Applied Physics B

, Volume 108, Issue 3, pp 475–478

Spectroscopy and laser operation of Nd-doped mixed sesquioxides (Lu1−xScx)2O3

Authors

    • Institute of Laser-Physics
  • M. Fechner
    • Institute of Laser-Physics
  • P. Koopmann
    • Institute of Laser-Physics
  • C. Brandt
    • Institute of Laser-Physics
  • K. Petermann
    • Institute of Laser-Physics
  • G. Huber
    • Institute of Laser-Physics
Article

DOI: 10.1007/s00340-012-5094-6

Cite this article as:
Reichert, F., Fechner, M., Koopmann, P. et al. Appl. Phys. B (2012) 108: 475. doi:10.1007/s00340-012-5094-6

Abstract

We report on crystal growth, spectroscopic investigations, crystal field tuning, and laser experiments of neodymium doped mixed sesquioxides (Lu1−xScx)2O3. Crystals were grown by the Nacken–Kyropoulos and the Heat-Exchanger method. Emission spectra for several mixing ratios are presented. Cw laser experiments were carried out with a 0.35 at.%-doped Nd:Lu1.82Sc0.18O3 crystal by using a Ti:sapphire laser as pump source, achieving a maximum slope efficiency of 47 % with respect to the absorbed pump power and a maximum output power of 356 mW at a wavelength of 952.7 nm. To the best of our knowledge, this represents the first continous wave (cw) laser operation of a Nd-doped mixed sesquioxide.

1 Introduction

In recent years, the developments in fabricating the sesquioxides Lu2O3, Sc2O3, and Y2O3 have allowed to employ these crystals as host materials for different active ions such as Yb3+, Tm3+, Er3+, and Ho3+ with great success [14]. But they also offer interesting features for other ions. Doping Nd3+ into Sc2O3 for example, leads to the highest known emission wavelengths of the transitions 4F3/2 → 4I9/2 and 4F3/2  → 4I11/2 at 966.6 nm and 1153.4 nm, respectively [5]. Furthermore, the emission peaks could be shifted by compositional tuning [6] with Lu2O3. For the 4F3/2 → 4I9/2-transition, this offers the possibility to reach a wavelength as low as 951.8 nm. This would allow to cover a large spectral range beyond that of the Nd3+-doped garnets.

Another spectroscopic feature which is interesting especially for a laser operating on the three-level transition is the ratio between the emission cross sections of the three- and the four-level transition. Where this value is only 0.13 in Nd:YAG, it reaches 0.20 in Nd:Sc2O3 and even 0.29 in Nd:Lu2O3. The larger values make it easier to prevent oscillation on the four-level transition.

In this paper, we investigate the emission characteristics of the mixed system Nd:(Lu1−xScx)2O3 and demonstrate for the first time cw laser action on the quasi- three-level transition. Crystals with various doping concentrations and mixing ratios were fabricated and laser experiments in the cw regime were carried out with a 0.35 at.%-doped Nd:Lu1.82Sc0.18O3 crystal yielding an emission wavelength of 952.7 nm and an output power of 356 mW. In the past, only pulsed laser operation of a Nd:LuScO3 has been reported [7].

2 Crystal growth

Small sized single crystalline samples for spectroscopic analyses were obtained by employing the Nacken–Kyro-poulos Method (NKM) [8] while large sized samples for laser experiments were fabricated with the Heat-Exchanger Method (HEM) [9, 10]. As basic materials Lu2O3, Sc2O3, and Nd2O3 powders with a purity of 99.999 % were used. These were mixed to yield (Lu1−xScx)2O3 crystals with 0≤x≤1. The Nd3+ concentrations given throughout this paper are given as they were weighted into the initial melt composition.

The crucibles used for both methods were made of rhenium in order to sustain the high temperatures (≈2450 °C) needed to melt sesquioxides. They had a diameter of 20 mm and a height of 20 mm (NKM) or 45 mm and 49 mm (HEM), respectively. The use of rhenium crucibles makes it necessary to work in a reducing atmosphere to prevent the formation of rhenium/oxygen compounds. These would be incorporated into the crystal and have a detrimental effect on the laser performance. The composition of the atmosphere was thus chosen to be 94.97 % N2 and 5 % H2. An amount of 300 ppm O2 with respect to the N2-concentration was added in order to avoid an oxygen deficiency in the crystals. The samples grown with the NKM had volumes of several cubic millimeters and exhibited stress due to the growth process. The samples grown with the HEM were up to a cubic centimeter in size.

It has to be taken into account that due to the low segregation coefficient of Lu2O3 in Sc2O3 and vice versa a gradient of the mixing ratio occurs throughout the as-grown boules. In order to determine the exact compositions of the samples, energy dispersive X-ray spectroscopy was performed with a Bruker Quantax EDX system.

3 Spectroscopy

In Fig. 1 and Fig. 2, the emission spectra of the transitions 4F3/2 →  4I9/2 and 4F3/2 → 4I11/2 in Nd:Lu2O3 and Nd:Sc2O3 are depicted [5]. In order to illustrate the splitting of the manifolds, the value Δλ may be defined as the distance between the two maxima of the transitions with the shortest and largest wavelength. From the values of Δλ given in Table 1, it can be seen that Nd:Sc2O3 offers a very broad splitting of the emission maxima corresponding to the 4F3/2 → 4I9/2 and 4F3/2  → 4I11/2 transitions. The maxima with the longest wavelength are at 966.6 nm and 1153.4 nm, respectively. To the best of our knowledge, no other known Nd3+-doped oxide or fluoride host matrix offers such long emission wavelengths.
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Fig. 1

Emission cross sections of the transition 4F3/2  → 4I9/2 in Nd:Lu2O3 and Nd:Sc2O3 [5]

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Fig. 2

Emission cross sections of the transition 4F3/2  → 4I11/2 in Nd:Lu2O3 and Nd:Sc2O3 [5]

Table 1

Values of Δλ for a selection of Nd3+-doped laser materials [5, 11]

Transition

Material

Δλ [nm]

4F3/2 → 4I9/2

Nd:Sc2O3

83 nm

Nd:Lu2O3

75 nm

Nd:YAG

77 nm

Nd:YLF

45 nm

Nd:YVO4

35 nm

4F3/2 → 4I11/2

Nd:Sc2O3

99 nm

Nd:Lu2O3

88 nm

Nd:YAG

70 nm

Nd:YLF

31 nm

Nd:YVO4

24 nm

By means of compositional tuning [6], the emission maxima can be tuned to specific wavelengths. The effect can be explained by taking into account that changing the lattice constants by adding smaller or larger ions changes the Coulomb as well as the spin-orbit interaction and also the Stark splitting. This leads to a shift of the emission lines.

In our experiments, Nd3+ was doped into a solid solution of (Lu1−xScx)2O3. The fluorescence properties of the mixed systems were then investigated by exciting samples of the various crystals with a Ti:sapphire laser tuned to 806.8 nm and recording the fluorescence light with a Bruker Equinox 55 Fourier-transform spectrometer with a resolution of 0.5 cm−1.

The shifts of the various peaks are displayed in Fig. 3 and Fig. 4. For increasing values of x, i.e., a higher Sc2O3 concentration, the emission maxima exhibit the expected redshift, where the shifting can be approximated by a linear function depending on x.
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Fig. 3

Wavelength shifts of a selection of emission peaks of the 4F3/24I9/2 transition demonstrating a linear dependency

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Fig. 4

Wavelength shifts of a selection of emission peaks of the 4F3/24I11/2 transition demonstrating a linear dependency

4 Laser experiments

As an example, laser experiments were performed with an uncoated 10.6  mm long Nd:Lu1.82Sc0.18O3 crystal which was polished plane parallel. It had an aperture of 5.5 mm×4.5 mm and a doping concentration of 0.35 at.%. The pump source was a cw Ti:sapphire-laser which was tuned to the absorption maximum at 806.8  nm (λP). The maximum output power of the Ti:sapphire laser was 2.7 W. From the emission spectrum of the crystal, the laser was expected to oscillate at a wavelength of 952.7  nm (λL) in free running mode.

A nearly concentric design was chosen for the resonator. The input (M1) and output coupling mirrors (M2) both had a radius of curvature of 50  mm. The input coupling mirror was AR coated for λP and HR coated for λL. Three different output coupling mirrors with transmission rates TOC for λL of 0.8 %, 1.8 %, and 3.3 % were available. Cooling of the crystal was achieved by contacting it to a copper block which was actively water cooled. The Ti:sapphire-laser beam was focused into the crystal by a lens (L) with a focal length of f=50 mm. The setup is depicted in Fig. 5. The absorbed pump power was measured at the threshold. Taking into account that the inversion density is constant above threshold, the ratio of incident to absorbed pump power should be constant too. Furthermore, the reflectivity of the output coupling mirrors for λP as well as the Fresnel reflection at the two facets of the crystal were taken into account.
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Fig. 5

Scheme of the resonator setup for the laser experiments

With this setup and a mirror with Toc=0.8 % laser action with a maximum output power of 356 mW at an absorbed pump power of 1.5 W was realized. The laser threshold in this case was at an absorbed pump power of \(P_{\mathrm{thr,abs}}=210\) mW. A maximum slope efficiency of 47 % was obtained by using a mirror with an output coupling rate of Toc=3.3 %. The corresponding laser characteristics are shown in Fig. 6. The emission wavelength of 952.7 nm was verified by an Ocean Optics HR2000 spectrometer. The progression of the output power with respect to the output coupling rates indicates that a further increase of the transmissivity of the mirror will likely result in higher slope efficiencies. However, suitable mirrors were not available. To the best of our knowledge, this represents the first cw oscillation of a Nd-doped mixed sesquioxide. So far, pulsed laser operation was demonstrated by Bagdasarov et al. [7] but no laser characteristics were reported.
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Fig. 6

Laser output power versus absorbed pump power of a 10.6 mm long Nd(0.35 at.%):Lu1.82Sc0.18O3 crystal

Until now, cw laser oscillation was only achieved in Nd:Lu2O3 and Nd:Sc2O3. The best slope efficiencies and output powers of lasers operating on the 4F3/2  → 4I9/2 transition in these cases were 7 % and 75 mW and 43 % and 430 mW, respectively [5]. The threshold pump powers were 620 mW and 90 mW. The better performance of the mixed sesquioxide compared with Nd:Lu2O3 may be twofold. One reason is most likely the improvement of the crystal quality due to the growth by the Heat Exchanger method. A second aspect is a stronger Stark splitting of the ground state due to the lattice shrinkage by Sc2O3 admixture. This is especially beneficial for quasi-three-level lasers, since it leads to a smaller Boltzmann population of the higher Stark levels of the ground state and thus to lower reabsorption, an effect that can also be seen by comparing the performance of Nd:Lu2O3 and Nd:Sc2O3 directly.

In order to evaluate the beam quality of the laser, the beam was collimated and focused with a pair of lenses with focal lengths of f=100 mm. The beam profile in front, at, and behind the focus was then recorded with a Coherent Beam Master knife-edge beam profiler. The resulting caustic together with an exemplary beam profile is depicted in Fig. 7. The beam quality factors were determined to be \(M^{2}_{x}=1.2\) and \(M^{2}_{y}=1.5\) at Pout=250 mW, the beam profile was nearly Gaussian.
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Fig. 7

Caustic and beam profile of the Nd:Lu1.82Sc0.18O3-laser with λL=952.7 nm

5 Summary

Crystal growth of Nd:(Lu1−xScx)2O3 with various mixing ratios by the Nacken–Kyropoulos and Heat-Exchanger method was performed. The composition of the obtained crystals was determined by energy dispersive X-ray spectroscopy and their emission characteristics in relation to the composition was investigated.

Cw laser oscillation of a Nd-doped mixed sesquioxide was demonstrated for the first time on the quasi-three-level transition 4F3/2  → 4I9/2 with a 10.6 mm long, 0.35 at.%-doped Nd:Lu1.82Sc0.18O3 crystal pumped by a Ti:sapphire laser. The laser was emitting on a wavelength of λL=952.7 nm and the maximum slope efficiency and output power were as high as 47 % and 356 mW, respectively. The minimal laser threshold was 210 mW. The beam quality factors were determined to be as low as \(M^{2}_{x}=1.2\) and \(M^{2}_{y}=1.5\).

Acknowledgement

The authors gratefully acknowledge the financial support of the German science foundation (DFG) within the framework of the graduate school 1355 “Physics with new advanced coherent radiation sources,” the Landesexzellenzinitiative “Frontiers in Quantum Photon Science” and the Joachim Herz Foundation.

Copyright information

© Springer-Verlag 2012