Optical properties of Yb3+-doped Sr3Y2(BO3)4 crystal

Yan Zhang; Guofu Wang

doi:10.1557/jmr.2012.210

Optical properties of Yb3+-doped Sr3Y2(BO3)4 crystal

Published online by Cambridge University Press: 05 July 2012

Yan Zhang and

Guofu Wang

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Yan Zhang*: Affiliation:
Department of Material Engineering, Shanghai Institute of Technology, Shanghai 200235, China: and Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
Guofu Wang: Affiliation:
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
*: a)Address all correspondence to this author. e-mail: yanzhang@sit.edu.cn

Article contents

Abstract
INTRODUCTION
EXPERIMENTS
RESULTS AND DISCUSSION
CONCLUSIONS
References

Abstract

Yb3+:Sr3Y2(BO3)4 crystals have been grown successfully by Czochralski method. The compound crystallizes in orthorhombic system, space group Pnma, with a = 7.4062(3) Å, b = 16.0030(7) Å, c = 8.7130(4) Å, α = β = γ = 90°, and Z = 4. Yb3+:Sr3Y2(BO3)4 has three cationic sites and two kinds of boron sites. The crystalline quality of the Yb3+:Sr3Y2(BO3)4 single crystal was verified by the width of the x-ray diffraction peak in the x-ray rocking curves measurement. The absorption spectrum, emission spectrum, and fluorescence lifetime of the Yb3+:Sr3Y2(BO3)4 crystal were detected at room temperature. The laser performance parameters βmin, Ipsat, and Imin were calculated.

Keywords

Crystal growth Yb3+-doped Sr3Y2(BO3)4 crystal Optical properties

Information

Type: Articles
Information: Journal of Materials Research , Volume 27 , Issue 16 , 28 August 2012 , pp. 2106 - 2110

DOI: https://doi.org/10.1557/jmr.2012.210 [Opens in a new window]
Copyright: Copyright © Materials Research Society 2012

I. INTRODUCTION

With the development of high-power InGaAs laser diode in the early 1990s, Yb³⁺-doped laser materials as promising materials for diode-pumped solid-state high-power lasers with wave lengths between 0.9 and 1.1 μm have been gained much attention.Reference Kolbas, Anderson, Laidig, Sin, Lo, Hsieh and Yang¹^–Reference Taira, Tulloch, Byer and Kobayashi³ Trivalent ytterbium has simple energy level scheme, i.e., the ground state ²F_7/2 and excited state ²F_5/2, which are separated by an energy gap of around 100,000 cm⁻¹. In comparison with the other laser ions such as Nd³⁺ and Er³⁺, it has some advantages, such as low quantum defect, no excited state absorption, no upconversion, and a relatively long fluorescence lifetime. Otherwise, Yb³⁺ ion exhibits an intense and broad absorption and emission band. Previously, the synthesis and structure of double borate family with formula M₃Re₂(BO₃)₄ (M = Ca, Sr; Re = Y, Gd, and La) were reported.Reference Mill, Tkachuk, Ershova, Mironov and Nikitichev⁴^–Reference Haumesser, Gaumé, Benitez, Viana, Ferrand, Aka and Vivien⁶ Recently, Nd³⁺-doped and Yb³⁺-doped Ca₃Re₂(BO₃)₄ crystal of growth and spectral properties were investigated.Reference Mill, Tkachuk, Belokoneva, Ershova and Mironov⁷^–Reference Wei, Lin and Wang⁹ This paper reports the crystal structure of Yb³⁺:Sr₃Y₂(BO₃)₄ and spectroscopic characterization as well as its laser parameters.

II. EXPERIMENTS

Yb³⁺:Sr₃Y₂(BO₃)₄ crystal was grown by the Czochralski method in a 2 kHz frequency furnace heating an iridium crucible in N₂ atmosphere. The crystal growth was carried out along the crystallographic directions [010], with the pulling rate in the range 1∼2 mm/h and the rotation rate of 15∼25 rpm. As a result, Yb³⁺:Sr₃Y₂(BO₃)₄ crystal with dimension Φ12.5 × 20 mm², free crack, and inclusion was obtained, as shown in Fig. 1. The x-ray powder diffraction (XRPD) method was used to determine the lattice structure and parameters of the as-grown Yb³⁺:Sr₃Y₂(BO₃)₄ crystal. The as-grown crystal of Yb³⁺:Sr₃Y₂(BO₃)₄ was ground into powder as a sample to be measured by the XRPD apparatus. The XRPD pattern of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal is shown in Fig. 2, which was performed using a D–max-rA type diffractometer with a powder, using Cu K_α radiation (λ = 1.54056 Å) by using a graphite monochromator at room temperature. The x-ray diffraction (XRD) data for structure refinement were collected in the step scan mode with steps of 0.020 (2θ). The obtained d values, Miller index, and the corresponding intensities of the peaks are shown in Fig. 2. The crystallinity of the Yb³⁺:Sr₃Y₂(BO₃)₄ samples was investigated by x-ray rocking curve (XRC) analysis for the (020) peak using a multipurpose advanced thin film x-ray system with an extra high resolution Rigaku diffractometer (Tokyo, Japan), which was evaluated based on the full width at half maximum (FWHM) of the peak. A sample of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal with dimensions 2.1 × 2 × 3 mm³ was cut from as-grown crystal for measurement of absorption and emission spectrum. The absorption spectrum was measured using a Perkin-Elmer UV–VIS–NIR (Lambda 35, Norwalk, CT) spectrophotometer at room temperature, which was measured in the range from 800 to 1100 nm. Photoluminescence spectrum was measured at room temperature using Edinburgh Analytical Instruments FLS 920 LifeSpec Ps spectrophotometer (Edinburgh, UK), which contained a Hamamatsu/R5509 photodetector (Hamamatsu, Japan).

FIG. 1.

The grown Yb³⁺-doped Sr₃Y₂(BO₃)₄ crystal.

FIG. 2.

The XRD pattern of Yb³⁺-doped Sr₃Y₂(BO₃)₄ crystal.

III. RESULTS AND DISCUSSION

Yb³⁺:Sr₃Y₂(BO₃)₄ crystallizes in the orthorhombic biaxial crystal system. The values of the unit cell constants are a = 7.4062(3) Å, b = 16.0030(7) Å, c = 8.7130(4) Å, α = β = γ = 90°. The crystal belongs to Pnma space group. According to the structure analysis, there are three cationic sites: two eight-coordinated sites are occupied by Sr(1) and Sr(2) ions, whereas the third seven-coordinated site is occupied by Y ions. Figure 3 shows three polyhedron sites. There are two kinds of boron sites, B(1) and B(2, 3), with 3-fold coordination. Moreover, the shortest distance for Y–Y interaction is equal to 3.896 Å, and it is not long enough to limit the energy transfer between the active ions. Thus, the quenching effect in Yb³⁺:Sr₃Y₂(BO₃)₄ is expected. Taking into account the compatibility of charge and ionic radii, the Yb ions will enter Y site for substitution, especially on the condition of low doping concentrations (Yb³⁺ < 5 at.%).

FIG. 3.

Cationic sites in Sr₃Y₂(BO₃)₄.

The crystalline quality of the Yb³⁺:Sr₃Y₂(BO₃)₄ single crystal can be characterized by the width of the XRD peak in the XRC measurement. Figure 4 shows the XRD rocking curve, which exhibits a FWHM at (020) peak is as low as 36 arcseconds, featuring a very symmetric diffraction peak. The result demonstrates excellent crystalline perfection of the grown crystal.

FIG. 4.

XRC for (020) reflection of Yb³⁺-doped Sr₃Y₂(BO₃)₄ crystal.

Figure 5 shows the absorption spectrum of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal. There are three main absorption bands centered at 922, 960, and 977 nm. The strongest absorption band is centered at 977 nm. The absorption cross section of ²F_7/2–²F_5/2 transition is determined from the optical density, thickness, and the concentration of the sample. It is calculated by:

(1)

${\rm{\sigma }}_{{\rm{abs}}} ({\rm{\lambda }}) = 2.303 \times {{{\rm{OD}}({\rm{\lambda }})} \over {lN_0 }},$

FIG. 5.

Absorption spectrum of Yb³⁺-doped Sr₃Y₂(BO₃)₄ crystal at room temperature.

where OD(λ) is the optical density at wave length λ, l is the thickness of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal, N ₀ is the concentration of Yb³⁺ in Yb³⁺:Sr₃Y₂(BO₃)₄ crystal, which was determined to be 2.72 × 10²⁰ cm⁻³ by the inductively coupled plasma method. The absorption coefficient at wave length of 977 nm is 5.91 cm⁻¹, so the absorption cross section at 977 nm is 2.17 × 10⁻²⁰ cm². Its FWHM is 8 nm. It is larger than that of Yb:GdCOB (2.6 nm)Reference Jiang, Wang, Zhang, Hu, Teng, Zhang and Wang¹⁰ and Yb:BOYS (6 nm).Reference Haumesser, Gaumé, Viana and Vivien¹¹ This characteristic is significant in the case of diode pump because this means the laser output power is less sensitive to the drift of the diode pump wave length caused by changes in temperature.

Figure 6 shows the fluorescence spectrum of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal excited with at 977 nm laser radiation. To illustrate the fluorescence bands, the fluorescence curve of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal is decomposed by Lorentzian multipeak fits, which correspond to the emission from the ²F_5/2 excited state manifold to the sublevels of the ²F_7/2 ground state manifolds. A broad emission band corresponding to ²F_5/2 → ²F_7/2 transition was observed in 900–1150 nm region with peaks at 981, 1003, 1027, and 1054 nm. The overall splitting of ²F_7/2 manifold is 747 cm⁻¹, which is larger than that of Yb:GdCOB (448.3 cm⁻¹) and Yb:YCOB (501.3 cm⁻¹).Reference Jiang, Wang, Zhang, Hu, Teng, Zhang and Wang¹⁰^, Reference Jiang, Wang, Zhang, Hu, Phil and James¹² Such large energy splitting will be available to lower laser thermal effect and reduce reabsorption losses. The spectra of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal are similar to absorption and emission ones obtained for Yb-doped glassy matrices. This is probably due to a particularly strong multisite character, as the host offer one Y site and two different strontium sites, which are also likely to be substituted with Yb³⁺ ions to some extent. Thus, the laser-active Yb³⁺ absorption and emission bands are broadened.

FIG. 6.

Fluorescence spectrum of Yb³⁺-doped Sr₃Y₂(BO₃)₄ crystal at room temperature (the dotted lines are Lorentz fitting line).

High-power directly diode-pumped femtosecond (fs) lasers are one of the hottest and most challenging current research fields. Ytterbium-doped crystals that typically have broad emission bands around 1 μm have been recognized as very attractive gain media for diode-pumped fs oscillation and amplification. Many Yb-doped materials have been already demonstrated as competitive laser gain media in the fs regime.Reference Mougel, Dardenne, Aka, Kahn-Harari and Vivien¹³^–Reference Berger, Greco and Schroeder²⁰ Among these new materials, Yb-doped glasses exhibit very broad and smooth emission spectra that permit possible generation of ultrashort pulses, but poor thermal conductivity and low stress fracture limit their applications in high-power laser systems. Yb³⁺:Sr₃Y₂(BO₃)₄ crystal exhibit a large fundamental manifold splitting up to 747 cm⁻¹, and a broad emission bandwidth in the range 1006–1100 nm with a FWHM of 40 nm. Broad gain bandwidth of the crystal has stimulated its applications for generation of ultrashort pulses in mode-locked regime.

Fluorescence decay curve of the ²F_5/2 multiplet of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal displayed in Fig. 7 was measured for an excitation at 977 nm and emission at 1027 nm. The decay curve shows a single exponential behavior and the fluorescence lifetime could be obtained from the slope of the fitting line k, i.e., τ_f = −(1/2.303 k). By linear fitting, the slope of the fitting lines is −0.368 and the fitted lifetime is about 1.18 ms. The long lifetime will allow for an enhance level of energy storage and be favorable to obtain a high inversion of the electronic population under diode pumping.

FIG. 7.

Room temperature fluorescence decay curve of Yb³⁺-doped Sr₃Y₂(BO₃)₄ crystal.

The parameter characterizing the potential laser performance of laser crystal is emission cross section, which can be calculated by either the reciprocity method or the F–L method. Since the reciprocity method relies on very weak absorption to predict emission at long wave lengths, its accuracy is inherently limited in this region. The emission cross section depended on the emission spectra line shape, so F–L method is much more accurate especially at longer wave length. The dependency between the emission cross sections and considered fluorescence spectra is described as:

(2)

${\rm{\sigma }}_{{\rm{em}}} ({\rm{\lambda }}) = {{{\rm{\lambda }}^5 } \over {8{\rm{\pi }}cn^2 }}{1 \over {{\rm{\tau }}_{\rm{r}} }}{{I({\rm{\lambda }})} \over {\int {{\rm{\lambda }}I({\rm{\lambda }})d{\rm{\lambda }}} }},$

where c is the speed of light in vacuum, I(λ) is the relative emission intensity, n is the crystal refractive index, which was measured roughly as 1.74 using the RHG-181 refractive index instrument (Fuzhou, China). τ_r is the radiative lifetime, which is estimated from the reciprocity prediction. The peak stimulated emission cross section and bandwidth predicted by the reciprocity method were used to estimate the radiative lifetime of the Yb³⁺ (²F_5/2) manifold, as calculated for the ²F_5/2 → ²F_7/2 transition by the following formula:

(3)

${1 \over {{\rm{\tau }}_{\rm{r}} }} = {{(2J + 1)} \over {(2J\prime + 1)}}{{8{\rm{\pi }}n^2 c} \over {{\rm{\bar \lambda }}^4 }}{1 \over {N_0 }}\int {k({\rm{\lambda }})d{\rm{\lambda }}} ,$

where n is the refractive index of the medium, c is the speed of light in vacuum, and the integral is taken the whole transition centered around ${\rm{\bar \lambda }}$ . J and J′ are the total momentum of ²F_7/2 manifold and ²F_5/2 manifold, respectively. We obtain a value of 1.04 ms for radiative lifetime τ_r, which is close to the 10 at.% sample fluorescence lifetime of 1.18 ms. The room temperature fluorescence lifetime estimate is reasonably consistent with the radiative lifetime inferred from the absorption data. Therefore, the radiative quantum efficiency is approximate to 1. The maximum emission cross section σ_e is 1.13 × 10⁻²⁰ cm² at 1027 nm.

Several important laser parameters, which can be derived from the absorption cross section, emission cross section, and lifetime, assess the performance of a quasi-three-level laser medium. The β_min represents the minimal fraction of Yb³⁺ ions in the excited state to achieve population inversion at the extraction wave length (λ_ext). So the quality β_min is important for quasi-three-level system such as Yb³⁺. It is given by:

(4)

${\rm{\beta }}_{{\rm{min}}} = {{{\rm{\sigma }}_{{\rm{abs}}} ({\rm{\lambda }}_{{\rm{ext}}} )} \over {{\rm{\sigma }}_{{\rm{abs}}} ({\rm{\lambda }}_{{\rm{ext}}} ) + {\rm{\sigma }}_{{\rm{ext}}} ({\rm{\lambda }}_{{\rm{ext}}} )}}.$

The pump saturation intensity I _psat, which is a measure of the ease with which the ytterbium ion population must be bleached to overcome the ground state absorption, can be given by:

(5)

$I_{{\rm{psat}}} = {{hc} \over {{\rm{\tau }}_{{\rm{em}}} {\rm{\lambda }}_{\rm{p}} {\rm{\sigma }}_{{\rm{abs}}} }},$

where τ_em is the fluorescence lifetime, λ_p is the pump wave length, σ_abs is absorption cross section at the pump wave length λ_p.

I _min is another laser parameter, which is the minimum absorption pump intensity to reach the threshold at the extraction wave length. It is utilized as a figure of merit. It also indicates the relative usefulness of Yb³⁺-doped media. It can be obtained from the above parameters as follows:

(6)

$I_{\min } = {\rm{\beta }}_{\min } \times I_{{\rm{psat}}} .$

Based on the absorption and emission cross section, the gain cross section σ_g of the Yb:Sr₃Y₂(BO₃)₄ crystal can be obtained by:

(7)

${\rm{\sigma }}_{\rm{g}} = {\rm{\beta \sigma }}_{\rm{e}} - (1 - {\rm{\beta }}){\rm{\sigma }}_{\rm{a}} ,$

where σ_a and σ_e are the absorption and emission cross section at the laser wave length, respectively. So the gain cross section spectra are derived for several population inversion rates β, see Fig. 8. It should also be noticed from this figure that σ_g is positive in a wide range of wave lengths. This fact clearly indicates that the possibility of obtaining tunable laser action is about 40 nm (from 1020 to at least 1060 nm). This tunability range is comparable to the one obtained in Yb:LSB crystals.Reference Romero, Johannsen, Mond, Petermann, Huber and Heumann²¹

FIG. 8.

The gain cross section of Yb³⁺-doped Sr₃Y₂(BO₃)₄ crystal.

The laser parameters of Yb:Sr₃Y₂(BO₃)₄ and other crystals are listed in Table I. Owing to special electric structure of ytterbium, the spectroscopic properties and laser characteristics of ytterbium are host dependent. As compared with the other Yb³⁺-doped borates, the absorption cross section of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal is larger than that of the other borate crystals except YAB crystal. At the same time, its emission cross section is similar to that of Yb³⁺-doped YCOB crystals. The important lasing parameter I _min is 0.32 kW/cm², which is lower than that of the other borate crystals except α-BYB crystal. It signifies that Yb³⁺:Sr₃Y₂(BO₃)₄ crystal has lower laser threshold. Thus, the studied Yb³⁺:Sr₃Y₂(BO₃)₄ crystal is very promising materials for active elements of solid-state laser operating in a microrange.

TABLE I.

Spectroscopic parameters of Yb³⁺-doped Sr₃Y₂(BO₃)₄ and other Yb³⁺-doped crystals.

^a This work.

^b Ref. Reference Wang, Dawes, Dekker, Chase, Knowles, Piper and Lu22.

^c Ref. Reference Haumesser, Gaumé, Viana and Vivien11.

^d Ref. Reference Pan, Hu, Lin and Wang23.

^e Ref. Reference Jiang, Wang, Zhang, Hu, Phil and James12.

IV. CONCLUSIONS

A transparency Yb³⁺:Sr₃Y₂(BO₃)₄ crystal with dimensions up to Φ12.5 × 20 mm² has been grown by the Czochralski method. Yb³⁺:Sr₃Y₂(BO₃)₄ crystallizes in the orthorhombic biaxial crystal system, in which there are two types of cationic sites occupied by Sr and Y ions, respectively. The FWHM at (020) peak in the XRC measurement is 36 arcseconds, which indicates excellent crystalline perfection of the grown crystal. The absorption and the luminescence spectra of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal are investigated in the vicinity of 1 μm. The spectral characteristics of Yb³⁺:Sr₃Y₂(BO₃)₄ crystal are compared with the respective characteristics of the known crystals. These results show that Yb³⁺:Sr₃Y₂(BO₃)₄ crystal is of considerable interest as a active medium of diode-pumped fs solid-state lasers operated in the 1 μm wavelength regime.

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