We report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 μm spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.
© 2017 Optical Society of America
Laser gain media doped with Tm are of great interest for obtaining coherent radiation around 2 μm due to the 3F4 → 3H6 transition of the Tm3+ ion. The emission of Tm-based lasers is “eye-safe” and it matches spectrally the strong absorption bands of such molecules as H2O, NO2, CO2 and NH3. Already established applications of Tm lasers include medical surgery , remote atmospheric sensing  and pumping of mid-infrared optical parametric oscillators (OPOs)  or holmium (Ho3+) based lasers .
Double tungstate crystals represent an important class of laser host materials and they were extensively investigated with Tm3+ doping in the regimes of continuous-wave (CW) operation, Q-switching, and mode-locking. The most profound examples are the ordered Tm3+-doped monoclinic potassium (rare-earth) double tungstates KRE(WO4)2 where RE = Gd , Y  or Lu , and the disordered tetragonal sodium double tungstates NaRE(WO4)2 where RE = Gd, Y, Lu or La ). The title compound, magnesium tungstate (MgWO4) belongs to another crystal family of monoclinic (sp. gr. P2/c) divalent metal monotungstates, having the general chemical formula MWO4 where M = Mg, Mn, Ni, Cd or Zn [9,10]. Earlier investigations of MgWO4 focused on its application as scintillator [11,12]. Only very recently, MgWO4 attracted attention as a promising laser host for doping with transition-metal ions, e.g. Cr3+ [13,14], and RE ions, e.g. Yb3+ . The difference in ionic radius between the divalent Mg2+ ion and trivalent RE ions induces distortion of the crystal field, which leads to broadening of the absorption and emission spectra [16–18]. This advantage is essential for broadly tunable laser operation and the generation of ultrashort pulses in the mode-locking regime. The monoclinic MgWO4 crystal is optically biaxial, thus polarization anisotropy of the spectroscopic properties of the RE dopant can be expected, leading to naturally polarized laser output. The thermal conductivity of the MgWO4 host crystal measured for an arbitrary orientation is ~8.7 W/mK  that is almost 3 times larger than that of monoclinic double tungstates (~3 W/mK).
Recently, we have demonstrated efficient laser operation with Yb3+-doped MgWO4 crystal . Motivated by this success and the diversity of applications of 2 μm laser sources, we extended this work aiming to develop a high quality MgWO4 crystal with Tm3+ doping. In the present work, we report on the crystal growth, spectroscopy and CW laser operation of monoclinic Tm:MgWO4 crystal, for the first time to the best of our knowledge.
2. Crystal growth
The Tm:MgWO4 crystal was grown by the top-seeded solution growth (TSSG) method with a flux of Na2WO4 in a vertical tubular furnace. The ratio of MgWO4:Na2WO4 in the melt was 5:7 (mol). The powder precursors were Na2CO3, MgO and WO3 with analytical grade of purity, and Tm2O3 with a purity of 99.99%. Stoichiometric amounts of raw materials with 10 at.% Tm2O3 were prepared by repeated solid state reactions. The mixtures of solute and flux were placed into a platinum crucible and sintered in a resistance furnace at 970 °C for 48 h. The crystal was grown at a cooling rate of 0.5–0.8°C/day and rotation rate of 10–15 rpm. The details of the furnace and parameters of the temperature controlling program can be found elsewhere [13,15]. The bulk Tm:MgWO4 crystal finally obtained had dimensions of 16 × 8 × 6 mm3, as shown in Fig. 1(a). The as-grown crystal had a yellowish coloration which originated from optical absorption in the 320–420 nm spectral range. This absorption is observed in undoped crystals as well as it is attributed to color centers [12,15]
To confirm the symmetry of the as-grown crystal, the X-ray powder diffraction (XRD) pattern was measured at room temperature by using a desktop X-ray diffractometer (Rigaku, MiniFlex 600) equipped with Cu Kα radiation. The experiment was carried out in the 2θ = 10–80° range with a step of 0.02° and a scan speed of 0.13°/min. Figure 1(b) shows the XRD pattern of a powdered as-grown crystal. The diffraction peaks are consistent with the standard pattern of undoped MgWO4 (crystallographic database, powder diffraction file #27-0789 for Huanzalaite). MgWO4 belongs to the monoclinic crystal class (point group 2/m, space group P2/c). Based on the XRD analysis, the lattice constants of Tm:MgWO4 are a = 4.697 Å, b = 5.678 Å, c = 4.933 Å, β = a^c = 90.77° (V = 131.54 Å3, Z = 2). They are larger than those of the undoped crystal (a = 4.686 Å, b = 5.675 Å, c = 4.928 Å, β = a^c = 90.3° ). This is attributed to the different ionic radii of Tm3+ and Mg2+ (0.88 and 0.72 Å, respectively, for the VI-fold O2-–coordination). The calculated crystal density of Tm:MgWO4 is 6.871 g/cm3.
The concentration of Tm3+ ions in the as-grown crystal was determined to be 0.89 at.%, i.e. 1.409 × 1020 at/cm3 by using the Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) method. The calculated segregation coefficient of the Tm3+ ion is KTm = 0.09.
3. Spectroscopic characterization
Monoclinic crystals are optically biaxial and their optical properties are described in the frame of the optical indicatrix with three mutually orthogonal principal axes X, Y and Z. Since the refractive indices of MgWO4 are still unknown, following  we have arbitrarily chosen the Y-axis to be parallel to the crystallographic b-axis which coincides with the 2-fold symmetry axis. The other two axes of the optical indicatrix, namely X and Z, are located in the a-c plane. The rotation of the optical indicatrix relative to the crystallographic frame is expressed by the angles a^X = 36.4° and c^Z = 37.1° . We have studied all the spectroscopic properties of Tm3+ in MgWO4 for the principal light polarizations E || X, Y and Z at room-temperature (RT, 293 K).
3.1 Optical absorption
In order to evaluate the optimum pump conditions for Tm:MgWO4 and to calculate the absorption and stimulated emission cross-sections, polarized absorption measurements were carried out in the 0.3–2.1 μm spectral range, Fig. 2(a). Tm3+ ions in MgWO4 exhibit strong anisotropy of the optical absorption. The observed absorption bands are assigned to the transitions from the 3H6 ground state of Tm3+ to the excited states 1G4, 3F2,3, 3H4, 3H5, and 3F4.
The band related to the 3H6 → 3H4 transition is suitable for pumping of Tm:MgWO4 with commercially available AlGaAs diode lasers emitting around 800 nm. The corresponding polarized absorption cross-sections, σabs, are shown in Fig. 2(b); they were calculated from the measured absorption coefficient as σabs = α/NTm. The maximum σabs is 2.04 × 10−20 cm2 at 802.6 nm for light polarization E || X and the full width at half maximum (FWHM) of this peak is ~14 nm. For E || Y, σabs = 1.45 × 10−20 cm2 at 796.4 nm with FWHM = 31 nm and for E || Z, σabs = 0.34 × 10−20 cm2 at 801.8 nm with FWHM = 25 nm. The absorption linewidths of Tm:MgWO4 around 800 nm are much broader compared to the intensively studied Tm3+-doped laser crystals, YAG and YLF, and broader than in the monoclinic double tungstates [5,7]. This relaxes the requirements for wavelength stabilization of the AlGaAs diode lasers acting as a pump source. We attribute this broadening of the spectral bands to the distortion of the crystal field symmetry when the Tm3+ ions are replacing the Mg2+ ones in the MgWO4 lattice [16–18].
The absorption spectra of Tm3+ in MgWO4 were analyzed in the frame of the standard Judd-Ofelt (J-O) theory [19,20] in order to determine the spontaneous emission probabilities. The absorption oscillator strengths were determined from the measured absorption spectra:Table 1.
The values of fexp were used to determine the J-O (intensity) parameters, Ωk, k = 2, 4, 6. For Tm:MgWO4, they are Ω2 = 7.609, Ω4 = 0.841 and Ω6 = 2.376 [10−20 cm2]. With these parameters, the absorption oscillator strengths f were calculated, showing a good agreement with the experiment:21]. J-O theory describes electric-dipole (ED) transitions. The contribution of magnetic-dipole (MD) transitions with J–J’ = 0, ± 1, fMD, was taken from the literature .
3.2 Luminescence (emission and lifetime)
The photoluminescence (PL) of Tm3+ in MgWO4 was characterized in terms of emission spectra and lifetime. The normalized polarized PL spectra were measured in the 1.3–2.1 μm spectral range under excitation at 802 nm, see Fig. 3. There are two bands in the spectra which correspond to the 3H4 → 3F4 and 3F4 → 3H6 transitions, respectively. Note that the PL spectra also exhibits strong polarization anisotropy.
The probabilities of spontaneous radiative transitions were calculated from the corresponding line strengths which, in turn, were derived from the J-O parameters Ωk and squared reduced matrix elements U(t), see Eq. (2b):22]. On the basis of probabilities of spontaneous transitions for the separate emission channels J→J’, we have calculated the total probability, Atot, the corresponding radiative lifetime of the excited-state, τrad, and the luminescence branching ratios for the emission channels, B(JJ’):Table 2. In particular, the radiative lifetime of the lowest excited-state, τrad(3F4) = 1.95 ms.
The stimulated emission, σe, cross-section spectra for the 3F4 → 3H6 transition of Tm3+ in MgWO4 were calculated from the measured absorption spectra by using the modified reciprocity method :Fig. 4. The maximum σe is 2.43 × 10−20 cm2 at 1877 nm for light polarization E || Y. For E || X, the emission band contains two local peaks at 1799.5 and 1848.5 nm with σe ~1.63 × 10−20 cm2. The maximum σe for E || Z polarization is 0.32 × 10−20 cm2 at 1884 nm. These values are comparable to the monoclinic double tungstates [5,7] but much higher compared to the commonly used Tm3+-doped YAG and YLF laser crystals. The anisotropy of the maximum stimulated emission cross-sections, σe(X): σe(Y): σe(Z), amounts to 5.1:7.6:1. Note that such a pronounced anisotropy will promote natural polarization selection and linearly polarized output from a Tm:MgWO4 laser.
The luminescence lifetime τlum of the 3F4 state of Tm3+ in MgWO4 was determined by a decay measurement. The sample was powdered in order to eliminate the effect of radiation trapping. By exciting the Tm3+ ions at 802 nm, we monitored the luminescence decay at ~1850 nm (from the 3F4 excited state). The results are presented in Fig. 5. The decay is clearly single-exponential and the dependence was fitted by τlum = 1.93 ms. The luminescence lifetime of the 3F4 state is very close to the radiative one determined from the J-O calculation. Therefore, the luminescence quantum yield ηq = τlum/τrad > 99% which indicates very weak non-radiative relaxation from this state. This is a consequence of the large energy-gap to the ground-state but also indicates very good optical quality of the as-grown crystal.
The laser emission range for Tm:MgWO4 can be predicted from the corresponding gain cross-section, σgain, spectra. Here, σgain = βσe – (1–β)σabs, β = N(3F4)/NTm represents the inversion ratio, i.e. the number of the Tm3+ ions in the excited state N(3F4) divided by the total ion density NTm. The σgain spectra for the 3F4 → 3H6 transition of Tm3+ in MgWO4 are shown in Fig. 6 for the three principal light polarizations E || X, Y and Z.
The maximum gain cross-sections correspond to light polarization E || Y. Thus, one may expect laser oscillation with this polarization for X-cut and Z-cut Tm:MgWO4 crystals. For low inversion ratios (β < 0.3), the local peak centered at ~2.02 μm dominates in the spectrum. For higher β, one may expect a jump of the emission wavelength to ~1.88 μm or even a dual-wavelength operation. For high inversion levels, one may expect tuning of the laser emission in the 1.85–2.06 μm range (i.e., >200 nm tunability). For Y-cut Tm:MgWO4, the laser will most probably operate in the E || X polarization. In the corresponding gain spectra, the position of the local peaks is at ~1.98 μm (for β < 0.2) and at 1.85 μm (for higher β).
3.3 Raman spectra
Mono- and double tungstate crystals are well-known for their Raman activity. Undoped crystals are used as Raman shifters  for sub-ns and ps pulses while RE-doped ones are used for self-Raman conversion in Q-switched and CW lasers. Recently, Tm3+-doped monoclinic double tungstates were found to be very suitable for vibronic laser operation relying on electron-phonon coupling with the low-energy Raman–active vibrational modes [25,26]. From this point of view, the study of the Raman spectra of the monoclinic Tm:MgWO4 crystal is of high interest for laser applications.
Polarized Raman spectra of Tm:MgWO4 were measured at room temperature, see Figs. 7(a)-7(c). Here, we use standard notations a(bc)d, where a and d correspond to the directions of propagation of incident and scattered light, and b and c – to their polarizations, respectively. The cases a = d = X, Y or Z are considered. The group analysis of the MgWO4 structure predicts 36 lattice modes of which 18 even (g) vibrations are Raman-active: 8Ag + 10Bg, see Table 3. For the WO6 octahedra, there are 6 active modes related to 6 W-O bonds: 4Ag + 2Bg. Typically, the internal vibrations of a tightly bound group of atoms have higher frequencies and can easily be assigned. An example is the scheelite structure, CaWO4, with two sets of modes at 0-409 and 797-912 cm−1 (the latter being internal). For MgWO4, the WO6 octahedra share two oxygen atoms and the ranges of external/internal modes may overlap. The internal modes are marked in Table 3 by asterisk, following [27,28]. The most intense mode at 916 cm−1 (FWHM = 13.6 cm−1) is assigned to the symmetric stretching W-O vibrations A1g, the modes at 812 and 709 cm−1 – to the asymmetric stretching Eg, and the modes at 683, 551 and 420 cm−1 – to the bending (Tg) of the regular octahedron. The remaining modes are external and they are related to the interaction of the WO6 groups with Mg atoms through the Mg-W, Mg-W-O and Mg-WOOW vibrations.
In Fig. 7(d), we compare the Raman spectrum of Tm:MgWO4 with those of two double tungstate crystals employed for Tm3+ doping: monoclinic KLu(WO4)2 and tetragonal scheelite-like NaGd(WO4)2. The spectra of the monoclinic crystals are similar due to the presence of double oxygen bridge vibrations. The appearance of low- and medium-energy modes for Tm:MgWO4 at 150-550 cm−1 may provide efficient vibronic laser operation at 2.1~2.2 μm.
4. Continuous-wave laser performance
For the laser experiments, a sample from the as-grown crystal was cut for light propagation along the Z-axis with an aperture of 1.86(X) × 3.96(Y) mm2 and a thickness of 3.05 mm. Both X × Y crystal faces were polished to laser quality and remained uncoated. The laser crystal was mounted in a Cu-holder and Indium foil was used to provide improved thermal contact from all 4 lateral sides. The holder was water-cooled to 12°C. A hemispherical laser cavity was used. The pump mirror (PM) was antireflection (AR) coated at 0.77-1.05 μm and high reflection (HR) coated at 1.80-2.08 μm. It was located at 1 mm from the laser crystal.
Several concave output couplers (OCs) with radius of curvature of ROC = 50 mm and transmission (TOC) of 1%, 3%, 5% and 15% at the laser wavelength were employed. The total geometrical cavity length was ~49 mm. The laser crystal was pumped through the PM by a fiber-coupled laser diode (LD) at ~802 nm (fiber diameter: 200 μm, N.A. = 0.22). The unpolarized pump beam was re-imaged into the laser crystal by a lens assembly with a 1:1 imaging ratio. The pump spot size in the crystal was wp = 100 μm (the beam propagation factor M2 of the diode laser output was calculated to be 86, and the confocal parameter 2zR was 1.9 mm). The single pass pump absorption of the crystal was 30%. The scheme of the Tm:MgWO4 laser is shown in Fig. 8.
The input-output characteristics and the emission spectra of the Tm:MgWO4 laser are presented in Fig. 9. For all the studied OCs, the laser emission was linearly polarized (E || Y); the polarization was naturally-selected by the anisotropy of the gain. No polarization-switching was detected. The output power dependence was clearly linear showing no detrimental influence of the thermal effects. The maximum output power was 772 mW at 2017-2029 nm (multi-peak spectrum) corresponding to a slope efficiency of η = 39% (with respect to the absorbed pump power), as measured with TOC = 5%. The laser threshold was at Pabs = 0.19 W and the optical-to-optical efficiency ηopt was 11% (with respect to the incident pump power). For smaller TOC, the laser performance was slightly inferior, resulting in decreased slope efficiency η = 38% (TOC = 3%) and η = 37% (TOC = 1%). For large TOC = 15%, the output power dropped further which is attributed to the increased upconversion losses. The laser emission spectra were weakly dependent on the output coupling, see Fig. 9(b), and the emission occurred in the 2017-2034 nm spectral range in agreement with the gain spectra for light polarization E || Y.
In conclusion, we report on the growth of monoclinic Tm:MgWO4 and its polarization-resolved spectroscopic characteristics and achieved for the first time laser operation with this new crystal. Absorption, stimulated-emission and gain cross-section spectra are obtained for the relevant transitions of the Tm3+ ion. Raman spectra are also measured. The Judd-Ofelt (J-O) analysis is performed to determine the spontaneous emission probabilities, luminescence branching ratios and radiative lifetimes. The luminescence of Tm3+ ions is characterized in terms of spectral and decay measurements. Laser operation of Tm in MgWO4 is realized in a compact cavity in the CW regime under diode-pumping.
The spectroscopic features of Tm:MgWO4 are determined by its low-symmetry structure and the distortion of the rare-earth site caused by the mismatch of the ionic radii of Mg2+ and Tm3+. This provides a substantial broadening of the absorption and emission spectral bands, as well as strong polarization-anisotropy of the transition cross-sections. The polarizations of interest in Tm:MgWO4 for reaching high pump efficiency and high gain are E || X and E || Y. For these polarizations, Tm:MgWO4 is promising for broad tuning of the laser emission (with expected tuning range of >200 nm) and generation of ultrashort pulses at wavelengths above 2 μm. The broad (FWHM ~20 nm) and intense absorption band related to the 3H6 → 3H4 transition relaxes the requirements for wavelength stabilization of the AlGaAs laser diodes employed for Tm:MgWO4 pumping. A relatively long lifetime (1.93 ms) of the upper laser level of Tm3+ in MgWO4 makes this material very promising also for generation of high pulse energies in the Q-switched operation mode. Further increase of the slope efficiency of Tm:MgWO4 lasers is possible for higher Tm doping levels by enhancing the efficiency of cross-relaxation for adjacent Tm3+ ions. MgWO4 has a rich Raman spectrum which may be used for extending the emission range of Tm:MgWO4 lasers to >2.1 μm with vibronic coupling. The codoping of MgWO4 with Tm3+ and Ho3+ ions may also offer another alternative for such long-wavelength emission.
The National Natural Science Foundation of China (No.11404332, No.61575199, No. 21427801); Key Project of Science and Technology of Fujian Province (2016H0045); the Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDB20000000); the National Key Research and Development Program of China (No.2016YFB0701002); the China Scholarship Council (CSC, No.201504910418 and No. 201504910629); the Instrument Project of Chinese Academy of Sciences (YZ201414); Spanish Government (MAT2016-75716-C2-1-R, MAT2013-47395-C4-4-R, TEC 2014-55948-R); Generalitat de Catalunya (2014SGR1358).
F.D. acknowledges additional support through the ICREA academia award 2010ICREA-02 for excellence in research. This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654148 Laserlab-Europe and under the Marie Skłodowska-Curie grant agreement No 657630. P.L. acknowledges financial support from the Government of the Russian Federation (Grant 074-U01) through ITMO Post-Doctoral Fellowship scheme.
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