Growth and Spectral Assessment of Yb3+-Doped KBaGd(MoO4)3 Crystal: A Candidate for Ultrashort Pulse and Tunable Lasers

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.


Introduction
With the development of high power InGaAs diode lasers, the Yb 3+ -doped laser materials have attracted great interest. The trivalent Yb 3+ ion has only two electronic manifold, i.e. the ground state 2 F 7/2 and the excited state 2 F 5/2 . The simple electronic-level scheme of Yb 3+ ion can reduce the excited state absorption, quantum defect and concentration, which is helpful to improve the laser efficiency. In addition, the Yb 3+ 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][2][3][4][5]. The powerful ultrashort pulse lasers have been achieved in some laser crystals, such as, Nd:SrGdGa 3 O 7 [6] and Nd:CLNGG crystals [7]. Thus, when the Yb 3+ -ion doped into the crystal with disordered structure, can it further improve the spectral properties of Yb 3+ -doped laser crystal materials.

Crystal Growth
Since KBaGd(MoO 4 ) 3 crystal incongruently melts at 1054uC [3], it is only grown by the flux method. The 15 at.% Yb 3+ -doped KBaGd(MoO 4 ) 3 crystal was grown from a flux of K 2 Mo 2 O 3 by the top solution seeding growth method (TSSG). The chemicals used were K 2 CO 3 , BaCO 3 and MoO 3 with purity 95%, La 2 O 3 and Yb 2 O 3 with purity of 99.99%. The starting materials consist of 17 mol% of solute (KBaGd(MoO 4 ) 3 ) and 83 mol% of solvent (K 2 Mo 2 O 3 ). 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 1050uC, 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 15uC/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 5uC above the saturation temperature for 30 min. The temperature was slowly cooled to 975uC to start growth. During the growth period, the crystal was slowly cooled at a cooling rate of 0.8,1.5uC/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 15uC/h. An Yb 3+ :KBaGd(MoO 4 ) 3 crystal with dimension of 5064069 mm 3 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 CuKa radiation (l = 1.054056Å ). The XRD pattern of Yb 3+ :KBaGd(MoO 4 ) 3 crystal can be indexed according to the crystal structure of KBaGd(MoO 4 ) 3 crystal, as shown in Fig. 2, which confirmed that the grown crystal belongs to the Yb 3+ :KBaGd(MoO 4 ) 3 crystal The Yb 3+ ion concentration in Yb 3+ :KBaGd(MoO 4 ) 3 crystal was measured to be 4.04 at.%, i. e. 1.494610 20 cm 23 by inductively coupled plasma atomic emission spectrometry (ICP-AES).

Spectral Properties
Since Yb 3+ :KBaGd(MoO 4 ) 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 4.662.3263.44 mm 3 was cut from as-grown Yb 3+ :KBaGd(MoO 4 ) 3 crystal along the principal X-, Yand Zaxes, 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-, Yand Z-axis, respectively.

Absorption Spectra
The polarized absorption spectra of Yb 3+ :KBaGd(MoO 4 ) 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-, Yand Z-polarization at about 979 nm, respectively. In comparison with the other Yb 3+ -doped crystals (Table 1), the FWHM of Yb 3+ :KBaGd(MoO 4 ) 3 crystal is almost 10,20 times broad than that of the other Yb 3+ -doped crystals. Such broad FWHM was further caused by the disordered structure of KBaGd(MoO 4 ) 3 crystal [3], except itself broad absorption and emission bands of Yb 3+ ion. Since the output wavelength of diode laser is increased at 0.2,.03 nm/uC 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   The absorption cross-sections were calculated to be 1.22610 220 cm 2 , 1.69610 220 cm 2 and 0.91610 220 cm 2 at 976 nm for the X-, Yand Z-polarization, respectively.

Fluorescence Lifetime
The radiative lifetime t rad of Yb 3+ ion in Yb 3+ :KBaGd(MoO 4 ) 3 crystal can be calculated Fortunately, it can be calculated from the absorption spectra by the follow formula [9]: where l mean is the mean wavelength of the absorption peak (976 nm), s abs (l) is the absorption cross-section at wavelength l, n is reflective index which is 2.0 [3]. Thus, the radiative lifetime is calculated to be about 272.8 ms. The fluorescence lifetime t f of the upper level was measured to be 523.7 ms, 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 2 F 5/2 to the 2 F 7/2 , so the fluorescence lifetime is longer than the real fluorescence lifetime of the 2 F 5/2 level. This calculated value is reliable when the re-absorption possibility is taken account. The re-absorption possibility of Yb 3+ ion in Yb 3+ :KBaGd(MoO 4 ) 3 crystal can be examined by the following formula [10] P~1{ exp½{s abs (l)N g l ð 2Þ where P is the re-absorption possibility, s abs (l) is the absorption cross-section at the same wavelength of the fluorescence photon.
Ng is the concentration of Yb 3+ 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 l X = l Z = 0.344 cm and l Y = 0.232 cm, respectively. Fig. 7 shows the re-absorption possibility of the X-, Yand Z-polarization in Yb 3+ :KBaGd(MoO 4 ) 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 Yb 3+ -dopping concentration is also important factors to affect the re-absorption possibility. To investigate this effect, the reabsorption possibilities as the function of the Yb 3+ 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 Yb 3+ -doped concentration and path length the photon fluorescence travels. The re-absorption possibility increases dramatically with the path length rise up at the same Yb 3+ -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 Yb 3+ -doped concentration propagates. For example, taking the l = 2 mm for the Ypolarization account, the possibility increase more than 3 times when the Yb 3+ -doped concentration rises form 1610 20 cm 23 to 5610 20 cm 23 . Circumstances are almost the same for the Xand Z-polarizations.

Fluorescence Spectra
The polarized emission spectra of Yb 3+ :KBaGd(MoO 4 ) 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 2 F 5/2 to the split 2 F 7/2 level, and the other two could be signed to the transitions of the secondary lowest level of upper 2 F 5/ 2 to first and third levels of 2 F 7/2 . Fig. 10 shows the energy levels of Yb 3+ :KBaGd(MoO 4 ) 3 crystal. To check the correction of identified stark energy-levels, a barycentres plot for various Yb 3+ -doped materials was presented in Fig. 11, including the Yb 3+ :KBaGd (MoO 4 ) 3 crystal [11,12]. The dot representing Yb 3+ :KBaGd (MoO 4 ) 3 crystal appears very closes to the fitted line, which indicates that the identification stark energy-levels in Yb 3+ :K-BaGd(MoO 4 ) 3 crystal is reliable.
The emission cross-section of 2 F 5/2 R 2 F 7/2 transitions of Yb 3+ :KBaGd(MoO 4 ) 3 crystal were usually calculate by the   [13][14][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: s em~l 5 I(l) In the RM method, Z l and Zu are partition functions for lower and upper levels, which can be calculated as follows: k is the Boltzmann's constant, and E ZL is the zero-line energy, which is defined as the energy separation between the lowest stark levels of 2 F 5/2 and 2 F 7/2 levels of Yb 3+ ions. So based on the absorption and emission spectra, the zero line energy is confirmed to be at 974.8 nm and the Z l /Z u 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   3 crystal, except itself broad emission of Yb 3+ 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, Yb 3+ :KBaGd(MoO 4 ) 3 crystal will be easier to achieve the output of ultra-short laser pulse than most reported Yb 3+ -doped crystals before.

Evaluation of Laser Potential
Based on the spectral parameters mentioned above, the important three laser performance parameters of b min , I sat and I min can be evaluated. The b min represents the minimum inversion fraction of Yb 3+ ions in the excited-state to achieve population inversion at the extraction wavelength. It was calculated by the following formula [16]: b min~s abs (l ext ) s abs (l ext )zs em (l ext ) ð7Þ The minimum inversion fraction b min of Yb 3+ ions in Yb 3+ :KBaGd(MoO 4 ) 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 I psat , which is a measure of the ease of bleaching the material, can be determined by the following equation [16]: Then I psat is calculated to be 60.9 KW/cm 2 , 44.1 KW/cm 2 and 81.65 KW/cm 2 at 976 nm for the X-, Y-and Zpolarization, respectively. I min is the minimum pump intensity to reach threshold at the extraction wavelength, which is important, too. The minimum pump intensity I min was derived by Then the minimum pump intensity I min at the wavelength of 1010 nm were calculated to be 10.5 KW/cm 2 , 7.6 KW/cm 2 and 14.1 KW/cm 2 for the X-, Y-and Z-polarization, respectively.
The gain cross-section s g is another important parameter to evaluate the possible tuning range of laser wavelength and it can be derived form following equation: s g~b s em (l){(1{b)s abs (l) ð10Þ Here b represents the excited state ions fraction. Since the Ypolarized emission spectrum has most broad and strong emission spectrum in Yb 3+ :KBaGd(MoO 4 ) 3 crystal, Fig. 13 gives the gain cross-section profiles for the Y-polarization. Yb 3+ :KBaGd(MoO 4 ) 3

Conclusion
A 4.04 at.% Yb 3+ :KBaGd(MoO 4 ) 3 crystal was grown by the TSSG method from the K 2 Mo 2 O 7 flux. The Yb 3+ :KBaGd (MoO 4 ) 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, Yb 3+ :KBaGd(MoO 4 ) 3 crystal can be regarded as a candidate for the ulstrashort pulse and tunable lasers.