Smooth and flat photoluminescence spectra of Nd<sup>3&#x002B;</sup> active ions in tri-doped CaF<sub>2</sub> single crystals

Xiaohui Li; Xiaohui Li; Xiaohui Li; Qianqian Hao; Dapeng Jiang; Dapeng Jiang; Dapeng Jiang; Qinghui Wu; Qinghui Wu; Zhen Zhang; Zhen Zhang; Zhen Zhang; Zhonghan Zhang; Zhonghan Zhang; Jie Liu; Liangbi Su; Liangbi Su; Liangbi Su; Liangbi Su

doi:10.1364/OME.383694

1. Introduction

Neodymium-doped fluorite-type crystals have attracted much attention as laser materials over the past few years. Alkaline-Earth fluoride crystals have low phonon energy which can obtain higher quantum efficiency than oxide crystal, and also exhibit small nonlinear refractive index which can weaken the nonlinear effect under intense pumping laser. Moreover, Nd-doped fluoride crystals specified for its broad emission bandwidth like glasses while remains a relatively high thermal conductivity [1] like YAG crystal. All the merits make Nd-doped fluoride crystals especially suitable for working as high-energy pulsed laser amplifiers for generating high-power ultrafast lasers. However, the clustering of Nd³⁺ ions in fluorite-type crystals have limited its emission quantum efficiency at 1.06µm for a long time because of the existence of the energy transfer between the Nd³⁺ ions in the Nd³⁺-Nd³⁺ clusters [2]. To break these clusters, co-doping Nd³⁺:CaF₂ crystal with regulating ions such as Y³⁺, La³⁺, Gd³⁺, Lu³⁺, Sc³⁺ have been studied [3–7], indicating these non-optically active ions have the potential to form the Nd³⁺-R³⁺ clusters around the Nd³⁺ ions, thus isolating Nd³⁺ ions from one another. The co-doping with regulating ions would improve the luminescence efficiency and laser property of Nd³⁺:CaF₂ crystal to a large extend. For example, Su et al. obtained the CW laser operation in Nd³⁺,Y³⁺:CaF₂ crystal for the first time [8]. Doualan et al. demonstrated the improvement of absorption, emission and laser properties of Nd³⁺:CaF₂ crystal by co-doping with Lu³⁺ ions [9]. Recently Wang et al. reported the amelioration of the spectral parameters and LD-pumped CW laser operation in Nd³⁺,La³⁺:CaF₂ crystal [10] and Nd³⁺,Gd³⁺:CaF₂ crystal [11].

The co-doping with regulating ions make Nd³⁺,R³⁺:CaF₂ (R = Y, La, Gd, Lu) crystals extremely suitable to generate high-power and ultra-short pulses. Q-switched laser operation have been obtained for Nd³⁺,Y³⁺:CaF₂ crystal [12], Nd,Lu³⁺:CaF₂ crystal [13] and Nd,Gd³⁺:CaF₂ crystal [14]. Furthermore, Nd³⁺,R³⁺:CaF₂ crystals also have potential to generating femtosecond mode-locked laser. Using Nd³⁺,Y³⁺:CaF₂ crystal as gain medium, Qin et al. achieved 103fs mode-locked pulses with an average output power of 89 mW [15], and Zhu et al. reported the 264fs mode-locked laser operation with an average output power of 180 mW [16]. Wang et al. obtained femtosecond passively mode-locked laser with pulse duration of 633fs and an average output power of 200 mW on Nd³⁺,La³⁺:CaF₂ crystal [17]. Soulard et al. demonstrated the femtosecond and dual-wavelength mode-locked operation with pulse duration of 435fs on Nd³⁺,Lu³⁺:CaF₂ crystal [18].

It is obvious that co-doping different non-optically active ions will improve the spectra properties and laser performance of Nd³⁺:CaF₂ crystal to varying degree because of the formation of different active sites. However, the generation of sub-100fs pulses need flatter and broader emission spectra, so the spectra of Nd³⁺:CaF₂ crystals need to be further tuned. Up to now only few works have done to co-doping several types of regulating ions into Nd³⁺:CaF₂ crystal, which have potential to obtain totally different absorption and emission spectra. In this paper, Nd³⁺ and two types of non-optically active ions were added into CaF₂ to grew tri-doped Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ (R₁, R₂= Y, La, Gd, Lu) crystals, aiming to produce a “combination effect” and thus optimize the spectral properties. Additionally, the preliminary CW laser experiment of Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal were very encouraging and exhibited a broad tuning range and quite high slope efficiency.

2. Experiments

Co-doped 0.5%Nd,5%R:CaF₂ crystals and tri-doped 0.5%Nd,2.5%R₁,2.5%R₂:CaF₂ crystals were grown using temperature gradient technique (TGT). Raw materials consist of CaF₂ (99.99%), NdF₃ (99.99%), YF₃ (99.99%), LaF₃ (99.99%), GdF₃ (99.99%), and LuF₃ (99.99%), powders, meanwhile PdF₂ (99.99%) was also added as oxygen scavenger. The TGT furnace was kept at high vacuum of about 10⁻³ Pa in the entire growth process. The samples used for spectral measurements was cut into slices with a thickness of 2mm and then polished. The actual concentrations were measured by ICP-OES (Agilent 725), as shown in Table 1.

Table 1. Actual concentrations for Nd³⁺:CaF₂ samples doped with different regulating ions

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Room temperature absorption spectra were tested in a UV/VIS/NIR spectrophotometer (PerkinElmer, Lambda 750). Fluorescence spectra were measured using a time-resolved fluorimeter (FLS980, Edinburgh Instruments, UK) pumped at 808nm by a laser diode, and the ﬂuorescence decay curves were recorded using a TDS 3052 oscilloscope pumped at 796nm by Xenon flashlamp. The fluorescence quantum efficiencies were also measured by FLS980 fluorimeter where a integrating sphere was used as sample chamber.

3. Results and discussions

3.1 Spectroscopic properties of co-doped CaF₂ crystals

Room temperature absorption and emission spectra were measured for the co-doped 0.5%Nd,5%R:CaF₂ (R = Y, La, Gd, Lu) crystals, as shown in Fig. 1. The spectra were all normalized for comparison. The absorption band around 800 nm shown in Fig. 1(a) which consists of two absorption peaks corresponds to the ⁴I_9/2→⁴F_5/2+²H_9/2 Nd³⁺ absorption transition. The absorption spectra are quite different co-doped Nd³⁺:CaF₂ crystals with different regulating ions. When choosing Y³⁺ or Lu³⁺ as regulating ions, the Nd³⁺,R³⁺:CaF₂ crystals have weaker absorption peaks around 790 nm and stronger peaks around 796 nm, whereas when choosing La³⁺ or Gd³⁺ as regulating ions, the absorption peaks around 790 nm are stronger than these around 796 nm.

Fig. 1. Comparison of (a) absorption and (b) emission spectra for 0.5%Nd,5%R:CaF₂ samples (R = Y, La, Gd, Lu).

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The room temperature emission spectra corresponding to the ⁴F_3/2→⁴I_11/2 emission transition of Nd³⁺ ions around 1060 nm of co-doped 0.5%Nd,5%R:CaF₂ crystals are shown in Fig. 1(b). The emission spectra around 1060 nm are quite similar when choosing La³⁺ or Gd³⁺ as regulating ions. The former has peaks at 1051 nm, 1064 nm and the latter has peaks at 1049 nm, 1064 nm, meanwhile both spectra have peak valleys at 1055 nm. On the other hand, the emission spectra of Y³⁺ or Lu³⁺ co-doped Nd³⁺:CaF₂ are quite different from that of La³⁺ or Gd³⁺ co-doped Nd³⁺:CaF₂. The former have two more adjacent emission peaks than the latter and no obvious peak valleys are observed.

The differences of the absorption and emission peaks of Nd³⁺,R³⁺:CaF₂ crystals can be explained as follows. In Nd³⁺:CaF₂ crystal, the substitution of the Ca²⁺ by Nd³⁺ is accompanied by charge compensation, interstitial fluorine ions F_i⁻ is thus formed. Three main optical centers in Nd³⁺:CaF₂ crystal have been identified, namely luminescence quenched M ([Nd³⁺-Nd³⁺]-F_i⁻₂) and N ([Nd³⁺-Nd³⁺]₂-F_i⁻₄) centers of orthorhombic symmetry, and luminescence non-quenched L (Nd³⁺-F_i⁻) centers of tetragonal symmetry [19,20]. When co-doping with regulating ions R³⁺ (R = Y, La, Gd, Lu), quasi-rhombic centers M’ ([Nd³⁺-R³⁺]-F_i⁻₂) and N’ ([Nd³⁺-R³⁺]₂-F_i⁻₄, [Nd³⁺₃-R³⁺]-F_i⁻₄, [Nd³⁺-R³⁺₃]-F_i⁻₄) as well as quasi-tetragonal L’ (F_i^–Nd³⁺-F_i⁻) centers are formed. Because of the difference of the binding energy for Y-F, Lu-F, La-F and Gd-F bonds, the effect of the R³⁺ (R = Y, La, Gd, Lu) to the optical centers are different, therefore the absorption and emission spectra for those Nd³⁺,R³⁺:CaF₂ crystals vary quite a lot. According to the characteristics of the absorption and emission spectra, the regulating ions can be classified into two categories, Y³⁺ and Lu³⁺ ions are in the same class while La³⁺ and Gd³⁺ ions are fall into another class. We expect that co-doping these two categories of regulating ions could produce a “combination effect” in tri-doped Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals, and the spectra of the crystals could thus be regulated.

3.2 Adjustment of the absorption spectra for tri-doped CaF₂ crystals

The comparison of absorption spectra of co-doped 0.5%Nd,5%R:CaF₂ crystals and tri-doped 0.5%Nd,2.5%R₁,2.5%R₂:CaF₂ crystals are shown from Fig. 2(a) to Fig. 2(f). The absorption spectrum of 0.5%Nd,5%Y:CaF₂ is very different from that of 0.5%Nd,5%La:CaF₂ crystal. The former has a weaker absorption peak at 790 nm and a stronger absorption peak at 797 nm, whereas the absorption peak of the latter is much stronger at 790 nm, as shown in Fig. 2(a). However, when co-doped with these two types of regulating ions, the absorption coefficient of these two absorption peaks for 0.5%Nd, 2.5%Y, 2.5%La:CaF₂ crystal are much closer.

Fig. 2. Comparison of absorption spectra for 0.5%Nd,5%R:CaF₂ samples and 0.5%Nd,2.5%R₁,2.5%R₂:CaF₂ samples (R, R₁, R₂=Y, La, Gd, Lu). The insert graphs shows the comparison of α₁/α₂ for tri-doped 0.5%Nd,2.5%R₁,2.5%R₂:CaF₂ crystals and its corresponding co-doped 0.5%Nd,5%R:CaF₂ crystals.

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It seems that the absorption spectra of the Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals are the combination of those of the Nd³⁺,R₁³⁺:CaF₂ crystals and Nd³⁺,R₂³⁺:CaF₂ crystals, which can be obviously observed from Fig. 2(a) to Fig. 2(d). Since the absorption spectrum of 0.5%Nd,5%La:CaF₂ crystal and 0.5%Nd,5%Gd:CaF₂ crystal is very similar to each other, when co-doping La³⁺ and Gd³⁺ together into the Nd:CaF₂ crystal, the absorption spectrum doesn’t change much, as shown in Fig. 2(e). Similar situation is also reflected in Fig. 2(f).

In order to further study the changing of the absorption spectra, α₁ and α₂ are defined as the absorption coefficient of the absorption peaks around 790 nm and 796 nm. According to the literature, the former peak corresponds to the M’ centers and the latter peak corresponds to the L’ centers in the crystal [21,22]. The α₁/α₂ ratios were calculated to compare the absorption spectra of tri-doped Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals with those of co-doped Nd³⁺,R₁³⁺:CaF₂ crystals and Nd³⁺,R₂³⁺:CaF₂ crystals, as shown in the insert graph of Fig. 2(a) to Fig. 2(f). The value of α₁/α₂ of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals are all fall in between that of the Nd³⁺,R₁³⁺:CaF₂ crystals and that of the Nd³⁺,R₂³⁺:CaF₂ crystals, indicating that the absorption spectra can be adjusted controllably by co-doping two different types of regulating ions.

3.3 Flattening of the emission spectra for tri-doped CaF₂ crystals

The room temperature emission spectra around 1060 nm of co-doped 0.5%Nd,5%R:CaF₂ crystals and tri-doped 0.5%Nd,2.5%R₁,2.5%R₂:CaF₂ crystals are shown in Fig. 3. When co-doping two different types of regulating ions, totally different emission spectra for tri-doped Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ (R₁=Y or Lu, R₂=La or Gd) samples were obtained, as shown in Fig. 3(a) to (d). For example, tri-doped Nd³⁺,Y³⁺,La³⁺:CaF₂ crystal possess two adjacent emission peaks at 1053 nm and 1062 nm, similar to that of the Nd³⁺,Y³⁺:CaF₂ crystal, however, as shown in Fig. 3(a), the emission spectrum of the former is much flatter and broader than the latter, which can be attributed to the introducing of another regulating ions La³⁺. Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal exhibits a pretty broad spectrum as Nd³⁺,Gd³⁺:CaF₂ crystal, but the peak of the former is much more smoothed, as illustrated in Fig. 3(b). The peak valley of the emission spectrum for Nd³⁺,Gd³⁺:CaF₂ is compensated, which is due to the introducing of the Y³⁺ ions. The changing of the emission spectra can ascribe to the formation of R₁³⁺-Nd³⁺ (R₁=Y or Lu) and R₂³⁺-Nd³⁺ (R₂=La or Gd) clusters in the crystal. These two different optical centers coexistence and work together in the crystal, generating quite different emission spectra. However, when co-doping same types of regulating ions, the emission spectra of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals didn’t change much compared to the single regulating ion doped crystals, as shown in Fig. 3(e) and Fig. 3(f), just as we conjectured.

Fig. 3. Comparison of emission spectra for 0.5%Nd,5%R:CaF₂ samples and 0.5%Nd,2.5%R₁,2.5%R₂:CaF₂ samples (R, R₁, R₂=Y, La, Gd, Lu).

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Table 2 shows the emission bandwidths of co-doped Nd³⁺,R³⁺:CaF₂ crystals and tri-doped Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals. The samples all exhibit emission bandwidth comparable to Nd:glasses, and the FWHM of emission spectra for different samples have some difference. In addition, when choosing Y³⁺ and Gd³⁺ as regulating ions, the emission bandwidth of Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal is obviously larger than that of Nd³⁺,Y³⁺:CaF₂ crystal and Nd³⁺,Gd³⁺:CaF₂ crystal, which means that the tri-doped CaF₂ crystals have potential to obtain broader emission spectrum.

Table 2. Comparison of emission bandwidths for Nd³⁺:CaF₂ samples doping with different regulating ions

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In order to validate the “combination effect” mentioned previously, the emission spectra of tri-doped Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals were reconstructed based on the emission spectra of Nd³⁺,R₁³⁺:CaF₂ and Nd³⁺,R₂³⁺:CaF₂. The red dash line in Fig. 4 are the reconstructed emission spectra of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals, which are the weighted linear combination of Nd³⁺,R₁³⁺:CaF₂ spectra and Nd³⁺,R₂³⁺:CaF₂ spectra. The reconstruction processes were based on the following equation:

(1)$${I_{1,2}} = {k_{1}}{C_{s1}}{I_{1}} + {k_{2}}{C_{s2}}{I_{2}}$$

Where I₁ and I₂ are the normalized emission intensity, C_s1 and C_s2 are the actual doping concentration, k₁ and k₂ are the weighting coefficients of Nd³⁺,R₁³⁺:CaF₂ and Nd³⁺,R₂³⁺:CaF₂ crystals. The weighting coefficients for Y³⁺, Lu³⁺, La³⁺, Gd³⁺ co-doped CaF₂ crystals were 1.0, 2.0, 0.9, 1.1, respectively. The reconstructed spectra agree well with the experimental spectra, as can be seen from Fig. 4(a) to Fig. 4(d). The coincidence of the reconstructed spectra and experimental spectra further confirms the “combination effect” of spectra mentioned previously, and it opens a new way for optimizing the composition of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals as needed. We believe that the emission spectra could be further tuned to be smoother and flatter by adjusting the proportion of the different types of regulating ions, which would be extremely suitable for the mode-locking ultrafast laser output.

Fig. 4. Comparison of reconstructed and experimental emission spectra for Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystal (R₁=Y or Lu, R₂=La or Gd).

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3.4 Fluorescence lifetime

Emission decay curves of the ⁴F_3/2 emitting energy level were recorded at 1060 nm when exciting at 796 nm, as represented in Fig. 5. Because of the existence of multiple optical centers in the crystal, not every decay curves can be well fitted using single-exponential model, so the “average lifetime” τ_em were calculated according to the following equation [23]:

(2)$${\tau _{em}} = \frac{{\int\limits_{t = 0}^\infty {I(t)} tdt}}{{\int\limits_{t = 0}^\infty {I(t)} dt}}$$

The comparison of calculated fluorescence lifetime for different regulating ions doped samples is shown in Table 3. The fluorescence lifetime of Nd³⁺,R₁³⁺:CaF₂ (R₁=Y or Lu) crystals are quite different from that of Nd³⁺,R₂³⁺:CaF₂ (R₂=La or Gd) crystals. The fluorescence lifetime of the single regulating ions co-doped samples is 479 µs for Nd³⁺,La³⁺:CaF₂ and 528 µs for Nd³⁺,Gd³⁺:CaF₂, much higher than the lifetime of Nd³⁺,Y³⁺:CaF₂ and Nd³⁺,Lu³⁺:CaF₂. When co-doped with these two different types of regulating ions, the fluorescence lifetime of the Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ (R₁=Y or Lu, R₂=La or Gd) samples is between that of Nd³⁺,R₁³⁺:CaF₂ samples and Nd³⁺,R₂³⁺:CaF₂ samples, which further confirms the “combination effect” mentioned before.

Fig. 5. Fluorescence decay curves of (a) co-doped 0.5%Nd,5%R:CaF₂ samples and (b) tri-doped 0.5%Nd,2.5%R₁,2.5%R₂:CaF₂ samples (R, R₁, R₂=Y, La, Gd, Lu).

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Table 3. Comparison of fluorescence lifetimes, quantum efficiency and peak emission cross sections for Nd³⁺:CaF₂ samples doping with different regulating ions

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The peak emission cross sections were also calculated based on the Fuchtbauer-Ladenburg expression:

(3)$${\sigma _{em}} = \frac{{\lambda _{peak}^4}}{{8\pi c{n^2}}} \cdot \frac{{{\beta _{2}}}}{{{\tau _{rad}} \cdot \Delta {\lambda _{em}}}}$$

Where λ_peak is the peak emission line of ⁴F_3/2→⁴I_11/2, β₂ and Δλ_em stand for the branching ratio and emission bandwidth of ⁴F_3/2→⁴I_11/2, τ_rad is the radiative emission lifetime which can be obtained by measuring the fluorescence quantum efficiency η:

(4)$$\eta = {\tau _{em}}/{\tau _{rad}}$$

The fluorescence quantum efficiency and the calculated peak emission cross sections are shown in the Table 3. The peak emission cross sections of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ (R₁=Y or Lu, R₂=La or Gd) samples are lower than that of Nd³⁺,R₁³⁺:CaF₂ samples but higher than that of Nd³⁺,R₂³⁺:CaF₂ samples, which also exhibit the “combination effect”.

3.5 Laser properties of tri-doped CaF₂ crystals

The CW laser experiment was carried out with the setup shown in Fig. 6 for Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal and Nd³⁺,Lu³⁺,Gd³⁺:CaF₂ crystal. The uncoated crystal with the dimension of 3×3×5 mm³ was pumped by a fiber-coupled diode laser at 790 nm with a core diameter of 100 µm. The laser beam was collimated into the crystal using a 1:2 coupling optics system. The input mirror M1 was a plane mirror with high-transmission coating at 790 nm, high-reflection coating at 1064 nm and anti-reflection coating at 808 nm. The transmission of the output concave mirror M2 with 100 mm radius of curvature was 2% at 1020-1090 nm. To effectively dissipate heat, the crystal was wrapped in indium foil and mounted in a 12°C heat sink. The slope efficiency η of Nd³⁺,Lu³⁺,Gd³⁺:CaF₂ crystal was 42.0% and the maximum output power was 1.3W. The slope efficiency of Nd³⁺,Y³⁺,Gd³⁺:CaF₂ laser was 49.4%, higher than that of Nd³⁺,Y³⁺:CaF₂ crystal laser [24] and Nd³⁺,Gd³⁺:CaF₂ crystal laser [11]. All of these laser experiments were performed under almost identical experimental conditions, using the same cavity design. The maximum output power of Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal was as high as 2.1W when the absorbed pumped power was 4.56W, which can attribute to the quite high quality of the crystal. The preliminary CW laser experiment was very encouraging and can provide valuable reference for the ultrafast laser output.

Fig. 6. Output power versus absorbed pump power for CW laser operation for 0.5%Nd,2.5%Y,2.5%Gd:CaF₂ crystal and 0.5%Nd,2.5%Lu,2.5%Gd:CaF₂ crystal.

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A V-type resonator was also employed to study the wavelength tunable performance of the Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal, as shown in Fig. 7. The input mirror M1 was a plane mirror with high-reflection coating at 1030-1080 nm. The concave mirror M2 with 200 mm radius of curvature was high-reflection coated at 1020-1090 nm, and the transmittance of the output mirror M3 was 2% at 1020-1090 nm. By inserting a birefringent filter (BF) into the cavity at Brewster’s angle, the CW tuning range from 1043 to 1077 nm was obtained, as shown in Fig. 8. The broad and smooth tuning range further prove that the tri-doped CaF₂ crystals have the potential to generate sub-100 fs pulses.

Fig. 7. Schematic of the CW tuning setup.

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Fig. 8. Wavelength tuning curve of 0.5%Nd,2.5%Y,2.5%Gd:CaF₂ crystal based on a quartz BF.

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4. Conclusions

The experimental results in this work indicate that the absorption and emission spectra of Nd³⁺:CaF₂ crystal can be tuned by introducing two kinds of non-optically active regulating ions. The regulating ions are classified into two classes, the emission spectra become more smoothed because of the “combination effect” of these two different types of regulating ions. The reconstructed emission spectra of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals agree well with the experimental spectrum, which prove that the spectra of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals is the weighted linear combination of that of Nd³⁺,R₁³⁺:CaF₂ crystals and Nd³⁺,R₂³⁺:CaF₂ crystals. The fluorescence decay properties of Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ samples further confirm the “combination effect” of the regulating ions. The preliminary CW laser operation was very encouraging. The slope efficiency and maximum output power of Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal reached up to 49.4% and 2.1W. A continuous tuning range of 34 nm (1043-1077 nm) was obtained for Nd³⁺,Y³⁺,Gd³⁺:CaF₂ crystal, indicates that the tri-doped CaF₂ crystals have the potential to generate sub-100 fs pulses. We believe that the emission spectra and bandwidth of the Nd³⁺,R₁³⁺,R₂³⁺:CaF₂ crystals could be further tuned by adjusting the ratios of regulating ions, which indicate that the tri-doped CaF₂ crystals are promising materials for generating sub-100 fs mode-locked ultrafast laser.

Funding

National Natural Science Foundation of China (61635012, U183010005); Shanghai Science and Technology Commission (18511109700); Shanghai Science and Technology Commission (18520744300).

Acknowledgments

We thank Lili Zheng, Shanghai Institute of ceramics, and Shiyu Sun, Shanghai Institute of Optics and Fine Mechanics in spectra measurements.

Disclosures

The authors declare no conflicts of interest.

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Sample	Real concentration (at.%)
Sample	Nd	Y	Lu	La	Gd
0.5%Nd,5%Y:CaF₂	0.49	4.67	–	–	–
0.5%Nd,5%Lu:CaF₂	0.50	–	4.87	–	–
0.5%Nd,5%La:CaF₂	0.69	–	–	7.61	–
0.5%Nd,5%Gd:CaF₂	0.46	–	–	–	5.42
0.5%Nd,2.5%Y,2.5%La:CaF₂	0.43	2.45	–	1.77	–
0.5%Nd,2.5%Y,2.5%Gd:CaF₂	0.42	2.25	–	–	2.23
0.5%Nd,2.5%Lu,2.5%La:CaF₂	0.52	–	2.23	2.08	–
0.5%Nd,2.5%Lu,2.5%Gd:CaF₂	0.45	–	2.40	–	2.44
0.5%Nd,2.5%Y,2.5%Lu:CaF₂	0.45	2.48	2.52	–	–
0.5%Nd,2.5%La,2.5%Gd:CaF₂	0.47	–	–	2.06	2.58

Regulating ions	Y	Lu	La	Gd	Y,Lu	La,Gd	Y,La	Y,Gd	Lu,La	Lu,Gd
τ_em (µs)	418	406	479	528	414	507	436	479	411	465
η(%)	79.9	70.2	63.8	77.9	75.2	75.7	71.7	80.1	74.8	86.5
σ_em(10⁻²⁰cm²)	3.12	3.13	1.82	2.04	3.10	1.95	2.18	2.13	2.59	2.52

Sample	Real concentration (at.%)
Sample	Nd	Y	Lu	La	Gd
0.5%Nd,5%Y:CaF₂	0.49	4.67	–	–	–
0.5%Nd,5%Lu:CaF₂	0.50	–	4.87	–	–
0.5%Nd,5%La:CaF₂	0.69	–	–	7.61	–
0.5%Nd,5%Gd:CaF₂	0.46	–	–	–	5.42
0.5%Nd,2.5%Y,2.5%La:CaF₂	0.43	2.45	–	1.77	–
0.5%Nd,2.5%Y,2.5%Gd:CaF₂	0.42	2.25	–	–	2.23
0.5%Nd,2.5%Lu,2.5%La:CaF₂	0.52	–	2.23	2.08	–
0.5%Nd,2.5%Lu,2.5%Gd:CaF₂	0.45	–	2.40	–	2.44
0.5%Nd,2.5%Y,2.5%Lu:CaF₂	0.45	2.48	2.52	–	–
0.5%Nd,2.5%La,2.5%Gd:CaF₂	0.47	–	–	2.06	2.58

Regulating ions	Y	Lu	La	Gd	Y,Lu	La,Gd	Y,La	Y,Gd	Lu,La	Lu,Gd
τ_em (µs)	418	406	479	528	414	507	436	479	411	465
η(%)	79.9	70.2	63.8	77.9	75.2	75.7	71.7	80.1	74.8	86.5
σ_em(10⁻²⁰cm²)	3.12	3.13	1.82	2.04	3.10	1.95	2.18	2.13	2.59	2.52

Smooth and flat photoluminescence spectra of Nd³⁺ active ions in tri-doped CaF₂ single crystals

Abstract

1. Introduction

2. Experiments

3. Results and discussions

3.1 Spectroscopic properties of co-doped CaF₂ crystals

3.2 Adjustment of the absorption spectra for tri-doped CaF₂ crystals

3.3 Flattening of the emission spectra for tri-doped CaF₂ crystals

3.4 Fluorescence lifetime

3.5 Laser properties of tri-doped CaF₂ crystals

4. Conclusions

Funding

Acknowledgments

Disclosures

References

Cited By

Figures (8)

Tables (3)

Equations (4)

Optical Materials Express

Abstract

1. Introduction

2. Experiments

3. Results and discussions

3.1 Spectroscopic properties of co-doped CaF2 crystals

3.2 Adjustment of the absorption spectra for tri-doped CaF2 crystals

3.3 Flattening of the emission spectra for tri-doped CaF2 crystals

3.4 Fluorescence lifetime

3.5 Laser properties of tri-doped CaF2 crystals

4. Conclusions

Funding

Acknowledgments

Disclosures

References

Cited By

Figures (8)

Tables (3)

Equations (4)

Optical Materials Express

3.1 Spectroscopic properties of co-doped CaF₂ crystals

3.2 Adjustment of the absorption spectra for tri-doped CaF₂ crystals

3.3 Flattening of the emission spectra for tri-doped CaF₂ crystals

3.5 Laser properties of tri-doped CaF₂ crystals