Journal of Crystal Growth 361 (2012) 11–15

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Journal of Crystal Growth

0022-02

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journal homepage: www.elsevier.com/locate/jcrysgro

Growth and optical properties of Co,Nd:LaMgAl11O19

Peng Xu a,b,n, Changtai Xia a,nn, Juqing Di a,b, Xiaodong Xu c, Qinglin Sai a,b, Lulu Wang a,b

a Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, Chinab Graduate School of Chinese Academy of Sciences, Beijing 100039, Chinac Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China

a r t i c l e i n f o

Article history:

Received 5 April 2012

Received in revised form

5 July 2012

Accepted 20 August 2012

Communicated by R.S. Feigelson1056 nm and 1082 nm with full-width at half-maximum (FWHM) of 6 and 8 nm, respectively. The large

4

Available online 4 September 2012

Keywords:

LaMgAl11O19

Self-Q-switched

Co,Nd co-doped

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jcrysgro.2012.08.031

esponding author at: Key Laboratory of Mat

ai Institute of Optics and Fine Mechanics, Ch

ai 201800, China. Tel.: þ86 21 69918719; fax

.: þ86 21 69918719; fax: þ86 21 69918607.

ail addresses: xupeng-512@163.com (P. Xu), x

a b s t r a c t

Nd,Co:LaMgAl11O19 (abbreviated as Co,Nd:LMA) was grown using the Czochralski method. The

structure, polarized absorption spectrum, fluorescence spectrum, and fluorescence decay time were

analyzed. The as-grown crystal had very wide absorption bands at 794 nm, which can be pumped by

GaAs laser diode without temperature stabilization. Two strong emission bands were present at

FWHM is due to the inhomogeneity of the Nd ion sites. The lifetimes of the F3/2 manifold of Co,Nd:LMA

at room temperature monitored at 905 nm, 1056 nm, and 1344 nm were 292, 288, and 350 ms,

respectively, which was caused by the different contribution of the three different sites with D3h and

C2v symmetry. The absorption band of Co is from 1.3 mm to 1.6 mm, and Co,Nd:LMA still has a

strong emission around the 1.38 mm, indicating that the Co,Nd:LMA can be applied as a potential

self-Q-switched material operating at 1.3 mm.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Passive shutters based on saturable absorption are of greatinterest due to their applications for Q switching and modelocking of near-infrared solid-state lasers to obtain light pulsesof high power and short/ultrashort duration [1]. PassivelyQ-switched solid-state lasers with high peak power and shortpulse width are widely used in optical communications, pollutionmonitoring, material processing, medical surgery, and so on [2,3].In recent years, self-Q-switched laser materials, which combinethe functions of the gain medium and the saturable absorber (SA),have attracted much attention, due to their compactness, lowloss, and simplicity in the laser design and application [4].Because self-Q-switched microchip lasers have simple, short,plane-parallel cavities, single-longitudinal-mode oscillation canbe easily obtained by adopting a suitable pump power intensity,which makes it a very good choice to study the effect oftransverse modes on the laser instabilities [5]. Several materialshave been demonstrated excellent output properties in 1–1.1 mmwavelength range, such as the Cr4þ ,Yb3þ:YAG [6], Cr4þ ,Nd3þ:YAG [7], Cr4þ ,Nd3þ:YVO4 [8], Cr4þ ,Nd3þ:GdVO4 [4], Yb3þ ,Naþ:CaF2 [9].

ll rights reserved.

erials for High Power Laser,

inese Academy of Sciences,

: þ86 21 69918607.

ia_ct@siom.ac.cn (C. Xia).

However, because the saturable absorption range of the Cr4þ

is from the 900 to 1200 nm only [6], the Cr4þ cannot be used asthe saturable absorption ions for the Nd3þ (4F3/2-

4I13/2 at1.3 mm) and the Er3þ (4I13/2-

4I15/2 at 1.5 mm) to form the self-Q-switched laser. The 1.3 mm lasers coincides with the low-dispersive as well as the low-loss spectrum of silica fiber, so ithas wide applications in many fields, such as medicine, commu-nications and light-sensing [10], etc. Therefore, the passivelyQ-switched and mode-locked lasers operating at 1.3 mm havebeen attracted a great deal of attention [11]. In recent years,tetrahedral Co2þ-doped crystals have attracted lots of attentionand interest from researchers, because these crystals have efficientabsorption saturation in the spectral range of 1.3–1.6 mm, whichindicates potential application of these tetrahedral Co2þ-dopedcrystals as a saturable absorber Q switch for the 1.3–1.6 mmlasers [12]. some Co2þ-doped oxide crystals, such as the MgAl2O4

[13], Y3Sc2Ga3O12 [14], LiGa5O8 [15], ZnGa2O4 [16], YAG [17],LaMgAl11O19 [12], and so on, have been demonstrated to be theeffectively passive Q switches for the Nd3þ and Er3þ doped lasercrystal and glass operating at the at 1.3 and 1.5 mm. Specifically, theCo:LaMgAl11O19 (abbreviated as Co:LMA) has shown outstandingperformance as the passive Q switches operating for Nd3þ (1.3 mm)and Er3þ (1.5 mm) in many experiments [18–20] since the Co:LMAwas initially found by Yumashev et al. [21].

On the other hand, Nd3þ-doped LaMgAl11O19 (abbreviated asNd:LMA) also has been demonstrated to be as a very useful lasergain medium with unique optical and laser properties [22], suchas the high quenching concentrations of Nd3þ , broad absorption

P. Xu et al. / Journal of Crystal Growth 361 (2012) 11–1512

bands of Nd3þ in the vicinity of the GaAs laser diode [23,24], longlifetime of the Nd3þ(4F3/2), and so on. Nd:LMA has already beenutilized as the microchip lasers [25,26] and the He magnet-ometers with low noise [24,27,28] in modern society due toits special optical merits. In the LMA, the absorption band(1.2–1.6 mm) of the Co2þ caused by the substitution of Mg2þ siteswith tetrahedral symmetry, as well as the emission of the Nd3þ

(4F3/2-4I13/2 at 1.3 mm) may enable the Co and Nd co-doped LMA

to be a potential self-Q-switched laser material. Therefore, in thispaper, Nd3þ and Co2þ ions co-doped LMA crystal was grown usingthe Czochralski method and the optical properties of the as-growncrystal were analyzed.

Fig. 1. Photograph of as-grown Co,Nd:LMA crystal boule. (For interpretation of the

references to color in this figure, the reader is referred to the web version of this

article.)

2. Experiments

According to the formula La0.95Nd0.05Mg0.99995Co0.00005Al11O19,the raw materials, Co3O4 (99.9985%), Al2O3 (99.999%), MgO(99.99%), and La2O3 (99.999%), were weighted and properlymixed together in an agate ball mill for 24 h. As La2O3 can absorbmoisture in the air up to as much as 20 wt% during weighting, theLa2O3 powder was heated in a alumina crucible in a mufflefurnace at 1200 1C for 5 h to eliminate water. It was cooled downinside the furnace to 300 1C at which the La2O3 powder wastransferred from the furnace and weighted quickly before it canreabsorb any moisture. After a thorough mixing and grinding, themixture powder was extruded into pellets by cold pressing at120 MPa. Then, the pieces were calcined in an alumina crucible at1200 1C for 10 h in a muffle furnace, and subsequently cooleddown to room temperature in the furnace. The polycrystallineCo,Nd:LMA formed by solid-state reaction was charged in aniridium crucible of 80 mm in diameter heated by coupling it witha 10 kHz motor-generator via copper coupling coil. The Co,Nd:LMA single crystal was grown by the Czochralski (CZ) methodunder an atmosphere of pure N2. As the (001) of the LMA is acleavage plane, LMA growth in o0014 direction will result intwinning, strain, and cleavage of the crystal [29]. Thus, to avoidthe defects and obtain a high quality crystal, the o1004direction was adopted as the growth direction. Therefore, arectangular o1004-oriented LMA single crystal bar with dimen-sions of 4�4�20 mm3 was used as the seed. The pulling ratewas 1–2 mm/h, and the rotation rate was 10–20 rpm. Thetemperature was controlled by EUROTHERM 818 controller/pro-grammer with a precision of 70.1 1C. The growth period wasextended to 50 h followed by a cooling down period of 60 h.

The Co,Nd:LMA crystal boule was blue in color because of theCo2þ and free from cracks, inclusions, and scattering centers. Itssize was 28 mm in diameter and 70 mm in length, as shown inFig. 1. However, the boule of the as-grown crystal was notcompletely cylindrical in shape. Only two very smooth parallelplanes were present at the two sides of the boule due to thecleavage of (001) and some small stairs-like platforms were presentalso due to the cleavage of (001). The quality of as-grown crystalwas investigated under semiconductor laser, and no obvious opticalpath was observed. This result showed that the as-grown Co,Nd:LMA crystal had excellent optical quality, and can be used foroptical applications.

Two wafers with dimensions of 15�15�1 mm3, cut fromCo,Nd:LMA crystal, were polished to spectral quality. One waferfor polarized absorption spectra was perpendicular to theo0104 axis and the other one for fluorescence spectrum wasperpendicular to o0014 . X-ray diffraction (XRD) was performedby an automated Ultima IV diffractometer (Rigaku, Japan) usingCu Ka radiation at a scan width of 0.021 over a range of 2y¼10–901. The Co2þ and Nd3þ concentrations in the as-grown crystalwere analyzed using an inductively coupled plasma atomic

emission spectrometer (ICP-AES). For both XRD and ICP-AES, fineground powder of the as-grown Co,Nd:LMA single crystal wasused as sample. The polarized absorption spectra of the Co,Nd:LMA crystal were recorded by a Lambda 900 spectrophotometer(Perkin-Elmer, Company) at room temperature. The fluorescencespectrum was recorded by a spectrofluorometer (Fluorolog-3, JobinYvon, Edison, USA) equipped with a R5509-72 photomultipliertube. An 808 nm diode laser was used as the excitation source.The fluorescence decay time was measured using a spectrophot-ometer (FL920, Edinburgh), whereas a microsecond flash lamp(uF900, Edinburgh) was used as the exciting source. The wave-length range of absorption spectra and fluorescence spectrum were300–2000 nm and 850–1500 nm, respectively. All measurementswere performed at room temperature.

3. Results and discussion

The XRD pattern of the as-grown Co,Nd:LMA crystal is shownin Fig. 2, and some evident peaks are assigned. The diffractionpeaks and relative intensity of the as-grown crystal are in goodagreement with JCPDS card (78-1845), which shows that theobtained phase is an LMA without any parasitic phase. The Co,Nd:LMA crystal has a slightly distorted magnetoplumbite PbFe12

O19 (M-type) structure with hexagonal space group P63/mmc.Based on the data and calculation, the cell parameters: a¼

b¼0.537 nm, c¼2.188 nm, slightly less than pure LMA. Accordingto Ref. [21], the perfect M-type Me2þAl12O19 to the LaMgAl11O19

occurs through the substitution of one of the Al3þ ion by Mg2þ

ions and the Me2þ ions by La3þ ions. The Co2þ ions replace theMg2þ ions with a distorted tetrahedral coordination.

The effective segregation coefficients of Co2þ and Nd3þ in theLMA crystal can be calculated from the following formula:

Km¼ Ct=C0

where Ct is the Co2þ and the Nd3þ concentrations at the growthstarting position in the crystals, and C0 is the initial Nd3þ andCo2þ concentrations in the melt. The segregation coefficients ofthe Co2þ and Nd3þ ions in the LMA crystal were calculated to be0.3686 and 0.5237, respectively. The concentration of Co2þ andNd3þ in the samples for spectroscopic measurements can be

Fig. 2. XRD pattern of the Co,Nd:LMA crystal.

Fig. 3. Polarized absorption spectra of the Co,Nd:LMA at room temperature, inset shows the polarized absorption spectra of the Co:LMA.

Fig. 4. Schematic energy level diagram of the Nd3þ and tetrahedral Co2þ . (For

interpretation of the references to color in this figure, the reader is referred to the

web version of this article.)

P. Xu et al. / Journal of Crystal Growth 361 (2012) 11–15 13

calculated by

Cs ¼ C0Kmð1�gÞ1�Km

where g is the crystallized fraction of the melt, and g is 35.19% inour experiment. Km is the segregation coefficient, and C0 is theinitial Co2þ ion and Nd3þ ion concentration in the melt. Then, thevolume concentration can be calculated as 1.366�1020 cm�3 and1.278�1017 cm�3 for Nd3þ and Co2þ , respectively.

The polarized absorption spectra of the Co,Nd:LMA at the roomtemperature is shown in Fig. 3. As LMA is a uniaxial material, twoabsorption spectra depending whether the radiation polarizationis either parallel or perpendicular to the optic axis. For compar-ison, the polarized absorption of the 0.005 Co:LMA is also shownin the inset of Fig. 3. The Co:LMA was grown by the same methodas that of the Co,Nd:LMA. The strongest absorption was at 580 nmwhich was assigned to the spin and electric-dipole-allowed4A2-

4T1(4P) transition of Co2þ [23], thus, the color of theas-grown Co,Nd:LMA is dark blue. The wide absorption band of1200–1600 nm, as shown in Fig. 4, is due to 4A2-

4T1(4F) of theCo2þ . The 4T1(4P) and 4T1(4F) states of the Co2þ ions split into

P. Xu et al. / Journal of Crystal Growth 361 (2012) 11–1514

various crystal field components because of the influence offour oxygen atoms surrounding Co2þ in the distorted tetrahedral(C3v local site symmetry) position, which accounted for thewide absorption in the near-infrared centered at 1350 nm [30].The Co2þ ions absorption was quite weak in the region of700–1100 nm, so the energy loss caused by the absorption ofCo2þ can be neglected in the pump band around 800 nm, as wellas the emission band of Nd3þ ion around 1050 nm. Many experi-ments and references have demonstrated that the absorption regionfrom 1200 nm to 1600 nm can be used as passive Q-switch for theNd3þ-doped laser crystal [31,32] and the Er3þ glass [33,34].

As the absorption of Co2þ is very weak in the 580–1200 nmregion, the remaining absorption peaks are assigned to transitionsof the Nd3þ ions. As shown in Fig. 3, the strong absorption linesoccur at 627, 737 and 794 nm, corresponding to transitions of4I9/2-

2G7/2þ4G5/2, 4I9/2-

4S3/2þ4F7/2, and 4I9/2-

2H9/2þ4F5/2 of

Fig. 5. Room temperature fluorescence spectrum of the Co,Nd:LMA in the near-

infrared range.

Fig. 6. Fluorescence decay curve of the 4F3/2 ma

the Nd3þ ions, respectively, as shown in the schematic energylevel diagram of Fig. 4. Some of the weak absorption peaks arealso shown in Fig. 3. The full-width at half-maximum (FWHM) ofthe absorption that peaked at the 794 nm is approximately20 nm, and the large width absorption band enables the Co,N-d:LMA to be pumped by LD without temperature stabilization.When the radiation is polarized parallel to the c-axis, the absorp-tion band is a little red shifted with respect to the absorptionband for radiation polarized perpendicular to the optic c-axis.More importantly, the absorption strength of p (E99c) polarizedradiation is one-third less than that of s (E?c) polarized radiation.The similar phenomenon was also mentioned in Ref. [35].

The fluorescence spectrum from 850 nm to 1500 nm of theCo,Nd:LMA excited at 808 nm by AlGaAs LD is shown in Fig. 5. Thefluorescent emission shows two broad, principal peaks centeredat 1056 nm and 1082 nm due to the transition of the 4F3/2 to 4I11/2

of the Nd3þ . The FWHM of the emission peaks centered at1056 nm and 1082 nm are about 6 nm and 8 nm, respectively,which means that the Nd3þ-doped LMA can be applied as atunable laser crystal. The broad bandwidth of the emissionindicates an inhomogeneous broadening behavior. Nd3þ ionshave been demonstrated to occupy three different sites, insteadof one site, in an ideal magnetoplumbite structure. Moreover, theLa3þ ions substituting for the Al3þ also distort the crystal field inthe hexagonal structure, leading to large split in the energy levelsof the dopants, which contributes to broadening the emissionspectra of the Nd3þ [24,25,35]. The emission at 1082 nm can beexploited in lasers for tuning to the resonant spectral lines of 3Heand 4He atoms for use in magnetometry [24,27]. The emission ofthe Co,Nd:LMA around 1.3 mm is due to 4F3/2-

4I13/2 transition ofthe Nd3þ ions. Four strong emission peaks exist at 1299, 1344,1380, and 1400 nm due to the 4I13/2 manifold of Nd3þ . Theabsorption region of the Co2þ ion overlaps the region of theemission by the 4F3/2-

4I13/2 of Nd3þ ions. The broad 4A2-4T1(4F)

absorption band of Co2þ ions makes it a saturable absorber forNd3þ emission at 1.3 mm. Moreover, the absorption of Co2þ ionsat the pump wavelength of AlGaAs LD (808 nm) is very weak, andthe absorption loss of the pump power can be neglected. Theemission of Nd3þ ions, as well as the absorption of Co2þ ions,

nifold of Co,Nd:LMA at room temperature

P. Xu et al. / Journal of Crystal Growth 361 (2012) 11–15 15

enables the Co,Nd:LMA to be a potential laser material used forself-Q-switched solid-state lasers operating at 1.3 mm. The fluor-escence decay curve of the 4F3/2 multiplet is shown in Fig. 6, inwhich the lifetime of 4F3/2 was monitored at the peaks centered at905, 1056 and 1344 nm. The curves follow a nearly single-exponential decay behavior. The fluorescence lifetime tf wasfitted to be 292 ms (905 nm), 288 ms (1056 nm), and 350 ms(1344 nm). The decay times of these peaks should be the same,but they are not. The reason of this phenomenon is due tothe inhomogeneous occupancy of the Nd3þ ions. We haveearlier mentioned that Nd3þ occupied three different sites.It has demonstrated that the decay times of the three sites aredifferent. The lifetimes of the two similar sites (the D3h symmetrywith a C2v distortion) are different but not by a large amount, andthe lifetime of the last one (C2v symmetry) is two times as long asthe other two sites (the D3h symmetry with a C2v distortion) [23].Therefore, the cause of the deviation is assumed to be due to thehigher ratio of the emission by the sites with the C2v symmetry at1344 nm, which means that the sites with the C2v symmetrycontribute more than the other two sites with D3h symmetry tothe emission at 1344 nm. The difference in the lifetimes mon-itored at the 905 nm and 1056 nm is due to the same reason.

4. Conclusion

High optical quality Co,Nd:LMA crystal was grown usingCZ-method in the o1004 direction. The segregation coefficientsof Co2þ and the Nd3þ in LMA crystal were calculated to be 0.3686and 0.5237, respectively. The broadening behavior of the absorp-tion and emission of the Co,Nd:LMA was due to the three differentsites occupied by Nd3þ ions. The wide absorption centered at793 nm can enable diode pumping without temperature stabili-zation, and the wide emission band can be utilized as the tunablelaser gain medium. More importantly, the emission of the peakcentered at 1082 nm can be employed for magnetometry. Thelifetimes of the 4F3/2 manifold of Co,Nd:LMA at room temperaturemonitored at 905 nm, 1056 nm, and 1342 nm were 292 ms,288 ms, and 350 ms, respectively, caused by the different contri-butions of the three different sites with D3h and C2v symmetries.The combination of the emission of Nd3þ due to the 4F3/2-

4I13/2

and the absorption of the Co2þ caused by 4A2-4T1(4F) may make

the Co,Nd:LMA to be a potential laser material used for compact,self-Q-switched, micro-chip solid-state lasers operating at 1.3 mm.

Acknowledgments

This work was supported by National Natural Science Founda-tion of China (No. 61078054), Science and Technology Commission

of Shanghai Municipality (No. 11DZ1140301) and Science andTechnology Innovation Project of Shanghai Institute of Ceramics,Chinese Academy of Sciences (No. Y24ZC5150G).

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