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Abstract
We report on the crystal growth, spectroscopy characterization and first laser operation of a new tetragonal disordered “mixed” calcium aluminate crystal, Tm:Ca(Gd,Lu)AlO4. The introduction of Lu3+ leads to an additional inhomogeneous broadening of Tm3+ absorption and emission spectra compared to the well-known Tm:CaGdAlO4. The maximum stimulated-emission cross-section for the 3F4 → 3H6 Tm3+ transition is 0.91 × 10−20 cm2 at 1813 nm for σ-polarization, and the emission bandwidth is more than 200 nm. A continuous-wave diode-pumped Tm:Ca(Gd,Lu)AlO4 laser generates 1.82 W at 1945 nm with a slope efficiency of 29%. Under Ti:Sapphire laser pumping, a continuous tuning of the laser wavelength from 1836 to 2083 nm (tuning range: 247 nm) is demonstrated. The Tm:Ca(Gd,Lu)AlO4 crystal is promising for tunable/femtosecond lasers at ~2 μm.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tetragonal (space group I4/mmm) calcium rare-earth aluminates CaLnAlO4 (where Ln = Gd or Y, abbreviated as CALGO and CALYO, respectively) are well-known disordered laser crystal hosts for Yb3+-doping [1,2]. They feature high thermal conductivity (~6.5 W/mK for CaGdAlO4) with a weak concentration-dependence and weak and positive thermal lensing originating from negative thermo-optic coefficients, dn/dT [3]. The structural disorder originates from a random distribution of Ca2+ and Ln3+ cations over a single type of sites (C4v) [2]. This leads to a significant inhomogeneous broadening of absorption and emission bands making these crystals attractive for sub-100 fs mode-locked (ML) lasers at ~1 μm in the case of Yb3+ doping [4,5]. Good thermo-mechanical properties of CaLnAlO4-type crystals enable power scaling of Yb3+ oscillators [6].
Considering the success of Yb:CaLnAlO4 crystals, the research interest turned to their doping with Thulium (Tm3+) ions. The latter are known for their eye-safe emission in the spectral range of ~2 μm (3F4 → 3H6 transition) [7]. The Tm3+ ions show efficient absorption at ~0.8 μm (to the 3H4 state), e.g., emission from Ti:Sapphire lasers or commercial powerful AlGaAs laser diodes. Efficient cross-relaxation for adjacent Tm3+ ions, 3H4 + 3H6 → 3F4 + 3F4, may raise the pump quantum efficiency up to 2 leading to high laser efficiency and reduced heat loading [8]. Due to the typically large Stark splitting of the ground-state (3H6), the ~2 μm Tm3+ emission band is broad opening the possibility of broadband wavelength tuning of the laser emission and ML laser operation.
In 1990s, Tm:CaYAlO4 crystals were initially studied [9,10]. In the recent years, Tm3+-doped CaLnAlO4 crystals were revisited regarding their growth, spectroscopic properties [11–13] and continuous-wave (CW) laser emission at ~2 μm [12,14]. In [14], a diode-pumped Tm:CaYAlO4 laser generated a maximum output power of 7.62 W at 1945 nm with a slope efficiency of 50.9% (vs. the absorbed pump power). Passive Q-switching of such lasers by Cr2+:ZnSe saturable absorbers has been reported [15]. Recently, an in-band pumping scheme (at ~1.7 μm, directly to the upper laser level, 3F4) was explored for Tm:CaYAlO4 leading to the generation of 6.8 W at 1968 nm with a slope efficiency of 52% (vs. the incident pump power) [16].
Regarding ML Tm:CaLnAlO4 lasers, a remarkable result is the generation of femtosecond pulses (650 fs) from a Tm:CaGdAlO4 laser ML by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM) [17]. The central wavelength of the laser spectrum was at 2021 nm with a full width at half maximum (FWHM) of 9.2 nm. Such a long laser wavelength (for Tm3+-doped crystals) was because a special “bandpass” output coupler was employed and the laser was constrained to oscillate above 2 μm. This had a key effect for achieving fs pulses. In [18], a SESAM ML Tm:CaYAlO4 laser operating at 1961 nm generated pulses with a duration as long as 35.3 ps.
Note that for most of the laser materials doped with Tm3+, the center of the 3F4 → 3H6 emission band is located slightly below 2 μm. This is unfavorable for ML because the laser emission spectrally overlaps with the atmosphere water vapor absorption bands preventing fs mode-locking. One way to solve this problem is to use special wavelength-selective cavity mirrors as described above to constrain the laser to operate at >2 μm (the corresponding part of the gain spectrum is related to the electron-phonon (vibronic) interaction [19]). Another option is to use a very limited number of host materials ensuring large Stark splitting of the 3H6 Tm3+ ground-state (e.g., cubic sesquioxides [20]). Recently, a new monoclinic Tm:MgWO4 crystal generating laser emission above 2 μm enabled the generation of 86 fs pulses at 2017 nm [21]. An alternative is to provide an additional inhomogeneous broadening of Tm3+ emission by structure disorder or compositional disorder (in a “mixed” crystal) of the host material affecting the crystal field. Recently, various Tm3+-doped crystals and ceramics with structure or compositional disorder were studied for fs ML lasers at ~2 μm [22,23].
In the present work, we aimed to grow and study the spectroscopic and laser properties of a novel calcium aluminate crystal, Tm:Ca(Gd,Lu)AlO4. The addition of optically passive Lu3+ ions is expected to provide additional compositional disorder leading to further spectral broadening which is attractive for fs pulse generation in ML lasers. Note that the existence of the CaLuAlO4 crystal has never been reported so far. The growth and spectroscopy of an Yb:Ca(Gd,Lu)AlO4 crystal was recently studied [24].
2. Crystal growth and structure
The Tm:Ca(Gd,Lu)AlO4 crystal melts congruently and thus it was grown by the conventional Czochralski (Cz) method using an Ar atmosphere in an Ir crucible. An automatic system was used to control the boule diameter. The polycrystalline samples were obtained by solid-state reaction from a mixture of the starting materials, 4N-pure CaCO3, Gd2O3, Lu2O3, Al2O3 and 5N-pure Tm2O3. They were placed in an Ir crucible and melted by an intermediate-frequency heater. A [001]-oriented CaGdAlO4 seed was used, the pulling rate was 0.5 mm/h and the rotation speed was 8 rpm. After the growth was completed, the crystal was cooled down to room temperature at a stepping rate of 15-25 K/h. A crack-free large-volume as-grown boule, Fig. 1(a), had a yellow coloration, attributed to interstitial oxygen atoms. The coloration was removed to a great extent by annealing at 950 °C for 24 h under N2 atmosphere with 5% H2.
Fig. 1 (a) Photograph of the as-grown Tm:Ca(Gd,Lu)AlO4; (b) X-ray powder diffraction (XRD) pattern (in black), numbers denote the Miller’s indices (hkl), standard XRD pattern of CaGdAlO4 is shown for comparison (in blue); (c) polarized Raman spectra of an a-cut crystal, a(xy)a are the Porto’s notations, numbers denote the peak Raman frequencies in cm−1, λexc = 514 nm.
The concentration of doping ions was measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to be 3.5 at.% Tm3+ and 5.2 at.% Lu3+. The segregation coefficients K = Ncrystal/Nmelt for these ions are 0.70 and 0.52, respectively. The stoichiometric crystal formula is CaGd0.913Lu0.052Tm0.035AlO4. The Tm3+ ion concentration is 4.31 × 1020/cm3 (crystal density: ρ = 6.01 g/cm3).
In CaGdAlO4, the Ca2+ and Gd3+ cations (ionic radii RCa = 1.180 Å, RGd = 1.107 Å) statistically occupy the same type of site (Wyckoff symbol: 4e, site symmetry: C4v, coordination number: IX). Tm3+ (RTm = 1.052 Å) and Lu3+ (RLu = 1.032 Å) cations are entering these sites. The local disorder results from the second coordination sphere of Tm3+ ions constituted by Gd3+|Lu3+ and Ca2+ ones, namely the charge difference of these ions and the different cation-cation distances. Note that doping of Ca(Gd,Lu)AlO4 by both Tm3+ and Lu3+ ions is expected to affect the crystal field because of the closeness of their ionic radii. In this way, one can argue that solely high Tm3+ doping can produce a similar effect to Tm3+, Lu3+ codoping. In the latter case, first, better crystal quality is achieved. Moreover, one avoids the unwanted energy-transfer upconverison related to very high Tm3+ concentration.
The structure and phase purity of Tm:Ca(Gd,Lu)AlO4 were determined by X-ray powder diffraction (XRD), Fig. 1(b). The crystal is tetragonal (space group I4/mmm - D174h, No. 139); the lattice constants are a = 3.6446 Å and c = 12.2157 Å. The XRD pattern is in agreement with that for undoped CaGdAlO4.
The vibronic properties of an a-cut crystal were studied by Raman spectroscopy, Fig. 1(c). The most intense Raman peak is found at 310 cm−1. It is red-shifted with respect to undoped CaGdAlO4 (330 cm−1) indicating a structure modification. The factor group analysis of the D174h unit cell predicts the following irreducible representations (k = 0): Γ = 2A1g + 2Eg + 4A2u + 5Eu + B2u of which the 2A1g and 2Eg modes are Raman-active. The band at 310 cm−1 is assigned as Eg. The highest phonon frequency hνph is 651 cm−1.
3. Optical spectroscopy
Tm:Ca(Gd,Lu)AlO4 crystals are optically uniaxial (the optical axis is parallel to the c-axis). There are two principal light polarizations, E || c (π) and E ⊥ c (σ). All spectroscopic studies were performed using an a-cut crystal at room-temperature (RT, 293 K).
The RT absorption spectra of Tm:Ca(Gd,Lu)AlO4 for π and σ polarizations are shown in Fig. 2(a). The broad absorption in the visible (300-550 nm) is related to the residual absorption of color centers [25]. The sharp band at 310 nm is due to the Gd3+ ions (the 8S7/2 → 6P7/2 transition). Note that the CaLnAlO4 crystals have an indirect bandgap Eg of ~4.2 eV [26] corresponding to the UV absorption edge at ~295 nm. In the spectrum, the bands related to Tm3+ transitions from the ground-state (3H6) to the excited ones (from 3F4 up to 1D2) are clearly resolved. The details about the 3H6 → 3H4 transition suitable for diode-pumping are shown in Fig. 2(b). The maximum absorption cross-section σabs = αabs/NTm of 1.66 × 10−20 cm2 at 792.3 nm corresponds to π-polarization and the full width at half maximum (FWHM) of the corresponding absorption peak is 17.5 nm. This is broader than for the 1.8 at.% Tm:CaGdAlO4 crystal studied for comparison (16.3 nm). For σ-polarization, σabs is 0.67 × 10−20 cm2 at 798.1 nm.
Fig. 2 Absorption of Tm:Ca(Gd,Lu)AlO4: (a) absorption spectrum of the annealed crystal (α: absorption coefficient), inset – photograph of the sample before (left) and after (right) annealing; absorption cross-sections, σabs, for the 3H6 → 3H4 Tm3+ transition. In (b), the spectra for 1.8 at.% Tm:CaGdAlO4 are also shown. The light polarization is denoted by π and σ.
The absorption spectra were analyzed within the standard Judd-Ofelt (J-O) theory [27,28], Table 1. The squared reduced matrix elements for Tm3+ ions U(k) were taken from [29]. The J-O (intensity) parameters are Ω2 = 2.933, Ω4 = 2.787 and Ω6 = 1.413 [10−20 cm2].
Table 1. Experimental and Calculated Absorption Oscillator Strengths for Tm:Ca(Gd,Lu)AlO4
The polarized luminescence spectra of the Tm:Ca(Gd,Lu)AlO4 crystal (the 3F4 → 3H6 Tm3+ transition) are shown in Fig. 3(a). The emission spectra are smooth and broad. The highest emission intensity corresponds to π-polarization. The FWHM of the emission spectrum is 187 (π) and 207 (σ) nm which is broader than for Tm:CaGdAlO4, 168 (π) and 186 (σ) nm. Thus, the introduction of Lu3+ ions leads to an additional spectral broadening due to the compositional disorder.
Fig. 3 Luminescence of Tm3+ in Ca(Gd,Lu)AlO4: (a) luminescence spectra for the 3F4 → 3H6 transition, λexc = 802 nm; (b) luminescence decay curve, circles – experimental data, line – single-exponential fit. In (a), the spectra for Tm:CaGdAlO4 are given for comparison. The light polarization is denoted by π and σ.
The luminescence decay of Tm3+ ions from the 3F4 multiplet was studied using the pinhole method [30] to diminish the effect of reabsorption on the measured lifetime. The decay curve corresponding to the smallest pinhole (diameter: ~100 μm) is shown in Fig. 3(b). It is clearly single-exponential; the decay time τlum is 3.2 ms.
The probabilities of spontaneous radiative transitions AΣ(JJ'), the luminescence branching ratios B(JJ') and the radiative lifetimes τrad were calculated using the J-O theory, Table 2. For the upper laser level (3F4), τrad = 2.46 ms. This value is shorter than the measured lifetime. The observed difference is attributed to the hypersensitivity of the 3H6 → 3F4 transition of Tm3+ ions [31] affecting the precision of the J-O calculations. For the 3H4 state, τrad = 0.44 ms.
Table 2. Calculated Emission Probabilities for Tm3+ in Tm:Ca(Gd,Lu)AlO4
The stimulated-emission (SE) cross-sections, σSE, for the 3F4 → 3H6 transition and π and σ polarizations were calculated using a combination of the modified reciprocity method (mRM) [32] and the Füchtbauer–Ladenburg (F-L) equation [33], see Fig. 4(a). The refractive indices were taken from [34], no = 1.904 and ne = 1.927 at ~1.8 μm. The maximum σSE = 0.91 × 10−20 cm2 at 1813 nm for σ-polarization. As Tm3+ ions represent a quasi-three-level laser scheme, the laser emission is expected at longer wavelengths. For a local peak in the SE cross-section spectra at 1946 nm for σ-polarization, σSE is 0.50 × 10−20 cm2.
Fig. 4 Transition cross-sections of Tm3+ in Ca(Gd,Lu)AlO4 crystal: (a) absorption, σabs, and stimulated-emission, σSE, cross-sections for the 3F4 ↔ 3H6 transition (π and σ light polarization); (b) gain cross-sections, σgain = βσSE – (1 – β)σabs, for various inversion ratios β = N2(3F4)/NTm (σ light polarization).
The gain cross-sections, σgain = βσSE – (1 – β)σabs, where β = N2(3F4)/NTm is the inversion ratio are plotted in Fig. 4(b) for σ-polarization. For small inversion ratios β < 0.10, the laser emission is expected at ~1950 nm. For β = 0.15, the gain bandwidth is 145 nm.
4. Laser operation
First, we studied CW operation of Tm:Ca(Gd,Lu)AlO4 crystal under diode-pumping without any wavelength-selective element in the laser cavity. The active element was a-cut (thickness: t = 3.4 mm, aperture: 3.0(c) × 3.0 mm2). It was polished to laser quality, wrapped with In foil and mounted in a water-cooled (12 °C) Cu-holder providing cooling from all 4 lateral sides. The compact (microchip-type) laser cavity consisted of a plane pump mirror coated for HT at 780-1000 nm and for HR at 1800-2100 nm, and a flat output coupler (OC) having a transmission TOC of 1.5%...20% at the laser wavelength. The pump source was a fiber-coupled (fiber core diameter: 200 μm, N.A. = 0.22) CW AlGaAs laser diode emitting up to 14 W of unpolarized light at 802 nm (FWHM = 3 nm). Its output was collimated and focused into the active element through the PM by a lens assembly (1:1 imaging ratio, focal length f = 30 mm). The radius of the pump beam waist wP and its Rayleigh length 2zR were 100 μm and 1.73 mm (M2 ~86), respectively. The pump absorption (double-pass) under lasing conditions was 62%.
The input-output dependences and typical laser emission spectra are shown in Figs. 5(b) and 5(c). The laser output was linearly polarized (σ); the polarization was naturally selected by the gain anisotropy. The maximum output power reached 1.82 W at 1945 nm with a slope efficiency η of 28.4% (with respect to the absorbed pump power Pabs) for TOC = 9%. The laser threshold was at Pabs = 0.80 W. With the increase of TOC, the emission wavelength blue-shifted from 1955 nm to 1931 nm, Fig. 5(c), in agreement with the gain spectra, Fig. 4(b). We have also studied laser performance of a similar c-cut crystal (t = 3.4 mm). The laser generated 1.22 W at 1944 nm with higher η = 34.4% (for TOC = 9%). The laser emission was unpolarized. The laser operation in the plano-plano cavity indicates positive thermal lens for both a-cut and c-cut crystals.
Fig. 5 (a-c) CW diode-pumped Tm:Ca(Gd,Lu)AlO4 laser: (a) laser set-up: LD – laser diode, PM – pump mirror, OC – output coupler; (a) input-output dependences, η – slope efficiency, inset – measured spatial profile of the laser beam for TOC = 9% and maximum Pabs; (b) laser emission spectra measured at the maximum Pabs. The crystal is a-cut and the laser polarization is σ.
An a-cut 1.8 at.% Tm:CaGdAlO4 crystal (t = 3.0 mm) was inserted in the same laser set-up for comparison yielding 1.16 W at 1944 nm with η = 31.8%. The laser emission was σ-polarized. Thus, the laser performance of Lu3+-codoped crystals is similar to that of Tm:CaGdAlO4.
The potential of Tm:Ca(Gd,Lu)AlO4 for ML laser operation was revealed by studying its wavelength tuning performance in an X-shaped laser cavity, Fig. 6(a). The active element (a-cut, t = 6.0 mm, aperture: 3.0 × 3.0 mm2) was antireflection (AR) coated for pump and laser wavelength. It was inserted in the cavity at normal incidence between two folding curved mirrors M1 and M2 (radius of curvature, RoC = 100 mm). M3 was a HR mirror and the transmission of the flat broadband OC was 0.5%...10%. The pump source was a CW Ti:Sapphire laser delivering >3 W at 798 nm (σ-polarization in the crystal). The pump was focused by a spherical lens (f = 70 mm) to a spot with wP = 30 μm. The pump absorption under lasing conditions measured near the laser threshold was 80.5% (single-pass).
Fig. 6 (a,b) Wavelength tuning of the Tm:Ca(Gd,Lu)AlO4 laser (a-cut crystal): (a) laser set-up: M1 and M2 – folding mirrors, M3 – HR mirror, OC – output coupler, L – lens; (b) wavelength tuning curves (Pabs = 2.6 W, laser polarization: σ).
Without any wavelength-selective element in the cavity, the free-running laser generated a maximum output power of 1.18 W at 1942 nm with a maximum η of 51.4% for σ-polarization (a-cut crystal, TOC = 10%). The laser threshold was at Pabs = 0.13 W. This high slope efficiency value indicates the effect of cross-relaxation for adjacent Tm3+ ions, 3H4 + 3H6 → 3F4 + 3F4, increasing the pump quantum efficiency above unity. Indeed, the Stokes efficiency for this laser ηSt is 40.8%.
For the wavelength-tuning experiment, a Lyot filter was inserted in the laser cavity near the OC, Fig. 6(a). It was a 3.2-mm-thick quartz plate with the optical axis at 60° to the surface. Using the a-cut Tm:Ca(Gd,Lu)AlO4 crystal, the laser wavelength was continuously tuned from 1836 to 2083 nm (tuning range Δλ = 247 nm, TOC = 0.5%) and from 1827 to 2071 nm (Δλ = 244 nm, TOC = 1.5%), Fig. 6(b). A similar experiment was also performed for the c-cut crystal resulting in a tuning range from 1814 to 2072 nm (Δλ = 258 nm, TOC = 0.5%). The laser polarization was σ in all cases.
For achieving fs pulses in ML thulium-doped lasers, it is desirable that the Tm3+ emission extends above 2 μm avoiding unwanted atmosphere water vapor absorption. This condition is satisfied in the Tm:Ca(Gd,Lu)AlO4 laser. The introduction of Lu3+ ions provided additional broadening of the emission spectra, Fig. 3(a). This effect is confirmed in the tuning curves. Indeed, for the Tm:CaGdAlO4 (a-cut), laser spectral tuning was limited to 2065 nm.
5. Conclusion
To conclude, we report on the growth, structural and spectroscopic characterization, and first CW and wavelength-tunable laser operation of a novel “mixed” disordered calcium aluminate crystal, Tm:Ca(Gd,Lu)AlO4. The introduction of Lu3+ preserves the tetragonal structure and induces additional spectral broadening for the Tm3+ absorption and emission bands with respect to Tm:CaGdAlO4. As a consequence, it is attractive for fs ML lasers at ~2 μm. Further work will focus on the codoping of these crystals with Tm3+ and Ho3+ ions, to shift the emission range further beyond 2 μm (due to the Ho ions), where water vapor absorption has a minor effect on broadband laser generation.
Funding
Spanish Government (MAT2016-75716-C2-1-R (AEI/FEDER,UE) and TEC 2014-55948-R); Generalitat de Catalunya (2017SGR755, 2016FI_B00844, 2017FI_B100158, and 2018 FI_B2 00123); Foundation of President of China Academy of Engineering Physics (YZJJLX2018005); the Government of the Russian Federation (074-U01); and ICREA (2010ICREA-02).
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