1. Introduction

Solid-state Yb-lasers presently receive much attention for high power applications in diode pumped thin disk [1], waveguide [2], and fiber [3] design configurations. The main reasons for this interest are: i) The availability of affordable, robust, and high power diode lasers emitting at 980 nm where Yb³⁺has rather high optical absorption. ii) The small quantum defect characteristic for the absorption and emission processes in Yb³⁺, which minimizes the heat transfer to the host. iii) The existence of only two ^2S+1L_J multiplets (²F_5/2 and ²F_7/2) for the Yb³⁺energy level scheme, which excludes optical losses by up-conversion, and the long lifetime of the excited ²F_5/2 state (typically ~1 ms), ensuring efficient energy storage. Moreover, due to the large number of electrons in the 4f¹³ configuration of Yb³⁺, these are less shielded from the host interaction than in other lanthanides, for instance in Nd³⁺, therefore Yb³⁺exhibits a significant broadening of its optical transitions, which makes it suitable for tunable lasers and for ultrashort pulse operation. This broadening can be further enhanced by the presence of local disorder for Yb³⁺ in a given host. The main drawback for room temperature laser operation of Yb³⁺remains the relatively large population of the terminal level of its quasi 3-level laser system. This promotes the search for new hosts with large splitting of the ground state multiplet.

Since the early days of solid state laser technology tungstate and molybdate crystals with the general formula DXO₄ (D being a divalent cation and X=W⁶⁺or Mo⁶⁺) have attracted attention as laser hosts due to their inert character. The incorporation of laser active trivalent lanthanide ions was achieved by the full substitution of the divalent D²⁺cation in the above formula by a pair of monovalent M⁺and trivalent T³⁺cations [4]. This led to the so-called double tungstate (DT) and double molybdate (DMo) crystals. The two principal crystalline structures developed for laser applications are the monoclinic C2/c, represented by KTW (T=Y, Gd, and Lu) compounds [5], and the tetragonal I4̄ structure characteristic of crystals such as NaTW (T=Y [6], La [7], Gd [8] and Lu [9]) or LiGdMo [10]. In these formulae, for the sake of simplicity, W and Mo denote (WO₄)₂ and (MoO₄)₂, respectively. The properties of Yb³⁺and other lanthanide lasants in such crystals have been studied extensively in recent years, but further improvements can be still expected by the development of novel DT and DMo crystal compositions. In this work we demonstrate an alternative partial substitution of the divalent cation leading to a large number of new compounds. Moreover, we present for the first time to our knowledge experimental results on Yb-laser operation in one of such compounds, Li_0.75Gd_0.75Ba_0.5(MoO₄)₂, hereafter LiGdBaMo, which is a monoclinic crystal with local disorder associated with the partial substitution of the divalent cation. The quantum defect for the laser operation of Yb³⁺in this host succeeds to be one of the smallest observed in a solid state laser.

2. Crystal growth and structural details

The full substitution of the divalent cation D²⁺mentioned above can be described by:

a) 2 [D²⁺XO₄]→M⁺T³⁺(XO₄)₂

Partial substitution of D can be achieved in several ways keeping constant the total number of cations and the total electric charge of the original D²⁺ions. Two possibilities can be schematized as shown below:

b) 4[D²⁺XO₄]→M⁺T³⁺D₂(XO₄)₄=2[M_0.5T_0.5D(XO₄)₂]

c) 8[D²⁺XO₄]→M₃ ⁺T₃ ³⁺D₂(XO₄)₈=4[M_0.75T_0.75D_0.5(XO₄)₂]

We followed the last approach, (c), in order to grow 10 at % Yb-doped crystals of Li_0.75Gd_0.75Ba_0.5(MoO₄)₂. The Top Seeded Solution Growth (TSSG) slow cooling method was applied using a Li₂Mo₂O₇ flux. The molar solute/flux ratio used was 2.66:97.34. The mixture was melted at 610°C in a Pt crucible and then held for 20 hours at ≈ 50°C above the melting temperature in order to achieve good homogenization. A Pt wire rotating at 12 rpm was used as the seed. For crystallization, the cooling interval was 11 0C and the cooling rate was 0.08°C/h. More details on the growth procedures can be found elsewhere [11].

The Yb:LiGdBaMo crystals obtained are monoclinic with the symmetry of the centrosymmetric space group C2/c. The single crystal X-ray diffraction refinement yielded the composition formula Li_0.715Gd_0.725Yb_0.06Ba_0.5(MoO₄)₂ [11]. The slight Li⁺deficiency is compensated by excess incorporation of trivalent Gd³⁺and Yb³⁺ions. This balance between monovalent and trivalent cations leads to extra positive electric charge, which is likely compensated by a small Mo⁶⁺deficiency, as reported in other DMo compounds [12]. The Yb³⁺content in the crystal is [Yb]=3.76×10²⁰ at/cm³ [11]. In the refined structure Gd³⁺(Yb³⁺) and Li(1)⁺share a single 8f site, while Ba²⁺and Li(2)⁺fully occupy different 4e sites, and Mo1, Mo2 and eight types of O are found in 8f sites. This ion distribution corrects a previous structural determination [13]. Our present results lead to the conclusion that only one crystallographic site with disordered local environment exists for Yb³⁺.

3. Yb³⁺optical spectroscopy and laser results

The disordered crystalline environment of Yb is reflected in a broad zero-line absorption, ²F_7/2(0)→²F_5/2(0’), with a full width at half maximum of FWHM=19 cm^-1. For comparison the FWHM of the same transition in ordered monoclinic KYW is ≈10 cm^-1. The sequence of Yb³⁺energy levels has been established (Ref. 11) as ²F_7/2=0, 223, 353, 460 cm^-1, and ²F_5/2=10248, 10413 and 10634 cm^-1. This gives a ratio of the partition functions of the upper and lower Yb³⁺multiplets close to unity.

The absorption of Yb³⁺is anisotropic with peak values of the absorption cross section σ_ABS equal to 1.92×10^-20 cm² for E//a, 2.58×10^-20 cm² for E//b, and 1.25×10^-20 cm² for E//c, at λ=976.6 nm in all cases. The corresponding peak emission cross sections, σ_EMI, calculated by the reciprocity principle were 1.01×10^-20 cm² for E//a, 2.81×10^-20 cm² for E//b, and 1.30×10^-20 cm² for E//c, at λ=1000 nm in all cases [11]. For the sake of completeness we have included in Fig. 1 the spectral dependence of the absorption and emission cross sections. It is worth to observe that the monoclinic Yb:LiGdBaMo exhibits much stronger anisotropy of the spectroscopic properties than the tetragonal DT or DMo crystals, while still retaining enhanced optical bandwidths.

Continuous wave (cw) laser operation of Yb:LiGdBaMo was studied in the ≈140 cm long astigmatically compensated four mirror (Z-shaped) optical cavity shown in Fig. 2, with single pass longitudinal pumping. We used a c-cut sample with a thickness of 2.4 mm. The uncoated sample was inserted between the two focusing mirrors M2 and M3 (radius of curvature equal to RC=-10 cm), under Brewster angle. Laser experiments were performed for the sample orientations with largest cross sections, i.e., polarization parallel to the a-axis (E//a) or to the b-axis (E//b), see Fig. 1(a)–(b). This was achieved by rotation of the sample while the pump polarization remained in both cases the same as that of the laser. The sample was not actively cooled. The pump source was a Ti:sapphire laser with a maximum power of ~1.5 W at the optimum pump wavelength, λ_P=976.6 nm. The pump beam was focused by a 6.28 cm ARcoated lens (FL). The transmission of the plane output coupler M1 was varied from 1% to 10%.

 

Fig. 1. 300 K polarized absorption (dots) and calculated emission (solid line) cross sections of Yb³⁺in LiGdBaMo laser.

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Fig. 2. Cavity used to study the cw and tunable laser operation of Yb:LiGdBaMo crystal

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The absorption measured at low incident pump power (0.5 W) was roughly 75%, both for E//a and E//b polarizations. Bleaching reduced this value to about 50% at the maximum incident power of 1.5 W applied, but when the laser was aligned, the absorption was enhanced by the recycling effect in the 3-level system of Yb³⁺and the same level of 75% was estimated almost independently of the incident pump power and the output coupler used.

The cw laser operation results obtained at room temperature are summarized in Fig. 3 and in Table 1. The output power is initially linearly proportional to the absorbed power, however, roll-off is observed at higher pump levels; this effect is more significant for the E//a polarization. Since the linear dependence was recovered using a chopper with a duty cycle of 25% for the pump beam, thermal effects should be responsible for this behaviour. The maximum output power obtained in the true cw regime for E//a polarization with an output coupler of T_OC=10 % was 295 mW, Fig 3(a), at an absorbed pump power of 1.06 W. The maximum slope efficiency with respect to the absorbed power, obtained for T_OC=10%, was η=47% (E//a) and η=43% (E//b). Note that although E//a polarization exhibited thermal degradation of the performance, it presented lower thresholds and higher slope efficiencies. The oscillation wavelength λ_L decreased with T_OC, from 1026 nm (T_OC=1%) to 1017–1019 nm (T_OC=10%). This behaviour is in agreement with the predictions of the gain cross section profiles with increasing inversion rate β [11].

Table 1. Slope efficiency η, laser wavelength λ_L, and pump power at laser threshold P_th, of the Yb:LiGdBaMo laser for several T_OC.

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Fig. 3. Output power versus absorbed pump power of the cw Yb:LiGdBaMo laser for E//a-and E//b-polarization and several output coupler transmissions T_OC: 1% (circles), 3% (squares), 5% (triangles) and 10% (diamonds). The linear fits shown give the slope efficiencies obtained for each T_OC value.

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The cw laser tunability has been studied with a two-plate birefringent filter inserted in the vicinity of the output coupler, see Fig. 2. Figure 4 shows the results obtained for the two polarizations considered. Continuous tuning was achieved from 1006 to 1039 nm for the E//apolarization with a FWHM of the tuning curve of 22 nm, and from 1003 to 1023 nm for the E//b-polarization with a FWHM of 9 nm. The shape of the tuning curves reflects the behaviour of the gain cross sections [11] or that of the emission cross sections shown in Fig. 1. The tuning ranges obtained are narrower than those typically observed with the tetragonal DT and DMo Yb-hosts of the NaTW or NaTMo-types, but they are characterized by rather short emission wavelengths, i.e., small quantum defects when pumped at the optimum absorption wavelength, 976.6 nm. Although in present experiments we have used an intracavity Lyot filter, the birefringence of Yb:LiGdBaMo would allow tuning by rotation of the current active laser crystal [14].

 

Fig. 4. Wavelength tunability of the Yb:LiGdBaMo laser for T_OC=3% and the two possible polarizations. The incident pump power is ≈1.43 W.

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As already mentioned, a small quantum defect (equal to (λ_L-λ_P)/λ_P) is a prerequisite for power scaling. Using a 125 µm plate of KYbW water-cooled crystal, a quantum defect of 1.6% was reported at room temperature [15], and even lower quantum defect of 0.6% was observed with a 130 µm plate of 10 at% Yb-doped KYW crystal, using eight pump passes [16]. It is obvious from Fig. 1 that Yb:LiGdBaMo can be pumped at wavelengths longer than the one corresponding to maximum absorption, thus we have explored this opportunity by increasing the pump wavelength to the limits set by the pump focusing optics (in particular, the mirror M2 in Fig. 2). The results obtained with T_OC=5% are shown in Fig. 5. As can be seen, for E//a polarization, the quantum defect achieved is about 2.8%. For E//b polarization, it was possible to obtain laser oscillation for pump wavelengths as long as λ_P=996.8 nm and the emission wavelength of λ_L=1003.8 nm gives then a quantum defect of only 0.7%. This result compares very well with the lowest values previously indicated for Yb-lasers, although no special cooling has been applied in the current case of Yb:LiGdBaMo. The very small quantum defect obtained in the present work with a bulk crystal of Yb:LiGdBaMo can be attributed to the specific anisotropy of the crystal host leading to favourable absorption and emission spectral profiles for polarization E//b, as shown in Fig. 1.

 

Fig. 5. Spectral record of the room temperature pump and emission wavelengths of the Yb:LiGdBaMo laser using a T_OC=5%.

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4. Conclusions

The monoclinic (space group C2/c) Yb:LiGdBaMo crystal, successfully grown in the present work by the TSSG method, has a single 8f site occupied simultaneously by Gd (72.5%), Yb (6%) and Li (21.5%). This leads to locally disordered environment around the Yb³⁺dopant ions, expressed in inhomogeneous broadening of the spectral features. The spectroscopic properties of the crystal are anisotropic with the largest absorption and emission cross sections for E//b-polarization. Room temperature laser emission in the cw regime was obtained both for E//a-and E//b-polarizations using a c-cut sample. The highest output power and slope efficiency obtained with a 10% output coupler and E//a are 295 mW and 47%, respectively, with a threshold of 412 mW (absorbed pump power). Tunability of 33 nm at the zero level is obtained for this polarization using a Lyot filter. However, the other polarization, E//b, has allowed operating the laser at extremely low quantum defect, 0.7%, despite the non-optimized cooling conditions. The Yb:LiGdBaMo crystal is a good candidate for scaling the output power in a thin disk laser configuration. Thus, the partial substitution of the divalent cation in the general host formula DXO₄ leads to new compounds with extraordinary optical properties for Yb³⁺.