Figures
Abstract
In order to explore new more powerful ultrashort pulse laser and tunable laser for diode-pumping, this paper reports the growth and spectral assessment of Yb3+-doped KBaGd(MoO4)3 crystal. An Yb3+:KBaGd(MoO4)3 crystal with dimensions of 50×40×9 mm3 was grown by the TSSG method from the K2Mo2O7 flux. The investigated spectral properties indicated that Yb3+:KBaGd(MoO4)3 crystal exhibits broad absorption and emission bands, except the large emission and gain cross-sections. This feature of the broad absorption and emission bands is not only suitable for the diode pumping, but also for the production of ultrashort pulses and tunability. Therefore, Yb3+:KBaGd(MoO4)3 crystal can be regarded as a candidate for the ultrashort pulse and tunable lasers.
Citation: Yu Y, Huang Y, Zhang L, Lin Z, Wang G (2013) Growth and Spectral Assessment of Yb3+-Doped KBaGd(MoO4)3 Crystal: A Candidate for Ultrashort Pulse and Tunable Lasers. PLoS ONE 8(1): e54450. https://doi.org/10.1371/journal.pone.0054450
Editor: Gerardo Adesso, University of Nottingham, United Kingdom
Received: September 30, 2012; Accepted: December 11, 2012; Published: January 22, 2013
Copyright: © 2013 Yu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work is supported by the National Natural Science Foundation of China (number 61108054), the National Natural Science Foundation of Fujian Province (number 2011J01376) and Scientific Research Foundation for Returned Scholars, Ministry of Education of China, respectively. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
With the development of high power InGaAs diode lasers, the Yb3+-doped laser materials have attracted great interest. The trivalent Yb3+ ion has only two electronic manifold, i.e. the ground state 2F7/2 and the excited state 2F5/2. The simple electronic-level scheme of Yb3+ ion can reduce the excited state absorption, quantum defect and concentration, which is helpful to improve the laser efficiency. In addition, the Yb3+ ion in the crystals exhibit strong and broad absorption and emission bands, which is beneficial to diode pumped ultrashort pulse lasers and tunable lasers. Recently, the laser crystals with disordered structure have been received much attention, because the disordered crystal structure can result in the broad absorption and emission bands of the laser crystals [1]–[5]. The powerful ultrashort pulse lasers have been achieved in some laser crystals, such as, Nd:SrGdGa3O7 [6] and Nd:CLNGG crystals [7]. Thus, when the Yb3+-ion doped into the crystal with disordered structure, can it further improve the spectral properties of Yb3+-doped laser crystal materials.
The KBaRe(MoO4)3 (Re = La-Lu, Y) compounds were first reported by N. M. Kozhevnikova et al [8]. The KBaGd(MoO4)3 crystal is a member of KBaRe(MoO4)3 (Re = La-Lu, Y) family. KBaGd(MoO4)3 crystal belongs to the monoclinic system with space group C2/c and cell unit parameters: a = 17.401 (11) Å, b = 12.226(8) Å, c = 5.324(4) Å, β = 106.19(1)°, V = 1087.73(373) Å3, Z = 4, D = 4.967 g.cm−3 [3]. Since the statistics of the structure have shown that the Ba and K ions in the crystal randomly occupy the same site with the same probability, KBaGd(MoO4)3 crystal has a high disordered structure [3]. This feature of structure could give further rise to the broad absorption and emission band of Yb3+ ion, which is beneficial to realize ultrashort pulse and tunable lasers. Here we report some preliminary results on Yb3+-doped KBaGd(MoO4)3 crystal.
Materials and Methods
1. Crystal Growth
Since KBaGd(MoO4)3 crystal incongruently melts at 1054°C [3], it is only grown by the flux method. The 15 at.% Yb3+-doped KBaGd(MoO4)3 crystal was grown from a flux of K2Mo2O3 by the top solution seeding growth method (TSSG). The chemicals used were K2CO3, BaCO3 and MoO3 with purity 95%, La2O3 and Yb2O3 with purity of 99.99%. The starting materials consist of 17 mol% of solute (KBaGd(MoO4)3) and 83 mol% of solvent (K2Mo2O3). The weighed raw materials were mixed and put into a platinum crucible. Then, the full charged crucible was placed in vertical tubular furnace and slowly heated up to 1050°C, and kept this temperature for 2∼3 days to let the solution melt completely and homogeneously. Then a platinum wire was used as a seed to contact the solution, and the solution was slowly cooled down at a cooling rate of 15°C/day. The small crystals grown on the platinum wire were obtained by spontaneous crystallization. Then, a good small crystal was selected as a seed to grow the larger crystal. After exactly determining the saturation temperature by repeated seeding, the seed contacted the solution at a temperature 5°C above the saturation temperature for 30 min. The temperature was slowly cooled to 975°C to start growth. During the growth period, the crystal was slowly cooled at a cooling rate of 0.8∼1.5°C/day and rotated at a rotating rate of 15∼20 rpm. When the growth ended, the crystals were drawn out of the solution and cooled down to room temperature at a cooling rate of 15°C/h.
An Yb3+:KBaGd(MoO4)3 crystal with dimension of 50×40×9 mm3 was obtained, as shown in Fig. 1(a). The grown crystal was confirmed by the powder X-ray diffraction (XRD) using a CAD4 diffractometer equipped with CuKα radiation (λ = 1.054056Å). The XRD pattern of Yb3+:KBaGd(MoO4)3 crystal can be indexed according to the crystal structure of KBaGd(MoO4)3 crystal, as shown in Fig. 2, which confirmed that the grown crystal belongs to the Yb3+:KBaGd(MoO4)3 crystal The Yb3+ ion concentration in Yb3+:KBaGd(MoO4)3 crystal was measured to be 4.04 at.%, i. e. 1.494×1020 cm−3 by inductively coupled plasma atomic emission spectrometry (ICP-AES).
2. Spectral Properties
Since Yb3+:KBaGd(MoO4)3 crystal belongs to monoclinic, the anisotropy of the crystal should be taken account. For the monoclinic crystal, the Y orthogonal principal crystallo-optic axe is parallel to the b Crystallography axe and the other two are in the ac plane. The orientation of the principal crystallo-optic axes X, Z to the ac axis was determined by using two crossed Glan–Taylor polarizer. Fig. 3 shows the sketch of the relationship between the optical axis and crystallography axis. A sample with dimension of 4.6×2.32×3.44 mm3 was cut from as-grown Yb3+:KBaGd(MoO4)3 crystal along the principal X-, Y- and Z- axes, as shown in Fig. 1(b). The sample was polished well and used for measuring the polarized absorption and fluorescence spectra at room temperature and low temperature. The polarized absorption spectrum was measured using a Perkin-Elmer UV-VIS-NIR spectrometer (Lambda-35) in a range of 900–1100 nm at room temperature. The polarized fluorescence spectra were recorded by a spectrophotometer (FLS920, Edinburgh) equipped with a xenon lamp as the excitation source. In the experiment the E-vector is parallel to the X-, Y- and Z-axis, respectively.
Results and Discussion
1. Absorption Spectra
The polarized absorption spectra of Yb3+:KBaGd(MoO4)3 crystal at room temperature is shown in Fig. 4, which exhibits a broad absorption feature. The absorption band has a very broad full-width at half-maximum (FWHM), which reaches to as higher as 45, 74 and 63 nm for the X-, Y- and Z-polarization at about 979 nm, respectively. In comparison with the other Yb3+-doped crystals (Table 1), the FWHM of Yb3+:KBaGd(MoO4)3 crystal is almost 10∼20 times broad than that of the other Yb3+-doped crystals. Such broad FWHM was further caused by the disordered structure of KBaGd(MoO4)3 crystal [3], except itself broad absorption and emission bands of Yb3+ ion. Since the output wavelength of diode laser is increased at 0.2∼.03 nm/°C with the operating temperature of the laser device, the temperature stability of the diode laser is needed to be crucially controlled. Therefore, such broad absorption band is very suitable for InGaAs diode laser-pumping. The absorption cross-sections were calculated based on the equation σabs = α/N where N is the Yb3+ ion concentration in Yb3+:KBaGd(MoO4)3 crystal, as shown in Fig. 5. The absorption cross-sections were calculated to be 1.22×10−20 cm2, 1.69×10−20 cm2 and 0.91×10−20 cm2 at 976 nm for the X-, Y- and Z-polarization, respectively.
2. Fluorescence Lifetime
The radiative lifetime τrad of Yb3+ ion in Yb3+:KBaGd(MoO4)3 crystal can be calculated Fortunately, it can be calculated from the absorption spectra by the follow formula [9]:(1)where λmean is the mean wavelength of the absorption peak (976 nm), σabs (λ) is the absorption cross-section at wavelength λ, n is reflective index which is 2.0 [3]. Thus, the radiative lifetime is calculated to be about 272.8 µs. The fluorescence lifetime τf of the upper level was measured to be 523.7 µs, as shown in Fig. 6. The fluorescence lifetime is longer than the radiative lifetime, which is caused by re-absorption phenomenon, particularly in the circumstance of the bulk crystal. The re-absorption phenomenon reduces the possibility of photon transition from the 2F5/2 to the 2F7/2, so the fluorescence lifetime is longer than the real fluorescence lifetime of the 2F5/2 level. This calculated value is reliable when the re-absorption possibility is taken account. The re-absorption possibility of Yb3+ ion in Yb3+:KBaGd(MoO4)3 crystal can be examined by the following formula [10]
(2)where P is the re-absorption possibility, σabs(λ) is the absorption cross-section at the same wavelength of the fluorescence photon. Ng is the concentration of Yb3+ ion in the ground state. The l represent the path length of fluorescence photo travels before it emits from the surface of the crystal sample, where lX = lZ = 0.344 cm and lY = 0.232 cm, respectively. Fig. 7 shows the re-absorption possibility of the X-, Y- and Z-polarization in Yb3+:KBaGd(MoO4)3 crystal. From Fig. 7 it is easy to note all of the re-absorption possibilities in the three polarizations almost rise up to 0.5 at the wavelength of about 980 nm. This result proves that the calculated radiative lifetime, nearly half of the measured fluorescence lifetime, is reasonable. On the other hand, from formula (2), the path length of fluorescence photo traveled and the Yb3+-dopping concentration is also important factors to affect the re-absorption possibility. To investigate this effect, the re-absorption possibilities as the function of the Yb3+ ion concentration and path length are drawn in Fig. 8. When the absorption cross-section is fixed at the wavelength of 980 nm, the absorption cross-section is largest. Fig. 8 clearly gives the relationship between the Yb3+-doped concentration and path length the photon fluorescence travels. The re-absorption possibility increases dramatically with the path length rise up at the same Yb3+-doped concentration, especially in the higher concentration range. Similarly, in an anisotropic path length sample, the re-absorption possibility also changes a lot when the Yb3+-doped concentration propagates. For example, taking the l = 2 mm for the Y- polarization account, the possibility increase more than 3 times when the Yb3+-doped concentration rises form 1×1020 cm−3 to 5×1020 cm−3. Circumstances are almost the same for the X- and Z-polarizations.
3. Fluorescence Spectra
The polarized emission spectra of Yb3+:KBaGd(MoO4)3 crystal at room temperature and un-polarized emission at 10 K are shown in Fig. 9. The emission spectra exhibited a broad emission bands. There is a sharp peak at about 976.4 nm in all of the polarized spectra, which is regarded as the zero-line. There are six peaks in the low temperature emission spectrum. Among them, four are corresponding to transitions from the lowest energy level of the 2F5/2 to the split 2F7/2 level, and the other two could be signed to the transitions of the secondary lowest level of upper 2F5/2 to first and third levels of 2F7/2. Fig. 10 shows the energy levels of Yb3+:KBaGd(MoO4)3 crystal. To check the correction of identified stark energy-levels, a barycentres plot for various Yb3+-doped materials was presented in Fig. 11, including the Yb3+:KBaGd(MoO4)3 crystal [11], [12]. The dot representing Yb3+:KBaGd(MoO4)3 crystal appears very closes to the fitted line, which indicates that the identification stark energy-levels in Yb3+:KBaGd(MoO4)3 crystal is reliable.
The emission cross-section of 2F5/2→2F7/2 transitions of Yb3+:KBaGd(MoO4)3 crystal were usually calculate by the reciprocity method (RM) and Füchtbauer–Ladenburg method (F–L) [13]–[15]. It is reason that the RM method can only be employed if there is significant absorption, i. e. only in the vicinity of the fundamental transition. In other words, the RM method is not accurate at long wavelengths. The RM method is only suitable at short wavelength region. However, the F-L method is suitable for the long wavelength region because the re-absorption effect is not intense [10], [14]. Both methods are expressed as following:(3)
(4)
In the RM method, Zl and Zu are partition functions for lower and upper levels, which can be calculated as follows:(5)
(6)k is the Boltzmann’s constant, and EZL is the zero-line energy, which is defined as the energy separation between the lowest stark levels of 2F5/2 and 2F7/2 levels of Yb3+ ions. So based on the absorption and emission spectra, the zero line energy is confirmed to be at 974.8 nm and the Zl/Zu is calculated to be 0.826. The emission cross-sections calculated by the two methods are shown in Fig. 12. Since above both methods are suitable for different range of wavelength, to calculate the emission FWHM needs to combine the results of both methods. In other words, the data of short wavelength is taken from the RM method and the data of long wavelength is taken from F-L method. Thus, Yb3+:KBaGd(MoO4)3 crystal has broad emission FWHM of 81 nm for Y-polarization. Again, the Y orientation exhibits larger emission cross-section than the other two orientations. Thus, the emission cross-sections at 1010 nm are 1.89, 3.17 and 0.97×10−20 cm2 for the X-, Y- and Z-polarization, respectively. In comparison with the other Yb3+-doped laser crystal materials (Table 1), Yb3+:KBaGd(MoO4)3 crystal has large emission cross-section and broad emission FWHM of 81 nm for Y-polarization. Such broad emission FWHM is caused by the disordered structure of Yb3+:KBaGd(MoO4)3 crystal, except itself broad emission of Yb3+ ion. As well known, the broadened emission band is the fundamental condition of realizing femtosecond laser. The broader the emission band is, the shorter laser pulse will be possible to obtain. Therefore, Yb3+:KBaGd(MoO4)3 crystal will be easier to achieve the output of ultra-short laser pulse than most reported Yb3+-doped crystals before.
4. Evaluation of Laser Potential
Based on the spectral parameters mentioned above, the important three laser performance parameters of βmin, Isat and Imin can be evaluated. The βmin represents the minimum inversion fraction of Yb3+ ions in the excited-state to achieve population inversion at the extraction wavelength. It was calculated by the following formula [16]:(7)
The minimum inversion fraction βmin of Yb3+ ions in Yb3+:KBaGd(MoO4)3 crystal was calculated to be 17.3% and 16.6% at 1010 nm for the RM and F-L methods, respectively.
The saturation pump intensity Ipsat, which is a measure of the ease of bleaching the material, can be determined by the following equation [16]:(8)
Then Ipsat is calculated to be 60.9 KW/cm2, 44.1 KW/cm2and 81.65 KW/cm2 at 976 nm for the X-, Y- and Z- polarization, respectively. Imin is the minimum pump intensity to reach threshold at the extraction wavelength, which is important, too. The minimum pump intensity Imin was derived by(9)
Then the minimum pump intensity Imin at the wavelength of 1010 nm were calculated to be 10.5 KW/cm2, 7.6 KW/cm2 and 14.1 KW/cm2 for the X-, Y- and Z-polarization, respectively.
The gain cross-section σg is another important parameter to evaluate the possible tuning range of laser wavelength and it can be derived form following equation:(10)
Here β represents the excited state ions fraction. Since the Y-polarized emission spectrum has most broad and strong emission spectrum in Yb3+:KBaGd(MoO4)3 crystal, Fig. 13 gives the gain cross-section profiles for the Y-polarization. Yb3+:KBaGd(MoO4)3 crystal exhibits broad gain cross-sections. The result indicates broad wavelength tunabilty. The FWHM of gain band at β = 0.8 are 52 nm and 45 nm for the RM and F-L method, respectively.
5. Conclusion
A 4.04 at.% Yb3+:KBaGd(MoO4)3 crystal was grown by the TSSG method from the K2Mo2O7 flux. The Yb3+:KBaGd(MoO4)3 crystal has broad absorption and emission bands, except the large emission and gain cross-sections. This feature is not only suitable for the diode pumping, but also for the production of ultra-short pulses. Therefore, Yb3+:KBaGd(MoO4)3 crystal can be regarded as a candidate for the ulstrashort pulse and tunable lasers.
Author Contributions
Conceived and designed the experiments: YY GW. Performed the experiments: YY ZL. Analyzed the data: YY GW. Contributed reagents/materials/analysis tools: YH LZ. Wrote the paper: YY GW.
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