1. Introduction

Perovskites are materials with a chemical formula of ABX₃ where A and B are cations, often of rather different sizes, and X is an anion. This class of materials is named after the mineral perovskite, calcium titanium oxide CaTiO₃. The perovskite structure is known for permitting diverse ionic substitutions at both cationic sites due to its high tolerance to structural distortions. Perovskites exhibit exciting compositionally driven ferro-, piezo-, and pyro-electrical properties as well as electro-optical effects, and they have been used as electric, magnetic, optical and catalytic materials in various applications [1,2].

Perovskite oxides, ABO₃, are relevant as host materials for doping with trivalent rare-earth ions for luminescent and laser materials [3]. In ABO₃ crystals, cations with a large ionic radius occupy A-sites and coordinate to XII oxygen atoms and those with a small ionic radius reside in VI-fold coordinated B-sites. The idealized form of ABO₃ compounds is cubic ($Pm\bar{3}m$), e.g., SrTiO₃ and BaTiO₃. More often, the structure is modified by cation displacement or by tilting of octahedra lowering the crystal symmetry. The orthorhombic CaTiO₃ phase (space group Pnma) is among the most common non-cubic variants. Orthorhombic yttrium aluminium perovskite, YAlO₃ (shortly YAP) is a well-known example of laser host material.

Among the ABO₃ perovskites, rare-earth (RE) orthoscandates AScO₃ with A = La to Ho have attracted a lot of attention in the past years. Their structures have been known since 1957 when they were presented by Geller [4] and further described by other authors [5–8]. For predicting the stability of ABO₃ perovskite structures, the Goldschmidt tolerance factor t is used [9]. The t factors for REScO₃ crystals are lower compared to other REBO₃ compounds suggesting that orthoscandates form highly distorted structures [7]. Gadolinium orthoscandate, GdScO₃, adopts the GdFeO₃ type structure (orthorhombic, sp. gr. Pnma, lattice constants: a = 5.745(1) Å, b = 7.929(2) Å, c = 5.481(1) Å [8]). Its Goldschmidt tolerance factor t is relatively low, 0.808, but well within the range of stable orthorhombic perovskites [7].

The structure of GdScO₃ is composed of a framework of corner-connected [ScO₆] octahedra and Gd³⁺ in interstices surrounded by eight [ScO₆] octahedra, see Fig. 1. The orthorhombic structure is derived from the ideal ABO₃ cubic (sp. gr. Pm$\bar{3}$m) aristotype by tilting about the axes of the cubic unit cell running parallel to the edges of the cubes formed by eight [ScO₆] octahedra in an a⁻b ⁺a⁻ pattern (Glazer notation [10]) as a response to the occupancy of the A-site by a cation which is smaller than that required to form the ideal cubic structure [7]. The octahedron tilting results in a symmetry reduction. It is also known that for rare-earth orthoscandates, the RE³⁺ coordination polyhedron in the A-site is best regarded as [REO₈] rather than [REO₁₂], i.e., a four-fold anti-prism [7].

 

Fig. 1. The crystal structure of an orthorhombic GdScO₃ crystal: (a) a fragment of the structure, black lines mark the unit-cell; (b) schematic of the [ScO₆] (left) and [GdO₈] (right) polyhedra showing the metal-to-oxygen (M – O) interatomic distances. The atomic coordinates reported by Veličkov et al. [8] are used.

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Very recently, GdScO₃ crystals gained increasing attention for doping with laser-active RE³⁺ ions targeting ultrafast laser development in the near-infrared. GdScO₃ melts congruently at a high temperature of ∼2100°C and can be grown by the standard Czochralski (Cz) method. As a host material, it features good thermo-mechanical properties, namely a relatively high thermal conductivity of about 5.5 Wm^-1K^-1 [11], and moderate average thermal expansion <α> of 10.9 × 10⁻⁶K^-1 [12] enabling efficient thermal management under high-power pumping. The low maximum phonon energy of 665 cm^-1 [13] suppresses non-radiative relaxation from the excited state of RE³⁺ ions. GdScO₃ is an optically biaxial crystal with a high refractive index (<n > = 2.07 at ∼1 µm [14]) and it features a notable polarization anisotropy of absorption and emission properties of the dopant RE³⁺ ions which leads to linearly polarized laser emission.

The spectroscopic properties of few RE³⁺ ions, including Nd³⁺ [14,15], Yb³⁺ [11], Tm³⁺ [16], Ho³⁺ [17], Er³⁺ [18], and Dy³⁺ [19], in GdScO₃ have been studied. Laser operation of RE³⁺-doped GdScO₃ crystals in the near infrared has also been described [20–24]. Li et al. reported on the crystal growth, optical spectroscopy and Judd-Ofelt analysis of Tm:GdScO₃ crystals [16]. Continuous-wave (CW) laser operation of this material was achieved with diode pumping yielding 4.53 W at 1988nm with a slope efficiency of 25% [21]. Liu et al. achieved a continuous wavelength tuning of a Tm:GdScO₃ laser from 1845 to 2006nm [23]. Recently, Zhang et al. demonstrated femtosecond mode-locked (ML) Tm:GdScO₃ laser operation: using a GaSb-based SEmiconductor Saturable Absorber Mirror, 44 fs pulses were generated at 2048nm with an average output power of 188 mW at a repetition rate of 77.6 MHz [24]. The growth, unpolarized spectroscopy and Judd-Ofelt analysis of Ho:GdScO₃ crystals was reported by Hu et al. [17].

Holmium ions (Ho³⁺) are appealing because of their emission around 2.1 µm due to the ⁵I₇ → ⁵I₈ electronic transition. This emission does not overlap with the structured absorption lines of water vapor in the atmospheric air enabling generation of femtosecond pulses from ML lasers [25]. The Tm³⁺,Ho³⁺ co-doping scheme is often used for achieving Ho laser emission [26,27]. It relies on the strong absorption of Tm³⁺ ions at ∼0.8 µm (the ³H₆ → ³H₄ transition) followed by a cross-relaxation (CR) process, ³H₄ + ³H₆ → ³F₄ + ³F₄, populating the Tm³⁺ metastable state (³F₄) in a “two-for-one” pump process, and a non-radiative energy-transfer (ET), ³F₄ (Tm³⁺) → ⁵I₈ (Ho³⁺), as illustrated in Fig. 2. The Tm³⁺,Ho³⁺ co-doping is also beneficial for combining the gain bandwidths of both ions supporting the generation of shorter pulses from ML lasers [28,29].

 

Fig. 2. A simplified energy level scheme of Tm³⁺ and Ho³⁺ ions showing processes relevant for laser operation around 2.1 µm: NR – multiphonon non-radiative relaxation, CR – cross-relaxation, ETU – energy-transfer upconversion, P₂₈ and P₇₁ – energy-transfer processes.

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In the present work, we report on the Cz growth, a detailed polarized room- and low-temperature spectroscopic study, and first laser operation of a Tm,Ho:GdScO₃ crystal. The Tm,Ho:GdScO₃ laser delivered a watt-level output at 2.1 µm with a high slope efficiency of about 50%.

2. Crystal growth

The Tm,Ho:GdScO₃ crystal was grown by the Cz method in an induction-heating furnace. The raw materials were oxide powders of Gd₂O₃, Sc₂O₃, Tm₂O₃ and Ho₂O₃ with the purity of 99.99%. The raw materials were accurately weighed in the stoichiometric ratios with the doping concentrations of Tm³⁺ and Ho³⁺ ions of 5 at.% and 0.5 at.% (with respect to Gd³⁺) in GdScO₃, respectively. The total weighed mass is 75% of the total mass that can be held in the crucible, considering that the growth charge expands as it melts. After thorough mixing, the raw materials were pressed into tablets with a diameter of 60 mm. Then, the tablets were sintered in air at 1500°C for 10 h. The crystal growth took place in an iridium crucible, with ZrO₂ as an isolator in argon atmosphere. The power used for crystal growth was about 8 kW, which corresponds to the melting point of Tm,Ho:GdScO₃ of about 2100 °C. The single crystal was grown using an [110] oriented seed from an undoped GdScO₃ crystal. The complete process included the following steps: furnace loading, vacuumation, argon charging, heating, melting, crystal growth and slow cooling down to room temperature for 20 h. The pulling rate was 1.0 mm/h and the rotation speed was 10 r.p.m. (revolutions per minute). A photograph of an as-grown Tm,Ho:GdScO₃ crystal is shown in Fig. 3(a). The boule had a cylindrical shape with the dimensions of Φ30 × 30 mm³. It was free of cracks, inclusions and bubbles and had a yellow-brown coloration due to color centers formed during the growth in an oxygen deficient atmosphere. This coloration was subsequently removed by annealing in air.

 

Fig. 3. Tm, Ho:GdScO₃ crystal: (a) a photograph of the as-grown crystal, the growth direction is along the [110] orientation; (b) X-ray powder diffraction (XRD) pattern, numbers denote the Miller’s indices, (hkl), black lines – the reference pattern of GdScO₃.

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The phase purity and the crystal structure of Tm,Ho:GdScO₃ were confirmed by X-ray powder diffraction (XRD), see Fig. 3(b). All the reflections in the diffraction pattern were assigned to the orthorhombic phase isostructural to undoped GdScO₃ (PDF card No. 79 - 0577). The characteristic GdFeO₃ type structure has four distorted perovskite units in the crystallographic cell (space group Pnma – D16 2 h, No. 62). According to previous studies, it is expected that the dopant Tm³⁺ and Ho³⁺ ions replace for the host forming Gd³⁺ cations in С_s symmetry sites (Wyckoff: 4c) [30–32].

The actual composition of the grown crystal was studied by Inductively Coupled Plasma Atomic Spectroscopy (ICP-AS). The RE doping levels were 3.76 at.% Tm, 0.35 at.% Ho (corresponding to ion densities of N_Tm = 6.25 × 10²⁰ at/cm³ and N_Ho = 0.58 × 10²⁰ at/cm³). The RE ion segregation coefficients, K_RE = C_crystal/C_melt, are K_Tm = 0.75 and K_Ho = 0.70.

3. Results and discussion

3.1 Raman spectra

The room-temperature polarized Raman spectra of the Tm,Ho:GdScO₃ crystal are presented in Fig. 4. Four polarization configurations, ${\boldsymbol a}({ij} )\bar{{\boldsymbol a}}$, where i, j = b and c (Porto’s notations) were used to identify vibration modes of different symmetry. For orthorhombic RE scandates (sp. gr. Pnma), according to the factor group analysis, four formula units consisting of 20 atoms per unit cell give rise to 60 vibrational modes at the center of the Brillouin zone (k = 0): Γ_op = 7A_g + 5B₁g + 7B₂g + 5B₃g + 8 Au + 9B_1u + 7B_2u + 9B_3u, of which 24 modes are Raman-active (A_g, B₁g, B₂g, and B₃g), 25 modes are IR-active (B_1u, B_2u, and B_3u) and 8 modes are silent (A_u), and Γ_ac = B_1u + B_2u + B_3u [13]. The Raman spectra of Tm,Ho:GdScO₃ are typical for orthorhombic perovskites and are strongly polarized. 22 active Raman modes were found and assigned out of 24 predicted ones. Note that to date, none of the perovskites has shown all the modes. The number of modes depends on the degree of distortion of the ideal perovskite structure. In the low-frequency range (below 200 cm^-1), the Raman modes are due to A-site (Gd³⁺) translations, while those at higher energies (above 200 cm^-1) are due to oxygen motions (bending, stretching, and tilting) in the [AO₈] units. In orthorhombic symmetry, the B cation (Sc³⁺) occupies a site of inversion symmetry (C_i) and thus, the Raman spectra of such perovskites should not have modes which are due to B cation translations or vibrations. The dominant Raman peak appears at 320 cm^-1 (A_g) and the maximum phonon energy is 665 cm^-1 (B₃g). The relatively low maximum phonon energy of GdScO₃ is beneficial for reducing the multiphonon non-radiative relaxation rate from the ⁵I₇ Ho³⁺ state.

 

Fig. 4. Polarized Raman spectra of an a-cut Tm,Ho:GdScO₃ crystal, λ_exc = 514 nm, numbers - Raman frequencies in cm⁻¹.

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3.2 Polarized absorption spectra

The orthorhombic GdScO₃ crystal is optically biaxial, and its optical indicatrix frame aligns with the crystallographic axes. The spectroscopic properties of Tm³⁺,Ho³⁺-codoped GdScO₃ were studied for the principal light polarizations E || a, b, c.

The room-temperature polarized absorption spectrum of Tm,Ho:GdScO₃ is presented in Fig. 5(a-c) in terms of the absorption coefficient α_abs. The assignment of electronic transitions of Tm³⁺ and Ho³⁺ ions follows that by Carnall et al. [33]. The broad absorption in the visible (320 – 500 nm) is assigned to color centers that are not entirely removed upon annealing in air [34]. Absorption bands related to transitions from the ³H₆ Tm³⁺ and ⁵I₈ Ho³⁺ ground states to the excited-states ranging from ³F₄ Tm³⁺ and ⁵I₇ Ho³⁺ up to ¹D₂ Tm³⁺ and ³L₉ + ⁵G₃ Ho³⁺ (wavelength range: 320 – 2150 nm) are observed. The UV absorption edge is observed at ∼320 nm (3.87 eV) whereas for undoped GdScO₃, the optical bandgap E_g is 5.8 eV [35].

 

Fig. 5. Polarized absorption properties of the Tm,Ho:GdScO₃ crystal: (a-c) absorption spectra; (d) absorption cross-sections, σ_abs, for the ³H₆ → ³F₄ Tm³⁺ transition, arrow indicates the pump wavelength.

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The absorption cross-sections, σ_abs = α_abs/N_Tm, for the ³H₆ → ³H₄ Tm³⁺ transition around 0.8 µm are shown in Fig. 5(d). This band is suitable for pumping Tm³⁺,Ho³⁺-codoped crystals by high-brightness Ti:Sapphire lasers or commercial high-power spatially multimode AlGaAs laser diodes. The maximum σ_abs value reaches 1.52 × 10⁻²⁰ cm² at 794.2 nm and the absorption bandwidth (full width at half maximum, FWHM) is as large as 8.0 nm (for light polarization E || b). For the other two polarizations, the absorption is weaker, σ_abs = 0.93 × 10⁻²⁰ cm² at 792.4 nm (for E || a) and σ_abs = 0.75 × 10⁻²⁰ cm² at 801.5 nm (for E || c). The relatively broad absorption of Tm³⁺ ions around 0.8 µm is beneficial for suppressing the effect of the temperature drift of the pump diode wavelength.

Recently, resonant (in-band) Tm³⁺ pumping was considered as a viable way for power scaling of Tm,Ho lasers. It corresponds to the ³H₆ → ³F₄ Tm³⁺ transition. Raman-shifted Er-fiber lasers emitting at 1.68 µm can address this absorption band. For Tm³⁺ ions in GdScO₃, the maximum σ_abs value is 0.98 × 10⁻²⁰ cm² at 1673.6 nm for light polarization E || b. For the other two polarizations, the absorption is weaker, σ_abs = 0.61 × 10⁻²⁰ cm² at 1670.7 nm (for E || a) and σ_abs = 0.67 × 10⁻²⁰ cm² at 1670.6 nm (for E || c).

3.3 Luminescence spectra and Tm³⁺ → Ho³⁺ energy transfer

The luminescence spectra of the Tm,Ho:GdScO₃ crystal in the near-IR are shown in Fig. 6(a), measured for the three principal light polarizations E || a, b, c under excitation at 794 nm (into the ³H₄ Tm³⁺ manifold). The spectra are strongly polarized and exhibit a notable spectral line broadening which is attributed to the strong electron-phonon coupling for this host crystal (note that homogeneous thermal line broadening is expected to be much stronger for Tm³⁺ ions than for Ho³⁺ ones in line with the measured spectra). The weak band at 1.37 – 1.53 µm is due to the ³H₄ → ³F₄ Tm³⁺ transition, and a broad band spanning from 1.55 to 2.3 µm is assigned to two spectrally overlapping transitions, namely ³F₄ → ³H₆ Tm³⁺ and ⁵I₇ → ⁵I₈ Ho³⁺. The latter emission is more intense despite the low Ho:Tm codoping ratio (1:10.7, according to the actual rare-earth ion densities), indicating an efficient Tm³⁺ → Ho³⁺ energy transfer. In the spectral range where laser operation is expected according to the quasi-three-level nature of the Ho³⁺ laser scheme with reabsorption from the ground-state (⁵I₈), higher emission intensity corresponds to light polarization E || c and several local peaks are found in the luminescence spectra centered at 2022, 2064, 2098, and 2111 nm.

 

Fig. 6. Emission properties of Tm,Ho:GdScO₃: (a) polarized luminescence spectra in the near-IR, light polarizations: E || a, b, c, arrows indicate the observed laser wavelengths (for light polarization E || c); (b) a comparison of unpolarized emission spectra of Tm³⁺,Ho³⁺-codoped and singly Tm³⁺-doped GdScO₃ crystals around 2 µm, λ_exc = 794 nm.

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The luminescence dynamics from the lower-lying excited states of Tm³⁺ and Ho³⁺ ions in the Tm,Ho:GdScO₃ crystal, which are responsible for laser emission around 2 µm, were studied under resonant Tm³⁺ excitation (at 1635 nm, to the ³F₄ state). The luminescence was detected at two wavelengths corresponding to almost pure Tm³⁺ (1870nm) and Ho³⁺ (2040nm) emissions. The measured luminescence decay curves from the ³F₄ Tm³⁺ and ⁵I₇ Ho³⁺ manifolds plotted on a semi-log scale are shown in Fig. 7. In the initial decay stage, a rapid rise of Ho³⁺ emission intensity is observed within a few hundred microseconds. At the same time, a corresponding fast drop in the intensity of Tm³⁺ luminescence is observed. Before the thermal equilibrium is reached between both active ions, this phenomenon corresponds to a direct energy transfer from Tm³⁺ to Ho³⁺. The luminescence decay rates of the ³F₄ Tm³⁺ and ⁵I₇ Ho³⁺ manifolds become almost identical at a longer time scale, about 1 ms after the excitation pulse. During this phase, energy transfer occurs in both directions. However, it is masked by a quasi-equilibrium between the respective multiplets.

 

Fig. 7. Luminescence decay curves from the ³F₄ Tm³⁺ and ⁵I₇ Ho³⁺ manifolds measured under resonant Tm³⁺ excitation (λ_exc = 1635 nm), circles – experimental data, curves – their fits using Eq. (1), τ₀ – thermal equilibrium decay time.

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The parameters of the bidirectional ³F₄(Tm³⁺) ↔ ⁵I₇(Ho³⁺) energy-transfer were estimated using the dynamical model developed by Walsh et al. [36]. The luminescence decay curves were fitted using the following equations:

Table 1. Thulium-Holmium Energy-Transfer Parameters for Various Laser Crystals

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The equilibrium constant for a Tm³⁺,Ho³⁺-codoped crystal can be independently estimated from the crystal-field splitting of Tm³⁺ and Ho³⁺ multiplets involved in the energy-transfer [36]:

3.4 Low-temperature spectroscopy

The dopant trivalent RE ions (Tm³⁺ and Ho³⁺, in our case) in GdScO₃ predominantly substitute for the Gd³⁺ host-forming cations in C_s symmetry sites with an VIII-fold oxygen coordination (Wyckoff: 4c). The corresponding ionic radii R_Gd = 1.053 Å, R_Tm = 0.994 Å and R_Ho = 1.015 Å. The Sc³⁺ sites (Wyckoff: 4b) feature a VI-fold oxygen coordination (R_Sc = 0.745 Å). The ^2S + 1L_J multiplets with integer total angular momentum J in the crystal-field of the symmetry C_s are split into a total of 2J + 1 Stark components. In the present work, we determined the crystal-field splitting of the two lowest multiplets of Tm³⁺ (³H₆ and ³F₄) and Ho³⁺ (⁵I₈ and ⁵I₇) in GdScO₃ as they are relevant for describing the laser emission properties around 2 µm and energy-transfer between these two dopant ions.

The low-temperature (LT, 12 K) absorption and luminescence spectra corresponding to the ³H₆ ↔ ³F₄ Tm³⁺ and ⁵I₈ ↔ ⁵I₇ Ho³⁺ transitions were measured for singly Tm³⁺-doped and Tm³⁺,Ho³⁺-codoped GdScO₃ crystals, respectively, using linearly polarized light, as shown in Fig. 8. The LT absorption spectra were used to resolve the crystal-field splitting of the RE³⁺ excited-states and they plotted vs. the photon energy (expressed in cm⁻¹). The LT emission spectra were measured to determine the Stark sub-level energies of the RE³⁺ ground-states and they are plotted as a function of (E_ZPL – photon energy, expressed in cm⁻¹). For labelling electronic transitions, we used the following phenomenological notations for Stark sub-levels: Y_i, Z_j, Y_k, Z_l for the ³F₄ (I = 1 - 9), ³H₆ (j= 1 - 13), ⁵I₇ (k = 1 - 15), and ⁵I₈ (l = 1 - 17) multiplets, respectively [43,44]. For the assignment of electronic transitions of Tm³⁺ and Ho³⁺ ions in GdScO₃, we followed the previous work on the isostructural GdAlO₃ crystal (sp. gr. Pnma) doped with Tm³⁺ and Ho³⁺ ions (Gd³⁺ site symmetry: C_s). The vertical dashes in Fig. 8 mark the electronic transitions for this reference crystals.

 

Fig. 8. Low-temperature (12 K) spectroscopy of Tm³⁺ and Ho³⁺ ions in the GdScO₃ crystal: (a,c) absorption spectra: (a) the ³H₆ → ³F₄ Tm³⁺ transition; (c) the ⁵I₈ → ⁵I₇ Ho³⁺ transition; (b,d) luminescence spectra: (b) the ³F₄ → ³H₆ Tm³⁺ transition; (d) the ⁵I₇ → ⁵I₈ Ho³⁺ transition, “+” indicate the assigned electronic transitions. Vertical dashes – experimental Stark splitting for Tm³⁺ and Ho³⁺ ions in C_s sites of the GdAlO₃ crystal (after [41,42]).

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Figure 9 presents the determined experimental crystal-field splitting of the (³H₆ and ³F₄) Tm³⁺ and (⁵I₈ and ⁵I₇) Ho³⁺ multiplets in GdScO₃. The ZPL energies E_ZPL are 5712 cm⁻¹ (Tm³⁺) and 5110 cm⁻¹ (Ho³⁺). The total Stark splitting of the ground-state amounts to ΔE(³H₆) = 681 cm⁻¹ (Tm³⁺) and ΔE(⁵I₈) = 382 cm⁻¹ (Ho³⁺). Table 2 lists the Stark sub-level energies for the considered multiplets.

 

Fig. 9. Experimental crystal-field splitting of two lowest multiplets of Tm³⁺ and Ho³⁺ ions in the GdScO₃ crystal. E_ZPL – zero-phonon line, Z₁, Z₂, Z₇ and Z₈ are the partition functions of the ³H₆, ³F₄ Tm³⁺ and ⁵I₇, ⁵I₈ Ho³⁺ multiplets, respectively.

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Table 2. Experimental Crystal-Field Splitting of the ³H₆, ³F₄,Tm³⁺ and ⁵I₈, ⁵I₇ Ho³⁺ Multiplets in the GdScO₃ crystal

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Both Tm³⁺ and Ho³⁺ ions in the GdScO₃ crystal exhibit broad and smooth absorption and emission spectra at room temperature for polarized light. The large spectral bandwidths may originate either from the homogeneous (thermal) broadening owing to a strong electron-phonon interaction with the host matrix or from the inhomogeneous broadening mechanisms.

Amanyan et al. studied Nd³⁺-doped GdScO₃ crystals and indicated that for this compound, the spectral linewidths at room temperature were substantially larger than those for ordered orthoaluminate (YAlO₃) and aluminium garnet (Y₃Al₅O₁₂) crystals [14]. These authors also suggested that the relatively large Stark splitting of RE³⁺ multiplets in GdScO₃ comes from the low-symmetry component in the crystal-field potential expansion indicating a strong low-symmetry distortion in the local symmetry of the activator center [14]. Guo et al. studied the LT fluorescence of Yb³⁺ ions in GdScO₃ and indicated that the energy transitions of Yb³⁺ were almost independent of the excitation wavelength revealing a single type of Yb³⁺ species [30]. Zhang et al. indicated that although the shape of [Gd|REO₈] polyhedra is the same through the GdScO₃ unit-cell, their orientations are different which may contribute to additional inhomogeneous spectral line broadening [11]. However, no evidence of the latter effect was provided.

In the GdScO₃ crystal, there are two possible sites for RE substitution, namely A-sites (Gd ones, C_s symmetry) and B-sites (Sc ones, C_i symmetry). The entering of RE³⁺ ions such as Tm³⁺ and Ho³⁺ into Sc sites looks very improbable due to the large difference in their ionic radii (R_Tm = 0.88 Å, R_Ho = 0.901 Å and R_Sc = 0.745 Å for VI-fold oxygen coordination), as pointed out by Piotrowski et al. [31]. Moreover, electric dipole (ED) transitions are forbidden for sites with a center of inversion, so that even if a small amount of RE³⁺ dopant ions are found in the Sc sites, they will not show themselves in the optical spectra (except of the transitions with a magnetic dipole component of the transition intensity). This situation is similar to the case of multi-site cubic sesquioxides (e.g., Y₂O₃) featuring a distribution of the dopant ions over C₂ and C_3i symmetry sites [45]. However, since both sites in the latter are occupied by the same host cations, 25% of the dopant ions remain optically inactive.

In the present work, no signs of multi-site behavior for eigher Tm³⁺ or Ho³⁺ ions were revealed by the LT spectroscopy and the measured LT emission spectra at 2 µm were almost independent of the excitation wavelength. To get further insight into this topic, we compared the LT (12 K) absorption spectra of Ho³⁺ ions (the ⁵I₈ → ⁵I₇ transition) for three crystals, namely ordered YAlO₃ (Y site symmetry: C_s), GdScO₃ (Gd site symmetry: C_s) and Y₂O₃ (Y site symmetry: C₂ and C_3i), as shown in Fig. 10. At 12 K, the temperature broadening of spectral lines is greatly suppressed. As seen from the figure, the spectral linewidths for all three crystals are comparable and they are much narrower than those for crystals with a structure and / or compositional disorder featuring a strong inhomogeneous spectral line broadening, such as Ca(Gd,Lu)AlO₄ [38] crystal or solid-solution (“mixed”) sesquioxides (Lu,Sc)₂O₃ [45].

 

Fig. 10. LT (12 K) absorption spectra of Ho³⁺-doped GdScO₃, Y₂O₃ and YAlO₃ crystals, the ⁵I₈ → ⁵I₇ transition.

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From the above-mentioned considerations, we conclude that the main reasons for observation of broad and structureless spectra of RE ions in GdScO₃ are (i) low-symmetry component in the crystal-field potential expansion leading to a large Stark splitting of RE³⁺ multiplets and (ii) a strong electron-phonon interaction leading to a large homogeneous thermal line widths at room temperature. The latter is in line with the fact that the spectral broadening is more significant for Tm³⁺ ions than for Ho³⁺ ions, as evidenced by the luminescence spectra at 2 µm, see Fig. 6(b). Indeed, it is known that the strength of the lattice-orbit interaction is not homogeneous across the lanthanide series and increases at its beginning and end being particularly strong for Tm³⁺ ions.

3.5 Laser operation

The layout of the laser setup is depicted in Fig. 11. The rectangular laser element was cut from the 3.76 at.% Tm, 0.35 at.% Ho:GdScO₃ crystal for light propagation along the a-axis (a-cut) with a thickness of 3.1 mm and an aperture of 3.0(b) × 3.0(c) mm². It was polished to laser quality on both sides, with good parallelism, and left uncoated. The laser element was mounted on a passively cooled copper holder using a thermally conductive silver paint for improved heat removal. The hemispherical laser cavity was formed by a flat pump mirror (PM) with high transmission (T = 80%) at 0.79 µm and high reflection at 1.86 – 2.31 µm and a series of concave output couplers (OCs) with a radius of curvature (RoC) of -100 mm and a transmission at the laser wavelength T_OC ranging from 0.1% to 10%. The crystal was placed near the PM with a small air gap of less than 1 mm. The geometric cavity length was ∼99 mm. The pump source was a CW Ti:Sapphire laser (3900S, Spectra Physics) emitting up to 3.9 W at 791 nm (to address the ³H₆ → ³H₄ Tm³⁺ absorption band) with a nearly diffraction-limited beam quality (M² ≈ 1). The pump power was adjusted by a half-wave plate with the polarization fixed by a polarizer to be E || b in the crystal. The pump radiation was focused into the crystal through the PM using an antireflection-coated aspherical lens with a focal length f of 75 mm. The diameter of the pump spot 2ω_p was 70 ± 10 µm. The pumping was done in a double-pass configuration due to the non-negligible reflectance of the OCs at 0.79 µm. The pump absorption efficiency under lasing conditions was weakly dependent on the output coupling and amounted to ∼82%. A long-pass reflective filter was placed after the OC to filter out the residual pump. The laser spectra were measured using an optical spectrum analyzer (AQ6376, Yokogawa) and a ZrF₄ fiber.

 

Fig. 11. Layout of the Tm,Ho:GdScO₃ laser: P – Glan-Taylor polarizer; FL – aspherical focusing lens; PM – pump mirror; OC – output coupler; F – long pass filter.

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The Tm,Ho:GdScO₃ laser generated a maximum output power of 1.16 W at ∼2067 and 2099nm (demonstrating dual-band emission) with a slope efficiency η of 50.5% (relative to the absorbed pump power) at the maximum absorbed pump power of 2.60 W for an intermediate T_OC of 5%, as illustrated in Fig. 12(a). The optical efficiency was 29.1% (relative to the pump power incident on the crystal). Upon increasing the output coupling from 0.1% to 10%, there was a gradual rise in the laser threshold from 52 to 290 mW. The input-output dependencies were linear within the investigated range of pump powers, and no indications of unwanted thermal effects or thermal fracture of the crystal were detected.

 

Fig. 12. Tm,Ho:GdScO₃ laser: (a) input-output dependences for various output coupling, a-cut crystal, η – slope efficiency, P_th – threshold pump power; (b) Findlay-Clay analysis for evaluating the round trip passive losses L; (c) typical spectra of laser emission measured well above the laser threshold, the laser polarization is E || c; (d) experimental crystal-field splitting of the ⁵I₇ and ⁵I₈ Ho³⁺ manifolds, arrows indicate the observed laser transitions.

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The round trip passive losses L in the Tm,Ho:GdScO₃ laser were evaluated by the Findlay-Clay analysis, i.e., by plotting the laser threshold power P_th vs. ln(1/R_OC) where R_OC = 1 – T_OC is the output-coupler reflectivity, Fig. 12(b). This yields L = 2.5 ± 0.5%, well below the Fresnel loss at the uncoated crystal surfaces owing to the etalon effect. The theoretical slope efficiency of the Ho laser is calculated as η = η_St,L × η_q × η_OC × η_mode, where η_St,L = λ_P/λ_L is the Stokes efficiency under lasing conditions, λ_P and λ_L are the pump and laser wavelengths, respectively, η_q is the quantum efficiency which reach 2 assuming a very efficient cross-relaxation for Tm³⁺ ions and efficient Tm³⁺ → Ho³⁺ energy transfer, η_OC = ln(1 - T_OC)/ln[(1 - T_OC) × (1 - L)] is the output-coupling efficiency and η_mode is the mode-matching efficiency. For T_OC = 5%, the result is η = 51.2% which is just above the measured slope efficiency.

Typical laser emission spectra measured well above the laser threshold, are depicted in Fig. 12(c). For a very low T_OC of 0.1%, the laser operated across a wide spectral range of 2097 – 2117 nm. For T_OC between 0.5% and 2%, the emission occurred at ∼2.10 µm. Upon increasing T_OC to 5% and 10%, the laser wavelength switched to ∼2.06 and 2.026 µm, respectively. For all studied OCs, the laser operated solely on the ⁵I₇ → ⁵I₈ Ho³⁺ transition without Tm³⁺ colasing confirming the properly selected Ho/Tm codoping ratio. The observed progressive blue-shift in the laser spectra can be attributed to the quasi-three-level nature of the ⁵I₇ → ⁵I₈ Ho³⁺ laser transition which involves reabsorption from the ground state. For all the studied OCs, the laser emission was linearly polarized (E || c), and the polarization state was naturally determined by the anisotropy of the gain, cf. Figure 6(a). Based on the determined crystal-field splitting of the ⁵I₇ (Y_k) and ⁵I₈ (Z_l) Ho³⁺ multiplets, we assigned the observed laser lines to the Y₁ → Z₁₇ (2115 nm), Z₁₆ (2100 nm), Z₁₃ (2067nm) and Z₇ (2024nm) electronic transitions, as illustrated in Fig. 12(d).

The polarization behavior of the Tm,Ho:GdScO₃ laser was studied in more detail in Fig. 13. By using a rotatory Glan-Taylor prism placed after the OC and the long-pass filter, we measured the polarization state of the output. The polar plot of the normalized laser power transmitted through the polarizer P vs. its orientation θ (θ = 0 for horizontal polarization, along the b-axis) is well fitted using the formula P_trans(θ) = P_u/2 + P_pcos²(θ – φ), Fig. 13(a), where P_u and P_p are the unpolarized and polarized power fractions, respectively, and φ is the preferential orientation of the electric field vector E, for P_u< 0.001 and φ = 90° suggesting linearly polarized light along the c-axis. The polarization degree P = P_p/(P_p + P_u) > 99.9%. Then, by inserting another AR-coated half-wave plate before the focusing lens, the orientation of the pump polarization E_P was changed between E || b and E || c. The output power of the Tm,Ho:GdScO₃ laser was monitored as a function of E_P at a fixed incident pump power, see Fig. 13(b). The strong absorption anisotropy of Tm³⁺ ions in the GdScO₃ at the pump wavelength of 791 nm, as shown in Fig. 5(d), leads to a notable variation of the pump absorption efficiency when rotating the pump polarization and, consequently, a change in the output power of the laser. This analysis confirms the preferential pumping with the E_P || b polarization.

 

Fig. 13. Polarization behavior of the Tm,Ho:GdScO₃ laser based on an a-cut crystal: (a) study of the polarization state of laser emission (E || c); (b) dependence of output power on the pump polarization E_P in the b-c plane, T_OC = 5%, P_inc = 1.1 W.

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The Tm,Ho:GdScO₃ laser generated a nearly circular output beam even at high pump powers. The corresponding 1D intensity distributions were well fitted by a Gaussian profile, as illustrated in Fig. 14(a,b). The laser was operating on the fundamental transverse mode (measured beam quality factors M²_x,y < 1.05), leading to good mode-matching efficiency.

 

Fig. 14. Spatial and temporal emission characteristics of the Tm,Ho:GdScO₃ laser: (a) a typical far-field beam profile; (b) the corresponding 1D intensity distributions (symbols) and their Gaussian fits (curves); (c) a typical switch-on oscilloscope trace, T_OC = 5%, P_abs = 2.0 W.

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A typical oscilloscope trace of the output emission from the Tm,Ho:GdScO₃ laser is shown in Fig. 14(c). It reveals a highly stable CW operation after the spiking following switching on of the pump, and no relaxation oscillations are observed.

The unpolarized spectra of visible and near-IR luminescence from the Tm,Ho:GdScO₃ crystal were measured under lasing and non-lasing conditions, see Fig. 15. The spectra are dominated by intense Tm³⁺ emission from the pump manifold according to the ³H₄ → ³H₆ transition, observed at 0.8 µm. The upconversion emissions are solely attributed to Ho³⁺ ions indicating an efficient Tm³⁺ → Ho³⁺ energy transfer draining the population of the Tm³⁺ metastable state (³F₄). In the visible, intense upconversion luminescence is observed in the green (the ⁵F₄ + ⁵S₂ → ⁵I₈ transition, at 544 nm) and red (the ⁵F₅ → ⁵I₈ transition, at 654 nm). Under lasing conditions, the intensity of Ho³⁺ upconversion luminescence is notably decreased compared to the case without lasing due to the rapid depopulation of the metastable ⁵I₇ Ho³⁺ state by the lasing process (⁵I₇ → ⁵I₈). In contrast, the intensity of Tm³⁺ emissions changes only marginally.

 

Fig. 15. Spectra of visible and near-IR luminescence (unpolarized emission) from the Tm,Ho:GdScO₃ crystal under lasing and non-lasing conditions, λ_P = 791 nm.

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4. Conclusion

To conclude, Tm³⁺,Ho³⁺-codoped gadolinium orthoscandate (GdScO₃) perovskite-type crystals are attractive for laser sources emitting around 2.1 µm due to their intense and broad absorption around 0.8 µm, broadband and polarized emission properties, predominantly unidirectional Tm³⁺ → Ho³⁺ energy transfer and a relatively large Stark splitting of the Tm³⁺ and Ho³⁺ ground manifolds assigned to the low-symmetry distortion in the local symmetry of the activator center. By employing low-temperature absorption and fluorescence spectroscopy, the crystal-field splitting of the two lowest manifolds of Tm³⁺ and Ho³⁺ ions were fully resolved. Our studies suggest that both rare-earth ions reside in a single type of sites (A-sites of the perovskite structure, C_s symmetry) and suggest that the spectral line broadening at room temperature mainly originates from the homogeneous (thermal) broadening owing to the electron-phonon interaction rather than some inhomogeneous mechanism. Highly-efficient, low-threshold and power scalable to the Watt-level continuous-wave laser operation of the Tm,Ho:GdScO₃ crystal was achieved. Considering their attractive spectroscopic properties, the Tm,Ho:GdScO₃ crystals are promising for sub-100 fs pulse generation from mode-locked lasers.