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Abstract
We report on the crystal growth, spectroscopic properties and visible laser performances of Pr3+-doped Sr0.7La0.3Mg0.3Al11.7O19 (ASL). ASL crystals doped with 2 at.% and 4 at.% Pr3+ were grown by Czochralski method. The laser experiments were carried out in a plane-concave resonator under excitation at 486 nm, provided from a 2ω-OPSL. Efficient laser emission was obtained with an 8 mm long sample of Pr(2at.%):ASL. The maximum output power amounted to 267 mW, 52 mW, and 318 mW at an absorbed power of 1.2W for red, orange, and deep red emission, respectively.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power solid-state laser sources in the visible spectral range are of great interest for several applications such as display technology, submarine communication, scientific spectroscopy, and biomedical treatment.
The energy level diagram of trivalent praseodymium shown in Fig. 1 indicates that Pr3+ is a very attractive ion for visible laser emission in various spectral regions such as blue (3P0→3H4), green (3P0,1→3H5), orange (3P0→3H6), red (3P0→3F2), and also deep red (3P0→3F4) [2]. Fluoride crystals such as Pr:LiYF4 are considered to be excellent host materials for Pr3+ doping. Pr:LiYF4 exhibits outstanding spectroscopic properties and highly efficient laser performance in the visible spectral range [3–7]. However, fluoride host materials possess limitations in their thermomechanical properties due to their low hardness and poor thermal conductivity. Moreover, the growth of fluoride host materials requires a more precise control of the growth atmosphere, which often contains aggressive and toxic reagents such as CF4 or even HF. On the contrary, many oxide host materials exhibit superior thermomechanical properties and can be easily grown by using the Czochralski method under N2 or ambient atmosphere. On the downside, they often possess a higher maximum phonon energy and stronger crystal field, which makes them less promising as host materials for trivalent praseodymium. However, some oxide hosts have a high-coordination number which leads to a long distance Pr-O, decreasing the crystal field strength. For instance, the perovskite YAlO3 (YAP) is considered to be an interesting host material also due to the high coordination number of 12 and low maximum phonon energy of 550 cm−1 [8, 9]. The mean bond length between Pr3+ and O2- ions in YAP is as high as 2.65 Å [10]. Pr:YAP crystals exhibit not only promising spectroscopic behaviors but also allow for efficient laser emission at 747 nm [11–13].
Fig. 1 Energy level diagram of trivalent praseodymium [1]. The colored arrows indicate the relevant transitions in absorption (upward arrow) and emission (downward arrow). The black and violet arrows indicate possible cross relaxation channels.
Since the availability of highly efficient blue pump sources in the late 1990s, laser operation has been demonstrated in various Pr3+-doped crystal hosts [14–16]. In recent years, efficient laser operation has also been demonstrated with the Pr3+-doped oxide material SrAl12O19 (SRA) [1, 17]. According to the binary phase diagram of SrO and Al2O3 [18], SRA exhibits an incongruent melting behavior. Thus, for the growth an excess of SrO is required in the melt [1]. By utilizing the Czochralski technique, single crystals with good optical quality can be obtained. Nevertheless, SrAl2O4 inclusions are incorporated in some parts of the as-grown boule, when the melt composition reaches the eutectic point. The host material LaMgAl11O19 (LMA) can be considered as a fully La-doped SRA crystal. The La3+-ion occupies Sr2+-sites, which requires codoping with the equivalent amount of Mg2+ on the Al3+ site for charge compensation. With Pr3+-doped LMA, laser operation was achieved, but the efficiencies did not reach the excellent values obtained with Pr3+-doped SRA [19]. Hence, it is very interesting to combine the excellent radiative properties of SRA with the congruent melting behavior of LMA in a solid solution Sr1-xLaxAl12-xMgxO19 (ASL). For composition values x between 0.2 and 0.4, ASL can be grown congruently and exhibits an optimal optical quality when the x value equals 0.3 [20]. Hence, this composition was chosen in this work for the growth of Pr:ASL single crystals. Here we report on its spectroscopic characterization and laser performance at several transitions in the visible.
2. Crystal structure
ASL crystallizes in the magnetoplumbite structure with the space group P63/mmc. Its lattice contains spinel blocs and planes (001), where the Sr2+/La3+ cation sites are located. These two sites have the symmetries D3h and C2v originated from SRA and LMA respectively. For x = 0.3, the dominant symmetry of the cation site is D3h. Lattice parameters were refined from XRD data of a Pr (2at.%):ASL sample. The hexagonal axis a exhibits a length of 5.573 Å, the c axis has a length of 22.005 Å. Pr3+-ions occupy the 12fold coordinated Sr/La sites with an average bond distance Pr-O of 2.773 Å. The minimum distance between two Pr3+-ions in ASL is 5.6 Å [21] and thus longer compared to Pr:LiYF4 (3.7 Å) [22]. Consequently, detrimental effects due to dipole-dipole exchange interaction are strongly reduced in ASL. The room temperature thermal conductivity was measured with a TCi Thermal Conductivity Analyzer by C-Therm to be 7.0 Wm−1K−1 and 4.7 Wm−1K−1 for undoped ASL and Pr (2 at.%):ASL samples (E⊥c), respectively. Both values are significantly higher than that of LiYF4 (1.7 Wm−1K−1) [23].
3. Crystal growth
Using the Czochralski technique, ASL crystals were grown with doping concentrations of 2 and 4 at% Pr3+ in the initial melt. The raw materials SrCO3, La2O3, Al2O3, MgCO3 and Pr6O11 with a purity of 4N were mixed according to the structural formula and pressed into pellets. The pellets were annealed under ambient air at 1550 °C for 24 h to form the single phase of ASL. The annealed pellets were filled into an iridium crucible and melted in a RF-heated furnace. The crystals were grown in the direction of their crystallographic c-axis by means of an oriented Pr(4at.%):ASL seed crystal at a pulling rate of 0.3 mm/h and 5 rpm rotation rate. The growth atmosphere consisted of pure N2. The grown 2 at.% and 4 at.% Pr:ASL crystals are shown in Fig. 2 (a) and (b), respectively.
Fig. 2 Left: As-grown single crystalline boules of Pr (2 at.%):ASL (a) and Pr (4 at.%):ASL (b) Right: Polished samples utilized in the laser experiments.
Both crystals were greenish transparent and exhibited high optical quality. With the naked eye neither inclusions, bubbles, nor cleaves were observed in any part of the crystals. However, using a Keyence VHX 5000 digital microscope, very small amounts of scattering centers with an average size of 2 μm were observed in both crystals.
We also investigated the maximum phonon energy of the hexa-aluminate crystals ASL and LMA. The room temperature Raman spectra were recorded for Pr (2at.%) and Pr (4at.%): ASL and compared with those of undoped ASL and LMA. Under a laser excitation at 532 nm, the Raman shift spectra were collected in the range between 300 cm−1 and 1000 cm−1 Raman shift by using Renishaw In-via Raman spectrometer equipped with an optical microscope. As presented in Fig. 3, all investigated hexa-aluminates exhibit the same characteristic Raman features, i.e. the same number of peaks and similar peak positions. The Raman spectrum of LMA shows slightly broadened linewidths compared to undoped and Pr3+-doped ASL. This is related to the disordered structure of LMA hosts. La3+ ions have significantly smaller ionic radius (1.36 Å) than Sr2+ ions (1.44 Å) with the same coordination number [24]. The work of A. Kahn et al. demonstrated that La3+ ions can be delocalized in three directions in the mirror plane (001) [25], leading to inhomogenously broadening of transmission lines. Considering that the composition of ASL with x = 0.3 is dominated by the SRA component, it is expected to find the characteristics of ASL including Raman linewidths more similar to those of SRA.
Fig. 3 Room temperature Raman spectra of undoped ASL (green line), undoped LMA (blue line), Pr (2at.%):ASL (red line), and Pr (4at.%):ASL (black line).
The maximum phonon energies were found at the same Raman shift of 869 cm−1 for all investigated samples. It should be noted that this value is somewhat higher than in previous reports (600 cm−1) [26]. However, it is still more than 4 times lower than the typical energy gap below the upper laser level Pr3+. The corresponding 5th order process for a non-radiative decay 3PJ → 1D2 is thus very unlikely [27] and a high radiative quantum efficiency can be expected.
Four samples were cut from the top, middle, and bottom part of the as-grown Pr(2at.%):ASL boule as well as the residual melt. Using energy dispersive microprobe analysis (EDX), an element analysis was performed. The stoichiometric composition of Sr, La, Pr, Mg, and Al at the different positions is listed in Table. 1.
Table 1. Stoichiometric composition of the Pr (2at.%): ASL sample at different positions in the boule and in the remaining melt.
The Pr3+ concentration in the top part in relation to the Pr3+ in the starting composition is a good measure for the segregation coefficient of Pr3+ in the melt. The resulting value of 0.8 matches the weighted average of both forming components (Pr:SRA (0.9) [8], Pr:LMA (0.5) [28]) of the solid solution and is much higher than the value of 0.2 in LiYF4 [22]. The segregation of La3+ and Mg2+ is higher than one. As a result, the concentration of Pr3+, Al3+, and Sr2+ in the melt and thus the growing crystal enriches while that of La3+ and Mg2+ depletes.
4. Spectroscopy
The spectroscopic properties were investigated for light polarized perpendicular (σ) and parallel (π) to the c-axis of the uniaxial ASL lattice. A 6 mm long sample from the middle part of the Pr (2at.%):ASL crystal was prepared in c-cut orientation for the spectroscopic investigations. With this sample the ground state absorption (GSA), fluorescence lifetime of the 3P0 and 1D2 multiplets, as well as fluorescence properties were investigated in the visible spectral range.
4.1 Ground state absorption
Transmission measurements of the Pr (2at.%):ASL sample were recorded for σ- and π-polarized light at room temperature in the spectral range between 400 and 1700 nm with a Varian Cary 5000i UV-VIS-IR spectrophotometer. The spectral resolution was 0.2 nm. From the results of the elemental analysis, the sample have a doping concentration of 1.6at.%Pr which corresponds to 0.6 × 1020 ions/cm3. The resulting GSA spectra, derived by the Beer-Lambert relation, are shown in Fig. 4 in the blue and green spectral range between 400 nm and 520 nm.
It can be seen that in this range Pr:ASL offers higher GSA cross sections in σ-polarization. Such characteristics are not only observed for the GSA spectra of Pr3+-doped hexaaluminates [13, 16], but also for other rare earth ions doped in hexaaluminates, e.g. Nd:ASL [29, 30] or Sm:SRA [31] and can be attributed to the coordination sphere of the rare earth ion [14, 32]. In σ-polarization the highest GSA cross sections occur at 444 nm, 464 nm, and 486 nm corresponding to the transitions 3H4 → 3P2, 3H4 → (3P1, 1I6), and 3H4 → 3P0, respectively. The peaks located at 444 nm and 486 nm are suitable for commercially available pump sources such as InGaN laser diodes and frequency-doubled optically pumped semiconductor lasers (2ω-OPSLs), respectively. Both peaks exhibit identical peak GSA cross sections of 1.3 × 10−20 cm2. The GSA cross section spectra of Pr:ASL are similar to those reported for Pr:SRA [1] and Pr:LMA [19]. The cross sections of Pr:hexaaluminates are considerably lower compared with fluoride host materials like LiYF4 and BaY2F8 approaching 10 × 10−20 cm2 and 6 × 10−20 cm2, respectively [33, 34].
4.2 Emission spectroscopy
Fluorescence spectra were recorded at room temperature in the visible spectral range between 450 nm and 750 nm in σ- and π-polarization. The measurement setup consisted of a Nd:YAG-pumped nanosecond pulsed optical parametric oscillator (OPO) tuned to 444 nm as excitation source, an Acton SP2300 monochromator from Princeton Instruments and a gated ICCD (PI-MAX4) camera from Hamamatsu. A polarizer was placed in front of the spectrometer entrance slit to obtain polarization dependent fluorescence spectra. The measured fluorescence intensity I was calibrated by means of a quartz tungsten halogen lamp from ORIEL Instruments. The stimulated emission cross sections σem were calculated with the Füchtbauer Ladenburg formula using the radiative lifetime of 38.3 µs [cf. section 4.3]. The stimulated emission cross section spectra are shown in Fig. 5.
Fig. 5 Room temperature stimulated emission cross sections of Pr (2at.%):ASL in the range from 450 nm to 760 nm in σ- and π-polarization.
The highest emission cross sections in the green, orange, red, and deep red spectral range were found at 542 nm, 620 nm, 643 nm, and 725 nm with values of 2.0 × 10−20 cm2, 3.0 × 10−20 cm2, 8.5 × 10−20 and, 11.0 × 10−20 cm2, respectively, in σ-polarization. In Table 2, these values are compared with those of isostructural hosts in the same orientation [1, 19] as well as YLF (E||c) [35].
Table 2. Emission cross sections of Pr3+ doped SRA, LMA, ASL and YLF in the visible spectral region.
4.3 Fluorescence dynamics
The Pr (2at.%) and Pr (4at.%):ASL samples were used to record the room temperature decays of the multiplets 3P0 and 1D2. The same OPO pump with a pulse duration of 5 ns was utilized to excite both samples at 444 nm and 588 nm, respectively. To separate the fluorescence signal of the two multiplets, a monochromator was installed in front of the detector. The monochromator was set to 486 nm and 615 nm, corresponding to the transitions 3P0 → 3H4 and 1D2 → 3H4, respectively. Figure 6 shows the room temperature fluorescence decay curves of both Pr:ASL samples in a semi-logarithmic scale as a function of time.
The decay dynamics of the 3P0 manifold can be expressed by a quasi-single exponential curve for both doping concentrations. A corresponding fit in the temporal range between 0 µs and 150 µs after the excitation pulse yield fluorescence lifetimes of the 3P0 manifold of 38 µs and 33 μs for the lower and higher doped sample, respectively. This room temperature lifetime of Pr (2at.%):ASL is higher than that of Pr (0.6at.%):LiYF4 (36 μs) [29]. The lifetime quenching observed at a doping concentration of 4at.% can be attributed to cross relaxation processes [cf. Figure 1 (black arrows)], but it should be noted that even at this comparably high doping concentration, promoting interionic processes, the lifetime remains as high as 32 µs. In order to determine the radiative lifetime of the 3P0 multiplet, we prepared Pr:ASL polycrystalline samples by solid state reaction for the lifetime measurement at two temperatures, 77 K and 300 K as shown in Table 3. At cryogenic temperatures (77 K), interionic processes between neighboring Pr ions become negligible. In this case, the radiative lifetime was found to be 38.3 µs from low Pr doped ASL samples (1at.% and 2at.%Pr).
Table 3. Fluorescence lifetime of 3P0 manifolds of ASL polycrystalline sample with various doping concentration.
Quenching was thus only observed when the doping concentration exceeded 3at.%. For example, 4at.% and 6at.%-doped Pr:ASL exhibited fluorescence lifetimes of 32.0 and 32.7 µs at room temperature, respectively, which shows that the quenching effect is not very strong for Pr:ASL samples. The ratio of the experimental and radiative lifetime yields a quantum efficiency η of 99% and 83% for ASL doped with 2at.% and 4at.%Pr, respectively, at room temperature.
Table 4 shows the radiative lifetimes of the 3P0 manifold of different Pr3+ doped hosts. As ASL is a solid solution of SRA and LMA with a La composition of 0.3, its spectroscopic properties including the fluorescence lifetime are obviously found to be very close to Pr:SRA. The fluorescence dynamics of the 1D2 multiplet emitting at 615 nm shows a non-single exponential behavior for both investigated doping concentrations as shown in Fig. 6 (b). This is related to the strong cross relaxation of 1D2 multiplet. The fluorescence lifetimes of 1D2 multiplet, averaged from the time-integrated fluorescence signal in the time interval 0 – 1000 μs divided by the initial signal were 214 µs and 116 μs when employing Pr (2at.%) and Pr (4at.%):ASL samples, respectively. This value is somewhat longer than the lifetime of the 3P0 due to the spin forbidden nature of the transition from the 1D2 into the ground state. Compared to the decreasing lifetime of the 3P0 state, the lifetime quenching with doping concentration is much stronger. This phenomenon can be attributed to more resonant cross relaxation processes of neighboring Pr3+ ions for this transition [cf. violet arrows in Fig. 1].
Table 4. Radiative lifetime of 3P0 manifolds of various hosts.
5. Laser performance
Laser experiments were performed with both Pr:ASL samples under 2ω-OPSL pumping. The pump source provided a maximum output power of 4 W at 486 nm with a good beam quality. The pump beam was focused in the crystal with a lens with a focal length of 50 mm. The hemispheric laser cavity consisted of two mirrors M1 and M2 as shown in Fig. 7. The input coupler mirror M1 was plane and highly reflective coated for the respective laser wavelength and antireflective for the pump beam. Various mirrors with transmissions between 0.8% and 13% for the laser wavelength were utilized as output coupling mirrors M2, which all exhibited a radius of curvature of 100 mm. Different Pr:ASL crystals with a quadratic aperture of 5 mm width were investigated subsequently in our experiments. We employed Pr (2at.%):ASL samples with 8 mm, 10 mm, and 12 mm length as well as a Pr (4at.%):ASL sample 5 mm length. All crystals were prepared in c-cut. The plane parallel polished facets were coated antireflective in the laser wavelength range between 520 nm and 730 nm. The crystals were placed on a water cooled copper holder held at 15 °C.
5.1 Red laser performance at 643 nm
Laser experiments utilizing the transition 3P0 →3F2 [cf. Figure 1] at a wavelength of 643 nm in the red were carried out using the three Pr (2at.%):ASL samples. At a medium output coupling rate of M2 of 3.6% we performed comparative measurements to find the optimal crystal length for this doping concentration. The measured single pass absorption efficiency of the 8 mm, 10 mm, and 12 mm long samples amounted to 40%, 49%, and 56% respectively. The corresponding input-output laser curves are presented in Fig. 8.
Fig. 8 Laser characteristics of Pr (2 at.%):ASL samples of different lengths at 643 nm in the red spectral region.
The best performance was obtained with the shortest sample of 8 mm length allowing for a slope efficiency of 24% with respect to the absorbed pump power of 1.2 W. The maximum output power of 234 mW corresponds to an optical-to-optical efficiency of 20%. The observed laser threshold of 200 mW was higher than the values typically observed in Pr3+-doped four-level-lasers based on fluoride host materials. The values for the longer samples fell behind the values observed with the 8 mm long sample, in the case of the 12 mm long sample we even observed an over-rolling of the output power towards highest pump power. Due to the reasonably low and similar pump absorption this behavior cannot be attributed to an inhomogeneous distribution of the pump light in the crystal nor thermal effects. Instead it points towards issues with the homogeneity of the crystalline quality.
In order to optimize the performance of the red laser, various output coupling rates between 0.8% and 5.7% were employed subsequently using the 8 mm long sample. The highest slope efficiency of 27% was obtained with an output coupler transmission of 5.7%. In this case, a maximum output power of 269 mW was realized [Fig. 9 (a)]. We also performed these experiments with the Pr (4 at.%):ASL sample, but as expected from the measurements of the fluorescence lifetime indicating quenching effects at this doping concentration, the efficiencies fell behind the values obtained with the Pr (2 at.%) doped sample as demonstrated in Fig. 9 (b).
Fig. 9 Output power of laser in red region with the respect of absorbed power for Pr(2 at.%):ASL (a) and Pr(4 at.%):ASL (b) with various %TOC.
The maximum slope efficiency was 19% at an output coupler transmission of 5.7% and the maximum output power 160 mW at an absorbed pump power of 1.5 W. Despite the very similar spectroscopic features, our results are about a factor of two below those reported in [11], where a slope efficiency of 59% was obtained at 644 nm using a 5.9 mm long Pr (2.7 at.%):SRA sample. This is another indicator for a probably non-optimized crystal quality of our ASL-samples.
5.2 Orange (620 nm) and deep red (725 nm) laser performances
The results of the laser experiments at the orange and deep red transition [cf. Figure 1] at 620 nm and 725 nm, respectively, are presented in Fig. 10 (a) and (b) for the same 8 mm long Pr (2at.%):ASL sample. At the orange laser transition, a maximum slope efficiency of 11% was realized. The maximum output power was 50 mW at an absorbed power of 1.1 W. We also achieved efficient laser operation in the deep red spectral region. At an output coupling rate of 2.8% the maximum output power was approaching 300 mW at an absorbed pump power of 1.0 W. The corresponding slope efficiency is 34%. Observing the highest slope efficiency at the longest laser wavelength – despite the lower Stokes-efficiency at this transition – is another indication for scattering centers in our laser crystals, as Rayleigh-scattering scales with λ−4 [37].
Fig. 10 Input-output laser characteristics of 8 mm long Pr (2 at.%):ASL in the orange (a) and deep red (b) spectral regions.
6. Summary
Utilizing the Czochralski technique, Pr:ASL crystals were congruently grown with an excellent optical quality. The segregation coefficient of Pr3+ in ASL was determined to be ~0.8. ASL is a solid solution between SRA and LMA and as such, Pr:ASL exhibits a moderate thermal conductivity of 4.7 Wm−1K−1, still being higher than that of typical fluoride crystals. The highest ground state absorption cross section of 1.45 × 10−20 cm2 is found at a wavelength of 466 nm in σ-polarization. However, due to its absorption features around 450 nm and 480 nm, Pr:ASL is suitable for pumping with InGaN laser diodes or frequency doubled optically pumped semiconductor lasers (2ω-OPSLs). The highest emission cross sections of 11.0 × 10−20 cm2 are located at 725 nm. Further emission peaks with values in the order of 10−20 cm2 are found at wavelengths of 521 nm, 620 nm, and 643 nm. By time dependent fluorescence measurements at cryogenic temperatures, the radiative lifetime was determined to be 38.3 µs.
Under 2ω-OPSL pumping laser operation of Pr:ASL was realized in the orange (620 nm), red (643 nm), and deep red (725 nm) spectral range for the first time. The highest slope efficiency of 37% with respect to the absorbed pump power was achieved at 725 nm with an 8 mm long Pr(2 at.%):ASL sample at an output coupling rate of 2.8%. With an absorbed pump power of 1.2 W, the output power exceeded 318 mW. Somewhat lower slope efficiencies of 11% and 27% were obtained at the laser wavelengths of 620 nm and 643 nm, respectively. To the best of our knowledge, these experiments represent the first demonstration of laser operation with Pr:ASL single crystals. Nevertheless, the optimal laser efficiencies were reached with a 8 mm long Pr:ASL which is the shortest Pr:ASL sample. This might be due to the non-optimized crystal quality. Therefore, further improvement of the Pr:ASL crystal quality by adjusting the growth parameters is required for better laser performances. This work on the growth conditions is now in progress.
Funding
Ministry of Science and Technology of Thailand (MOST).
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