Broadly tunable laser operation near 2μm in a locally disordered crystal of Tm^3+-doped NaGd(WO_4)_2

Abstract

A single crystal of NaGd(WO 4) 2 with disordered structure was grown by the Czochralski method with 5 mol % of Tm 3+-doping in the melt. This crystal host belongs to the I 4 tetragonal space group. The Tm 3+ optical absorption and emission spectroscopy revealed strong polarization dependence in accordance with the S 4 local site symmetry of the uniaxial host. The Stark energy levels of Tm 3+ in this host were determined from 5 K optical absorption and photoluminescence measurements. The 300 K lifetime of the upper laser level for the 3 F 4 → 3 H 6 transition amounts to 1.35±0.20 ms. The maximum ground state absorption cross sections for the 3 H 4 level of interest for diode pumping near 795 nm are 2.9×10-20 and 1.18×10-20 cm 2 for the σ and π polarization, respectively. The maximum emission cross sections amount to 10.6×10-21 cm 2 at 1796 nm and 9.5×10-21 cm 2 at 1847 nm, for the σ and π polarization, respectively. Laser operation in the cw regime has been achieved at room temperature under Ti:sapphire and diode laser pumping using an uncoated sample. A maximum output power of ≈300 mW was obtained at 1925 nm for the σ polarization under Ti:sapphire laser pumping. Slightly higher powers were obtained with diode pumping but the threshold pump power also increased. In this case the laser naturally selected the σ polarization. The optical bands of Tm 3+ in this crystal have inhomogeneous contribution and their FWHM reaches, e.g. for the 3 H 6 → 3 F 4 transition at 5 K, about 60 cm-1. This is related to Na and Gd (or Tm) local disorder between the two nonequivalent lattice sites of the host. The full cw laser tunability range at 300 K, from 1813 to 2025 nm, corresponds to ≈17 THz, and is one of the broadest achieved with a Tm 3+-doped crystalline material, despite the relatively low efficiency obtained with this first sample of Tm:NaGd(WO 4) 2 .

Thulium doped monoclinic KLu(WO4)2 single crystals: growth and spectroscopy

“…The tuning range was similar in Tm:Yb:KYW [27]. For other well-known hosts such as YLF, a tuning range of 162 nm is possible in the cw mode [28] and NGdW with 212 nm tuning range [21].…”

“…These peaks are suitable for diode-pumping and their σ a are slightly higher than those of Tm:KGd(WO 4 ) 2 (KGdW) [18], Tm:KY(WO 4 ) 2 (KYW) [19] and Tm:KYb(WO 4 ) 2 (KYbW) [20] and almost twice as the high as that the disordered tungstate Tm:NaGd(WO 4 ) 2 (NGdW) [21,22].…”

Spectroscopy of tetragonal Eu:NaGd(WO4)2 crystal

“…In particular, efficient laser operation with Nd 3þ [6], Yb 3þ [7,8], Tm 3þ [9,10] and Ho 3þ ions [11] has been reported. These materials have an important advantage as compared with their monoclinic counterparts where Na þ is replaced by K þ , i.e.…”

Spectral properties of Tm3+-doped LiLa(MoO4)2 crystal

“…To obtain the ∼2.0 m laser, Tm 3+ -doped media become very promising because the 3 F 4 → 3 H 6 transition of Tm 3+ ions is an effective channel [4] and the broad emission band of the transition also makes tunable laser operation possible [5]. In addition, the broad and strong absorption band of the 3 H 6 → 3 H 4 transition around 795 nm is suitable for the AlGaAs diode laser pumping [6]. Another attractive character of Tm 3+ ions is the high efficiency for the ∼795 nm diode pumping because in principle one pump photon can excite two ions into the 3 F 4 upper laser level through cross-relaxation between Tm 3+ ions [7,8].…”