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Abstract
Er:GdScO3 crystal was grown by the edge-defined film-fed (EFG) method for the first time. The rocking curve showed that the as-grown crystal has a high crystalline quality. The peak absorption cross section at 980 nm is 0.38×10−20 cm2 with full width at half maximum (FWHM) of 19.5 nm and the peak emission cross section at 2720 nm is 0.93×10−20 cm2 with FWHM of 149.0 nm. The fluorescence lifetimes of 4I11/2 and 4I13/2 were fitted to be 2.24 ms and 4.57 ms, respectively. At inversion ratio P≥0.5, the positive local peak of gain cross-section at 2.7 µm was obtained. Continuous-wave laser operation of Er:GdScO3 crystal was demonstrated. The maximum output power of 863 mW was obtained with a slope efficiency of 11.25%. The results indicated that Er:GdScO3 crystal is a potential laser material for 2.7 µm laser.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared (MIR) laser around 2.7 µm has attracted increasing interest in recent years owing to its wide applications including medical surgery, atmospheric environment monitoring, pump source of optical parametric oscillators (OPO) and directional infrared countermeasure, etc [1–4]. The 2.7 µm MIR laser can be obtained in erbium-doped laser materials based on luminescence transition of 4I11/2→4I13/2. However, the 2.7 µm laser radiation of Er3+ is restricted by the self-terminating effect due to the accumulation of population in the 4I13/2 energy level, which results in much longer lifetime of the lower level 4I13/2 than the upper level 4I11/2. The energy transfer upconversion (ETU) process (4I13/2 + 4I13/2 → 4I15/2 + 4I9/2) is an effective method to overcome this negative effect, and two-photon fluorescence at 2.7 µm is achieved, and quantum efficiency is greatly improved [5]. To date, the 2.7 µm lasers have been reported in Er3+ doped laser host materials like CLGO [6], BLGO [7], LuGG [8], YSGG [9], YAG [10], YAP [11], Lu2O3 [12], Y2O3 [13], CaF2 [14], SrF2 [15], LiYF4 [16], ZBLAN [17], etc.
An ideal host material should have the characteristics of high optical damage threshold, chemical stability and high transparency in the spectral range for 2.7 µm MIR laser. Moreover, the low phonon energy and disordered local crystal field are essential to enhance 2.7 µm laser performance. Gadolinium scandate (GdScO3) is a kind of efficient laser host material. GdScO3 crystal has the structure of perovskite belonging to orthorhombic system with the space group of Pnma (no. 62) [18]. The GdScO3 crystal has robust thermal stability and low phonon energy (∼452 cm-1), which decreases non-radiative transition probability of 4I11/2→4I13/2. The rare earth ions doped GdScO3 crystals have been reported in recent years. A. Yamaji et al. investigated optical properties and radiation response of Ce:GdScO3 single crystal in 2012 [19]. Strong yellow emission at 578 nm of Dy:GdScO3 and Dy,Tb:GdScO3 laser crystal was reported by F. Peng et al [20,21] in 2018. In 2019, D.H Wang et al. demonstrated a potential Cr:GdScO3 crystal for near-infrared band tunable laser [22]. The Tm:GdScO3 crystal exhibits strong emission at 1460 nm and 2300 nm reported by Q. Li et al [23] in 2020. However, 2.7 µm continuous-wave laser based on Er:GdScO3 crystal has never been reported.
In this work, Er:GdScO3 crystal was grown by edge-defined film-fed growth (EFG) method for the first time. The spectroscopic properties of Er:GdScO3 were studied and analyzed. The gain cross-section was studied to evaluate the potential of 2.7 µm laser output. Continuous-wave laser operation of Er:GdScO3 crystal has been demonstrated at 2.7 µm.
2. Experimental procedure
Er3+-doped GdScO3 single crystal has been grown by the EFG method, simply shown in Fig. 1. We use a radio frequency (RF) heating source. A tungsten crucible with height of 40 mm and diameter of 70 mm was placed on a ZrO2 pedestal which is supported by a vertical quartz tube. The crucible was surrounded by two sets of ZrO2 ceramics covered with graphite felt. At high temperature tungsten material is sensitive to oxidizing environment. Tungsten oxidation was successfully minimized in the thermal construction by vacuum carbon deoxidation process of zircon ceramics. Since ZrO2 reacts with tungsten material at high temperature, any physical contact between tungsten material and ZrO2 is not allowed in the hot zone construction. Therefore, three W rods were welded directly to the crucible as holders. In this way the hot crucible is completely surrounded by gas and has no contact to the insulating zirconia pedestal. The setup assures the good thermal insulation of the hot zone and allows the adjustment of the appropriate temperature gradients near the crucible.
The Gd2O3 (5N), Sc2O3 (5N) and Er2O3 (5N) powders were weighed according to the stoichiometric composition Er0.05Gd0.95ScO3 and mixed. The mixed powders were pressed into bulks and sintered in air atmosphere at 1500°C for 12 h. The raw materials were melted in the tungsten crucible and rose to the crystallization front within the capillary channel. The crystal was grown along the crystalline a-axis in high-purity nitrogen atmosphere with the atmosphere pressure of 1.1-1.2 atm and the pulling rate of 5-25 mm/h. The as-grown Er:GdScO3 single crystal with a dimension of 1×3×14 mm3 is shown in Fig. 2.
The crystal phase was measured by Ultima IV diffractometer from 10° to 90°. The rocking curve was measured by a X'pert Pro MPD diffractometer with a hybrid Kα1 monochromatic. The actual concentration of Er3+ ions in Er:GdScO3 crystal was measured to be 7.06×1020 cm-3 by inductively coupled plasma-atomic emission spectrometry (ICPAES). The absorption spectrum in a range of 400-1700 nm was measured by a Jasco V-570 UV/VIS/NIR spectro-photometer with step size 1nm. The fluorescence spectrum and fluorescence decay curve were measured by an Edinburgh Fluorescence pectrophotometer (FSP920) under 976 nm excitation. The laser output power was measured by the power meter (30A-SH-V1 Israel). All the measurements of Er:GdScO3 crystal along a crystallographic axis were carried out at room temperature.
The experimental setup schematic diagram of CW Er:GdScO3 laser is shown in Fig. 3. A commercial 976 nm laser diode (LD) with a maximum power of 15 W was adopted as pump source. The gain medium is 5at.% Er:GdScO3 crystal with a length of 6 mm and a cross section of 1×3 mm2. The Er:GdScO3 crystal was wrapped in indium foil and fixed in a copper holder cooled by a thermoelectric cooler. In the laser linear cavity, a input mirror M (R=50 mm) with high transmission (>90%) at 976 nm and high reflectivity (>99.5%) at 2.7 µm and a output coupler (OC) with transmittance TOC of 1%, 3%, 5% at 2.7 µm were employed to obtain 2.7 µm laser output.
3. Results and discussion
3.1 Crystal structure
The XRD pattern of Er:GdScO3 crystal with the standard pattern of GdScO3 phase are shown in Fig. 4. All the diffraction peaks of Er:GdScO3 crystal consistent well with the standard pattern JCPDS#79-0577 of GdScO3 phase. Doped Er3+ ions do not change crystal phase of GdScO3 crystal, and Er:GdScO3 crystal grown by EFG method belongs to orthorhombic system. The lattice parameters of Er:GdScO3 were calculated to be a=5.739 Å, b=7.925 Å, c=5.462 Å, which are smaller than those of GdScO3 crystal (a=5.742 Å, b=7.926 Å, c=5.482 Å). The smaller lattice cell mainly results from the smaller ion radius of Er3+ (0.89 Å) than that of Gd3+ (0.938 Å).
The rocking curve of the wafer along the (100) face is shown in Fig. 5. The FWHM is 0.043°, which indicates high crystalline quality of as-grown Er:GdScO3 crystal.
3.2 Spectroscopic properties
The absorption spectrum of Er:GdScO3 crystal in a range of 400-1700 nm is shown in Fig. 6. Ten absorption bands correspond to the transitions from ground state 4I15/2 to the excited states were assigned. The strong absorption peak at 980 nm matches well with the commercial InGaAs laser diode (LD) pump source. The absorption cross-section could be calculated by the following formula [13]:
Where σabs is the absorption cross-section, L is the thickness of crystal sample, OD is the optical density and Nc is actual concentration of Er3+ ions in Er:GdScO3 crystal. The absorption cross-section at 980 nm was calculated to be 0.377×10−20 cm2, and the FWHM was 19.5 nm. The large FWHM is beneficial for reducing the temperature dependence and improving absorption efficiency of 976 nm commercial laser diode pump.
Fig. 6. Absorption spectrum of Er:GdScO3 crystal with inset showing absorption spectrum around 980 nm at room temperature.
The MIR fluorescence spectrum of Er:GdScO3 crystal in the spectral range of 2.5-3.0 µm is shown in Fig. 7. A broad emission band with single emission peak at 2720 nm corresponds to luminescence transition 4I11/2→4I13/2 of Er3+ ion. The emission cross-section is calculated by the Fuchtbauer–Ladenburg (F–L) formula [14]:
The fluorescence decay curves of 4I11/2 (a) and 4I13/2 (b) multiplet are shown in Fig. 8. The reabsorption of 4I11/2→4I13/2 transition will result in that the fluorescence lifetime of 4I13/2 is larger than the actual value. In order to decrease the influence of reabsorption, a thin sample with thickness of 1 mm was used to measure the fluorescence lifetime. With a single exponential function, the fluorescence decay curves of 4I11/2 and 4I13/2 multiplet were calculated to be 2.24 ms and 4.57 ms. The fluorescence lifetime of 4I11/2 multiplet is longer than that of other Er doped oxide crystals shown in Table 1. Table 1 shows the spectroscopic properties of Er:GdScO3 crystal and other Er3+ doped oxide crystals. The longer fluorescence lifetime of 4I11/2 energy level and smaller fluorescence lifetime ratio of τ(4I13/2)/τ(4I11/2) in Er:GdScO3 crystal are beneficial to achieve continuous wave laser output around 2.7 µm.
Table 1. Spectroscopic properties of Er:GdScO3 and other Er-doped oxide crystals in MIR.
The relationship between absorption cross-section and emission cross-section of 4I11/2 →4I13/2 transition of Er3+ can be expressed as [15]:
Gain cross-section σgain is an important parameter to evaluate the laser performance of Er3+ quasi-three-level system. The gain cross-section can be calculated by the following formula [15]:
where P is the population inversion ratio. The calculated gain cross-section σgain of Er:GdScO3 crystal with different P from 0 to 1 is shown in Fig. 9. As inversion ratio P≥0.5, the gain cross-section is observed to be positive at the peak of 2720 nm, which indicates Er:GdScO3 crystal is a potential laser crystal for 2.7 µm MIR laser.3.3 Laser performance
The 2.7 µm CW laser output powers of Er:GdScO3 crystal as a function of absorbed pump power at different TOC of 1%, 3% and 5% are shown in Fig. 10(a). The laser threshold was 2.4 W in CW laser operation at TOC of 5%. Using the output coupler with transmission of 5%, the maximum output power of 863 mW was obtained under an absorbed pump power of 12.3 W, which led to a slope efficiency of 11.25% by linear fit. Figure 10(b) shows the laser wavelength registered at maximum output power with a peak at 2726 nm. The CW output power of Er:GdScO3 crystal could be further increased under higher pump power according to linear increased trend of output power at the maximum pump power.
Fig. 10. (a) CW laser output powers of Er:GdScO3 crystal versus absorbed pump power. (b) Spectrum of CW laser.
4. Conclusion
5at.% Er:GdScO3 crystal was successfully grown by EFG method for the first time. The rocking curve showed that the as-grown crystal has a high crystalline quality. The absorption cross section at 980 nm is 0.38×10−20 cm2 with FWHM of 19.5 nm. The emission cross section at 2720 nm is 0.93×10−20 cm2 with FWHM of 149.0 nm. The fluorescence lifetimes of 4I11/2 and 4I13/2 energy level were fitted to be 2.24 ms and 4.57 ms. The gain cross-section was calculated and analyzed at 2.7 µm. At inversion ratio P≥0.5, the positive local peak was obtained at 2.7 µm. The maximum output power of 863 mW was obtained with a slope efficiency of 11.25%. All these features illustrate that the Er:GdScO3 crystal is a potential and excellent candidate for 2.7 µm MIR laser.
Funding
National Key Research and Development Program of China (2016YFB1102202); National Natural Science Foundation of China (61621001, 61805177, 61861136007); Key Laboratory of Research on Chemistry and Physics of Optoelectronic Materials (2008DP173016).
Disclosures
The authors declare no conflicts of interest.
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