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We report on laser operation in the orange and red spectral range in samarium (Sm3+)-doped fluoride and oxide crystals at 300 K. Sm3+-doped LiLuF4 (LLF) and SrAl12O19 (SRA) crystals were grown by the Czochralski-technique and utilized for spectroscopic investigations and laser experiments. The spin-forbidden transitions of Sm3+exhibit low cross sections the order of 10−21 cm2, but high radiative upper state lifetimes of several ms in both crystal systems. Under 2ω-OPSL-pumping at 480 nm, orange laser operation was achieved with Sm:LLF and Sm,Mg:SRA at lasing wavelengths of 606 nm and 593 nm, respectively. Furthermore laser oscillation was demonstrated at 648 nm in the red and 703 nm in the deep red spectral range with Sm:LLF and Sm,Mg:SRA, respectively. Output power levels of several 10 mW were obtained at slope efficiencies of up to 15 %. Most of the realized lasers were operating in a strongly modulated or even self-pulsing regime.
© 2015 Optical Society of America
1. Introduction
The trivalent samarium ion (Sm3+) exhibits fluorescence in the visible spectral range (cf. Fig. 1) [1]. The most dominant transition occurs typically in the orange spectral range around 600 nm. Furthermore emission in the red, deep red, and green spectral range can be observed. These fluorescence properties make Sm3+ interesting for rare-earth based solid-state lasers operating directly in the visible spectral range without the need for any nonlinear frequency conversion. Visible laser radiation may find applications in medicine, spectroscopy, and quantum optics (e.g. in optical clocks [2]). Another interesting field of application for lasers at a wavelength of 589 nm is found in astronomy in sodium-guide star applications [3]. Furthermore solid-state lasers emitting in the visible spectral range allow for the generation of coherent UV radiation by simple intracavity frequency doubling [4].
Fig. 1 Energy level scheme of Sm3+(after [1]). Colored upwards arrows indicate absorption, colored downward arrows indicate emission, and black arrows indicate cross relaxation processes.
The energy level scheme of Sm3+ is depicted in Fig. 1. The large number of closely spaced energy levels in the energy range between the ground state and ≈11,000 cm−1 causes different cross relaxation channels as shown at the right hand side of Fig. 1 [5]. They are detrimental for laser operation.
Spectroscopic investigations on Sm3+-doped materials like single crystals [6, 7], ceramics [8, 9], nanocrystals [10, 11], and glasses [12, 13] have shown the potential of Sm3+ as an optically active ion in solid-state materials. Nevertheless only a few Sm3+-based lasers are reported. The first laser oscillation of a Sm3+-doped active material was achieved in 1979 by Kazakov et al. utilizing Sm3+-doped TbF3 operating at 593 nm [14]. The crystal was pumped by a Xe-flash lamp and cooled to 116 K. In the following, continuous-wave (cw) laser operation with few 10 mW of output power was obtained with a Sm3+-doped silicate glass fiber [15]. Laser operation is also reported for Sm3+in LiTbF4 at 605 nm with a slope efficiency of 20 % [16]. The excitation of the Sm3+-ions was based on an energy transfer process originating from the Tb3+-ions, which were pumped by a Ar-ion laser system at 488 nm. In the same configuration laser operation was also demonstrated at 651 nm with a maximum output power of 28 mW and a slope efficiency of 13 % [17].
To the best of our knowledge, we presented the first laser operation of Sm:LiLuF4 [18] and with Sm,Mg:SrAl12O19 the first realization of laser operation of any Sm3+-doped oxide host material [19]. Here we report in-depth on the growth, spectroscopy, and laser operation of the Sm3+-doped fluoride host material lithium lutetium tetrafluoride (LiLuF4, LLF) and the Sm3+- doped oxide host material strontium hexaaluminate (SrAl12O19, SRA). Fluoride host materials often exhibit lower phonon energies and higher band gap energies compared to oxides, but they often have worse thermo-mechanical properties. In contrast, oxide host materials show better thermo-mechanical properties, but due to the higher binding energies of the O2−-anions compared to the F−-anions, they typically exhibit higher phonon energies and lower band gap energies. The latter increases the tendency towards non-radiative decay of the excited state and the possibility of excited state absorption.
The most established fluoride host materials are LiYF4 (YLF) and the isomorphic LiLuF4 (LLF). Both have been shown to be suitable host materials for visible lasers [2, 20, 21]. According to its phase diagram LLF can be grown from a stoichiometric melt composition [22], while YLF requires a slightly non-stoichiometric melt composition [23]. To obtain YLF crystals with very high optical quality, the growth has to be performed in pure CF4 atmosphere [24]. Apparently, the growth of high quality LLF-crystals is less demanding in terms of growth atmosphere. Therefore, we chose LLF as the fluoride-host material for Sm3+ in this work.
The host material SRA has, compared to other oxide materials, low phonon energies below 600 cm−1 [25] and a high band gap energy of 7.5 eV [26]. The suitability of SRA as a host material for efficient visible lasers in bulk and waveguide geometry has been demonstrated in several experiments with Pr,Mg:SRA [27, 28, 29]. Based on these results SRA is considered to be a suitable host material also for visible lasers based on Sm3+-doping.
2. Crystal growth
Both crystals, LLF and SRA, can be grown by the Czochralski technique. Three LLF crystals from melts mixed according to Sm0.02:LiLu0.98F4, Sm0.04:LiLu0.96F4, and Sm0.10:LiLu0.90F4 were grown in our laboratory. In accordance with the quasi-binary phase diagram of LiF and LuF3 [22] all crystals were grown congruently. Before each growth run, the furnace was evacuated to approximately 5 · 10−5 mbar to remove atmospheric water and oxygen. During this procedure the crucible was heated to approximately 300° C. Afterwards the growth chamber was filled with a gas mixture of 60 % Ar and 40 % CF4 to a pressure of 1.3 bar. No gas flow was applied. An iridium wire was used as the seed with a pulling speed of 0.7 mm/h and a rotation of 5 rpm.
In contrast, due to the quasi-binary phase diagram of SrO and Al2O3 [30], the growth of SRA requires a slightly non-stoichiometric melt with a SrO:Al2O3 ratio of ≈16:84 wt.%, compared to the stoichiometric ratio of 14:86 wt.%. Trivalent rare-earth ions occupy the divalent Sr-site in SRA. Therefore, a charge compensation with Mg2+ co-doped on the Al3+-site is required in the same ionic amount. Two crystals according to melt compositions of Sm0.006,Mg0.006:Sr1.14Al11.78O19 and Sm0.09,Mg0.09:Sr1.06Al10.92O19, corresponding to doping concentrations in the melt of 0.5 at.% and 7.8 at.%, respectively, were fabricated. The crystals were grown with a pulling speed of 2.0 mm/h and 0.7 mm/h and rotations of 15 rpm and 7 rpm for the lower and higher doped crystal, respectively. Both crystals were grown from Ir-crucibles with an Ir-wire used as a seed in a 99 % N2/1 % O2 atmosphere with a gas flow of 3–5 l/h.
From the crystals grown from melt compositions according to Sm0.04:LiLu0.96F4 and Sm0.09,Mg0.09:Sr1.06Al10.92O19 samples were taken from the bottleneck-shaped upper part and analyzed with energy dispersive x-ray analysis. The doping concentrations were determined to be 1.2 at.% and 6.7 at.%, respectively. Assuming the growth of the initial bottleneck did not yet significantly change the composition of the melt, the segregation coefficients can be estimated to be 0.30 and 0.86 for Sm:LLF and Sm,Mg:SRA, respectively. The value for LLF is in good accordance with the segregation coefficient of 0.1–0.2 reported for Pr3+ with its even larger ionic radius [31]. The result for SRA also matches the value of 0.91 reported for Pr3+ [27], because at these high segregation coefficients no strong dependence on small changes of the ionic radius is expected. The main growth parameters are summarized in Table 1.
Table 1. Crystal growth parameters for Sm:LiLuF4 (Sm:LLF) and Sm,Mg:SrAl12O19 (Sm,Mg:SRA).
3. Spectroscopy
Samples of the Sm(1.2 at.%):LLF boule and the Sm,Mg(6.7 at.%):SRA boule were cut and polished with facets corresponding to the ac-plane of the tetragonal (LLF) and hexagonal (SRA) lattice to perform spectroscopic investigations. With a Varian CARY 5000 spectrophotometer polarization dependent transmission spectra were measured. The samples had a thickness of 3.3 mm and 1.2 mm, respectively. The ground state absorption (GSA) cross sections σGSA were determined by the Beer-Lambert law taking the measured doping ion concentrations of each crystal into account. The spectra are shown in Fig. 2 (left) and Fig. 2 (right) for Sm:LLF and Sm,Mg:SRA, respectively.
Fig. 2 Polarization dependent ground state absorption cross sections of Sm(1.2 at.%):LiLuF4 (left) and Sm,Mg(6.7 at.%):SrAl12O19 (right). (Please note the different scales. For presentation the cross sections of Sm,Mg:SRA are divided by a factor of 10 for λ ≤ 450 nm.)
Throughout the visible range Sm:LLF exhibits higher cross sections for π-polarization, while Sm,Mg:SRA offers the higher cross sections for σ-polarized light. For SRA this feature is due to the orientation of the coordination sphere of the RE3+-site with respect to the crystal lattice [27]. 12 oxygen ions form a cuboctahedron with the RE3+-site placed in the D3h reflection plane with the c axis as surface normal [32, 27]. The higher cross sections for σ-polarization can be beneficial e.g. for using polarization beam combining for laser diode pumped laser operation [33]. LLF does not exhibit such a symmetry obviously favoring one polarization and thus typically for other rare-earth ions, e.g. Pr3+, no dominating polarization is observed [34, 35].
For both systems the strongest absorption occurs around 400 nm with GSA cross sections of 2.2 · 10−20 cm2 and 8.5 · 10−20 cm2 for Sm:LLF and Sm,Mg:SRA, respectively. This absorption can be attributed to transitions into the 6P3/2- and 6P5/2-multiplets, originating from the 6H5/2 ground state. These transitions are spin-allowed and exhibit therefore high absorption cross sections of several 10−20 cm2. GaN-laser diodes emitting in this spectral region are available and may be an efficient pump source in future experiments. In contrast, the transitions in the cyan-blue wavelength range around 480 nm are spin-forbidden. This results in at least one order of magnitude lower GSA cross sections. The absorption into the emitting 4G5/2-multiplet at ≈ 560 nm is not shown in Fig. 2, as it exhibits GSA cross sections in the order of 10−22 cm2 or even less. Thus, this transition is not suitable for in-band pumping. Our GSA cross sections for Sm:LLF are in a good agreement with those reported in [36] or those for Sm:YLF [37]. The values determined for Sm,Mg:SRA are in accordance with the values for the isostructural Sm:Sr1−xLaxMgxAl12 xO19 [38].
It is worth mentioning, that we did not observe absorption around 700 nm, which would indicate the incorporation of Sm2+ [39]. It can thus be concluded that no significant amount of Sm2+ is incorporated in our crystals and the GSA cross section spectra represent the absorption features of pure Sm3+doped into the respective host crystal. The obtained GSA cross sections were utilized to determine the doping concentrations of samples from all other grown boules. The results are in good agreement with the segregation coefficients in all cases.
Using the same samples, we measured polarized fluorescence spectra of Sm:LLF and Sm,Mg:SRA. As an excitation source, we utilized an InGaN-laser diode and a frequency doubled optically pumped semiconductor laser (2ω-OPSL) at 443 nm and 480 nm, respectively. To apply lock-in technology the excitation was modulated by a mechanical chopper blade with a duty cycle of 1:1 and a frequency of 300 Hz. The fluorescence was imaged onto the entrance slit of a SPEX 1000M 1 m monochromator and detected by a Si-photodiode. The spectra were recorded with a spectral resolution of 0.3 nm and corrected with respect to the optical setup and the spectral response of the Si-diode.
The determination of the stimulated emission cross sections σSE via the polarization dependent Füchtbauer-Ladenburg equation [40] requires the knowledge of the radiative lifetime of the emitting level. For this purpose, time resolved fluorescence measurements were performed. The crystals were excited by 20 ns pulses with a repetition rate of 10 Hz from a Solar Systems OPO (optical parametric oscillator) at a wavelength of 480 nm. The time resolved fluorescence signal of the orange emission was separated from the excitation light by a 0.5 m-spectrometer and detected with a Hamamatsu S1 photomultiplier tube. In this way, the fluorescence lifetime of the emitting 4G5/2-multiplet of a 0.5 at.%-doped Sm,Mg:SRA-sample was determined to be 3.4 ms. Due to the low cation-density in SRA, the doping concentration of 0.5 at.% corresponds to a Sm-density of only 1.7 · 1019 cm−3 (corresponding to 0.12 at.% e.g. in YAG). Therefore, no concentration quenching is expected and the measured fluorescence lifetime is assumed to be in good agreement with the radiative lifetime of the emitting level. The measurements for the decay time of the 4G5/2-multiplet of a 0.6 at.%-doped Sm:LLF sample resulted in a fluorescence lifetime of 4.8 ms. Even though the cation-density is about a factor of 5 higher in this case, this result is in good correlation with the radiative lifetime of Sm:LLF determined in [36]. This indicates a relatively low sensitivity towards concentration quenching of the 4G5/2-decay time and allowed us to utilize the measured value in the Füchtbauer-Ladenburg-equation also in the case of Sm:LLF.
The resulting stimulated emission cross section spectra of Sm:LLF and Sm,Mg:SRA (cf. Fig. 3) show the characteristic fingerprint of Sm3+ in the visible spectral range. The strongest emission occurs in the orange spectral region at 605.5 nm and 592.9 nm with cross sections of 1.2 · 10−21 cm2 and 1.3 · 10−21 cm2, respectively. This emission can be attributed to the transition 4G5/2 → 6H9/2. Further visible emission with cross sections in the order of several 10−22 cm2 can be found around 645 nm in the red and around 700 nm in the deep red, corresponding to transitions into the higher lying 6H9/2 and 6H11/2-multiplets (see Fig. 1). Also in the green spectral range around 560 nm, corresponding to the transition into the ground state 6H5/2, emission cross sections of few 10−22 cm2 are present.
Fig. 3 Polarization dependent emission cross sections of Sm:LiLuF4 (Sm:LLF, left) and Sm,Mg:SrAl12O19 (Sm,Mg:SRA, right).
4. Laser experiments
Laser experiments were performed with Sm:LLF and Sm,Mg:SRA. The doping concentrations of the prepared samples were calculated from transmission measurements and the corresponding GSA cross sections (cf. sec. 3) to 1.2 at.% and 6.8 at.% for Sm:LLF and Sm,Mg:SRA, respectively. Both samples were prepared in a-cut geometry with lengths of 7.0 mm and 6.8 mm, respectively. Hemispherical resonators with different lengths were set up with plane input and curved output coupling mirrors M1 and M2, respectively. The radii of curvature of M2 allowing for the best laser results were 50 mm for Sm:LLF and 100 mm for Sm,Mg:SRA. The lengths of the cavities were aligned close to the stability limit and optimized for maximum output power at each data point. The input coupling mirror M1 was highly reflective for the respective laser wavelength and highly transmissive for the pump wavelength. For the output coupling mirror M2 different mirrors with transmission rates for the laser wavelength between 0.1 % and 3.0 % were utilized. As a pump source a continuous-wave 2ω-OPSL with a maximum output power of 4 W and a central emission wavelength of 479.6 nm was applied. The beam was focused with lenses of different focal lengths into the respective laser crystals. The best results were obtained for Sm:LLF utilizing a lens with a focal length of 40 mm. In the case of Sm,Mg:SRA a lens with a focal length of 100 mm allowed for the best results. In order to state slope efficiencies with respect to the absorbed pump power, the absorbed pump power was determined at the laser threshold. The absorption efficiency was assumed to be constant above the laser threshold. The single pass absorption efficiency was in the range of 20 % to 25 %, but a double pass due to a reflectivity of up to 25 % of M2 for the pump light was taken into account, resulting in a total absorption efficiency of ≈ 25 % in the 7.0 mm Sm(1.2 at.%):LLF sample and ≈ 30 % in the 6.8 mm Sm,Mg(6.8 at.%):SRA sample.
In our experiments we obtained laser operation of Sm:LLF at the wavelengths 606 nm and 648 nm, as well as at 593 nm and 703 nm with Sm,Mg:SRA. The laser output from Sm:LLF at λlas = 606 nm in the orange spectral region corresponds to the strongest emission peak of the 4G5/2 → 6H9/2-transition in this material (cf. Figs. 1 and 3). The highest output power of 86 mW was achieved at an output coupling rate of TOC = 1.7 %. The corresponding slope efficiency was ηsl = 13 % with respect to the absorbed pump power (cf. Fig. 4, left). The output power was strongly modulated in this case, thus the output power refers to the average output power. An increased output coupling rate resulted in a higher laser threshold, but the slope efficiency did not improve. Furthermore, laser emission was obtained at the transition 4G5/2 → 6H9/2 at a wavelength of 648 nm. Despite the lower peak emission cross sections in this wavelength range, a slightly higher slope efficiency of 15 % was achieved at an output coupler transmission of TOC = 0.7 % for the laser wavelength. We did not succeed to obtain laser oscillation around 700 nm in this 1.2 at.% doped LLF sample, nor at any wavelength in samples prepared from the 3.0 at.%-doped Sm:LLF boule. Lifetime measurements of the upper laser level at this doping concentration showed a strong increase of non-radiative relaxation processes. The lifetime decreased to 2.6 ms, which is in good agreement with the reported lifetimes of higher doped Sm:LiYF4 [5]. The decreasing lifetime can mainly be attributed to cross relaxation processes, which become more likely with higher doping concentrations due to shorter distances between the doping ions.
Fig. 4 Laser characteristics of Sm(1.2 at.%):LiLuF4 (Sm(1.2 at.%):LLF, left) in the orange and red at 606 nm and 648 nm, respectively, and Sm,Mg(6.8 at.%):SrAl12O19 (Sm,Mg(6.8 at.%):SRA, right) in the orange and deep red at 593 nm and 703 nm, respectively. To obtain laser operation at 593 nm, the pump beam was modulated with a mechanical chopper (dashed lines). Due to a strong self-modulation of the laser output power in most cases, the average output powers are stated.
Also with the oxide crystal Sm,Mg(6.8 at.%):SRA laser experiments were conducted. Again, the laser results at the wavelength of highest emission cross sections around 593 nm fell behind those obtained at the longest wavelength emission band around 703 nm shown in Fig. 3.
Stable laser oscillation at 593 nm in the orange was only obtained under chopped pumping by modulating the pump beam with a duty cycle of 1:1 and a frequency of 150 Hz by means of a mechanical chopper. With an output coupling rate of 1.2 % an output power of only 7 mW was achieved with a slope efficiency as low as 1 %. Better results under true cw pumping were obtained at a deep red lasing wavelength of 703 nm. In this case, a maximum output power of 45 mW was achieved at an output coupling rate of 1.3 %. The obtained slope efficiencies did not exceed 10 % for any output coupling rate. Moreover, in contrast to the results obtained with Sm:LLF, we did not succeed in obtaining laser operation at 641 nm, corresponding to the 4G5/2 → 6H11/2
Independently on whether the crystals were pumped cw or chopped, the output power of most of the realized lasers showed a strongly modulated or even self-pulsing behavior (spiking). Exemplary output traces are depicted in Fig. 5 for Sm:LLF (top) and Sm,Mg:SRA (bottom). The pulse durations were in the order of µs with repetition rates in the kHz range and thus significantly shorter than the upper laser level lifetimes of several ms. Consequently, a self-terminating behavior caused by trapping of the radiation in one of the energy levels below 11,000 cm−1 seems unlikely. We observed an increasing repetition rate with increasing pump power. Hence, the lasers showed a Q-switching like behavior. Only at a low output coupler transmission of 0.1 %, true cw laser operation was achieved for Sm:LLF at 606 nm. The self-pulsing behavior could be a kind of saturable absorption with inadequate saturation parameters. However, according to the GSA spectra (cf. Fig. 2) this absorption cannot start from the ground state as there is no absorption at any of the laser wavelengths. This phenomenon could also be caused by excited state absorption (ESA) starting from the upper laser level and ending in short-living levels with energies exceeding 30,000 cm−1, but it should be noted that ESA-measurements were performed and no such transition could be detected. Nevertheless, even very weak ESA could seriously influence the laser properties. It thus cannot be excluded that such a weak ESA-process is responsible for the self-pulsing. As this behavior was observed in two very different host materials as the fluoride LLF and the oxide SRA, we believe that this is an intrinsic feature of the Sm3+-ion and it may be very difficult to find the right conditions or a proper host material for true cw laser operation.
Fig. 5 Exemplary laser output traces of Sm(1.2 at.%):LiLuF4 (top) and Sm,Mg(6.8 at.%):SrAl12O19 (bottom). The output traces of Sm:LLF are shown for λlas = 606 nm and different output coupling ratios TOC at maximum output power. The output traces of Sm,Mg:SRA are shown for different laser wavelengths λlas at their respective maximum average output powers.
5. Conclusion
Sm3+-doped lithium lutetium tetrafluoride (LiLuF4, LLF) and strontium hexaaluminate (SrAl12O19, SRA) crystals were grown by the Czochralski-method. The segregation coefficient of Sm3+was estimated to be 0.30 in the fluoride host LLF, while it was 0.86 in the oxide host material SRA. The spin-forbidden ground state absorption in the cyan-blue spectral range around 480 nm exhibits cross sections in the order of few 10−21 cm2, while the spin allowed GSA-transitions around 400 nm exhibit one order of magnitude higher cross sections of several 10−20 cm2. The Sm3+-ion exhibits four main fluorescence bands in the green, orange, red, and deep red spectral range. The highest stimulated emission cross sections of 1.2·10−21 cm2 and 1.3 · 10−21 cm2 were found in the orange spectral range at 606 nm and 593 nm for Sm:LLF and Sm,Mg:SRA, respectively. Time-dependent fluorescence measurements resulted in lifetimes of 4.8 ms in Sm:LLF and 3.4 ms in Sm:SRA for the 4G5/2-multiplet, which are believed to be close to the intrinsic radiative lifetimes of these systems.
Laser operation was obtained in both crystal systems. With Sm:LLF a maximum output power of 86 mW and a slope efficiency of up to 13 % with respect to the absorbed pump power were realized at 606 nm in the orange spectral range. At 648 nm in the red spectral region a maximum output power of 93 mW and a slope efficiency of 15 % were obtained. By applying Sm,Mg:SRA as the active medium, laser oscillation was achieved in the orange at 593 nm and deep red at 648 nm with maximum output powers of 7 mW and 45 mW, and slope efficiencies in the order of 1 % and 10 %, respectively. By modulating the pump beam with a mechanical chopper the average output power was stable over time. Even though the on-time of the pump was in the order of magnitude of the radiative lifetime of Sm3+ in SRA, this did not change the temporal output characteristics. In most cases the output power was strongly modulated or even exhibited self-pulsing behavior (spiking) with a Q-switching like character. A possible explanation for this phenomenon is excited state absorption at the laser wavelengths. However this behavior is under further investigation. Neither with Sm:LLF nor with Sm,Mg:SRA the slope efficiency exceeded 20 %. As Sm3+ shows similar behavior in very different applied host crystals, we believe it might be difficult to find a suitable host material for Sm3+. Nevertheless a more efficient laser operation with respect to the incident pump power can be expected with laser diodes at 400 nm as a pump source.
To the best of our knowledge, these experiments represent the first demonstration of laser operation of Sm:LLF and with Sm,Mg:SRA the first realization of laser operation of a Sm3+-doped oxide host material.
Acknowledgments
The authors gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft (DFG) within the graduate school 1355 “Physics with new coherent radiation sources” and the excellence cluster “The Hamburg Centre for Ultrafast Imaging - Structure, Dynamics and Control of Matter at the Atomic Scale”.
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