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Abstract
We report on the crystal growth, structure, Raman and optical spectroscopy of novel “mixed” tetragonal vanadates, Yb:Lu1-x-yYxLayVO4. Optical absorption, stimulated-emission, and gain cross-section spectra of Yb3+ are determined for π and σ polarizations. For a Yb:Lu0.74Y0.23La0.01VO4 crystal, the absorption bandwidth is >10 nm, the σSE is 1.1 × 10−20 cm2 at 1013 nm, the gain bandwidth is >40 nm (for π-polarization), and the radiative lifetime of the 2F5/2 state is ~305 μs. The Stark splitting of the Yb3+ multiplets is determined using low-temperature (6 K) spectroscopy. A diode-pumped a-cut 2 at.% Yb:Lu0.74Y0.23La0.01VO4 laser generated 5.0 W at 1044 nm with a slope efficiency of 43%. The developed materials are promising for sub-100 fs mode-locked lasers at ~1 µm.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tetragonal (zircon (ZrSiO4) type, sp. gr. I41/amd) orthovanadate crystals, REVO4 where RE = Gd, Y, or Lu, are very suitable hosts for rare-earth laser-active ions. These materials are optically uniaxial (the optical axis is parallel to the crystallographic c-axis) and offer linearly polarized laser output [1-2], relatively high transition cross-sections for the dopant ions [3] and good thermo-mechanical and thermo-optical properties (high thermal conductivity, low thermal expansion, positive dn/dT coefficients and weak thermal lensing) [4-5]. Recently, REVO4 crystals have been studied for Yb3+ doping [1,3,6–8] resulting in efficient continuous-wave (CW) [1], Q-switched [9] and sub-100 fs mode-locked (ML) oscillators [10]. Optical bistability has been also observed in CW Yb:REVO4 lasers [11]. In ref [1], a diode-pumped CW a-cut Yb:LuVO4 laser generated 8.3 W of π-polarized output at 1031 nm with a slope efficiency of 80% (with respect to the absorbed pump power). In ref [10], a semiconductor saturable absorber mirror (SESAM) was used to generate 58-fs-long laser pulses from a ML Yb:LuVO4 laser at 1036 nm at a repetition rate of 94 MHz.
In the REVO4 crystals, there is a single site for accommodating the RE3+ ions (D2d symmetry, VIII-fold O2- coordination). The Yb3+ ions replace the RE3+ ones resulting in a RE1-xYbxVO4 composition. Stoichiometric tetragonal YbVO4 crystals also exist but the fluorescence quenching effect is determental [12]. Besides the ordered Yb:REVO4 compounds, “mixed” crystals, e.g. Yb:Gd1-xYxVO4 or Yb:Lu1-xGdxVO4, can be grown with good optical quality [3,13,14]. Due to the inhomogeneous broadening of the Yb3+ spectral bands as a result of the local disorder leading to slightly varying crystal field strengths, such crystals are of interest for shortening of the pulse from ML lasers. In ref [13], a SESAM ML Yb:Gd0.64Y0.36VO4 laser generated 100-fs pulses at ~1021 nm (shorter compared to the ordered Yb:YVO4 crystal) with a spectral bandwidth of ~33 nm (for π-polarization).
In the present work, we have grown, characterized and demonstrated CW laser operation of novel “mixed” vanadate crystals, Yb:Lu1-x-yYxLayVO4, featuring a strong broadening of the spectral bands due to the particular “mixture” of passive ions (Lu3+, Y3+ and La3+) with pronounced ionic radius difference.
2. Crystal growth and structure
Crystals with two different compositions of 2.6 at.% Yb:Lu0.51Y0.45La0.01VO4 and 2 at.% Yb:Lu0.74Y0.23La0.01VO4 were grown by the Czochralski method in N2 + 2 vol.% O2 atmosphere using Ir crucibles and [100]-oriented undoped YVO4 seeds. The pulling rate was 2-3 mm/h and the rotation rate was 10-30 rpm (revolutions per minute). The as-grown crystals were cooled down to room temperature (RT, 293 K) at 30–80 °C/h and annealed in air at 1200 °C for 20 h. The obtained boules with dimensions of Ø25 × 20 mm3, Fig. 1(a),(b), were of high optical quality. The as-grown crystals had yellowish coloration which could be partially removed by annealing in air. For the spectroscopic and laser studies, 3 mm-thick rectangular samples were cut along the a-axis with an aperture of 3 × 3(c) mm2. They provided access to both principal light polarizations of the uniaxial vanadates, π (E || c) and σ (E ⊥ c)
Fig. 1 (a,b) Photographs of the as-grown crystals (a) 2 at.% Yb:Lu0.74Y0.23La0.01VO4 and (b) 2.6 at.% Yb:Lu0.51Y0.45La0.01VO4 crystals; (c) X-ray powder diffraction (XRD) patterns of these crystals (the numbers denote the Miller’s indices (hkl)).
The structure and the phase purity of the grown crystals were confirmed by X-ray powder diffraction analysis, Fig. 1(c). Both crystals are tetragonal (sp. gr. I41/amd – D194h, No. 141, formula units per unit-cell Z = 4, point group 4/mmm) with the following lattice parameters: a = b = 7.076 Å, c = 6.266 Å (for the Yb:Lu0.51Y0.45La0.01VO4 crystal) and a = b = 7.056 Å, c = 6.253 Å (for the Yb:Lu0.74Y0.23La0.01VO4 one). The composition of the crystals and the Yb3+ doping concentration were determined by Inductively Coupled Plasma (ICP) atomic spectroscopy: NYb = 3.31 × 1020 cm−3 and 2.57 × 1020 cm−3 for the 2.6 at.% Yb-doped and 2 at.% Yb-doped crystals, respectively.
The vibrational properties of the crystals were studied by polarized Raman spectroscopy, Fig. 2. For the point group 4/mmm, the irreducible representations at the center of the Brillouin zone (k = 0) are: Г = (2A1g + 2B1u) + (B1g + A1u) + (A2g + B2u) + (4B2g + 4A2u) + (5Eg + 5Eu) of which 12 (2A1g + B1g + 4B2g + 5Eg) are Raman-active [15]. For the studied crystals, a total of 9 modes are clearly resolved in the spectra and the maximum phonon frequency hνph is 898 cm−1 (Yb:Lu0.74Y0.23La0.01VO4) and 895 cm−1 (Yb:Lu0.51Y0.45La0.01VO4). These lines are assigned as ν1(A1g) and correspond to internal symmetric vibrations of the tetrahedral [VO4]3- groups [16].
Fig. 2 Polarized Raman spectra for the a-cut (a) 2 at.% Yb:Lu0.74Y0.23La0.01VO4 and (b) 2.6 at.% Yb:Lu0.51Y0.45La0.01VO4 crystals, λexc = 514 nm.
3. Optical spectroscopy
The absorption cross-section spectra (σabs) for the studied crystals are shown in Fig. 3(a, c) with the polarizations along π and σ. For Yb:Lu0.74Y0.23La0.01VO4, the maximum σabs corresponds to π-polarization, 5.2 × 10−20 cm2 at 984.5 nm (zero-phonon line, ZPL, the full width at half maximum (FWHM) of the absorption peak is 10.4 nm). For σ-polarization, the peak σabs value is 2.7 times lower and it is reached at 969.6 and 984.6 nm. For the Yb:Lu0.51Y0.45La0.01VO4 crystal, σabs for π-polarization is slightly lower, namely 4.4 × 10−20 cm2 at 984.7 nm while the FWHM of the absorption peak is broader, 13.1 nm. Similarly, the absorption spectrum for σ-polarization features two peaks at 969.9 and 984.7 nm with 2.4 times lower σabs. The determined FWHM values for the ZPL are broader than those in Yb:REVO4 (RE = Gd, Y and Lu) crystals and in the previously studied “mixed” vanadates Yb:Gd1-xYxVO4 and Yb:Lu1-xGdxVO4 [3]. The addition of large La3+ ions (ionic radius: 1.16 Å compared to 1.053 Å for Gd3+, 1.019 Å for Y3+, and 0.977 Å for Lu3+ in VIII-fold O2- coordination) is expected to contribute to the distortion of the crystal field and the observed spectral broadening. Indeed, it is known that LaVO4 is monoclinic (monazite ((Ce,La)PO4) type, sp. gr. P21/n) [17].
Fig. 3 Room temperature (293 K) optical spectroscopy of (a, b) 2 at.% Yb:Lu0.74Y0.23La0.01VO4 and (c,d) 2.6 at.% Yb:Lu0.51Y0.45La0.01VO4 crystals: (a,c) absorption cross-sections, σabs, and (b, d) stimulated-emission cross-sections, σSE, for π and σ light polarizations.
The stimulated-emission cross-sections, σSE, were calculated with a combination of the reciprocity method (RM) [18] and the Füchtbauer-Ladenburg (F-L) formula [19]. The latter method was applied for the long-wavelength part of the σSE spectra (>1050 nm). In the RM, the determined Yb3+ Stark splitting was considered (see Fig. 6). In the F-L method, the measured polarized luminescence spectra and the calculated radiative lifetimes, τrad (2F5/2), were used (see Fig. 3 and 4). The results obtained by the two methods were in reasonable agreement having in mind the effect of reabsorption on the measured luminescence spectra. The results for σSE are shown in Fig. 3(b, d). In the spectral range where laser operation is expected, σSE amounts to ~1.1 × 10−20 cm2 at 1013 nm (π) or 1010 nm (σ) (for Yb:Lu0.74Y0.23La0.01VO4). For the Yb:Lu0.51Y0.45La0.01VO4 crystal, the corresponding σSE are slightly lower, 1.0 × 10−20 cm2 at 1012 nm (π) or 1009 nm (σ).
Fig. 4 Room-temperature (293 K) luminescence decay curves for powdered (a) 2 at.% Yb:Lu0.74Y0.23La0.01VO4 and (b) 2.6 at.% Yb:Lu0.51Y0.45La0.01VO 4 crystals, λlum = 1010 nm, λexc = 980 nm. Symbols are the experimental data, red lines are their single-exponential fits.
The measured luminescence decay curves (excitation at 980 nm, luminescence at 1010 nm), see Fig. 4, are clearly single-exponential for both crystals and the decay time τlum is 363 μs (for the Yb:Lu0.74Y0.23La0.01VO4 crystal). For the Yb:Lu0.51Y0.45La0.01VO4 crystal, τlum is longer, 384 μs, explaining the difference in the transition cross-sections. Both lifetimes determined lifetimes are longer than those for Yb:REVO4 (RE = Gd, Y and Lu) crystals, for which τlum = 247-345 μs [3].
The Stark splitting of the Yb3+ multiplets has been determined with low-temperature (LT, 6 K) absorption and emission spectroscopy for polarized light assuming J + 1/2 splitting of each multiplet, see Fig. 5.
Fig. 5 Low-temperature (6 K) spectroscopy of (a,b) 2 at.% Yb:Lu0.74Y0.23La0.01VO4 and (c,d) 2.6 at.% Yb:Lu0.51Y0.45La0.01VO4 crystals: (a, c) absorption spectra and (b, d) photoluminescence (PL) spectra, λexc = 973 nm, for π and σ light polarizations.
The absorption and emission lines corresponding to the 0 → j' and 0' → i transitions are assigned (here, the indices i = 0-3 and j' = 0'-2' are the sub-levels of the 2F7/2 ground-state and 2F5/2 excited-state, respectively). This allowed us to plot the scheme of the energy levels of Yb3+ in the studied crystals, Fig. 6. In this figure, the calculated partition functions [18] for both multiplets Z1(2) are indicated, i.e., for Yb:Lu0.74Y0.23La0.01VO4, Z1 (2F7/2) = 1.82 and Z2 (2F5/2) = 1.48, so that Z1/Z2 = 1.23.
Fig. 6 Stark structure of the energy levels of Yb3+ in (a) Lu0.74Y0.23La0.01VO4 and (b) Lu0.51Y0.45La0.01VO4 crystals, Z1(2) are the partition functions, numbers denote the energies of the 0-3 (2F7/2) and 0'-2' (2F5/2) sub-levels (in cm−1), arrows indicate the transitions in absorption (blue) and emission (red) at low temperature and the corresponding wavelengths
In Fig. 7, the so-called barycenter plot is presented for different Yb3+-doped crystals [20] where the barycenter energy of the 2F5/2 excited-state is plotted vs. the barycenter energy of the 2F7/2 ground-state. For RE3+, the barycenter of any 2S + 1LJ(4fn) multiplet shows a linear variation with the barycenter of any other isolated 4fn multiplet. For Yb3+, this dependence is expressed by the formula ‹E›(2F5/2) = 10166.6 + 0.997‹E›(2F7/2) cm−1 [20]. The determined Stark splitting is well in line with this trend.
Fig. 7 A barycenter plot for Yb3+ in different crystals (barycenter energy ‹E› for the 2F5/2 upper laser level vs. ‹E› for the 2F7/2 ground-state): symbols – experimental data, line – their linear fit. Crystals: KGW – KGd(WO4)2, YLF – LiYF4, YAP – YAlO3, YAB – YAl3(BO3)4, YSO – Y2SiO5, CAS – Ca2Al2SiO7, YAG – Y3Al5O12, YCOB –YCa4O(BO3)3.
Based on the determined Stark splitting and using the modified reciprocity method [21], we calculated the radiative lifetimes of the 2F5/2 state as τrad = 305 μs (Yb:Lu0.74Y0.23La0.01VO4) and 325 μs (Yb:Lu0.51Y0.45La0.01VO4). The difference in τrad and τlum (cf. Figure 4) reflects the effect of radiation-trapping on the measured luminescence decay curves. Still, the radiative lifetime for Yb:Lu0.51Y0.45La0.01VO4 is longer and both values are longer than those for the Yb:REVO4 crystals [6].
According to the quasi-three-level nature of the Yb3+ laser, the gain cross-sections, σgain = βσSE – (1–β)σabs, were calculated, see Fig. 8. Here, β = N2(2F5/2)/NYb is the inversion ratio. For both π and σ light polarizations, the σgain spectra are smooth and broad. The gain bandwidth (FWHM) is 41 nm (π) or 33 nm (σ) for Yb:Lu0.74Y0.23La0.01VO4 and 39 nm (π) or 29 nm (σ) for Yb:Lu0.51Y0.45La0.01VO4 (for β = 0.2) indicating high suitability of the grown crystals for broadly tunable and sub-100-fs ML lasers. Indeed, the determined gain bandwidths are broader than those calculated for Yb:YVO4 and Yb:Gd0.64Y0.36VO4 at the same inversion level for π-polarization (32 nm and 33.5 nm, respectively) [13].
Fig. 8 (a, b) Gain cross-sections, σgain = βσSE – (1–β)σabs, for the 2F5/2 → 2F7/2 transition of Yb3+ in Lu0.74Y0.23La0.01VO4 for π (a) and σ (b) polarizations, β = N2(2F5/2)/NYb is the inversion ratio. Figure 8. (c, d) Gain cross-sections, σgain = βσSE – (1–β)σabs, for the 2F5/2 → 2F7/2 transition of Yb3+ in Lu0.51Y0.45La0.01VO4 for π (c) and σ (d) polarizations.
4. Laser operation
For the laser experiments, the crystals were oriented for light propagation along the a-axis (a-cut). The dimensions were 3 (a) × 3 (c) × 3 (a) mm3; both input and output faces were polished to laser quality and remained uncoated. The crystals were wrapped using In foil to improve the thermal contact from all 4 lateral sides and mounted in a Cu-holder water-cooled to 12 °C. Two laser cavities were designed.
Cavity #1 (compact plane-concave) was formed by a concave (R = 100 mm) pump mirror (PM) highly reflective (HR) coated for 1.01-1.2 μm and with high transmission (HT) at the pump wavelength (~980 nm), and a flat output coupler (OC) having a transmission TOC of 5% at 1.01-1.1 μm. The crystal was placed close to the PM. The total cavity length Lcav was ~20 mm. Cavity #2 (microchip-type) utilized a flat PM with a similar coating and a set of flat OCs with TOC = 1%, 2.5%, 5% or 10%. Both PM and OC were placed close to the crystal surfaces resulting in Lcav ~3 mm. The crystal was pumped by fiber-coupled (fiber core diameter: 200 μm, numerical aperture, N.A. = 0.22) InGaAs laser diodes emitting unpolarized output at ~978 nm (up to 25 W and 17 W for cavity #1 and #2, respectively). A lens assembly (1:1 re-imaging ratio, f = 30 mm) was used to collimate and focus the pump radiation. The pump spot radius in the crystal wp was 100 μm and the confocal parameter 2zR was 1.8 mm.
The input-output dependences for cavity #1 are shown in Fig. 9. For both crystals, the laser output was linearly polarized (π) with the polarization naturally-selected by the anisotropy of the gain. The Yb:Lu0.74Y0.23La0.01VO4 laser generated 5.0 W at ~1044 nm with a slope efficiency η of 43% (with respect to the absorbed pump power Pabs). The laser threshold was at Pabs = 4.3 W. The output performance for the Yb:Lu0.51Y0.45La0.01VO4 laser was inferior: The maximum output power reached 4.15 W at lower η of 33% and higher threshold, (4.9 W). For both crystals, the input-output dependences was linear at least up to Pabs = 16 W.
Fig. 9 Input-output dependences of cavity #1 for CW Yb:Lu1-x-yYxLayVO4 lasers, η – slope efficiency.
The results obtained with the microchip-type cavity and the Yb:Lu0.74Y0.23La0.01VO4 crystal are shown in Fig. 10 (a, b). Microchip laser operation with the Yb:REVO4 crystals is possible due to the positive dn/dT coefficients, and, consequently, positive sign of the thermal lens [4]. The maximum output power was 2.01 W at 1033-1038 nm with η = 37% (for TOC = 5%). Further power scaling was limited by the available pump power. With the increase of TOC, the emission wavelength shortened from 1045 to 1050 nm (for TOC = 1%) to 1023-1027 nm (for TOC = 10%), in agreement with the gain spectra, see Fig. 8(a). The multi-peak emission from the microchip-type laser was related to etalon effects. The output of the microchip laser was also naturally π-polarized.
Fig. 10 Input-output dependences and typical laser emission spectra measured at maximum Pabs for CW Yb:Lu1-x-yYxLayVO4 lasers with microchip-type laser cavity #2, η – slope efficiency.
5. Conclusion
The tetragonal Yb:Lu1-x-yYxLayVO4 crystals offer strongly polarized spectral bands (absorption and emission) with enhanced inhomogeneous broadening compared to previously reported “mixed” orthovanadates. For Yb:Lu0.74Y0.23La0.01VO4, the FWHM of the ZPL absorption peak is >10 nm, the stimulated-emission cross-section σSE is 1.1 × 10−20 cm2 at ~1013 nm and the gain bandwidth is >40 nm (for π-polarization). Moreover, this crystal features relatively long radiative lifetime of the upper laser level, ~305 μs. Multi-watt CW laser output at ~1044 nm is demonstrated in the Yb:Lu1-x-yYxLayVO4 crystals under diode-pumping at 978 nm. The new crystals are very promising for sub-100 fs ML oscillators and broadly tunable lasers around 1 µm.
Funding
National Natural Science Foundation of China (No.61575199, No.11404332, No.J1103303, No.61775039); Key Project of Science and Technology of Fujian Province (2016H0045); Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDB20000000); Instrument Project of Chinese Academy of Sciences (YZ201414) and the Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2008DP173016); Spanish Government (MAT2016-75716-C2-1-R, (AEI/FEDER,UE), TEC 2014-55948-R); Generalitat de Catalunya (2014SGR1358).
Acknowledgments
F. D. acknowledges additional support through the ICREA academia award 2010ICREA-02 for excellence in research. P. L. acknowledges financial support from the Government of the Russian Federation (Grant No. 074-U01) through ITMO Post-Doctoral Fellowship scheme.
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