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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 8 — Apr. 11, 2011
  • pp: 7640–7645
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Efficient mid-infrared laser operation of Li3Lu3-xTmxBa2(MoO4)8 disordered crystal

Mauricio Rico, Xiumei Han, Concepción Cascales, Fátima Esteban-Betegón, and Carlos Zaldo  »View Author Affiliations


Optics Express, Vol. 19, Issue 8, pp. 7640-7645 (2011)
http://dx.doi.org/10.1364/OE.19.007640


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Abstract

Tm-doped Li3Lu3Ba2(MoO4)8 monoclinic (C2/c) crystals were grown by the TSSG-method. Details of the crystal growth and Tm3+ spectroscopy are presented. 514 mW of laser light at 1940 nm was obtained with 71.4% of slope efficiency in quasi-cw operation mode. The laser was tuned in the 1853-2009 nm range. The crystal shows local disorder due to the shared occupancy by Li and Lu of the same 8f lattice site, this confers potential applications for mode-locked sub-200 fs laser pulses.

© 2011 OSA

1. Introduction

The development of ultrafast (fs) solid state lasers (SSLs) has been based on gain media with large optical bandwidths. The effort focused first in Ti3+ (3d1 electronic configuration) whose interactions with the vibrational lattice environment provide the physical support for broad-band emission, however it requires excitation with green light which presently is not efficiently achieved by direct diode laser (DL) pumping. Later on, a lot of work has been conducted on Yb3+ (4f13 electronic configuration) which exhibits smaller bandwidth than Ti3+ but can be pumped at 980 nm by direct InGaAs DL emission. At this stage the importance of disordered single crystals (crystals with a spatially variable Crystal Field on the lasant ion) for fs laser pulse production was recognized, and several of such crystals showed the shortest pulses obtained with Yb-SSLs [1

1. Y. Zaouter, J. Didierjean, F. Balembois, G. L. Leclin, F. Druon, P. Georges, J. Petit, P. Goldner, and B. Viana, “47-fs diode-pumped Yb3+:CaGdAlO4 laser,” Opt. Lett. 31(1), 119–121 (2006). [CrossRef] [PubMed]

,2

2. A. García-Cortés, J. M. Cano-Torres, M. D. Serrano, C. Cascales, C. Zaldo, S. Rivier, X. Mateos, U. Griebner, and V. Petrov, “Spectroscopy and lasing of Yb-doped NaY(WO4)2: Tunable and femtosecond mode-locked laser operation,” IEEE J. Quantum Electron. 43(9), 758–764 (2007). [CrossRef]

].

As the applications of fs lasers expand, the demand for new laser wavelengths grows beyond the Ti3+ (λ≈0.75-1.0 μm) and Yb3+ (λ≈1.03-1.065 μm) tuning ranges. Presently, particular attention is devoted to the 1.8-3.5 μm mid-infrared region where medical and environmental applications exist. Options for such fs SSLs include Tm3+, Ho3+ and Cr2+ ions.

The 3F43H6 emission of Tm3+ (4f12 electronic configuration) tunable in the ≈1.8-2.0 μm range exhibits favorable conditions for fs laser applications, i.e. large emission bandwidth, strong absorption of AlGaAs DL ≈800 nm emission, high radiative efficiency due to the cross relaxation between 3H4 and 3F4 multiplets and a long 3F4 lifetime, which provides low laser threshold. In Tm-doped ordered crystals Fourier-limited mode-locked pulses longer than 1 ps are typically obtained, for instance, 35 ps were obtained for Tm:YAG [3

3. J. F. Pinto, L. Esterowitz, and G. H. Rosenblatt, “Continuous-wave mode-locked 2- µm Tm: YAG laser,” Opt. Lett. 17(10), 731–732 (1992). [CrossRef] [PubMed]

]. The inclusion of Tm3+ in disordered crystals is expected to reduce the achievable laser pulse duration.

Tm3+ is also of relevance as sensitizer of the 5I75I8 electronic transition of Ho3+ (4f10 electronic configuration), which emits at slightly longer wavelengths, typically ≈2.05 μm. Ho3+ has narrow optical emission bandwidth and therefore it is not well suited to support fs laser pulses. By hosting Ho3+ in a disordered NaY(WO4)2 crystal and by codoping with Tm3+ for direct DL pumping at ≈800 nm, the shortest (191 fs) mode-locked laser pulses at λ≈2.060 μm with a significant average output power (82 mW) were obtained [4

4. A. A. Lagatsky, X. Han, M. D. Serrano, C. Cascales, C. Zaldo, S. Calvez, M. D. Dawson, J. A. Gupta, C. T. A. Brown, and W. Sibbett, “Femtosecond (191 fs) NaY(WO4)2 Tm,Ho-codoped laser at 2060 nm,” Opt. Lett. 35(18), 3027–3029 (2010). [CrossRef] [PubMed]

]. Beyond these wavelengths, Cr2+ in chalcogenides (ZnSe, ZnS) emits over λ≈ 2-3 μm and it has been used to produce tunable [5

5. E. Sorokin, I. T. Sorokina, M. S. Mirov, V. V. Fedorov, I. S. Moskalev and S. B. Mirov, “Ultrabroad continuous-wave tuning of ceramic Cr:ZnSe and Cr:ZnS lasers,” in Advanced Solid State Photonics, Technical Digest (Optical Society of America 2010) paper AMC2.

] and mode-locked fs lasers [6

6. M. N. Cizmeciyan, H. Cankaya, A. Kurt, and A. Sennaroglu, “Kerr-lens mode-locked femtosecond Cr(2+):ZnSe laser at 2420 nm,” Opt. Lett. 34(20), 3056–3058 (2009). [CrossRef] [PubMed]

], but it requires optical pump in the 1.5-2 μm region, and typically Tm-doped either fiber or crystalline lasers are used for this purpose.

2. Crystal growth and composition analysis

5 at% and 10 at% Tm-doped Li3Lu3Ba2(MoO4)8 crystals were grown using Pt crucibles by the Top Seeded Solution Growth (TSSG) method using a Li2MoO4 flux. The raw reagents for the growth were 99% Li2CO3, 98% BaCO3, 99.95% MoO3, 99.99% Lu2O3, and 99.99% Tm2O3. The molar ratio between the flux (solvent) and the compound to grow (solute) was 5:1.

The saturation temperature was found at 799 °C. The crystal growth was induced by supersaturation cooling at a rate of 0.05 °C/h for an interval of 19 °C. A b-oriented seed of the same crystal was used to induce crystal nucleation. After growth finish the crystal was removed from the melt and cooled to 25 °C at 6 °C/h. The grown crystals have square cross section shape with dimensions about 30×20 mm2. Figure 1a
Fig. 1 (a) As-grown 5 at% Tm-doped Li3Lu3Ba2(MoO4)8 crystal. (b) Anisotropy in the ac plane of the 3H4 optical absorption (α) of Tm3+ and position of a´ and c´ optical axes. (c) 25 °C absorption cross sections (σA) of the 3H4 multiplet for light polarized along the optical axes.
shows one of the crystals grown.

Li, Lu and Tm composition of the 10 at% Tm-doped crystal with nominal formula Li3Lu2.7Tm0.3Ba2(MoO4)8 was analyzed by Inductively Coupled Plasma (ICP) emission spectrometry by using a Perkin Elmer (Optima 2100 DV) equipment. The crystal formula resulted Li2.44±0.15Lu2.78±0.01Tm0.35±0.01 Ba2(MoO4)8.

3. Single crystal X-ray diffraction analysis

The crystal structure was determined by X-ray diffraction, and these results also provided an independent assessment of the crystal composition. A suitable single crystal cut from the 10 at% Tm-doped Li3Lu3Ba2(MoO4)8 grown crystal was mounted on a Bruker SMART CCD diffractometer equipped with a normal focus 3 kW sealed tube. Details of the data collection and analytical treatment were similar to those described previously [7

7. A. García-Cortés and C. Cascales, “Crystal growth and optical and spectroscopic characterization of the ytterbium-doped laser molybdate Yb-Li3Gd3Ba2(MoO4)8,” Chem. Mater. 20(12), 3884–3891 (2008). [CrossRef]

].

The single crystal has the monoclinic structure with space group C2/c (No. 15) with two molecules per unit cell. The unit cell parameters are a= 5.1672(4) Å, b= 12.5858(10) Å, c= 19.0744(15) Å and β= 91.521(1) °. In this structure Lu(Tm) and Li1 are sharing a same 8f crystal site with occupancy factors of 0.795(3) and 0.205(3), respectively. Ba and Li2 fully occupy each one a different 4e site, and Mo1, Mo2 and the eight types of O are located in different 8f sites. Because of the very similar electron density, an unique population for Lu and Tm ions over the shared site was refined. The structure refinement yielded R 1 = 0.0461, with anisotropic displacement parameters for all atoms. From this refinement the formula of the grown crystal should be written as Li2.82(Lu,Tm)3.18Ba2(MoO4)8. This confirms the crystal Li deficiency and the total Lu and Tm composition agrees the sum of the individual compositions obtained by ICP.

4. Tm3+ spectroscopy

The optical absorption coefficient (α) of oriented samples was recorded at 25 °C by using a Varian spectrophotometer, model Cary 5E. The crystal transparency extends in the ultraviolet up to ≈390 nm (at the α= 1 cm−1 absorption). The position of the optical principal axes (,,) have been determined from the intensity variation of the 3H4 Tm3+ absorption, see Fig. 1b. Due to the monoclinic crystal structure = b, is rotated 20±5° in the anti-clock direction respect to c as the crystals is observed from the +b axis, finally a´ is orthogonal to and . Figure 1c shows the 3H4 Tm3+ absorption cross section (σA= α/[Tm]) which is used for pumping at λ≈796 nm the 3F43H6 ≈1.95 μm laser transition. The strongest absorption is obtained for light polarized parallel to , and it is characterized by two overlapped peaks at 794.5 nm and 797 nm. The intensity of these peaks decreases for and polarized light and new absorptions appear at both sides, 782 and 804 nm.

Figure 2
Fig. 2 Absorption (dashed line), emission (points) cross sections and luminescence (line) of 5 at% Tm doped Li3Lu3Ba2(MoO4)8 crystal for light polarized parallel to the three optical axes (a) , (b) b´=b and (c) .
shows the 300 K 3F4 absorption cross section of Tm-doped Li3Lu3Ba2(MoO4)8 crystals. The emission cross sections (σE) of the 3F4 multiplet have been calculated by using the reciprocity method [8

8. D. E. McCumber, “Einstein relationships connecting broadband emission and absorption spectra,” Phys. Rev. 136(4A), A954–A957 (1964). [CrossRef]

] as

σE=σAZlZuexp[EzlhυkBT]
(1)

The partition function ratio can be approximated by that obtained for Tm3+ in NaLa(WO4)2, with similar crystal field intensity: Zl/Zu≈ 1.42 [9

9. J. M. Cano-Torres, X. Han, A. García-Cortés, M. D. Serrano, C. Zaldo, F. J. Valle, X. Mateos, S. Rivier, U. Griebner, and V. Petrov, “Infrared spectroscopic and laser characterization of Tm in disordered double tungstates,” Mater. Sci. Eng. B 146(1-3), 22–28 (2008). [CrossRef]

]. Ezl ≈ 0.69 eV is taken from the spectra in Fig. 2, kB is the Boltzmann constant and T the sample temperature.

The calculated σE values are compared in Fig. 2 to the 3F4 photoluminescence excited through the 3H4 absorption with a Ti-sa laser at 800 nm. The luminescence was dispersed by a SPEX (f= 34 nm) spectrometer and detected using a 77 K cooled InSb photovoltaic detector (Hamamatsu, model P5968-060) connected to a lock-in amplifier. It can be observed that the 3F4 photoluminescence is strongly affected by fluorescence reabsorption.

To minimize this effect in the measurement of the 3F4 lifetime ground crystals were dispersed in ethylenglycol for refractive index matching. The crystals were excited at 800 nm with a Quanta-Ray MOPO-HF optical parametric oscillator and the detected signal was analyzed with a 500 MHz oscilloscope. The 3F4 lifetime obtained for the 5 at% Tm-doped crystal was 1.10 ms and it decreases to 0.97 ms for the 10 at% Tm-doped crystal.

For the quasi-three level Tm3+ laser, the gain cross section σG=β×σE-(1-β)×σA (β is the inversion ratio factor) provides a first estimation of the laser operation and tuning range. Figure 3
Fig. 3 Gain cross sections (σG) of Tm3+ in Li3Lu3Ba2(MoO4)8 crystal for different inversion ratios (β) and light polarized parallel to the three optical axes (a) , (b) b=b´ and (c) .
shows the results obtained for the three orientations. In all cases a reduction of the laser wavelength is expected for operation conditions requiring larger β values.

5. Laser experiments

The Tm-doped Li3Lu3Ba2(MoO4)8 laser was pumped at λEXC= 796.2 nm by a chopped (50% duty cycle) Ti:sa laser. Figure 4
Fig. 4 Cavity setup of the 10 at% Tm-doped Li3Lu3Ba2(MoO4)8 laser excited by a Ti-sapphire laser at λpump= 796.2 nm. λ/2: plate polarization control. L: focusing lens. M1: plane total reflector. OC: output coupler. For tuning experiments, a single plate birefringent filter (Lyot filter) was introduced in the cavity under Brewster angle.
shows the experimental setup used for laser demonstration. The pump beam was focused by a f=70 mm lens to a spot with a Gaussian waist of ≈40 μm. The hemispherical resonator is formed by a mirror M1 designed for high transmission at the pump wavelength (T>98%) and high reflectivity in 1800-2100 nm range and ended by output couplers (OC) with 100 mm of radius and different transmissions (TOC) at 2000 nm. A λ/2 plate was used to control the pump polarization state. Uncoated samples were held on a copper block at 25 °C without active cooling. A 2.04 mm thick a-cut 10 at% Tm-doped Li3Lu3Ba2(MoO4)8 crystal was examined for pump light parallel to the b=b´ and c axes, see Figs. 5a
Fig. 5 Input-output power characteristics in quasi-cw regime of the 10 at% Tm-doped Li3Lu3Ba2(MoO4)8 laser. The symbols are the experimental results and the lines are fits for calculation of the slope efficiency. (a) a-cut sample pumped parallel to b for several TOC. (b) Same sample pumped parallel to c. (c) b-cut sample pumped parallel to a´(▲), c´ (∆) and c (▲),TOC= 4%.
and 5b. Another b-cut sample with thickness 1.913 mm was examined for light polarized parallel to the a´, c´ and c axes, see Fig. 5c. The pump absorption of the samples at low pump power and under non-lasing conditions was ≈80% and ≈95%, respectively.

The laser efficiency of the b-cut sample was inferior due to the lower crystalline quality of the sample examined. Slightly better efficiency is found for pump light parallel to the a´ axis, and very little difference is found between c´and c configurations, which is due to the close optical cross sections along the optical and crystallographic frames, see Fig. 1b. The laser emission was polarized parallel to a´ independently of the pump polarization. This is ascribed to the larger gain cross section along the a´axis for the laser wavelength, λ≈ 1930 nm.

Laser tuning was studied by inserting in the cavity a single-plate Lyot filter (1-mm thick quartz plate) placed at Brewster angle. Figure 6
Fig. 6 Laser tuning of a 10 at% Tm-doped Li3Lu3Ba2(MoO4)8 crystal obtained with a Lyot filter. The pump light at 796.2 nm was polarized along the crystal c (○) or b (●) axes but the emission was always b polarized. TOC= 2.4%, Pinc= 1.12 W.
shows the results obtained with the a-cut sample pumped parallel to the c and b axes for TOC= 2.4%. The emission was a single longitudinal mode and the peak wavelength shifted form 1853 nm to 2009 nm. This tuning range is only slightly lower to that obtained with disordered double tungstate crystals [10

10. J. M. Cano-Torres, M. D. Serrano, C. Zaldo, M. Rico, X. Mateos, J. Liu, U. Griebner, V. Petrov, F. J. Valle, M. Galán, and G. Viera, “Broadly tunable laser operation near 2 μm in a locally disordered crystal of Tm3+-doped NaGd(WO4)2,” J. Opt. Soc. Am. B 23(12), 2494–2502 (2006). [CrossRef]

].

The spectral distributions under free-running laser emission for different TOC contain several longitudinal modes, typically between 3 and 7 modes, but the peak positions were not stable. By recording sequential distributions, the envelop of the multimode emissions could be envisaged. For the a-cut sample the average laser emission wavelength systematically decreases with increasing output coupler transmission: 2005 nm for TOC= 0.6%, 1985 nm for TOC= 1.1%, 1965 nm for TOC= 2.4%, 1940 nm for TOC= 4% and 1925 nm for TOC= 8%. This behaviour is consistent with the behaviour of the gain cross section: When the cavity losses increase the stimulated emission requires of larger inversion ratio β to achieve positive σG and therefore the average wavelength shifts towards shorter wavelengths as the gain curve does, see Fig. 3.

The envelope of these multimode emissions gives an idea about the potential of the crystal for mode-locked laser experiments. Taken as reference the FWHM= 22 nm of the mode envelop obtained for TOC= 1.1% and assuming a hyperbolic secant pulse shape, pulse duration shorter than 200 fs can be expected from these disordered crystals.

6. Conclusions

Room temperature laser operation of a 10 at% Tm-doped Li3Lu3Ba2(MoO4)8 crystal grown by the TSSG method has been demonstrated for the first time. Under present experimental conditions, without active cooling, no antireflective sample coatings and in quasi-cw pumping regime, a maximum output power of ≈515 mW around 1940 nm was obtained with a slope efficiency of ≈70%. To our knowledge this is the best laser performance so far obtained for a Tm-doped disordered crystal. Tuning between 1853 and 2009 nm was also demonstrated. The disorder in Li/Lu occupancy in the crystalline structure and the observed large free running laser bandwidth suggest potential application in ultrafast (fs) mode-locked laser below 200 fs.

Acknowledgments

Work supported by Spain under MAT2008-06729-C02-01 and RYC2005-1922 projects.

References and links

1.

Y. Zaouter, J. Didierjean, F. Balembois, G. L. Leclin, F. Druon, P. Georges, J. Petit, P. Goldner, and B. Viana, “47-fs diode-pumped Yb3+:CaGdAlO4 laser,” Opt. Lett. 31(1), 119–121 (2006). [CrossRef] [PubMed]

2.

A. García-Cortés, J. M. Cano-Torres, M. D. Serrano, C. Cascales, C. Zaldo, S. Rivier, X. Mateos, U. Griebner, and V. Petrov, “Spectroscopy and lasing of Yb-doped NaY(WO4)2: Tunable and femtosecond mode-locked laser operation,” IEEE J. Quantum Electron. 43(9), 758–764 (2007). [CrossRef]

3.

J. F. Pinto, L. Esterowitz, and G. H. Rosenblatt, “Continuous-wave mode-locked 2- µm Tm: YAG laser,” Opt. Lett. 17(10), 731–732 (1992). [CrossRef] [PubMed]

4.

A. A. Lagatsky, X. Han, M. D. Serrano, C. Cascales, C. Zaldo, S. Calvez, M. D. Dawson, J. A. Gupta, C. T. A. Brown, and W. Sibbett, “Femtosecond (191 fs) NaY(WO4)2 Tm,Ho-codoped laser at 2060 nm,” Opt. Lett. 35(18), 3027–3029 (2010). [CrossRef] [PubMed]

5.

E. Sorokin, I. T. Sorokina, M. S. Mirov, V. V. Fedorov, I. S. Moskalev and S. B. Mirov, “Ultrabroad continuous-wave tuning of ceramic Cr:ZnSe and Cr:ZnS lasers,” in Advanced Solid State Photonics, Technical Digest (Optical Society of America 2010) paper AMC2.

6.

M. N. Cizmeciyan, H. Cankaya, A. Kurt, and A. Sennaroglu, “Kerr-lens mode-locked femtosecond Cr(2+):ZnSe laser at 2420 nm,” Opt. Lett. 34(20), 3056–3058 (2009). [CrossRef] [PubMed]

7.

A. García-Cortés and C. Cascales, “Crystal growth and optical and spectroscopic characterization of the ytterbium-doped laser molybdate Yb-Li3Gd3Ba2(MoO4)8,” Chem. Mater. 20(12), 3884–3891 (2008). [CrossRef]

8.

D. E. McCumber, “Einstein relationships connecting broadband emission and absorption spectra,” Phys. Rev. 136(4A), A954–A957 (1964). [CrossRef]

9.

J. M. Cano-Torres, X. Han, A. García-Cortés, M. D. Serrano, C. Zaldo, F. J. Valle, X. Mateos, S. Rivier, U. Griebner, and V. Petrov, “Infrared spectroscopic and laser characterization of Tm in disordered double tungstates,” Mater. Sci. Eng. B 146(1-3), 22–28 (2008). [CrossRef]

10.

J. M. Cano-Torres, M. D. Serrano, C. Zaldo, M. Rico, X. Mateos, J. Liu, U. Griebner, V. Petrov, F. J. Valle, M. Galán, and G. Viera, “Broadly tunable laser operation near 2 μm in a locally disordered crystal of Tm3+-doped NaGd(WO4)2,” J. Opt. Soc. Am. B 23(12), 2494–2502 (2006). [CrossRef]

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.5680) Lasers and laser optics : Rare earth and transition metal solid-state lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: November 10, 2010
Revised Manuscript: January 21, 2011
Manuscript Accepted: February 1, 2011
Published: April 6, 2011

Citation
Mauricio Rico, Xiumei Han, Concepción Cascales, Fátima Esteban-Betegón, and Carlos Zaldo, "Efficient mid-infrared laser operation of Li3Lu3-xTmxBa2(MoO4)8 disordered crystal," Opt. Express 19, 7640-7645 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-8-7640


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References

  1. Y. Zaouter, J. Didierjean, F. Balembois, G. L. Leclin, F. Druon, P. Georges, J. Petit, P. Goldner, and B. Viana, “47-fs diode-pumped Yb3+:CaGdAlO4 laser,” Opt. Lett. 31(1), 119–121 (2006). [CrossRef] [PubMed]
  2. A. García-Cortés, J. M. Cano-Torres, M. D. Serrano, C. Cascales, C. Zaldo, S. Rivier, X. Mateos, U. Griebner, and V. Petrov, “Spectroscopy and lasing of Yb-doped NaY(WO4)2: Tunable and femtosecond mode-locked laser operation,” IEEE J. Quantum Electron. 43(9), 758–764 (2007). [CrossRef]
  3. J. F. Pinto, L. Esterowitz, and G. H. Rosenblatt, “Continuous-wave mode-locked 2- µm Tm: YAG laser,” Opt. Lett. 17(10), 731–732 (1992). [CrossRef] [PubMed]
  4. A. A. Lagatsky, X. Han, M. D. Serrano, C. Cascales, C. Zaldo, S. Calvez, M. D. Dawson, J. A. Gupta, C. T. A. Brown, and W. Sibbett, “Femtosecond (191 fs) NaY(WO4)2 Tm,Ho-codoped laser at 2060 nm,” Opt. Lett. 35(18), 3027–3029 (2010). [CrossRef] [PubMed]
  5. E. Sorokin, I. T. Sorokina, M. S. Mirov, V. V. Fedorov, I. S. Moskalev and S. B. Mirov, “Ultrabroad continuous-wave tuning of ceramic Cr:ZnSe and Cr:ZnS lasers,” in Advanced Solid State Photonics, Technical Digest (Optical Society of America 2010) paper AMC2.
  6. M. N. Cizmeciyan, H. Cankaya, A. Kurt, and A. Sennaroglu, “Kerr-lens mode-locked femtosecond Cr(2+):ZnSe laser at 2420 nm,” Opt. Lett. 34(20), 3056–3058 (2009). [CrossRef] [PubMed]
  7. A. García-Cortés and C. Cascales, “Crystal growth and optical and spectroscopic characterization of the ytterbium-doped laser molybdate Yb-Li3Gd3Ba2(MoO4)8,” Chem. Mater. 20(12), 3884–3891 (2008). [CrossRef]
  8. D. E. McCumber, “Einstein relationships connecting broadband emission and absorption spectra,” Phys. Rev. 136(4A), A954–A957 (1964). [CrossRef]
  9. J. M. Cano-Torres, X. Han, A. García-Cortés, M. D. Serrano, C. Zaldo, F. J. Valle, X. Mateos, S. Rivier, U. Griebner, and V. Petrov, “Infrared spectroscopic and laser characterization of Tm in disordered double tungstates,” Mater. Sci. Eng. B 146(1-3), 22–28 (2008). [CrossRef]
  10. J. M. Cano-Torres, M. D. Serrano, C. Zaldo, M. Rico, X. Mateos, J. Liu, U. Griebner, V. Petrov, F. J. Valle, M. Galán, and G. Viera, “Broadly tunable laser operation near 2 μm in a locally disordered crystal of Tm3+-doped NaGd(WO4)2,” J. Opt. Soc. Am. B 23(12), 2494–2502 (2006). [CrossRef]

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