Synthesis, spectroscopy, and efficient laser operation of &#x201C;mixed&#x201D; sesquioxide Tm:(Lu,Sc)<sub>2</sub>O<sub>3</sub> transparent ceramics

Wei Jing; Pavel Loiko; Josep Maria Serres; Yicheng Wang; Elena Vilejshikova; Magdalena Aguiló; Francesc Díaz; Uwe Griebner; Hui Huang; Valentin Petrov; Xavier Mateos

doi:10.1364/OME.7.004192

1. Introduction

Cubic (C-type, bixbyite structure) rare-earth sesquioxides A₂O₃ (where A = Lu, Y, Sc, etc.) are well known host crystals for trivalent laser-active lanthanide ions (Ln³⁺) such as Yb³⁺, Tm³⁺ or Ho³⁺ [1]. They feature good thermal and thermo-optical properties (high thermal conductivity, κ = 12.8 W/mK for Lu₂O₃ which is weakly dependent on the Ln³⁺ concentration [2], and positive dn/dT coefficients [3]), broad transparency range (0.22-8 µm) and high refractive index (~1.91 for Lu₂O₃ at ~1 µm) [1], one of the lowest maximum phonon frequencies among the oxide matrices [1], as well as broad spectral bands for the Ln³⁺ dopant ions [1,4,5]. Efficient, wavelength-tunable and power-scalable near-IR Ln-doped sesquoxide lasers operating in the continuous-wave (CW) [1,6,7] and mode-locked (ML) regimes (at ~1 and ~2 µm) [8–10] have already been reported. These crystals have also been recognized to be very suitable for thin-disk oscillators [11,12].

Cubic A₂O₃ crystals are in particular attractive for thulium lasers. The Tm³⁺ ion (electronic configuration: [Xe]4f¹²) is known for its emission around 2 µm due to the ³F₄ → ³H₆ 4f-4f electronic transition [13]. This eye-safe emission is used in range-finding, environmental sensing (LIDAR) and medicine. The Tm³⁺ ions can be excited by commercial AlGaAs laser diodes at ~0.8 µm (into the ³H₄ state) while the possible cross-relaxation process, ³H₄ + ³H₆ → ³F₄ + ³F₄, increases the excitation quantum efficiency [14]. In cubic A₂O₃ crystals they feature a strong Stark splitting of the ground state (³H₆), > 800 cm⁻¹, leading to very broad emission and gain spectra extending up to ~2.1 µm [1,5,6] which is much longer than for many other Tm³⁺-doped oxide crystals. Moreover, this feature can be enhanced in “mixed” Tm:(Lu_1-_x,Sc_x)₂O₃ crystals exhibiting a compositional disorder. A 1 at.% Tm:LuScO₃ sesquioxide crystal was previously studied in [15] generating a CW output power of 705 mW at ~2.1 µm with a slope efficiency of 55% (under Ti:Sapphire laser pumping). A 155 nm-broad (1960-2115 nm) tuning range of the laser emission was demonstrated [15]. In the ML regime, 105 fs pulses were achieved at 2010 nm using a semiconductor saturable absorber mirror (SESAM) [16]. This corresponded to the shortest pulse duration ever reported for any ML Tm³⁺:A₂O₃ oscillator, proving the potential of “mixed” crystals for fs ML lasers.

Due to the high melting point of the cubic A₂O₃ crystals (2450 °C for Lu₂O₃), their growth typically requires the use of expensive rhenium (Rh) crucibles [17]. Rh can be the source of crystal coloration. The growth of large-volume highly Tm³⁺-doped single crystals with high optical quality is complicated. The technology of Tm³⁺-doped sesquioxide transparent ceramics was proposed as an alternative allowing for an easier and size-scalable production. There are multiple reports on Tm:Lu₂O₃ transparent ceramics [18,19]. In [18], a 2 at.% Tm:Lu₂O₃ ceramic laser generated 26 W at 2066 nm with a slope efficiency of 42%. Bulk fs ML lasers [20] and thin-disk lasers [21] based on Tm:Lu₂O₃ ceramic are also known.

In the present work, we focused on a novel “mixed” Tm:(Lu_2/3Sc_1/3)₂O₃ transparent ceramic exhibiting promising potential for ultrafast lasers and amplifiers. We present structural and detailed spectroscopic characterization and compare this ceramic with a Tm:LuScO₃ single-crystal. In addition to the ease of fabrication, we demonstrate that this new ceramic material can provide superior laser characteristics in comparison with the previously studied “mixed” Tm:LuScO₃ ceramics [22] and single-crystals [15].

2. Fabrication of the Tm:(Lu,Sc)₂O₃ ceramics

The Tm:(Lu,Sc)₂O₃ ceramics were fabricated by the Hot Isostatic Pressing (HIP) sintering method using powders of Sc₂O₃, Lu₂O₃, and Tm₂O₃ (purity: 99.99%, Alfa Aesar) as raw materials. The raw materials, with a stoichiometric amount of 100 at.% Lu + Sc (taken in a proportion of Lu:Sc = 2:1) and 5 at.% Tm over it, were mixed uniformly by ball milling for 24 h, dried for 6 h at 70 °C, sieved, dry-pressed at 10 MPa, and cold isostactically pressed at 200 MPa. The green bodies of Tm:(Lu,Sc)₂O₃ ceramics were firstly pre-sintered at 1750 °C for 10 h under vacuum (pressure, P < 10⁻³ Pa) to densify the preforms. For further densification, the pre-sintered Tm:(Lu,Sc)₂O₃ ceramic samples were post-sintered by HIP at 1800 °C for 2 h in an Ar atmosphere (P = 195 MPa) to eliminate the closed pores around the grain boundaries. Finally, the ceramics were annealed at 1500 °C for 10 h in an O₂ atmosphere to eliminate the oxygen vacancies and remove internal stresses. Samples of Tm:(Lu,Sc)₂O₃ ceramics with a diameter of 15 mm and a thickness of 5 mm were obtained. Some polished samples are shown in Fig. 1(a). For the spectroscopic experiments, 3 × 3 × 3 mm³ cubes were cut from the prepared samples and polished to laser-grade quality. The composition of the ceramics can be represented as 4.76 at.% Tm:(Lu_2/3Sc_1/3)₂O₃.

Fig. 1 Some of the fabricated 4.76 at.% Tm:(Lu,Sc)₂O₃ ceramics (laser-grade-polished ceramic disks) (a); X-ray diffraction (XRD) pattern (in blue) compared to the standard pattern of ScLuO₃ (in red) (b); Raman spectrum, excitation wavelength: 514 nm (c).

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The polycrystalline phase of the samples was analyzed at room-temperature (RT, 293 K) by X-ray diffraction (XRD) (D2 PHASER, Bruker Co. Ltd), using Cu Kα radiation. The experiment was carried out in the 2θ = 10-80° range with a step of 0.02° and a scan speed of 0.1°/min. As shown in Fig. 1(b), the measured diffraction pattern agrees with the JCPDS card No. 04-002-0541 of cubic LuScO₃ (C-type structure, sp. gr. $I a \bar{3}$ - T⁷_h, No. 206). Based on the position of the XRD peaks, the lattice constant of the Tm:(Lu,Sc)₂O₃ ceramic is a = 10.3683 Å (Z = 16, calculated density, ρ_calc = 7.493 g/cm³), which is larger than that for the standard LuScO₃ crystal (a = 10.105 Å). This is attributed to the larger fraction of Lu³⁺ with larger ionic radius (R_Lu = 0.861 Å for VI-fold O^2- coordination) as compared to Sc³⁺ (R_Sc = 0.745 Å), as well as the Tm³⁺ doping (R_Tm = 0.88 Å).

The vibronic properties of the ceramics were studied with Raman spectroscopy, Fig. 1(c). We used a Renishaw inVia confocal Raman microscope with an excitation wavelength λ_exc = 514 nm. The Raman spectrum of the Tm:(Lu,Sc)₂O₃ ceramic is typical for cubic sesquioxides A₂O₃ [23] while significant broadening of the Raman bands is observed as compared to single crystals. The most intense Raman band is centered at 399 cm⁻¹. It has an asymmetric shape with a full width at half maximum (FWHM) of 32 cm⁻¹ and is composed of several poorly resolved components. A factor group analysis predicts the following irreducible representations at the Г-point: Г_total = 4A_g + 4E_g + 14F_g + 5A_u + 5E_u + 17F_u of which 22 modes (A_g, E_g and F_g) are Raman-active, 17 modes (F_u) are IR-active and the rest are silent [24]. We have resolved 16 Raman bands as indicated in Fig. 1(c). While the strongest band is typically ascribed to the A_g + F_g vibrations, the assignment of the modes is complicated [24].

The morphology of a polished surface and a fracture surface was characterized by field-emission scanning electron microscopy (SEM, Inspect F, FEI). The Tm:(Lu,Sc)₂O₃ sample is almost pore-free, see Fig. 2. The grain boundaries are clean and there is almost no secondary phase at the grain boundaries or inner grains. The average grain size in our samples is of 5-6 μm, as determined by the linear intercept method. The grain size distribution is uniform over the sample cross-section.

Fig. 2 Field-emission scanning electron microscopy (SEM) images of a polished (a) and a fracture (b) surface of the 4.76 at.% Tm:(Lu,Sc)₂O₃ ceramic. The scale bar in (a,b) is 20 µm.

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3. Optical spectroscopy

3.1 Optical absorption

The absorption spectrum of the Tm:(Lu,Sc)₂O₃ ceramic was measured using a Varian CARY 5000 spectrophotometer (Agilent) with a resolution of 0.2 nm. In addition to RT studies, we measured the absorption spectra at 6 K using an Oxford Instruments Ltd. cryostat (Model SU 12) with He-gas close-cycle flow. From the absorption coefficient α_abs measured at RT, we calculated the RT absorption cross-section, σ_abs = α_abs/N_Tm, where N_Tm = 13.8 × 10²⁰ cm⁻³ corresponds to 4.76 at.% Tm doping.

The transmission of the laser-grade polished 3 mm-thick Tm:(Lu,Sc)₂O₃ ceramic sample measured at 2.15 µm (out of the absorption band of Tm^{3 +}) was ~77% which is slightly lower than the value arising purely from the Fresnel losses expected for this material (81.1%, if assuming a refractive index n of 1.92). No noticeable distortion or scattering of the probe He-Ne laser beam passing through the sample were detected.

The RT absorption spectrum is shown in Fig. 3. The absorption band at 1.4-2.1 µm is associated with the ³H₆ → ³F₄ transition of Tm^{3 +}. The maximum σ_abs for this transition is 4.26 × 10^-21 cm² at 1622 nm. This band is suitable for in-band pumping of Tm^{3 +} ions, e.g., by Raman-shifted Er fiber lasers [25]. The band at 0.74-0.83 µm corresponds to the ³H₆ → ³H₄ transition and it renders the Tm:(Lu,Sc)₂O₃ ceramics suitable for pumping with AlGaAs laser diodes. Its maximum σ_abs is 2.80 × 10⁻²¹ cm² at 793 nm with a FWHM of ~25 nm. At shorter wavelengths, the transitions to the ³F_2,3 (0.64-0.71 µm), ¹G₄ (0.45-0.49 µm), ¹D₂ (0.35-0.37 µm) and ³P_0-2 + ¹I₆ (0.27-0.30 µm) excited states were also observed. The last one overlaps with the UV absorption edge of the Tm:(Lu,Sc)₂O₃ ceramic at λ_g = 282 nm (E_g = 4.4 eV).

Fig. 3 (a-f) Room-temperature (at 293 K, in red) and low-temperature (at 6 K, in blue, not in scale) absorption spectra of the 4.76 at.% Tm:(Lu,Sc)₂O₃ ceramic.

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The low-temperature absorption spectra of the Tm:(Lu,Sc)₂O₃ ceramic are also shown in Fig. 3 (in blue, not in scale with respect to the RT spectra). Due to the compositional disorder, the absorption spectra are very broad even at 6 K and the transitions to the individual Stark levels are not completely resolved.

The absorption bands of the Tm³⁺ ion in the (Lu,Sc)₂O₃ ceramic related to the transitions from the ³H₆ ground-state to the excited states from ³F₄ to ¹D₂ were analyzed using the standard Judd-Ofelt (J-O) theory [26,27]. The absorption oscillator strengths were determined from the measured absorption spectra:

f_{Σ}^{\exp} (J J') = \frac{m_{e} c^{2}}{π e^{2} N_{Tm} {〈 λ 〉}^{2}} Γ (J J'),

where m_e and e are the electron mass and charge, respectively, c is the speed of light, Г(JJ’) is the integrated absorption coefficient within the absorption band and 〈λ〉 is the “center of gravity” of the absorption band. The results are summarized in Table 1.

Table 1. Experimental and Calculated Absorption Oscillator Strengths for 4.76 at.% Tm:(Lu,Sc)₂O₃ Ceramic

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The values of f_Σ^exp were used to determine the J-O (intensity) parameters, Ω_k, k = 2, 4, 6: Ω₂ = 2.429, Ω₄ = 1.078 and Ω₆ = 0.653 [10⁻²⁰ cm²]. Using these parameters, the absorption oscillator strengths were calculated showing a good agreement with the experiment (as indicated by a moderate root-mean-square (rms) deviation, 0.35):

f_{Σ}^{calc} (J J') = \frac{8}{3 h (2 J' + 1) 〈 λ 〉} \frac{{(n^{2} + 2)}^{2}}{9 n} S_{ED}^{calc} (J J') + f_{MD}^{calc} (J J'),

S_{ED}^{calc} (J J') = \sum_{k = 2, 4, 6} U^{(k)} Ω_{k}, where U^{(k)} = {〈 (4 f^{n}) S L J | | U^{k} | | (4 f^{n}) S' L' J' 〉}^{2} .

Here, S_ED^calc are the line strengths, h is the Planck constant, n is the refractive index of the crystal and U^(k) are the squared reduced matrix elements [28]. The J-O theory describes the electric-dipole (ED) transitions. The contribution of the magnetic-dipole (MD) transition with J–J’ = 0, ± 1 (for the ³H₆→³H₅ transition), f_MD^calc, was taken from [29].

3.2 Optical emission

The probabilities for spontaneous radiative transitions were calculated from the corresponding line strengths which were determined from the J-O parameters Ω_k and the squared reduced matrix elements U^(k), see Eq. (2b) [28]:

A_{Σ}^{calc} (J J') = \frac{64 π^{4} e^{2}}{3 h (2 J' + 1) {〈 λ 〉}^{3}} n {(\frac{n^{2} + 2}{3})}^{2} S_{ED}^{calc} (J J') + A_{MD}^{calc} (J J') .

The MD contributions A_MD^calc were taken from [29]. Using the values of A_Σ^calc for the separate emission channels J→J’, the total probability, A_tot, the corresponding radiative lifetime of the excited state, τ_rad, and the luminescence branching ratios for the emission channels, B(JJ’) were determined:

τ_{rad} = \frac{1}{A_{tot}} and B (J J') = \frac{A_{Σ}^{calc} (J J')}{A_{tot}}, where A_{tot} = \sum_{J'} A_{Σ}^{calc} (J J') .

The results are shown in Table 2. The calculated radiative lifetime of the lowest excited state is τ_rad(³F₄) = 4.01 ms. Previously, luminescence decay studies of a 1 at.% Tm:LuScO₃ single crystal [30] and a 2 at.% Tm:LuScO₃ ceramic [22] revealed a lifetime τ_lum(³F₄) of 3.84 and 3.2 ms, respectively. The J-O modelling also predicts the radiative lifetime of the ³H₄ pump level, τ_rad(³H₄) = 0.72 ms. In [30] for a 1 at.% Tm:LuScO₃ crystal, τ_lum(³H₄) was measured to be 55 µs while no values for the LuScO₃ crystals with lower doping are available (e.g., for 0.2 at.% Tm:Lu₂O₃ crystal, τ_lum(³H₄) = 300 µs).

Table 2. Calculated Emission Probabilities for Tm³⁺ in (Lu,Sc)₂O₃ Ceramic

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The stimulated emission (SE) cross-sections, σ_SE, for the ³F₄ → ³H₆ transition of Tm³⁺ in (Lu,Sc)₂O₃ ceramic were calculated from the measured absorption spectrum, see Fig. 3(a), using the modified reciprocity method [31]:

σ_{SE}^{} (λ) = \frac{1}{8 π n^{2} τ_{rad} c} \frac{σ_{abs}^{} (λ) e^{- h c / (k T λ)}}{\int λ^{- 4} σ_{abs}^{} (λ) e^{- h c / (k T λ)} d λ} .

Here, k is the Boltzmann constant, T is the crystal temperature (RT), τ_rad is the radiative lifetime of the emitting state (³F₄). To reduce the error in σ_SE at long wavelengths (> 2 µm) originating from the exponential term in Eq. (5), we additionally used the Füchtbauer–Ladenburg (F-L) equation [32] which is adequate because of the negligible reabsorption in this spectral range, Fig. 3(a). The σ_SE spectrum is shown in Fig. 4(a). The maximum σ_SE amounts to 7.15 × 10⁻²¹ cm² at 1951 nm. Above 2 µm where laser operation is expected for low-loss cavities, a local maximum of σ_SE = 2.38 × 10⁻²¹ cm² is found at 2090 nm.

Fig. 4 Spectroscopy of Tm³⁺-doped (Lu,Sc)₂O₃ ceramic: (a) absorption, σ_abs, and stimulated-emission, σ_SE, cross-sections for the ³H₆ ↔ ³F₄ transition; (b) gain cross-sections, σ_g = βσ_SE – (1 – β)σ_abs, for the ³F₄ → ³H₆ transition. β = N(³F₄)/N_Tm is the inversion ratio.

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The gain cross-section, σ_g = βσ_SE – (1 – β)σ_abs, where β = N(³F₄)/N_Tm is the inversion ratio, is typically calculated to estimate the expected laser wavelength, see Fig. 4(b). For low β < 0.06, broad and smooth gain spectra are observed centered at ~2.10 µm. For higher β, a peak at 1.97 µm and further one at 1.95 µm dominate the spectrum.

4. Continuous-wave laser operation

4.1 Laser set-up

The laser experiments were performed in a compact microchip-type cavity, Fig. 5(a). The 2.91 mm-thick polished and uncoated Tm:(Lu,Sc)₂O₃ ceramic active element (AE) with an aperture of 3.01 × 3.06 mm² was mounted in a water cooled (12 °C) Cu-holder. Indium foil was used to provide a better thermal contact from all 4 lateral sides of the AE. The plano-plano cavity consisted of a flat pump mirror (PM) antireflection (AR) coated for 0.77-1.05 μm and high reflection (HR) coated for 1.80-2.12 μm, and a flat output coupler (OC) with transmission of T_OC = 1.5%, 3% or 5% at 1.84-2.12 μm. Both PM and OC were placed close to the AE with minimum air gaps, so that the geometrical cavity length was ~3 mm. The AE was pumped through the PM by a fiber-coupled AlGaAs laser diode (LuOcean model Lu0808D200, Lumics, fiber core diameter: 200 μm, NA: 0.22) emitting randomly polarized output at 802 nm (the wavelength was stabilized by water-cooling of the diode). We used a lens assembly (focal length: 30 mm, 1:1 imaging ratio) to focus the pump light into the AE. The radius of the pump beam in the crystal was 100 μm (confocal parameter of the pump beam 2z_R = 1.9 mm). The OCs provided a partial reflection (R ≈40%) at the pump wavelength. The total pump absorption under lasing conditions was 48%.

Fig. 5 Tm:(Lu,Sc)₂O₃ ceramic microchip laser: (a) scheme of the laser, LD – laser diode, PM – pump mirror, OC – output coupler; (b) laser beam profile captured at P_abs = 5.05 W, corresponding to an output power of 1.01 W for T_OC = 3%.

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The laser emission spectra were measured with a compact spectrometer (WaveScan, sensitivity: 1.0-2.6 µm, APE GmbH). The beam profile was captured in the far-field using a FIND-R-SCOPE near-IR camera (model 85726, sensitivity: 0.4-2.2 µm), see Fig. 5(b).

4.2 Microchip laser operation

Microchip laser operation has been achieved with all OCs. This indicates a positive thermal lens (and positive dn/dT coefficient) for Tm:(Lu,Sc)₂O₃ ceramics. The laser output was randomly polarized. The input-output dependences are shown in Fig. 6(a). The maximum output power reached 1.01 W at 2095-2102 nm with a slope efficiency η = 24% (with respect to the absorbed pump power, P_abs) for T_OC = 3%. The laser threshold was at P_abs = 0.86 W and the optical-to-optical efficiency η_opt was 10% (with respect to the incident power). For T_OC = 1.5%, slightly lower η = 19% was observed. For both OCs, the output dependence was linear up to P_abs ~5 W where thermal fracture was observed. For T_OC = 5%, the laser output deteriorated (96 mW with η = 8%) which is attributed to stronger upconversion losses and the associated heating. Typical laser emission spectra are presented in Fig. 6(b). The laser emission occurred at ~2.09 µm in agreement with the gain spectra of Tm^{3 +} for low inversion ratios β < 0.06, Fig. 4(b). A small β is expected for the utilized low output coupling. The multi-peak spectral behavior is due to etalon effects.

Fig. 6 Tm:(Lu,Sc)₂O₃ ceramic microchip laser: (a) input-output dependences, η – slope efficiency; (b) typical laser emission spectra measured at P_abs = 5.05 W (for T_OC = 1.5% and 3%) and 2.40 W (T_OC = 5%).

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The Tm:(Lu,Sc)₂O₃ ceramic microchip laser generated a nearly-circular output beam with a measured M²_x,y < 1.2 (at P_abs = 5.05 W, for T_OC = 3%), see Fig. 5(b).

As compared with a previous report on a 2 at.% Tm:LuScO₃ ceramic emitting at 1982 nm [22], the laser characteristics in the present work are considerably improved. In [22], under diode-pumping at 790 nm, the maximum output power was limited to 211 mW (at P_abs ~3.5 W) by thermal roll-over with a slope efficiency of only 8%. Here, despite the higher Tm doping, the ceramic quality allowed to work at higher pump levels to achieve an output power as high as ~1 W at 3 times higher slope efficiency.

5. Comparison of ceramics with a single crystal

For the comparative spectroscopic and laser studies, we used a 1.0 at.% Tm:LuScO₃ crystal grown by the heat-exchanger method (HEM) [30].

At first, we compare the spectroscopic properties of the single crystal and the ceramic. The absorption cross-section spectra for the ³H₆ → ³H₄ transition are shown in Fig. 7(a). The spectra are similar in shape while less structured for the ceramics. The maximum σ_abs of 2.60 × 10⁻²¹ cm² for the Tm:LuScO₃ single crystal is observed at 793 nm. It is known that for the Tm³⁺ ion in a “mixed” LuScO₃ crystal, σ_abs is lower compared to Tm:Lu₂O₃ and Tm:Sc₂O₃ [30] due to a compositional disorder. A similar effect is expected for “mixed” ceramics.

Fig. 7 Spectroscopy of Tm³⁺ in a 4.76 at.% Tm:(Lu_2/3Sc_1/3)₂O₃ ceramic and in a 1 at.% Tm:LuScO₃ single-crystal: (a) σ_abs spectra for the ³H₆ → ³H₄ transition, (b) σ_SE spectra for the ³F₄ → ³H₆ transition. The noise of the black curve in (b) is due to water absorption affecting the measured luminescence spectrum.

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The SE cross-section spectra for the ³F₄ → ³H₆ transition for the single crystal and the ceramic are compared in Fig. 7(b). A slight reduction of σ_SE for the ceramic is observed: for the Tm:LuScO₃ single crystal, the maximum σ_SE is 8.0 × 10⁻²¹ cm² at 1956 nm. The σ_SE value for the single crystal determined with the F-L method can be overestimated as compared to that obtained by the reciprocity method in analogy to Tm:Lu₂O₃ [30]. The physical reason is the shorter measured luminescence decay time τ_lum for Tm:A₂O₃ crystals than the radiative one τ_rad. The peak in the SE cross-section spectra at >2 µm is shifted to a longer wavelength for the Tm:LuScO₃ single-crystal (2098 nm) due to the larger fraction of Sc³⁺ ions as compared to the ceramic.

For the laser experiments with the Tm:LuScO₃ single-crystal, we cut a 2.91 mm-thick cubic active element with an aperture of 2.82 × 3.13 mm². It was polished to laser-grade quality and remained uncoated. The experiments were performed in the set-up depicted in Fig. 5(a). Microchip laser operation has been achieved and the results are presented in Fig. 8. Due to the observed degradation of the Tm:LuScO₃ laser output for T_OC > 3%, we additionally studied a high-reflective OC (T_OC = 0.2%). The laser emission was randomly polarized. The output power reached 221 mW at 2092-2095 nm with η = 21% for T_OC = 1.5%. The laser threshold was at P_abs = 0.38 W and η_opt was 7%.

Fig. 8 Tm:LuScO₃ single crystal microchip laser: (a) input-output dependences, η – slope efficiency; (b) typical laser emission spectra measured at P_abs = 1.41 W.

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Previously, a similar crystal with a thickness of 3 mm was studied under Ti:Sapphire pumping at 796 nm [15,30]. The output power was 324 mW at 1978-1992 nm with η = 40% (for T_OC = 1.5%). This is comparable to our result considering the better quality of the pump beam from the Ti:Sapphire laser. Using a 7.7 mm-thick sample, the same authors extracted 705 mW at 2090-2115 nm with η = 55% (at a maximum P_abs = 1.4 W) [30].

6. Conclusion

“Mixed” sesquioxide Tm:(Lu,Sc)₂O₃ transparent ceramics produced by the HIP method are promising materials for broadly tunable CW and ultrashort (femtosecond) ML oscillators at 1.95-2.1 µm. They allow for high Tm³⁺ doping levels by keeping the optical quality high, size-scalable synthesis, and similar spectroscopic properties as compared to those for HEM-grown Tm:LuScO₃ single crystals. In the present work, we report on the first transparent ceramics with composition (Lu_2/3Sc_1/3)₂O₃ doped with 4.76 at.% Tm. Their morphology, structure and vibronic properties are studied. The transition probabilities of Tm³⁺ ions are determined within the J-O theory. The ceramic provides broad and smooth gain spectra at 1.9-2.15 µm, as well as relatively high stimulated-emission cross-section at >2 µm, namely 2.38 × 10⁻²¹ cm² at 2090 nm. Using the Tm:(Lu,Sc)₂O₃ ceramic in a compact diode-end-pumped microchip laser, watt-level output at ~2.1 µm has been achieved with a slope efficiency of 24%, representing the highest output power demonstrated for any Tm³⁺-doped (Lu,Sc)₂O₃ crystalline or ceramic laser material.

Funding

Spanish Government (MAT2016-75716-C2-1-R, (AEI/FEDER,UE); MAT2013-47395-C4-4-R, TEC 2014-55948-R); Generalitat de Catalunya (2014SGR1358).

Acknowledgments

W. J. acknowledges financial support from the Key Laboratory of Science and Technology on High Energy Laser, CAEP. F.D. acknowledges additional support through the ICREA academia award 2010ICREA-02 for excellence in research. P. L. acknowledges financial support from the Government of the Russian Federation (Grant No. 074-U01) through ITMO Post-Doctoral Fellowship scheme. P. L. expresses his gratitude to Dr. Christian Kränkel (Institut für Laser-Physik, Universität Hamburg) for providing a Tm:LuScO₃ single-crystal and the information about its spectroscopic properties, as well as for the fruitful discussion.

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Transition	‹λ›, nm	‹E›, cm⁻¹	Г, cm⁻¹nm	f_Σ^exp × 10⁶	f_Σ^calc × 10⁶
³H₆→³F₄	1709	5851	716.3	2.007	2.053^ED
³H₆→³H₅	1190	8401	381.1	2.202	1.223^ED + 0.525^MD
³H₆→³H₄	797.52	12539	160.2	2.061	2.150^ED
³H₆→³F₂ + ³F₃	684.70	14605	118.7	2.072	2.465^ED
³H₆→¹G₄	467.25	21402	19.9	0.746	0.723^ED
³H₆→¹D₂	360.05	27774	33.4	2.110	1.897^ED
rms deviation					0.35

Excited state	Terminal state	‹λ›, nm	A_Σ^calc(JJ’), s⁻¹	B(JJ’), %	A_tot, s⁻¹	τ_rad, ms
³F₄	³H₆	1709	249.15^ED	100	249.15	4.01
³H₅	³F₄	3921	7.25^ED + 1.39^MD	2.4	363.35	2.75
	³H₆	1190	252.51^ED + 102.19^MD	97.6
³H₄	³H₅	2417	21.74^ED + 12.78^MD	2.5	1383.5	0.72
	³F₄	1495	98.19^ED + 28.49^MD	9.2
	³H₆	797.5	1222.3^ED	88.3
³F₂ + ³F₃	³H₄	4840	5.66^ED + 0.31^MD	0.2	2549.2	0.39
	³H₅	1612	344.62^ED	13.5
	³F₄	1142	67.08^ED + 73.34^MD	5.5
	³H₆	684.7	2058.2^ED	80.8
¹G₄	³F₂	1619	15.72^ED	0.6	2647.8	0.38
	³F₃	1471	52.08^ED + 4.22^MD	2.1
	³H₄	1128	244.76^ED + 41.22^MD	10.8
	³H₅	769.2	671.99^ED + 188.91^MD	32.5
	³F₄	643.0	168.92^ED + 11.48^MD	6.8
	³H₆	467.3	1248.5^ED	47.2

Transition	‹λ›, nm	‹E›, cm⁻¹	Г, cm⁻¹nm	f_Σ^exp × 10⁶	f_Σ^calc × 10⁶
³H₆→³F₄	1709	5851	716.3	2.007	2.053^ED
³H₆→³H₅	1190	8401	381.1	2.202	1.223^ED + 0.525^MD
³H₆→³H₄	797.52	12539	160.2	2.061	2.150^ED
³H₆→³F₂ + ³F₃	684.70	14605	118.7	2.072	2.465^ED
³H₆→¹G₄	467.25	21402	19.9	0.746	0.723^ED
³H₆→¹D₂	360.05	27774	33.4	2.110	1.897^ED
rms deviation					0.35

Excited state	Terminal state	‹λ›, nm	A_Σ^calc(JJ’), s⁻¹	B(JJ’), %	A_tot, s⁻¹	τ_rad, ms
³F₄	³H₆	1709	249.15^ED	100	249.15	4.01
³H₅	³F₄	3921	7.25^ED + 1.39^MD	2.4	363.35	2.75
	³H₆	1190	252.51^ED + 102.19^MD	97.6
³H₄	³H₅	2417	21.74^ED + 12.78^MD	2.5	1383.5	0.72
	³F₄	1495	98.19^ED + 28.49^MD	9.2
	³H₆	797.5	1222.3^ED	88.3
³F₂ + ³F₃	³H₄	4840	5.66^ED + 0.31^MD	0.2	2549.2	0.39
	³H₅	1612	344.62^ED	13.5
	³F₄	1142	67.08^ED + 73.34^MD	5.5
	³H₆	684.7	2058.2^ED	80.8
¹G₄	³F₂	1619	15.72^ED	0.6	2647.8	0.38
	³F₃	1471	52.08^ED + 4.22^MD	2.1
	³H₄	1128	244.76^ED + 41.22^MD	10.8
	³H₅	769.2	671.99^ED + 188.91^MD	32.5
	³F₄	643.0	168.92^ED + 11.48^MD	6.8
	³H₆	467.3	1248.5^ED	47.2

Synthesis, spectroscopy, and efficient laser operation of “mixed” sesquioxide Tm:(Lu,Sc)₂O₃ transparent ceramics

Abstract

1. Introduction

2. Fabrication of the Tm:(Lu,Sc)₂O₃ ceramics