Na Modification of Lanthanide Doped Ca3Nb1.5Ga3.5O12-Type

Article pubs.acs.org/crystal

Na Modification of Lanthanide Doped Ca3Nb1.5Ga3.5O12-Type Laser Garnets: Czochralski Crystal Growth and Characterization Elena Castellano-Hernández, María Dolores Serrano,* Rafael J. Jiménez Riobóo, Concepción Cascales, and Carlos Zaldo Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, c/Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain

Andrzej Jezowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Okolna Str., 50-422 Wroclaw, Poland

Pavel A. Loiko Center for Optical Materials and Technologies, Belarusian National Technical University, 65/17 Nezavisimosti Avenue, 220013 Minsk, Belarus S Supporting Information *

ABSTRACT: The Na+-Yb3+ (or Er3+) co-substitution of Ca2+ in Ca3Nb1.5Ga3.5O12 (CNGG) laser crystal is studied. In contrast to other garnets whose structural disorder is exclusively based on the presence of differently sized cations on the same crystal sites, Na+ incorporated in the dodecahedral site (a site also shared by Ca2+ and trivalent lanthanides) creates diverse electric charge distributions over the dodecahedral sublattice, which adds to the disorder associated with Nb5+ and Ga3+ simultaneous occupation of the octahedral and tetrahedral sites. The currently determined cationic compositions of Czochralski grown congruent CNGG and Na-modified CNGG crystals show that Na+ incorporation reduces the cationic vacancy concentration on dodecahedral and octahedral sites but does not affect that in tetrahedral sites. Physical properties of interest for laser design (optical transmission, elastic constants, hardness, specific heat, thermal conductivity, thermal expansion, refractive index dispersion, group velocity dispersion, and thermo optic coefficients) have been systematically determined at cryogenic temperatures and above room temperature. Na+ incorporation into CNGG decreases the crystal growth temperature, promotes Yb3+ doping, and importantly, increases the Yb3+ optical bandwidth, offering good prospects for the implementation of ultrashort pulses in mode-locked laser oscillators.

1. INTRODUCTION

Ln:YAG. The use of crystals with large absorption bandwidth as laser media was first motivated by the minimization of the unstabilities associated with the emission wavelength thermal drift of diode lasers, DL, used for optical pumping, but more recently, great interest in Yb doped disordered crystals has arisen associated with the production of ultrashort (fs) laser pulses. Mode-locked ultrashort laser pulsed systems are usually built in two stages: the oscillator and the amplifier. At the oscillator stage pulses with very short duration are generated although the energy per pulse is rather low (∼10−9 J). Energy is added to the

Trivalent lanthanide, Ln, doped single crystals with large absorption and fluorescence bandwidths have been known since the early days of solid state laser technology. Most representative crystals of this class are tetragonal double tungstates and double molybdates like NaY(W/MoO4)2, solid state mixtures of RE3(Al/Ga)5O12 (RE= Sc, Y, and Ln) garnets, Ln doped CaF2 fluorite, Y3CaGdAlO4, Ca4GdO(BO3)3, SrY4(SiO4)3O apatite, LuScO3 sexquioxide, or Gd1−xYxVO4. However, the interest for these crystals as optical gain media in lasers was not initially recognized because large optical bandwidths are inherently associated with a reduction of peak optical cross sections;1 thus, in continuous wave, cw, operation the laser efficiencies of these crystals are lower than those of crystals with a unique Ln center, like Ln:Y3Al5O12 garnet, © XXXX American Chemical Society

Received: November 14, 2015 Revised: January 7, 2016

A

DOI: 10.1021/acs.cgd.5b01607 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

physical properties. Actually, only an X-ray diffraction study22 has explicitly reported the composition of CNGG crystals as {Ca2.95□0.05}[Ca0.07Nb1.62Ga0.275□0.025](Ga2.925Nb0.05□0.025)O12 (it is worth noting that in the translation of ref 22, the Ga population in the tetrahedral site is wrongly written as 2.295), i.e., vacancies in the three cationic crystal sites, small amounts of Ca and Ga in the octahedral site, and residual Nb population in tetrahedral sites were found. In the present work we explore, for the first time to our knowledge, the charge balanced substitution of two divalent Ca ions by a monovalent Na-trivalent Yb pair in the CNGG garnet structure. This cosubstitution maintains charge neutrality while the sum of monovalent Na (1.18 Å in VIII coordination) and trivalent Yb (0.985 Å in VIII coordination) radii is roughly twice the divalent Ca ionic radius (1.12 Å in VIII coordination); thus, little crystal elastic stress should be introduced. Hereafter, CNGG crystals incorporating Na will be named as CNNGG. We show that this strategy promotes ytterbium incorporation and enlarges the optical bandwidth of the Yb Stark−Stark transitions. The crystalline structure of CNGG is revisited by using modern equipment and analysis techniques for single crystal X-ray diffraction, scXRD, and the effects of Na−Yb codopands on it and on physical properties are studied. It is found that Na sits on dodecahedral sites, and thus it behaves differently from Li that, supposedly, fills cationic vacancies.

pulses in the amplifier. Optical gain crystals used in oscillators must have a very large fluorescence bandwidth to obtain ultimate short pulse duration, τ, since this and pulse bandwidth, Δν, are inversely related; for instance, Δν × τ = 0.441 for a Gaussian shaped pulse. Ln in perfectly ordered single crystals have narrow room temperature bandwidths (typically ≤50 cm−1) and therefore cannot support laser pulses shorter than ∼250 fs. Thus, inhomogeneously broadened Ln doped crystals are being sought out for this laser application. From the previously mentioned crystals only garnet, sexquioxide, and fluorite structures have isotropic physical properties, which is desirable for laser design and manufacture. However, only small sexquioxide crystals with limited optical quality have been grown so far due to the very high (∼2730 K) growth temperature required. On the other hand, the low phonon energies of fluorides produce a discontinuous tuning over the whole bandwidth of the Ln even in inhomogeneously broadened cases. Cubic (space group Ia3̅d, no 230, Z = 8) garnets with {M}3[N]2(R)3O12 formula, where {M}, [N], and (R) are dodecahedral (24c, 8 coordinated), octahedral (16a, 6 coordinated), and tetrahedral (24d, 4 coordinated) sites, respectively, are one of the best options to produce inhomogeneously broadened Ln doped laser crystals because hard single crystals can be grown in large sizes by the Czochralski, CZ, technique. The most common strategy used so far to induce inhomogeneous broadening in garnets has been the isovalent (3+) substitution over any of the three cationic sites of the structure, like in {Ln-Sc}3[Sc-Me]2Me3O12 (Me = Ga or Al),2,3 or a combined aliovalent substitution over the dodecahedral (2+) and tetrahedral (4+) sites, like Ca3Ga2Ge3O12, CGGG, and other more complex Ge garnets.4 The common feature of all these substitutions is that the three cationic positions are fully occupied. An alternative situation is found for the aliovalent substitution over the octahedral site giving rise to Ca3[Nb2‑yGay]Ga3□O12, hereafter CNGG, crystals. The congruent CNGG crystal composition differs from the stoichiometric one (y= 0.5) and cationic vacancies, □, are present to maintain charge neutrality in the congruent case. This particular feature broadens all Stark−Stark transitions between 2S+1LJ multiplets of the Ln and induces glass-like optical absorption and fluorescence, which has been well documented in Nd:CNGG single crystals.5,6 Laser operation of CNGG crystals was first studied in flash lamp pumped optical cavities when codoped with Cr3+ (as activator) and Nd,7 Tm,8 Ho,9 or Er.10 Similarly, laser properties were also found for the isostructural Ca3(Ta1.5Ga0.5)Ga3O12, CTGG, crystal as well as for Li modifications of the two crystals above, i.e., CLNGG and CLTGG. Later on, a significant amount of work has been devoted to exploring the laser properties of these crystal garnets under DL pumping. Tunable cw and Q-switch laser operation have been shown for Nd,11−13 Tm,14 and Yb15,16 doped CNGG, CLNGG, and CTGG crystals. Further, laser pulses were produced by modelocking first in the ps range12,17 and later in the fs time domain,18−21 both in Yb or in Tm doped crystals. So far, the shortest (55 fs with ⟨POUT⟩ = 60 mW) laser pulses reported at the maximum gain wavelength for a garnet has been obtained with Yb:CLNGG crystal,20 showing the great potential of these crystals to generate ultrashort laser pulses in oscillators. Despite the interest in CNGG type crystals for laser applications, limited information is available regarding their defect structure and on the effects of Ln doping on growth and

2. EXPERIMENTAL TECHNIQUES Crystals were grown in air or under pure oxygen atmosphere by using a RF(9.2 kHz)-heating Cyberstar pulling apparatus with a ceramic after-heater and platinum crucibles (40 mm in diameter and height). The system monitors the crystal weight to control its diameter. The phase composition of the synthesized starting materials was determined by powder X-ray diffraction, pXRD, at 300 K. The melting behavior under argon atmosphere of the single crystals and powders was studied by differential scanning calorimetry, DSC, in a Setaram equipment, model Setsys 16 Evolution, at a heating/cooling rate of 10 K/min. Crystal structures were determined from scXRD data collected at 300 K with a Bruker Kappa Apex II diffractometer equipped with a fine-focus sealed tube working at 50 kV/30 mA. Mo Kα radiation (0.71073 Å) was selected with a graphite monochromator. Small prismatic single crystals cut from grown crystal boules were selected in order to minimize the possibility of twinning. Data were corrected for absorption using the multiscan method (SADABS). All calculations were performed using the SHELXTL program.23 More details of the experimental procedures and data analyses can be found in the Supporting Information, SI, (Tables SI.1−SI.5). Further elemental analyses were made by wavelength dispersive X-ray fluorescence spectroscopy in a MagiX spectrometer equipped with a 2.4 kW Rh anode generator. Quantitative analyses were based on calibration curves made with specifically developed standards. Surface mechanical tests were made at 300 K with a Micro Materials instrument using a three-sided Berkovich diamond nanoidenter. The penetration depth was varied between 50 and 800 nm. The hardness, H, and reduced modulus, Er, of CNGG crystal were obtained from the charge vs displacement hysteresis curves by the Oliver-Pharr method.24 Specific heat at constant pressure, Cp, was measured for T < 397 K with a Quantum Design Physical Property Measurement system. The evolution of the thermal conductivity with temperature, κ(T), was measured by the steady-state longitudinal heat-flow method in the T = 4−360 K range using prismatic (∼3 × 3 × 10 mm3) crystal bars mounted in a liquid He cryostat for the study of heat propagation along the largest sample dimension. Thermal expansion coefficient, α, was measured with a horizontal dilatometer Netzsch 402PC in the T = 573−773 K range with the precision of ∼0.1 × 10−6 K−1. B



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Dispersion of the refractive index, n(λ), has been determined at 300 K by the minimum deviation angle method using a prism, a goniometer, and several laser systems (N2, dye, Ar, Ti-sapphire, and MOPO lasers) to cover the λ = 0.35−2 μm range. The uncertainty of this method (Δn≈ 10−3) mainly arises from the determination of the prism angle. In order to improve the accuracy of our measurements we deduced this angle from more accurate (Δn ≈ 10−5) measurements using a Metricom system and a calibrated prism (n = 2.8652 at λ = 0.6328 μm). This latter technique was also applied for n measurements in Er or Yb doped CNGG and CNNGG crystals. For thermo-optic characterization of CNGG crystal, a prismatic (4.26 × 3.52 × 11.6 mm3) sample with all faces perpendicular to ⟨100⟩ directions was used. The thermal coefficient of the optical path, TCOP, W = dn/dT + (n − 1)α, was measured at T ≈ 313 K by the laser beam deviation method for a medium with a linear thermal gradient25 (dn/dT is the thermo-optic coefficient, TOC). The precision of TOC measurements was ∼0.5 × 10−6 K−1. Brillouin spectra were obtained at 300 K in transmission configuration at an incident angle of 45°. The scattered light was analyzed using a Sandercock-type 3 + 3 tandem Fabry−Pérot interferometer; more details about Brillouin measurements can be found in the SI. Raman spectra were collected at 300 K exciting with an Ar laser tuned at λ = 0.488 μm. Other spectroscopic measurements were performed in the T = 6−300 K range with the help of a closed cycle He cryostat: Optical absorption, OA, measurements in the visible and near-infrared (λ < 2 μm) were determined by using a Varian spectrophotometer, model Cary 5E. Beyond 2 μm, OA was determined with a FFT Bruker spectrophotometer, model IFS66vIS with 2 cm−1 of resolution; for this purpose ground crystals were dispersed in KBr pellets. Yb lifetime, τ, measurements were performed at 300 K exciting with a MOPO system tuned at λ = 0.9715 μm. The λ = 1.035 μm fluorescence dispersed with a grating monochromator was detected with a Peltier cooled Hamamashu photomultiplier, model R2658, and averaged with a Lecroy oscilloscope. Preliminary laser characterization was also performed under Ti-sapphire laser pumping. These results can be found in the SI.

Figure 1. θ−2θ pXRD 300 K scans of solid state synthesized powders of (a) Yb:CNGG and (b) Yb:CNNGG, compounds. Peaks corresponding to the Ca2Nb2O7 phase are labeled (*). (c) Scan corresponding to the grown Ca3Nb1.6875Ga3.1875O12 (in the melt) powdered single crystal and the JCPDS file 04−006−0824 corresponding to Ca3.02Nb1.67Ga1.67O12 composition are given for reference.

For crystal growth, the garnet phase was first obtained by solid state reaction as described above. A platinum wire or a [111] oriented YAG crystal were used as seeds. The seed was rotated (15−30 rpm) and pulled (1−2 mm/h) simultaneously. A neck of about 10 mm in length and 5 mm in diameter was first grown, and afterward a shoulder region was grown to obtain a diameter about 16−20 mm, and finally the body of the crystal was pulled. Total crystal length varied between 25 and 60 mm. After growth, the crystal was cooled down in two stages, at the highest temperature the RF heating power was reduced slowly (2−3 days), and then to zero over the next 1−2 days. Figure 2 shows pictures of CNGG and Yb:CNGG grown

3. CRYSTAL GROWTH Preliminary to crystal growth, we studied the coexistence of the garnet phase with foreign phases for Yb:CNGG and Yb:CNNGG compounds synthesized by solid state reaction. For this purpose precursor products, CaCO3 (Alfa Aesar 99.5%), Na2CO3 (Alfa Aesar 99.5%), Nb2O5 (Aldrich 99.9%), Ga2O3 (Aldrich ≥99.99%), Er2O3 (Alfa Aesar 99.99%), and Yb2O3 (Alfa Aesar 99.998% -Yb5N-, 99.99% -Yb4N-, and 99.9% -Yb3N-), were mixed together and heated to 1648−1673 K for 6 h. After cooling to 300 K the powder was ground and newly heated to the same temperatures during the next 6 h. The top heating temperature was limited by melting and depends on dopant concentration. At this stage it was qualitatively observed that compounds with Na and Yb have lower melting temperatures that those singly doped with Yb. Figure 1 shows the θ−2θ pXRD scans of the resulting powders. The garnet phase is always present, but in Yb:CNGG a foreing peak at 2θ = 30.1° is observed for Yb concentration ≥6 at%. This peak, corresponding to the cubic pyroclore Ca2Nb2O7 phase, is also observed in Yb:CNNGG products but only for Yb concentration higher than 10 at%. Further DSC analyses determined that the melt of Yb:CNNGG powders is congruent up to 20 at% Na:20 at% Yb:CNNGG and the melting temperature of the latter composition is 20 K lower than that of CNNG. Therefore, Na codoping helps to incorporate Yb into the garnet phase and reduces the growth temperature.

Figure 2. Images of some CZ grown garnet single crystals. (a) CNGG crystal nucleated on a platinum wire. (b) 8.9 at% Yb:CNGG crystal nucleated on a [111] YAG seed. (c) CNGG crystal prism used for the determination of thermo-optic coefficients. (d) Polished crystal slab sliced from a 7.6 at% Yb:CNGG crystal. C



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Table 1. Overview of Growth Conditions and Compositions of CNGG and CNNGG Crystals Obtained by the Czochralski Methoda I

II

III

Ca3Nb1.6875Ga3.1875O12b,f,h Ca2.955Nb1.556Ga3.207O11.655c,j Ca3.041Nb1.668Ga3.180d,j Ca3.115Nb1.647Ga3.179d,m Ca2.76Yb0.24Nb1.6875Ga3.1875O12b,f,i

8/7.98e Yb3N

Ca2.76Yb0.24Nb1.6875Ga3.1875O12b,f,i

8/7.6e Yb4N

Ca2.7Yb0.3Nb1.6875Ga3.1875O12b,f,i

10/8.9e Yb5N

Ca2.65Yb0.35Nb1.6875Ga3.1875O12b,f,h Ca2.55Yb0.45Nb1.6875Ga3.1875O12b,f,h Ca2.514Yb0.475Nb1.269Ga3.505O11.657c Ca2.65Na0.175Yb0.175Nb1.6875Ga3.1875O12b,f,h

11.6/9.3e Yb3N 15/15.8e Yb3N

Ca2.46Na0.36Yb0.18Nb1.6875Ga3.1875O12b,f,i

12/ND Na 6/6.15e Yb4N

Ca2.61Na0.18Yb0.21Nb1.6875Ga3.1875O12b,f,i

6/ND Na 7/5.9e Yb4N

Ca2.52Na0.24Yb0.24Nb1.6875Ga3.1875O12b,f,h

8/ND Na 8/8.3e Yb4N

Ca2.28Na0.48Yb0.24Nb1.6875Ga3.1875O12b,f,h Ca2.367Na0.354Yb0.279Nb1.341Ga3.479O11.534c,j Ca2.373Na0.384Yb0.243Nb1.294Ga3.526O11.491c,m Ca2.454Na0.293Yb0.253Nb1.6875Ga3.1875O12b,g,i

16/11.7j−12.8c,m Na 8/9.3j−8.1c,m Yb4N

Ca2.28Na0.36Yb0.36Nb1.6875Ga3.1875O12b,f,h Ca2.352Na0.168Yb0.483Nb1.558Ga3.319O12.034c

12/5.6c Na 12/16.1c Yb4N

Ca2.9877Er0.0123Nb1.6875Ga3.1875O12b,f,h Ca3Nb1.6875Ga3.1875O12+Er2O3b,f,h Ca2.931Er0.054Nb1.363Ga3.411O11.536c

0.41/0.35e Er ND/1.8c−3.7d Er

IV 20

30.8

1.5j 1.8k,l,m 1.4j 1.5k,l,m 1.8j,k 2l,m 1.1 2

20

60.3

30j,k,l 25m 20j,k,l 15m 20 20

30.9

1.4j 1.7k 1.9l,m 1.5j 1.8k 2l,m 1.8j 1.8k 2l,m 1.4j 1.5k 1.7l,m 1.6j 1.8k 2l,m 1.6j 1.8k 2.1l,m 1.2j 1.4k 1.7l,m 2 2

5.8/ND Na 5.8/6.6e Yb4N

9.8/ND Na 8.4/10.2e Yb4N

V

2

22.1 13.6

30j,k 25l 20m 30j,k 25l 20m 20j,k 20l 15m 35j,k 30l 2m 30j,k 25l,m

35.3

30j,k 25l,m

43.3

15

19.4

20 20

ND ND

130

64.8

51.8

18.5

a

I = Melt and calculated compositions. II = Melt/Crystal dopant concentrations (at%). III = Pulling rate (mm/h). IV = Rotation rate (rpm). V = Color, ∫ α(λ)dλ from λ = 0.55 to 0.8 μm (×10−7). ND = No determined. bMelt composition. cscXRD calculated composition. dX-ray fluorescence calculated composition. eOptical absorption calculated composition. fGrown in air. gGrown in pure oxygen. hPt seed. iYAG seed. jNeck. kShoulder. l Body. mTail.

Table 2. Unit Cell Parameter, a (Å), Oxygen Atomic Coordinates, x, y, z (×104) and Cationic Occupancy Factors, OF, for Shared 24c, 16a, and 24d Sites in CNGG, Ln(Yb,Er):CNGG, and Yb:CNNGG Crystalsa OF 24c - dodecahedral site crystal CNGG 15.8 atom %Yb:CNGG 11.7 atom %Na:9.3 atom % Yb:CNNGG 12.8 atom %Na:8.1 atom % Yb:CNNGG 5.6 atom %Na:16.1 atom % Yb:CNNGG 3.7 atom %Er:CNGG a

a

Ca

2+

+

Na

Ln

3+

12.4969(1) 12.4603(2) 12.4842(1)

0.985 0.838 0.789

0.118

0.157 0.093

12.4890(1)

0.791

0.128

12.4611(1)

0.784

0.056

12.4922(1)

0.977

16a - octahedral site □

Ga1

3+

24d - tetrahedral site

□

Ga23+

Nb25+

□

R1

0.003 0.002

0.821 0.921 0.935

0.102 0.005 0.005

0.077 0.074 0.061

0.0109 0.0116 0.0127

0.625 0.627 0.663

0.372 0.371 0.337

0.081

0.638

0.362

0.934

0.006

0.060

0.0139

0.161

0.773

0.227

0.955

0.004

0.041

0.0131

0.620

0.378

0.885

0.041

0.074

0.0118

0.018

0.015 0.005

Nb1

5+

0.005

0.002

Final R1 factors [reflections with I > 2σ(I)] of corresponding structure refinements are indicated.

D



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Table 3. Selected Bond Lengths and Intermetallic Distances (Å) in CNGG, Ln(Yb,Er):CNGG, and Yb:CNNGG Crystals Ca/Ln/Na−O 24c crystal CNGG 15.7 atom %Yb:CNGG 11.7 atom %Na:9.3 atom % Yb:CNNGG 12.8 atom %Na:8.1 atom % Yb:CNNGG 5.6 atom %Na:16.1 atom % Yb:CNNGG 3.7 atom %Er:CNGG

Nb1/Ga1−O Ga2/Nb2−O 16a

24d

Ca/Ln/Na−Nb1/Ga1

Ca/Ln/Na−Ga2/ Nb2

Ca/Ln/Na−Ca/ La/Na

24c-16a

24c-24d

24c-24c

×4

×4

×6

×4

×4

×4

×2

×4

2.413(1) 2.397(1) 2.407(1)

2.539(1) 2.515(1) 2.531(1)

1.987(1) 1.995(1) 1.990(1)

1.851(1) 1.848(1) 1.849(1)

3.4930 3.4828 3.4894

3.8264 3.8152 3.8225

3.1242 3.1151 3.1210

3.8264 3.8152 3.8225

2.409(1)

2.536(1)

1.988(1)

1.849(1)

3.4908

3.8240

3.1223

3.8240

2.399(1)

2.519(1)

1.992(1)

1.848(1)

3.4830

3.8154

3.1153

3.8154

2.410(1)

2.535(1)

1.988(1)

1.850(1)

3.4917

3.8249

3.1230

3.8249

crystals. Table 1 summarizes all crystals studied in this work, their growth conditions and the compositions determined. Crystals nucleated on platinum wire have a preferential orientation with the [100] crystalline direction parallel to the pulling axis and well developed lateral faces perpendicular to [100] and [110] directions, the cross section perpendicular to the pulling axis is near square, but up to 15° of random departure from this nominal orientation has been found. Crystals nucleated on [111] YAG seeds replicate the seed orientation and have hexagonal cross sections. Undoped CNGG crystals appeared free of macrodefects and cracks. Single Ln doped CNGG crystals show that some cracks developed presumably upon crystal cooling; Na codoping did not change this situation significantly.

unchanged. Additional incorporation of monovalent Na fully removes the cationic vacancies in (24c) and (16c) sites, but has little effect on the vacancy concentration in the tetrahedral (24d) site. Table 3 summarizes the metal−oxygen bond lengths for the three kinds of coordination polyhedra, as well as main intermetallic distances ( 2σ(I)] for each analyzed crystal. Positive values of the anisotropic thermal displacements have been obtained for all atoms in these crystals (see SI, Table SI.5). Further details and results of structure refinements can be consulted in the SI. Table 2 shows that the partial substitution of Ca (ionic radius of VIII coordinated divalent Ca is 1.12 Å) in CNGG crystals by smaller Er (ionic radius of VIII coordinated trivalent Er is 1.004 Å) or Yb (ionic radius of VIII coordinated trivalent Yb is 0.985 Å) ions leads to the decrease of the unit cell parameter a (Å) from 12.4969(1) to 12.4922(1), in the first case, and more substantially to 12.4603(2), for the 15.8 at% Yb doped crystal. The simultaneous incorporation of Na (ionic radius of VIII coordinated monovalent Na is 1.18 Å), which replaces Ca at dodecahedral 24c sites, and Yb reverses this trend, and the lattice parameter of Yb:CNNGG approaches that of CNGG when Na incorporation was increased, i.e., a reaches 12.4890(1) Å in a 12.8 at% Na:8.1 at% Yb:CNNGG crystal. Current data indicate that the incorporation of Ln (Er or Yb) in dodecahedral (24c) sites reduces by a factor 3 and 1.5 the concentration of cationic vacancies in this (24c) and in the octahedral (16a) sites, respectively, while the concentration of vacancies in tetrahedral (24d) positions remains basically

Figure 3. Room temperature Raman spectra of CNGG, Yb:CNGG, and Yb:CNNGG crystals. The spectra have been arbitrarily shifted in the y-axis to facilitate understanding of the figure.

shifted by 771/791 and 831/846 cm−1 correspond to coupling with vibration modes of tetrahedral [GaO4] and [NbO4] complexes, respectively. The double structures observed in both cases were ascribed to the modes of a regular oxygen tetrahedron (either with Ga or Nb) and to an oxygen tetrahedron with a nearby cationic vacant position. This structure can be observed in Figure 3 for our CNGG crystal. Similar spectra obtained in Yb:CNGG show a decrease of the intensity of the satellite peaks at 791 and 846 cm−1, which according to the above model could be understood as a decrease of the nearby vacancy concentration. This conclusion agrees with the result of the scXRD structural analysis (Table 2) that shows a significant reduction of vacancies in the dodecahedral and octahedral garnet sites with Yb incorporation. E



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to (110) and to (100) crystal faces, but changes are smaller than 10%. The largest hardness occurs for (100) face with H ≈ 13 GPa and Er ≈ 200 GPa. 5.2. Thermophysical Properties. 5.2.a. Specific Heat. Figure 5 shows the thermal evolution of the specific heat, Cp, of

The Raman spectra of Yb:CNNGG crystals are broader than the previous ones, and the double peak structure is not resolved. Since Na incorporation practically eliminates vacancies in dodecahedral and octahedral sites, the origin of these structureless peaks must be attributed to distortions of the regular tetrahedra most likely caused by the spatially variable distribution of Ca, Na, and Yb at dodecahedral positions, which are located at rather short intermetallic distance, ∼3.12 Å, from these tetrahedra (see Table 3).

5. PHYSICAL CHARACTERIZATION 5.1. Mechanical Properties. 5.1.a. Thermal Expansion Coefficient. The average value of the determined thermal expansion coefficient, α, of CNGG in the T = 300−500 K range is α = 7.8 × 10−6 K−1. This result is close to recently reported values for Nd:CNGG, α = 7.9 × 10−6 K−1,26 and Nd:CLNGG, α = 8.2 × 10−6 K−1.27 5.1.b. Elastic Properties. Elastic constants were obtained from the acoustic wave velocity as cij = ρv2, where ρ is the density of the medium, ρ = 4814 Mg/m3 for our CNGG crystal (see SI, Table SI.1). Brillouin spectroscopy provides the velocity of acoustic waves as v = λ0Δν/2 sin θ, where λ0 is the vacuum wavelength of the light, Δν the frequency shift of the Brillouin signal, and in our experimental setup the incident angle is θ = 45°. Brillouin measurements were made on a CNGG plate cut perpendicularly to the [001] crystal axis and rotated around this axis by an azimuthal angle. Brillouin spectra can be found in the SI (see Figure SI.2). Figure 4 shows the evolution of the quasi-

Figure 5. Temperature dependence of the specific heat, Cp, (△) of CNGG crystal and thermal conductivity, κ(T), of CNGG (●) and 3.7 at% Er:CNGG (○).

CNGG crystal from 2 to 396 K. It increases monotonously with temperature. At 300 K Cp = 0.560 J/gK, which is very close to the value previously reported for 0.5 at% Nd:CNGG, Cp = 0.595 J/gK at 303 K.26 5.2.b. Thermal Conductivity. Figure 5 also shows the κ(T) dependence of CNGG (sample dimensions 3.36 × 3.51 × 10.864 mm3) and 3.7 at% Er:CNGG (sample dimensions 3.59 × 4.45 × 5.62 mm3) crystals in the temperature range 5 to 300 K. For both samples the qualitative evolution is similar: κ increases at the lowest investigated temperatures, reaches a maximum, and decreases for higher temperatures. For dielectric crystals, formation of such a maximum is the result of an interplay between increasing (with temperature) energy of phonons and increasing intensity of three-phonon scatterings in so-called U-processes, leading to strong thermal resistivity.29 The initial low temperature increase follows a κ ∼ Tk law, with k = 2 and k = 1 for CNGG and 3.7 at% Er:CNGG crystals, respectively. At the maximum κ = 81 W m−1 K−1 for CNGG and κ = 19 W m−1 K−1 for 3.7 at% Er:CNGG. Er shifts the temperature of the maximum from 13 to 28 K. The indicated differences are due to phonon scattering at Er point centers, decreasing κ. At the temperatures just above the maximum κ drops exponentially in both cases. At higher temperatures the change becomes gradually weaker and finally attains κ ∼ T−1 dependence. Here the U-processes are much more significant than point (Er atoms) phonon scatterings; therefore, at high temperatures κ for both samples become close each other, attaining at 300 K a unique value of 4.3 W m−1 K−1. This measured value is slightly higher than the value previously calculated by the laser flash method in a 0.5 at% Nd:CNGG crystal, κ = 3.43 W m−1 K−1.26 5.3. Optical Trasmission. Figure 6 shows several optical absorption features of CNGG crystals. The 300 K bandgap associated with the garnet structure has an absorption edge (corresponding to an optical absorption coefficient ξ = 200 cm−1) at λ = 0.2848 μm (4.35 eV) which shifts to λ = 0.2808 μm (4.42 eV) at 6 K (see Figure 6a). The greenish to pale yellow color observed in the crystals is related to much weaker

Figure 4. Velocity of quasi-longitudinal (■) and quasi-transverse (○) acoustic waves propagating at 300 K in different directions of a CNGG crystal. The lines are the fits.

longitudinal and quasi-transverse acoustic wave velocities for different azimuthal angles of the sample. A 90° periodicity is observed with maxima and minima corresponding to the [100] and [110] crystal directions. The experimental azimuthal angle dependences of the quasi-longitudinal and quasi-transverse acoustic branches were fit with Christoffel’s equations given in the SI. The elastic constant set obtained from a minimum square deviation fit for the CNGG crystal is c11= 236.9 ± 4 GPa, c12= 82.3 ± 4.5 GPa, and c44= 72.10 ± 2 GPa. 5.1.c. Surface Hardness. Knowledge of the hardness, H, of garnets is desirable for optimization of polishing procedures. In garnets H is known to depend slightly on crystal orientation.28 The results of hardness and reduced modulus, Er, for three orientations of CNGG crystal can be found in the SI (see Figure SI.3). Both mechanical parameters increase from (111) F



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An important issue to obtain short laser pulses is the control of the pulse chirping that enlarges the pulse duration over the round trip in the cavity. This is related to the group velocity dispersion, GVD, of the different optical elements interacting with the pulse, and in particular with the GVD of the crystal used as gain medium. GVD is defined as GVD =

λ 3 ⎛ d2n ⎞ ⎜ ⎟ 2πc 2 ⎝ dλ 2 ⎠

(2)

therefore, GVD can be calculated from the Sellmeier parameters. A graphic representation of GDV(λ) can be found in the SI (Figure SI.4). At λ = 1.040 μm (typical emission of Yb), GVD = 152.4 fs2/mm. 5.4.b. Effect of Dopants. Figure 7 shows the refractive index determined by the prism coupling method for the crystals

Figure 6. Optical absorption of CNGG crystal. (a) Absorption coefficient, ξ, near to the bandgap obtained with a 68-μm-thick plate. (b) Bands related to the crystal coloration measured at 6 K (solid line) and 300 K (dashed line). (c) Infrared absorption of ground crystal (solid line), as well as 10 at% Na:10 at% Yb:CNNGG (blue dashed line) and 20 at% Na:20 at% Yb:CNNGG (red dashed line) solid state synthesized powders. Figure 7. Refractive index, n, at λ = 0.6328 μm of CNGG (○), Er:CNGG (▲), Yb:CNGG (◇), and Yb:CNNGG (□). Crystals with largest green coloration are labeled with G.

isolated absorption bands in the λ = 0.55−0.80 μm spectral region (see Figure 6b). This coloration cannot be diminished by after growth annealing in air, and their thermal dependence is weak: At 6 K three well resolved bands are observed at λ = 0.740, 0.664, and 0.625 μm, but with increasing temperature the intensity of the two former bands decreases in benefit of a broader band at λ = 0.677 μm. CNGG crystal coloration has often been attributed to oxygen lack induced during growth. In our experience, the intensity of these bands is mainly determined at the synthesis step of the powder garnet phase, and only secondarily related with the growth oxygen atmosphere and presence of Ln or Na dopants. Their physical origin could be tentatively assigned to small polaron centers trapped at cationic vacancies.30 The infrared transparency of the CNGG and CNNGG garnets shown in Figure 6c extends up to λ ≈ 8 μm. The small absorption observed near λ = 2.9 μm corresponds to residual moisture of the KBr used to form the pellet. Thus, the CNGGtype garnet structure is particularly well suited for near and mid-infrared lasers. 5.4. Refractive Index. 5.4.a. Dispersion, n(λ). The spectral dispersion of the 300 K refractive index of CNGG was measured in the λ = 0.35−2 μm range. A graphic representation of these results can be found in the SI (Figure SI.4). The fit of our n(λ) data to the Sellmeier formula: n2 = A +

Bλ 2 − Dλ 2 λ2 − C2

included in Table 1. Although the availability of Er doped crystals provides a rather reduced range of Er concentration, the measurements clearly show an n increase (∼4 × 10−4 per at % Er) with respect to that of CNGG. The incorporation of Yb has an inverse effect, i.e., n decreases with Yb concentration (∼ −1.7 × 10−4 per at% Yb). Within the experimental uncertainty, the incorporation of Na or the crystal coloration does not add any further n modification with respect to Yb:CNGG crystals. 5.4.c. Thermal Dependence, n(T). Figure 8 shows the experimental TCOP dispersion, W(λ). For CNGG W = 15.0 × 10−6 K−1 (at λ = 1.06 μm) that is slightly lower than for YAG, ∼16 × 10−6 K−1. The dispersion of the TOC for CNGG is derived taking into account the previously determined thermal

(1)

provides the A = 2.2125, B = 1.5810, C = 0.1928 μm, D = 1.3531 × 10−2 μm−2 parameter set. The results show only small differences with two previous data points: n(λ = 0.5461 μm) = 2.003 and n(λ = 1.060 μm) = 1.92.31

Figure 8. Dispersion of the thermal coefficient of the optical path, W (●), and thermo-optic coefficient, dn/dT(■), for CNGG crystal. Symbols are the experimental data and curves are their modeling. G



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expansion coefficient, α. TOCs are positive, dn/dT = 7.8 × 10−6 K−1 (at λ = 1.06 μm) and similar to that of YAG, dn/dT = 7.8 × 10−6 K−1 at the same wavelength. TCOP and TOC values decrease for larger wavelengths, having small dispersion in the near-IR range. The dispersion of TOCs can be modeled taking into account two contributions to the temperature dependence of the refractive index, namely, the volumetric thermal expansion (expressed by αvol coefficient) and temperature variation of the bandgap Eg (expressed by dEg/dT). Explicit expressions of this model can be found in the SI (eq SI.4). Following this model, Figure 8 includes the calculations for W and dn/dT extended to the region of interest for Er, Tm, and Ho lasers, i.e., λ = 1.5 to 2 μm, where experimental results are not available. The free parameters for the fit were the bandgap and its temperature derivative. The obtained best-fit values are Eg = 5.4 ± 0.2 eV (λg = 0.230 ± 0.010 μm) and dEg/dT = 1.7 × 10−4 eV/K. The obtained λg value is smaller than the UV absorption edge previously determined, and in accordance with the value obtained with reflection spectroscopy for Gd3Ga5O12 crystal,32 λg ≈ 0.230 μm at 300 K. 5.5. Ytterbium Spectroscopy. The electronic configuration of Yb is simply constituted by two multiplets, ground 2 F7/2 and excited 2F5/2, split by the crystal field into 4 and 3 Stark energy levels, respectively. At room temperature electronic transitions from several Stark levels of the ground multiplet to the excited multiplet, 2F7/2(0, 1, 2, 3) → 2F5/2(0′, 1′, 2′), can be observed; moreover, most of these transitions are phonon assisted, giving rise to a spectrum rich in bands. Figure 9a shows a comparison of 300 K OA spectra of 7.6 at%

and lattice cell volume, but it is important to note that in the present determination the ratio between σABS at λ = 0.9717 and 0.933 μm is larger than in previous reports. In order to determine the possible effect of Na incorporation on the Yb optical bandwidth, we collected the 2F7/2(0) → 2 F5/2(0′) OA at 6 K with high spectral resolution, i.e., 1.5 × 10−4 μm. Moreover, thin samples (thickness ≈ 0.15 mm) were used to ensure the linearity of the spectrophotometer response. Figure 9b shows the results for Yb:CNGG and Yb:CNNGG. In both cases a prominent band at λ = 0.9709 μm (I) and two minor side bands at λ = 0.9730 (II) and λ ≈ 0.9698 μm (III) are observed. It is obvious that the incorporation of Na does not change the number of observed centers with regard to single Yb:CNGG, which is according to the results shown in Table 2, but the peak absorption cross section decreases and the full width at half-maximum, fwhm, bandwidth increases with Na incorporation. Taken as reference, the most intense band associated with center I, the 7.6 at% Yb:CNGG crystal has a fwhm = 1.02 × 10−3 μm (10.8 cm−1), which compares with a fwhm = 1.51 × 10−3 μm (16.0 cm−1) for the 12.8 at%Na:8.1 atom %Yb:CNNGG crystal. 2 F5/2 Yb lifetime in CNGG and CNNGG crystals has been determined at 300 K as a function of Yb concentration and raw Yb2O3 purity. For brevity the details of these results can be found in the SI (Figure SI.5). No significant difference was observed with Na incorporation. The largest value found for a 2 at% Yb:CNNGG ceramic sample was τ = 590 μs. The increase of the Yb concentration and the reduction of the Yb2O3 purity reduce the Yb lifetime. With the aim of comparing the response of CNGG and CNNGG crystals as laser gain media, we tested ∼8 at% Yb doped crystals under similar experimental conditions. CNNGG crystals show better laser behavior than CNGG such as higher laser efficiency and longer laser wavelengths, implying a lower inversion ratio for lasing. These results can be found in the SI (Figure SI.7).

6. DISCUSSION Yb has attracted great attention for the generation of ultrashort laser pulses due to a fluorescence spectrally broader than that other Ln, the absence of upconversion losses, its efficient absorption of DL emission at λ = 0.980 μm and to the low quantum defect between absorption and fluorescence bands (reduced thermal load to the crystal). A clear crystallographic model of CNGG garnets is essential for the understanding and design of optical bandwidth of Yb. From the results of Table 2 it is worth noting the qualitative similarities of cationic compositions derived for our congruent CNGG crystal, {Ca2.955□0.045}[Nb1.250Ga0.744□0.006](Ga2.463Nb0.306□0.231), and that previously reported,22 {Ca2.95□0.05}[Ca0.07Nb1.62Ga0.275□0.025](Ga2.925Nb0.05□0.025): (i) Minoritary populations of Ga in octahedral sites and Nb in tetrahedral sites are found in both studies. (ii) Vacancies are found at the three cationic sites. However, some quantitative differences are also encountered: (i) From the current structure refinement Ca is present only at dodecahedral (24c) positions. (ii) The Ga amount at octahedral (16a) positions and the Nb at tetrahedral (24d) sites are considerably higher in the present refinement than in the previous analysis. The solubility of Ln in garnets with trivalent cations in the dodecahedral site (like YAG) is large; in fact, Yb3Al5O12 single crystals are known. However, in Ca garnets, like CGGG or

Figure 9. Comparison of the Yb optical absorption cross section, σABS, for 7.6 at% Yb:CNGG (solid line) and 12.8 at% Na:8.1 at% Yb:CNNGG (dashed line) crystals: (a) 300 K and (b) 6 K measurements.

Yb:CNGG and 12.8 at% Na:8.1 at% Yb:CNNGG crystals. The 300 K peak Yb absorption cross section (at λ = 0.9717 μm) for the Yb:CNGG crystal, σABS ≈ 2.35 × 10−20 cm2, is slightly larger than that corresponding to the Yb:CNNGG, i.e., σABS ≈ 1.99 × 10−20 cm2. The σABS values now measured are similar to previous determinations, namely, (2−3.3) × 10−20 cm2 for congruent Yb:CNGG.15,33 Differences are most likely due to the different procedures for determination of Yb concentration H



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Yb:CNNGG crystals SYb is usually larger than 1 (SYb:CNNGG = 0.87−1.34). This shows how Na−Yb codoping promotes the Yb incorporation in CNGG crystals. Garnets with co-substitutions on each one of three different cationic lattice sites have been probed as disordered crystals to produce Ln bandwidth enlargement. A comparison between the different approaches based on Yb spectroscopy is difficult because 300 K fluorescence and optical absorption involve overlapped transitions starting and ending in several different Stark levels; thus, the observed envelop bandwidths are more representative of the crystal field splitting of the 2F7/2 and 2F5/2 Yb multiplets than of the coexistence of several Yb centers, and on the other hand, spectroscopic information at low (4−10 K) temperature is not generally available. In order to display the relative importance of the different substitution approaches Figure 10 shows a summary of the short laser pulse achievements obtained with Yb doped garnet crystals.

CNGG, the substitution of divalent Ca by trivalent Yb supposes an electric charge increase that limits Yb incorporation if not properly compensated either with the creation of cation vacancies or with the incorporation of cations with different electric charge, like trivalent Ga/tetravalent Ge in CGGG, or pentavalent Nb/trivalent Ga in CNGG. According to the results of Table 2 in single Yb (or Er) doped CNGG, the latter occurs essentially over the populations of the tetrahedral site. The Yb−Na cosubstitution of Ca, not requiring charge compensation, does not modify the population distributions of octahedral and tetrahedral sites with regard to Yb:CNGG. This suggests that the increase of bandwidth in Yb:CNNGG crystals shown in Figure 9 should be associated with the incorporation of Na in dodecahedral sites. The crystal growth procedures may lead to garnet crystals with different Ln bandwidth properties; for instance, garnets grown from fluxes exhibit a more perfect structure and smaller Ln bandwidths than those grown from the CZ method.34 CNGG type single crystals have been grown from its melt, either by Bridgman,35 μ-pulling down,36 or, most commonly, by the CZ method. Details of the CZ growth procedures can be found in the literature for CNGG,37 CLNGG,38 CTGG,39 and CLTGG.40 Previous experiences determined the melting temperature of congruent CNGG and CTGG crystals as 1743 and 1819.2 K,39 respectively. Li incorporation reduces the melting temperature to 1723 K for CLNGG (Li0.275),36 but not for CLTGG (Li0.20), 1821 K.40 We have found that Na modification also reduces the melting temperature of CNNGG crystals by ∼1 K/at%Na, with regard to CNGG. Other disordered garnets require higher melting temperature, for instance, Lu3Al5O12 (mp 2333 K)−Y3Al5O12 (mp 2213 K) mixtures, Gd3Ga5O12 (mp 2003 K), Gd3Sc2Ga3O12 (mp 2123 K), or {Y2.93Sc0.07}[Sc1.36Ga0.64](Ga)3O12 (mp 2150 K). The CZ growth at these high temperatures requires Ir crucibles and reducing growth atmosphere. Although this has been circumvented in some cases by the use of the optical floating zone growth method, the crystals obtained have small diameters, which is little suitable for applications. Only CGGG (mp 1633 K) shares with CNGG (mp 1743 K) the low temperature melting characteristics and the capability of being grown by CZ technique in air. The low melting temperature of CNGG type crystals is a significant advantage over other laser garnets melting at higher temperature. First, it allows the use of Pt crucibles, cheaper and more stable than Ir ones, which in turn allows heating in air avoiding the formation of crystal color centers associated with oxygen deficiency. Lower heating temperature and the preliminary synthesis of the CNGG phase allows minimization of Ga volatility and potential composition changes found in other garnets. As shown in Table 1, the melt and crystal compositions of CNGG type single crystals are basically the same, i.e., the incorporation of Er or Yb ions in CNGG was found with segregation coefficients near unity. This aids a uniform distribution of the laser ions in the crystal. In fact, it has been observed that although the Yb concentration decreases by about 10% from the initial part of the crystal neck to the crystal shoulder, which is associated with the change of crystal diameter and thus the growth kinetics at the solid−liquid interface, later it is constant along the entire crystal body (see SI, Figure SI.1). Although the Yb segregation coefficient in CNGG and CNNGG crystals was always close to 1, it is interesting to note that for Yb:CNGG crystals most often SYb < 1 occurs (SYb:CNGG = 0.8−1.05), while in

Figure 10. Summary of SESAM (full symbols) and KL (open symbols) mode-locked laser pulses obtained with garnets.

This comparison is limited to lasing results at the wavelength corresponding to peak gain cross section, i.e., λ ≈ 1.030−1.040 μm. It is worth noting that sub 40 fs laser pulses were obtained by Kerr lens, KL, mode-locking with Yb:YAG;41 however, in this case laser operation was forced to the low energy tail of the material optical gain (λ = 1.060 μm) and therefore it compromises operation efficiency. Such a result along with those obtained with thin disk cavity designs (producing much larger output energy) are ignored in Figure 10 in order to compare only results obtained under similar experimental conditions. It can be observed that the shortest laser pulses have been generally obtained with the disordered CNGG type garnets. Only Yb:Y3Ga5O12 garnet showed sub 100 fs pulses, but these results were newly obtained by KL mode-locking and therefore are not limited by the SESAM spectral response, as happens in the rest of the cases shown. The results of Figure 10 show the prevalence of CNGG-type crystals over other ordered and disordered laser garnets for the purpose of ultrashort laser pulse generation. In order to understand this situation the garnet structure should again be considered. Each YbO8 dodecahedron (Yb at 24c site) shares edges with four other dodecahedra at ∼3.82 Å of distance (see their dense packing in Figure 11a); it is also surrounded by four corner-sharing octahedra (16a site) at ∼3.49 Å, Figure 11b, and by six tetrahedra (24d site), two of them at ∼3.12 Å sharing edges and the four remaining linked by shared corners at ∼3.82 Å (see Figure 11c). Let us consider the potential influence of these coordinations on the crystal disorder around Yb: I



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Figure 11. Polyhedra coordinations around the dodecahedral position. (a) Dodecahedral−dodecahedral coordinations. (b) Dodecahedral− octahedral coordinations. (c) Dodecahedral−tetrahedral coordinations.

heat of CNGG is Cp = 0.560 J/gK, while for YAG it is 0.620 J/ gK; i.e., both are rather similar. The thermal conductivity of undoped YAG is generally considered satisfactory for laser applications: its 300 K value in undoped YAG amounts κ = 10− 11 Wm−1K−1, but this value is largely reduced by dopands, for instance, κ = 5.7 W m−1 K−1 for 5 at% Yb:YAG. The thermal conductivity of CNGG is undoubtedly lower than that of YAG, but at 300 K it is not sensitive to the presence of impurities because its value, κ = 4.3 W m−1 K−1, is determined by phonon scattering processes. This value is similar to or even higher than that of other disordered crystals successfully used as optical media for ultrashort pulsed oscillators: 8.9 at% Yb:CaF2, κ = 4.1 W m−1 K−1, 6.9 at% Yb:NaY(WO4)2, κ = 1.6−1.8 W m−1 K−1, or 7 at% Yb:Ca4GdO(BO3)3, κ = 1.6−2.7 W m−1 K−1. The refractive index dispersion measured for CNGG allowed us to determine a GVD = 152.4 fs2/mm at λ ≈ 1.040 μm, which is about twice that of YAG, GVD ≈ 65 fs2/mm. Although Yb incorporation decreases the CNGG refractive index value, the calculated GVD can be taken as a good approximation for the optical cavity design either using prism pairs or chirped mirrors for dispersion compensation. The positive TOCs obtained (see Figure 8) advance some beam thermal lensing associated with the temperature distribution induced by the absorption of the pumping beam. This is typically quantified with a “generalized” thermo-optic coefficient, χr,ϑ; the indices r and θ denote the direction parallel to the light polarization and its orthogonal direction in the plane normal to the light propagation, respectively. Under the “plane stress” approximation typically used for the description of a diode-pumping44

a. Due to the low concentration of Nb2 (

Na Modification of Lanthanide Doped Ca3Nb1.5Ga3.5O12-Type

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