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
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 [1,2].
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 3F4→3H6 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 . 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 5I7→5I8 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 . Beyond these wavelengths, Cr2+ in chalcogenides (ZnSe, ZnS) emits over λ≈ 2-3 μm and it has been used to produce tunable  and mode-locked fs lasers , 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.
Therefore, efficient Tm-doped SSL crystals and in particular disordered Tm-doped crystals are needed to reduce the laser pulse duration in the mid-infrared region and to pump other SSLs. Previous work on Yb-doped Li3Gd3Ba2(MoO4)8  showed that Li and Gd share the same lattice site with occupancy factors of 0.215 and 0.785, respectively, introducing an inhomogeneous broadening of Yb3+ transitions. The present work extends this knowledge to the Li3Lu3Ba2(MoO4)8 crystal doped with Tm. Lu is used instead of Gd to improve the Tm incorporation to the crystal, since the ionic radii of both elements are closer. The present work describes the crystal growth, crystalline structure, Tm3+ spectroscopy, and laser operation under Ti-sapphire (Ti-sa) laser pumping of the title crystal.
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 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 .
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 (a´,b´,c´) have been determined from the intensity variation of the 3H4 Tm3+ absorption, see Fig. 1b. Due to the monoclinic crystal structure b´= b, c´ 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 b´and c´. Figure 1c shows the 3H4 Tm3+ absorption cross section (σA= α/[Tm]) which is used for pumping at λ≈796 nm the 3F4→3H6 ≈1.95 μm laser transition. The strongest absorption is obtained for light polarized parallel to a´, and it is characterized by two overlapped peaks at 794.5 nm and 797 nm. The intensity of these peaks decreases for c´ and b´ polarized light and new absorptions appear at both sides, 782 and 804 nm.
Figure 2 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  as
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 . 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 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 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 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.
Figure 5 shows the output (Pout) versus absorbed (Pabs) power characteristics of these lasers under different OCs and crystal orientations. For //b and //c pump configurations of the a-cut sample, the maximum Pout and slope efficiency (η) versus Pabs were obtained with TOC= 4%, i.e. Pout= 514 mW, η=71.4% and Pout= 519 mW, η=69.3%, respectively. It is remarkable that the output-input characteristics of the laser remain linear up to the maximum available pump power, Pinc= 1.12 W. Pump laser thresholds (Pth) as small as 68 mW and 83 mW were obtained for these pump configurations, respectively. In all cases, the laser output was polarized parallel to the crystal b axis. This is related to the larger σG of b axis, see Fig. 3.
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 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 .
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.
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.
Work supported by Spain under MAT2008-06729-C02-01 and RYC2005-1922 projects.
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