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2Tm3+, Li+ ZnWO4: novel 2-pm laser crystal
D. Lis1*, K. Subbotin12, Yu. Zimina12, Ya. Didenko2, S. Pavlov2, A. Titov2, E. Zharikov1
1-Prokhorov General Physics Institute, Russian Academy of Sciences, 38 Vavilova St., 119991 Moscow, Russia
2- Mendeleev University of Chemical Technology of Russia, 9 Miusskaya Sq., 125047 Moscow, Russia
We report on the crystal growth, spectroscopic investigation, and laser performance of Tm3+-doped monoclinic zinc tungstate (Tm:ZnWO4). Tm3+-doped ZnWO4 crystals containing charge compensator (Li+ ions) were grown using the Czochralski (Cz) method in air using a Pt crucible. The actual Tm3+ doping level for this second crystal was 1.59 at.% and the segregation coefficient is almost 0.4 owing to the positive effect of Li+ codoping. Meanwhile, the actual Li+ content in the crystal was measured to be 1.65 at.%, and the segregation coefficient is only ~ 0.2.
ZnWO4 belongs to the monoclinic class adopting the C2/1 - P2/c space group and the 2/m point group, with a general multiplicity Z of 2. The lattice constants are a = 4.692A, b = 5.721A, c = 4.928A, and the monoclinic angle P = a A c = 90.632°. ZnWO4 optical properties are described within the optical indicatrix frame, featuring three mutually orthogonal principal axes denoted as Np, Nm, and Ng. One of them (Np) is parallel to the crystallographic b-axis, aligned with the 2-fold symmetry axis. The other two axes of the optical indicatrix, Nm and Ng, lie in the a-c plane.
The polarized absorption spectra reveal a strong polarization anisotropy of absorption properties. The maximum absorption cross-sections oabs is 1.09x10-20 cm2 at 803.6 nm and the corresponding absorption bandwidth is 16 nm for light polarization E || Ng. For the other two polarization states, oabs is smaller, amounting to 0.83x10"20 cm2 at 807.7 nm (for E || Np), and 0.24x10"20 cm2 at 802.7 nm (for E || Nm).
A Judd-Ofelt analysis is conducted, the spontaneous emission probabilities, luminescence branching ratios and radiative lifetimes are determined. The crystal-field splitting of the 3H and 3F4 Tm3+ multiplets was achieved using low-temperature spectroscopy. The ZnWO4 crystal exhibits a relatively large total Stark splitting of the Tm3+ ground state, AE(3H6) of 644 cm-1, evidencing a relatively strong crystal-field for this material. This leads to the longest wavelength of a purely electronic transition 3F4 of 2028 nm, i.e., above 2 ^m, which is rarely observed for commonly
used laser host crystals.
Polarized luminescence spectra and decay kinetic are obtained. Tm3+ ions in ZnWO4 exhibit a significant polarization anisotropy of their emission properties that is favorable for achieving linearly polarized laser output. The maximum stimulated-emission (SE) cross-sections, ose reaches 2.93x10"2° cm2 at 1871 nm for light polarization E || Np. In the long-wave spectral region where laser action is expected to be supported by the reabsorption from the ground-state for quasi-three-level 2-^m Tm lasers, the peak SE cross-sections are 0.77x10"20 cm2 at 2015 nm and 0.70x10"20 cm2 at 1971 nm also for light polarized E || Np. Tm3+ ions exhibit smooth and broad emission spectral profiles extending beyond 2 ^m, positioning Tm3+-doped ZnWO4 as a promising candidate for generation of femtosecond pulses in this spectral range which is well detuned from the structured absorption of water vapors in the air.
The decay of 3F4 level is well described by the single-exponential law, yielding a luminescence lifetime Hum of 1.57 ms for the powdered sample, as compared to 2.08 ms for the bulk crystal.
The laser element was cut from the annealed crystal for light propagation along the Ng optical indicatrix axis (Ng-cut). The continuous-wave Tm3+, Li+: ZnWO4 laser generated a maximum output power of 282 mW at 1964-1983 nm (exhibiting a broad laser spectrum) with a slope efficiency n of 14.7% with respect the absorbed power and a laser threshold of 188 mW. The combined attributes of large Stark splitting, polarized emission, spectral broadening, and prolonged luminescence lifetime position Tm-doped ZnWO4 crystals as promising candidates for advanced laser systems.
This work has been supported by Russian Scientific Fund (grant №23-22-00416).
