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We present, to the best of our knowledge, the first report on the crystal growth, spectroscopic investigation, and laser performance of Yb-doped MgWO4 monoclinic crystal. A top seeded solution growth method was employed to develop a high quality monoclinic crystal. The spectroscopic properties were characterized in terms of absorption, photoluminescence, and lifetime measurement. Corresponding emission and gain cross-sections were calculated. Lasing behavior has been investigated in continuous-wave regime with the maximum output power of 2.52 W and a slope efficiency of 52.8%.
© 2016 Optical Society of America
1. Introduction
Ytterbium-doped laser gain media have raised an extremely intense interest in the development of high-efficiency diode-pumped solid-state lasers operating around 1 μm region [1]. In contrast to trivalent neodymium ion (Nd3+), trivalent ytterbium ion (Yb3+) has a very simple electronic level structure which consist only two manifolds (the 2F7/2 ground state and the 2F5/2 excited state.). This merit precluded it from most of the parasitic effects included upconversion, cross relaxation and excited-state absorption. Yb3+-doped crystals are attractive as effective active media because they offered the combination features of small quantum defects limit thermal problems, a long fluorescence lifetime increasing the energy-stroage time, broadband emission suit for tunable laser or femtosecond oscillator and broadband absorption make it possible pump with commercial high power InGaAs laser diodes emission at near 980 nm [2].
The characteristics of the Yb3+ absorption and emission strongly depend on the chosen host material. Among the Yb3+-doped crystalline materials, monoclinic tungstates especially the monoclinic double tungstates, e.g. Yb:KG(WO4)2, Yb:KY(WO4)2 and Yb:KLu(WO4)2, are exceptionally suitable hosts for ytterbium doping [3–6 ]. Compared to the widely used Yb:YAG crystal, the main advantages of monoclinic tungstates are related to the very high values of the absorption and emission cross section, broad emission linewidths and the strong anisotropy due to the low-symmetry structure of these kinds of biaxial crystals. These make them very competitive to other ytterbium-doped crystals for the application of waveguide laser [7–9 ], mode-locked oscillator [10–12 ] and amplifier [13]. Nevertheless, relatively narrow absorption bandwidth (≤ 4 nm) and small thermal conductivity were the main limit factors of Yb3+-doped double tungstates. Therefore, exploration of new host with good optical physical properties for Yb3+-doped laser gain medium was still significant in the research field of materials science.
AWO4 (A = Cd, Zn Mg) belongs to another monoclinic tungstates family for laser host materials. Some studies confirmed that trivalent rare earth ions can replace divalent A2+ ions [14–16 ]. Differences of the ion radii and charge between trivalent and divalent may produce lattice distortion and vacancy in crystal, which would possibly lead to a reduction of crystal field symmetry and change the spectral intensity or even broaden of the absorption and emission spectra [17–19 ]. Monoclinic magnesium tungstate, a biaxial crystal with high refractive index and clear birefringence, possesses a NiWO4 type structure with space group P2/c and cell dimensions of a = 4.686 Å, b = 5.675 Å, c = 4.928 Å, β = 90.3°, Z = 2 [20,21 ]. With trivalent chromium ion (Cr3+) doped, we have grown large-dimension and high-quality chromium ion (Cr3+) doped monoclinic magnesium tungstate crystal (Cr3+:MgWO4). After characterized the spectral and thermal properties, MgWO4 was confirmed to be a promising host for laser gain media doping with rare earth ions [20–23 ].
In the present paper, we have successfully developed Yb3+-doped monoclinic MgWO4 bulk crystal with Top Seeded Solution Growth (TSSG) method for the first time to the best of our knowledge. The anistropic spectroscopic characterizations were determined in terms of optical absorption, photoluminescence, and lifetime measurements at room temperature. Lasing action was evaluated in the CW regime with the maximum output power of 2.52 W, which corresponding to an optical-to-optical efficiency of 40% and a slope efficiency of 52.8%.
2. Crystal growth of bulk Yb:MgWO4
The Top Seeded Solution Growth was used to grow Yb:MgWO4 crystal, and Na2WO4 was used as flux. The ratio of MgWO4: Na2WO4 in the melt is 5:7 (mol). The chemicals used were MgO, Na2CO3, WO3 and Yb2O3 with purity of 99.99%. The stoichiometric amounts of raw materials with 10 at% Yb2O3 were mixed thoroughly and put into a platinum crucible. Growth process was performed in a homemade vertical furnace with a nickel-chrome wire as the heating element. The solution was kept at 980°C for 2 days to melt homogeneously. The crystal was grown at a cooling rate of 0.6~1°C/d and rotated at a rotating rate of 10 rpm in the range of 945~920°C. The detail of the furnace and parameters of temperature controlling program were the same as described in Ref [22].
After a period of about 30 days, a transparent and free of cracking Yb:MgWO4 bulk crystal sample with dimension of 25 × 13 × 8 mm3 was obtained and it was shown in Fig. 1 inset. We attributed the yellowish coloration of the sample to the impurities or defects in the crystal with the strong absorption in the region of 320-420 nm, which is similar to the Ref [21]. The X-ray diffraction pattern of as-grown Yb:MgWO4 crystal was obtained by using a Rigaku MiniFlex II powder diffractometer with Cu Kα radiation (λ = 0.154187 nm) at room temperature, which was shown in Fig. 1. The result was consistent with the standard pattern of MgWO4 (PDF#27-0789), which confirms that the as-grown crystal was MgWO4 and without the significantly changes in crystal structure after introducing the Yb3+ doped.
Fig. 1 The X-ray power diffraction pattern of Yb:MgWO4 and photograph of the Yb:MgWO4 single crystal (inset).
Since doping ion has certain concentration gradient in the crystal during the process of crystal growth, it is highly significant to measure average concentration of as-grown crystal. The average Yb3+ ion concentration of Yb:MgWO4 crystal was determined by ICP-AES with three samples which were cut from the top, middle and bottom of as-grown crystal. The measured average concentration was 1.819 × 1020 cm−3 (1.25 at.%). The effective segregation coefficient of Yb3+ ion was determined to be 0.13, which is was calculated based on the formula from Ref [23]:
where is measured average concentration, is doping concentration in the crystal molten.3. Spectroscopic characterization of Yb:MgWO4 crystal
The monoclinic MgWO4 crystal is an optically biaxial host crystal. Its optical properties were described in the frame of three mutually orthogonal principal optical axes X, Y and Z. The orthogonal dielectric frame (X,Y,Z) of monoclinic crystal does not correspond to the axes of the crystallographic coordinate system (a,b,c). Only one of the principal optical axes coincides with the 2-fold symmetry axis. In the case of MgWO4 this is corresponding to the crystallographic b-axis. And the two other principal optical axes are in the (a, c) plane but rotated around the b-axis from the a-c ones. We determined their orientation by means of transmission measurements with polarizing microscope. Figure 2 shows the orientation of the optical ellipsoid with respect to the crystallographic frame (both frames a, b, c and X, Y and Z are right-handed). The principal optical axis Z is located at 37.1° with respect to the c crystallographic axis and X is located at 36.4° with respect to the a crystallographic axis.
Fig. 2 (a) Orientation of the principal optical axes (X,Y,Z) with respect to the crystallographic frame (a,b,c) of monoclinic Yb:MgWO4.
A polished sample with a dimension of 3.56 × 5.02 × 4.52 mm3 was employed for the spectroscopic investigation. And its parallel faces were normal to the principal optical axes of X, Y and Z, respectively. Polarized optical absorption and fluorescence spectra measurements were carried out at room temperature for different directions of propagation parallel to the principal optical axes to obtain the optimal conditions for pumping of such lasers and to calculate the corresponding emission cross-section. The polarized absorption spectra were recorder by using the Perkin–Elmer UV–VIS–NIR (Lambda 35) spectrophotometer in the range of 800-1100 nm. Figure 3(a) showed the experimentally-determined absorption cross-section spectra for the single 2F7/2→2F5/2 transition of Yb3+ in MgWO4 crystal in the range of 850-1050 nm at three polarization directions. The maximum absorption line was observed at around 975 nm for light polarization parallel to the X principal optical axis with a line width (FWHM) of around 7.1 nm, which indicated that Yb:MgWO4 crystal is very suitable for pumping with commercial available InGaAs diode laser operating near 980 nm. The room temperature absorption cross-section spectra note the strongly optical anisotropy of Yb:MgWO4 crystal, which is maximal for E//X and minimum for E//Z. The absorption cross-sections at 975 nm were calculated to be 3.76 × 10−20 cm2 (E//X), 2.43 × 10−20 cm2 (E//Y) and 1.08 × 10−20 cm2 (E//Z), respectively. The lifetime was measured by using Lifespec-ps system of Edinburgh Instruments Ltd. The excitation light source is a commercial tunable picosecond pulsed Ti: sapphire laser. The wavelength of the excitation light is of 910 nm, and the detection wavelength is of 1055 nm. The measurement of radiative lifetime was performed with a powder sample so as to avoid the effect of radiation trapping yielded a decay curve that could be fitted by a single exponential corresponding to a time constant of 366 µs at room temperature, which was shown in Fig. 3(b). Polarized emission cross-sections spectra can be calculated from the combination of radiative lifetime and fluorescence spectra with Füchtbauer–Ladenburg (F–L) formula [24, 25 ], which were shown in Fig. 3(c). The emission cross-sections at 1055 nm were calculated to be 2.13 × 10−20 cm2 (E//X), 2.99 × 10−20 cm2 (E//Y) and 2.11 × 10−20 cm2 (E//Z), respectively. The polarized fluorescence spectra were measured in the range of 920-1200 nm by using the Xenon lamp as excitation light source in combination with the Edinburgh Analysis Instruments FLS920 spectrophotometer, which were shown in Fig. 3(d). The emission bands observed in the range of 920–1120 nm with peaks center at 975 nm, 1003 nm, 1021 nm and 1055 nm, respectively. They were corresponding to the emission from the excited 2F5/2-manifold to the four sublevels of the ground state manifold 2F7/2. Polarized fluorescence spectra suggested the strong optical anisotropy with monoclinic Yb:MgWO4 crystal. The emission bandwidths at around 1055 nm are 33 nm (E//X), 21 nm (E//Y) and 27 nm (E//Z). Such broad band emission supported the potential applications of Yb:MgWO4 crystal in tunable lasers and femtosecond oscillators. From the absorption and emission measurements at 295 K, we derived the energies position of the four sublevels of the ground state multiplet 2F7/2 and the three sublevels of the excited state multiplet 2F5/2 of Yb3+ in MgWO4, which was inserted in Fig. 3(d). The Stark splitting of the 2F5/2 mutiplet is indicative of strong crystal field, which is advantageous for tunable and ultra-short pulse generation.
Fig. 3 (a) Room temperature absorption cross-section spectra for the of Yb:MgWO4 crystal with polarized light E//X, E//Y and E//Z, respectively; (b) Decay curve of the Yb3+ ion emission from the 2F5/2 excited state. (c) Room temperature emission cross-section spectra of Yb:MgWO4 crystal with polarized light E//X, E//Y and E//Z, respectively . (d) Room temperature fluorescence emission spectra of Yb:MgWO4 crystal with polarized light E//X, E//Y and E//Z, respectively. Inset: Schematic diagram of Stark levels and transitions of Yb:MgWO4 crystal.
The potential gain bandwidth for tunable or mode-locked operation of Yb3+-doped quasi-three-level laser can be determined by calculating the gain cross-section which depends on the inversion rate. The gain cross-section , which is used for evaluating the possibility of tuning range of the laser wavelength. It can be obtained from the following equation [25].:
Where represents the inversion fraction of Yb3+ ion in the excited state to achieve population inversion at the extraction wavelength. The gain cross-section , calculated for X, Y and Z polarization were illustrated in Fig. 4 (a), (b) and (c) , respectively. Laser gain was expected to occur only at >0. For Yb:MgWO4 crystal, when exceed 0.4 the positive gain appears. It meant that laser emission could be realized from 970 nm to 1080 nm while β≥0.4.Fig. 4 Calculated gain cross-section for polarization along the (a) X, (b) Y and (c) Z axes of Yb:MgWO4 at different population inversion rates β.
4. Laser performance
The schematic of Yb:MgWO4 laser was shown in Fig. 5(a) . Laser action of Yb:MgWO4 crystal was evaluated by using a simple two-mirrors linear plano-concave resonator consisted of a plane high-reflecting (HR) dichroic input mirror and a 100 mm radius of curvature output coupler. An uncoated sample of 1.25 at.% Yb:MgWO4 with end faces of 3 × 3 mm2 and 3.7 mm long, which was cut along X-axis for lasing action. It was wrapped with indium foil and mounted on a copper heat sink with a thermal-electric cooler module (TEC) to improve the condition of thermal dissipation. An 8 W fiber-coupled diode laser emitting at 975 nm with a core-diameter of 200 μm and N.A. of 0.22 worked as pumping source. The pump beam was reimaged onto the laser crystal by a pair of plane-convex focusing lenses, which provided a spot radius of around 100 μm. The absorption of pump power was measured around 65.5%. The typical cavity length was 20 mm. Figure 5(b) illustrated the power scaling characteristics of the Yb:MgWO4 laser by using two plano-concave output couplers with transmission at laser wavelength (Toc) of 2% and 5%, respectively. The inserted far-field spatial intensity profile of the output beam was measured at the maximum output power. The lser polarization was always linearly and parallel to the Y-axis throughout the entire incident pump power level. The maximum output power was 2.52 W for Toc = 5% at the threshold pump power of 1.48 W, resulting in an optical-to-optical efficiency of 40%, whereas the slope efficiency was 52.8%. With employing the output coupler of Toc = 2%, the maximum output power was 1.94 W and the threshold pump power at 1.21 W, which corresponding to the optical-to-optical efficiency of 30.8% and a slope efficiency of 37.9%. Figure 5(c) was depicted the experimental result for the lasing spectrum at the maximum incident pump power by using a fiber-coupled optical spectrometer (Ocean Optics HR4000 with the optical resolution of 0.1 nm). Output spectrum was centered at near 1059.9 nm with the FWHM of 1.3 nm. The output power grew linearly without rollover which indicated the possibility of further power scaling by using higher power laser diode.
Fig. 5 (a) Scheme of the Yb:MgWO4 laser; (b) Input-output characteristics of Yb:MgWO4 laser, insert: spatial intensity profiles of the output beam; (c) laser spectrum of Yb:MgWO4 laser.
5. Discussion and conclusion
High-quality monoclinic magnesium tungstate crystal was grown with size sufficient for its characterization in the field of spectroscopy and laser operation with Yb3+-doping. The main advantages of this Yb:MgWO4 crystal as laser gain meidum are very high values of the absorption and emission cross-section, strong anisotropy and relatively broad absorption and emission bandwidths. The main absorption peak of Yb:MgWO4 crystal is at around 975 nm which is slightly bule shifted with respect to the Yb3+ doped monoclinic potassium double tungstates (e.g. Yb:KG(WO4)2 [26], Yb:KY(WO4)2 [26] and Yb:KLu(WO4)2 [5] at around 981 nm) and similar to the tetragonal double tungstates (e.g. Yb:NaY(WO4)2 [27], Yb:NaGd(WO4)2 [28] and Yb:NaLu(WO4)2 [29]). The peak value of absorption cross-section of Yb:MgWO4 crystal is 3.76 × 10−20 cm2 (E//X) which is more than that of YbYAG (8.2 × 10−21 cm2 [24]). The emission cross-section spectra have a main peak around 1055 nm, with a peak value 2.99 × 10−20 cm2 (E//Y) which is similar to those of Yb:KG(WO4)2 (1025 nm, 3 × 10−20 cm2) [26], Yb:KY(WO4)2 (1023 nm, 2.8 × 10−20 cm2) [26] and Yb:KLu(WO4)2 (1026 nm, 2.63 × 10−20 cm2 [5]). The measured maximum absorption bandwidth of Yb:MgWO4 crystal is 7.1 nm at around 975 nm which is much broader than those of Yb3+-doped monoclinic potassium double tungstates (e.g. 3.5 nm of Yb:KG(WO4)2 of [26], 3.7 nm of Yb:KY(WO4)2 [26] and 4 nm of Yb:KLu(WO4)2 [5]). The measured maximum emission bandwidth of Yb:MgWO4 crystal is 33 nm (E//X) which is much broader than that of Yb:YAG (9 nm [30]) and again broader than those of Yb3+-doped monoclinic potassium double tungstates (e.g. 20 nm of Yb:KG(WO4)2 of [26], 16 nm of Yb:KY(WO4)2 [26] and 28 nm of Yb:KLu(WO4)2 [5]). The value of the upper level lifetime of Yb:MgWO4 crystal is 366 μs which is more similiar to that of Yb:KLu(WO4)2 [5]. Concerning with the continuous-wave laser slope efficiency of Yb:MgWO4 crystal, exceeding 50% indicates that the optical quality of as-grown sample is very good.
In conclusion, we have successfully developed an ytterbium-doped, monoclinic magnesium tungstate crystal with dimension of 25 × 13 × 8 mm3 by the top seeded solution growth method for the first to the best of our knowledge. The spectroscopic characteristics of Yb:MgWO4 was determined in terms of polarized optical absorption, photoluminescence, and lifetime measurements. Calculated emission and gain cross sections were presented. Lasing action in continuous-wave regime was also evaluated. A maximum output power 2.52 W was acquired with corresponding to an optical-to-optical efficiency of 40% and a slope efficiency of 52.8%. The attractive spectral features that well spectral matched between crystal absorption band and the spectra emission band of commercial InGaAs laser diode is highly significant to further power scaling of such laser. Compared to other Yb3+-doped double tungstate or YAG crystals, Yb:MgWO4 possesses broader emission and absorption FWHM due to the lattice distortion and vacancy. And it also signified with large absorption and emission cross-sections. Therefore all these make Yb:MgWO4 to be a promising candidate for sub-100 fs oscillator in mode-locked regime.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (11404332, 61575199, 61108054, 61308085, 61475158, 61275177, and 61405171), the National High-Tech R&D Program of China (Grant No.2013AA014202), and Key Project of Science and Technology of Fujian Province (2014H0052, 2016H0045). This work was partly supported by the China Scholarship Council (CSC) No.201504910418.
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