C.Boyer and F.C.J.M.van Veggel,Nanoscale,2,1417–

1419(2010).

[24]S.K.W.MacDougall,A.Ivaturi,J.Maques-Heuso,K.W.

Kr¨a mer,and B.S.Richards,Opt.Exp.20,A879–A887 (2012).

C 2013by WILEY-VCH Verlag GmbH&Co.KGaA,Weinheim https://www.wendangku.net/doc/4c14606232.html,