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Growth, thermal conductivity, spectra, and 2 µm continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal
Author’s Accepted Manuscript Growth, thermal conductivity, spectra, and 2 μm continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal Shanming Li, Lianhan Zhang, Chun Li, Yuxin Leng, Mingzhu He, Guangzhu Chen, Yilun Yang, Shulong Zhang, Peixiong Zhang, Zhenqiang Chen, Min Xu, Yin Hang
PII: DOI: Reference:
www.elsevier.com/locate/jlumin
S0022-2313(18)32346-9 https://doi.org/10.1016/j.jlumin.2019.02.003 LUMIN16261
To appear in: Journal of Luminescence Received date: 17 December 2018 Revised date: 1 February 2019 Accepted date: 2 February 2019 Cite this article as: Shanming Li, Lianhan Zhang, Chun Li, Yuxin Leng, Mingzhu He, Guangzhu Chen, Yilun Yang, Shulong Zhang, Peixiong Zhang, Zhenqiang Chen, Min Xu and Yin Hang, Growth, thermal conductivity, spectra, and 2 μm continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.02.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Growth, thermal conductivity, spectra, and 𝟐 𝛍𝐦 continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal Shanming Li1, 2, Lianhan Zhang1, Chun Li3,4, Yuxin Leng4, Mingzhu He1, Guangzhu Chen1, 2, Yilun Yang1, 2, Shulong Zhang1, 2, Peixiong Zhang5, Zhenqiang Chen5, Min Xu1, Yin Hang*1 1
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2 3
4
University of Chinese Academy of Sciences, Beijing 100039, China
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 5
Department of Optoelectronic Engineering, Jinan University, Guangzhou, Guangdong 510632, China *Corresponding author: [email protected]
Abstract The Tm3+ and Ho3+ ions co-doped LaF3 crystal was grown successfully by the Bridgman method for the first time. X-ray diffraction, thermal conductivity, absorption, and fluorescence properties were investigated systematically. The thermal conductivity is 1.99 W/(mK) at room temperature. The absorption cross-section at 792 nm and emission cross-section at 1987 nm are 0.18 × 10−20 cm2 and 0.32 × 10−20 cm2 , respectively. The Tm,Ho:LaF3 crystal exhibits wide absorption band (38 nm) and emission band (140 nm), as well as long fluorescence lifetime (17.56 ms). With a 10-mm-long sample, the output power of 574 mW at 2047 nm was obtained for the incident power of 3.53 W with a slope efficiency of 18.5 %. It can be proposed that Tm,Ho:LaF3 crystal can be a potential material for LD-pumped tunable and ultra-short pulse laser applications operating at around 2 μm. Keywords: Tm,Ho:LaF3 crystal; spectroscopic property; 2 μm laser 1. Introduction In recent years, Tm3+ and Ho3+ ions co-doped laser materials operating at around 2 μm have attracted much attention in lots of areas. Lasers in this range are eye-safe and match well with the absorption lines of NO2, CO2, and H2O, which makes it possible for the applications of light detection and ranging (LIDAR), free-space optical communication, atmospheric sensors, and the laser microsurgery [1-4]. In addition, 2 μm lasers can be pumping sources of optical parametric oscillators (OPOs) in mid- and far-infrared spectral regions [5]. It is the key point to find appropriate gain media in this wave band. The gain media require the following advantages: (1) low phonon energy to ensure the high fluorescence efficiency; (2) good infrared transmission characteristics; (3) large emission cross-sections and long fluorescence lifetime. Furthermore, wide and smooth emission bands are necessary to realize the ultra-short pulse generation. A lot of research has been done on the exploration of gain media [6-16]. At present, our research group focuses on the low-phonon-energy LaF3 crystal [17-20]. The maximum phonon energy of LaF3 crystal is about 360 cm-1 which is lower than most of the reported hosts [17]. No charge compensation exists when doping rare earth ions because the doping ions replace the site of La3+ ions. What’s more, there is high transmittance at the mid-infrared wavelength. Actually, several works have been done on the exploration of 2 μm emission in LaF3 host [19-20]. However, there is no relevant report on the Tm3+ and Ho3+ co-doped LaF3 crystal. For the first time, Tm3+ and Ho3+ ions were introduced into the LaF3 crystal to explore the properties of 2 μm emission in this paper. The Tm,Ho:LaF3 crystal was grown with the Bridgman method. Structural, thermal, and spectroscopic characteristics were measured and analyzed systematically. It exhibits wide emission band and long fluorescence lifetime, which are advantageous for generation of
the ultra-short pulse with high energy. Above all, laser output at around 2 μm was obtained from Tm,Ho:LaF3 crystal to indicate a potential use in the ultra-short infrared field. 2. Experimental sections 2.1 Crystal growth A novel Tm3+ and Ho3+ co-doped LaF3 crystal was grown by the Bridgman method in the medium-frequency induction furnace. The raw materials were fluoride powder of the LaF 3, TmF3, and HoF3 provided commercially with the high purity of 99.99%. The doping concentrations of Tm3+ and Ho3+ ions introduced in the raw materials are 5 at.% and 0.5 at.%, respectively. The crystal crystallized spontaneously in a graphite crucible under the atmosphere of high-purity Argon (80 %) and Carbon tetra-fluoride (20 %) during the whole procedure. The temperature zones consists of high-temperature zone (raw material melting) and low-temperature zone (crystal growth and annealing). The temperature of the high-temperature zone is higher than 1500 ℃. At first, the crucible was placed at the high-temperature zone with the heating rate of 80 ℃/h. Notably, the heating rate shouldn’t be slow when the temperature is high due to the sublimation of LaF3 at ~ 1350 ℃. Heat preservation time of the raw materials at the high-temperature zone stayed for 4 hours. Then the crucible was pulled down to the low-temperature zone at the speed of 1 mm/hour. After a 70 mm-long journey, the as-grown crystal was annealed for 20 hours at the same atmosphere to release the heat stress. Finally, it took 20 hours to cool down to room temperature. The as-grown crystal was obtained with the size of ∅20mm × 30mm, shown in the inset of Fig.1. 2.2 Measurement conditions The concentrations of the Tm3+ and Ho3+ were measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The crystal structure identification was undertaken on a D⁄max2550 X-ray Diffraction (XRD) using Cu Kα1 radiation. The crystal was cut into several samples with the size of ∅12.6 × 1.5 mm3 and polished to spectral quality for further thermal and spectral tests. It is worth mentioning that all of the samples are in the c-axial direction during the experimental procedure for thermal and spectroscopic investigation. The thermal diffusivity in the range of 300 - 600 K was detected by using a xenon flash apparatus (LFA 457 Netzsch). The absorption spectra of the grown crystals were recorded by a Lambda 1050 UV/VIS/NIR spectrophotometer (Perkin-Elmer). The fluorescence spectra of the crystal were measured with Edinburgh Instruments FLS920 under excitation of 808 nm. The fluorescence decay curves were measured at 2 μm under excitation of the 808 nm pulse laser. The pump source was a laser diode centered at 793 nm with the fiber core diameter of 200 um and the numerical aperture of 0.22. The pumped beam passes through a coupled system into the cavity. The cavity consisted of two mirrors: an input mirror (M 1) with a curvature of 100 mm and a flat output mirror (M2). M1 mirror had high transmission at 793 nm and high reflection at around 2 μm. M2 mirror was used with different transmittances (2 % and 5 %) at output wavelength to optimize the output laser efficiency. In addition, a third mirror (M3) was used to separate the pump laser (transmitted) and gained laser (reflected). The Tm,Ho:LaF3 crystal sample with the size of 3 × 3 × 10 mm3, polished end faces and uncoated, was mounted in a copper holder with an indium foil. A water-cooled heat sink permits the temperature of the crystal to be maintained at 20 ℃. 3. Results and discussion 3.1 Segregation and XRD analysis Table 1: The concentrations of Tm and Ho in the as-grown Tm,Ho:LaF3 crystal
Chemical element
Atomic ratio
Lattice concentration
Segregation coefficient
Tm
2.93 at. %
3.9 × 1020 cm−3
0.58
Ho
0.277 at. %
0.37 × 1020 cm−3
0.55
The inset of Fig.1 shows the as-grown Tm,Ho:LaF3 crystal and the polished sample. As can be seen from the polished sample, no bubbles are found inside the crystal. The crystal is well-crystallized and transparent, despite the matte surface. The matte surface results from incomplete crystallization of the surface. The process of crystal growth removes impurities when the segregation coefficient is smaller than 1. Therefore, there is a high concentration of the Tm3+ and Ho3+ ions at the end of the crystal growth in the Bridgman method. Table 1 shows the concentrations of Tm3+ and Ho3+ ions. The lattice concentration and segregation coefficient of Tm3+ ions are 3.9 × 1020 cm−3 and 0.58, respectively. The lattice concentration and segregation coefficient of Ho3+ ions are 0.37 × 1020 cm−3 and 0.55, respectively. Both of the segregation coefficients are smaller than 1, which is the main reason for the matte surface. Fig.1 shows the XRD pattern of powder of Tm,Ho:LaF3 crystal. After calculation by commercial software Jade, the cell parameters are a = 7.185 Å and c = 7.332 Å, respectively. Compared with the standard card (JCPDS #32-0483): a = 7.187 Å, c= 7.350 Å, it is relatively reduced, which is also caused by the fact that the radiuses of Tm3+ and Ho3+ ions are less than La3+. These doping doesn’t change the overall structure of LaF3 crystal as all peaks match perfectly.
(111)
Tm,Ho:LaF3 crystal
20
30
40
50
70
(413) (116) (332)
(411) (224) (412)
(223)
60
(304)
(222)
(311)
(114)
(115)
(113) (302) (221)
(202) (211)
(112)
(002) (110)
(300)
PDF #32-0483
80
90
2 ()
Fig.1 XRD patterns of the as-grown Tm,Ho:LaF3 crystal and the database JCPDS 32-0483; the inset shows the as-grown Tm,Ho:LaF3 crystal and sample. 3.2 Thermal conductivity
0.57
0.70
0.56
2
Specific heat (J/(gK))
0.55
Thermal diffucivity (mm /s)
0.65
0.60 0.54 0.53
0.55
0.52 0.50 0.51 0.45 0.50 0.49
0.40 300
350
400
450
500
550
600
Temperature (K)
Fig.2 Thermal diffusivity and specific heat capacity versus temperature of the as-grown Tm,Ho:LaF3 crystal
Thermal conductivity (W/(mK))
2.0
1.9
1.8
1.7
1.6
1.5 300
350
400
450
500
550
600
Temperature (K)
Fig.3 The calculated thermal conductivity versus temperature of the as-grown Tm,Ho:LaF3 crystal. Thermal conductivity plays an important part in laser crystals. To evaluate the thermal performance of Tm,Ho:LaF3 crystal, the thermal diffusivity and the specific heat capacity versus temperature in the range of 300 – 600 K were measured and shown in Fig.2. With the obtained thermal diffusivity and specific heat capacity, the thermal conductivity thermal diffusivity,
is the density and
can be calculated according to Eq. (1) (
is the
is the specific heat). × ×
As shown in Fig.3, the thermal conductivity is 1.99 3+
(1) ⁄ m
value of the pure LaF3 crystal [21]. The doped Tm and Ho
3+
at 300 K, which is smaller than the ions result in the lattice distortion. The
consequently increased phonon scattering centers bring down the mean free path of phonons. That’s the
main reason for the decrease in thermal conductivity. Besides, as the temperature rises, the thermal conductivity decreases. It is because that the intrinsic scattering of phonons decreases with increasing temperature. The thermal conductivity goes to a minimum value because of the limited mean free path by the lattice spacing. Therefore, the slope of the thermal conductivity becomes flat when the temperature rises [22]. 3.3 Absorption spectra Fig.4 shows the absorption spectra of Tm,Ho:LaF 3 crystal in the range of 300 – 2100 nm. There are five main absorption peaks centered at around 356 nm, 688 nm, 792 nm, 1212 nm, and 1696 nm, corresponding to the transitions from Tm3+: 3H6 to Tm3+: 1D2, 3F3, 3H4, 3H5, and 3F4, respectively. In addition, five main absorption peaks centers at 488 nm, 536 nm, 640 nm, 1150 nm, and 1926 nm, corresponding to the transitions from Ho 3+: 5I8 to Ho3+: 5F1+5G6, 5F4+5S2, 5F5, 5I6, and 5I7, respectively. The overlap of the peaks between 1696 nm and 1926 nm confirms the energy transfer from Tm 3+: 3F4 to Ho3+: 5I7 energy level. The peak at 792 nm is important as it matches well with the emission of the commercial AlGaAs LD. The absorption coefficient (α) at 792 nm is 0.71 cm-1. Calculated by Eq. (2), the absorption cross-section is 0.18 × 10−20 cm2, where NTm is the lattice concentration of Tm in Tm,Ho:LaF3 crystal: α⁄NTm
σabs
(2)
Notably, the full width at half maximum (FWHM) is about 38 nm. It is advantageous to possess such a broad absorption band for the crucial temperature stability of the output wavelength of the AlGaAs LD.
-1
Absorption coefficient (cm )
1.4
1.2
3
1
Tm: F3
Tm: D2
3
Tm: H5 1.0 5
5
F1+ G6
3
Tm: H4
0.8
3
5
Tm: F4
5
F4+ S2
0.6
5
I7
5
F5
0.4
5
I6
0.2
0.0 400
600
800
1000
1200
1400
1600
1800
2000
Wavelength (nm)
Fig.4 Room temperature absorption spectra of Tm,Ho:LaF3 crystal 3.4 Fluorescence properties
1.0
Fitted curve Exp. data
0.8
Intensity (a.u.)
Normalized intensity (a.u.)
1600
0.6
1200
3+ 5
Ho , I7: =17.56 ms Adj. R-Square: 0.9995
800
400
0 0
10
20
30
40
50
60
Time (ms)
0.4
0.2
0.0 1600
1700
1800
1900
2000
2100
2200
Wavelength (nm)
Fig.5 Fluorescence spectra of the Tm,Ho:LaF3 crystal under 808 nm excitation；The inset shows the fluorescence lifetime of 2 μm emission excited by 808 nm pulse laser. The fluorescence spectrum in the range of 1600 – 2200 nm under the 808 nm laser excitation was shown in Fig. 5. Five main peaks were recorded corresponding to the transitions of Tm3+: 3F4 - 3H6 (1843 nm) and Ho3+: 5I6 - 5I7 (1927 nm, 1949 nm, 1987 nm, and 2039 nm). The crystal exhibits a broad fluorescence band with the FWHM of near 140 nm which is advantageous for the wide turning range and short pulse generation. The emission cross-section was calculated with the help of the F-L equation: σem where I
5
I
8πcτ ∫ n2
I
d
(3)
is the fluorescence intensity, τ is the measured lifetime of the upper laser level, n is the
refractive index, c is the velocity of light, and cross-section at 1987 nm is approximately 0.32 × 10
is the emission wavelength. The emission −20
cm2 , smaller than the singly doped Ho:LaF3
crystal. This is due to the fact that the energy transfer up-conversion excites the 5I7 energy level to 5I5 energy level, which has been reported in several pieces of literature. The fluorescence lifetime of 2 μm emission was also measured under an 808 nm pulse laser and fitted by the single-exponential function. The powder sample was used in this measurement process to avoid the effect of radiation trapping and the re-absorption can be limited to a large extent. The measured fluorescence lifetime of 2 μm emission was 17.56 ms, which is longer than that of the laser crystals, such as 7 ms of Ho:YAG [23], 2.44 ms of Tm,Ho:YVO4 [24], and 16 ms of Ho:LLF [25]. This is mainly benefited from the lower phonon energy of the host (LaF3 crystal) which is reported to be 360 cm-1. As a consequence, the Tm,Ho:LaF3 crystal can be a potential for the high-power pulse laser material due to the perfect property of energy storage. 3.5 Laser performance
0.6
laser spectrum
Output power (W)
0.5
0.4
0.3
0.2
T=2% T=5%
0.1
0.0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
Absorbed pump power (W)
Fig.6 Output power versus incident power for different output couplers. Fig. 6 shows the output power as a function of the absorbed pump power for different transmissions (2% and 5 %) of M2. The laser spectrum is also shown in the inset of Fig.6. Obviously, the laser emission exhibits better performance when the mirror of 2 % transmission is used. The peak of the laser emission is 2047 nm when the transmission is 2%. The threshold value was 0.418 mW with the incident pump power of 1.585 W. The low threshold arises from the facts that the mechanism of Tm-Ho co-doping is advantageous for the population conversion. Moreover, the lower threshold also indicates the lower loss of the cavity. The maximum output power of 574 mW was obtained with an absorbed pump power of 3.53 W, corresponding to the slope efficiency of 18.5 %. This may be caused by the following reasons: (1) the emission cross-section is not very high (0.32 × 10−20 cm2 ) at the current ratio (5: 0.5) of Tm3+ and Ho3+ ions; (2) the absorbed pump power is calculated by the power difference before and after the Tm,Ho:LaF3 crystal, which also involves scattering power; (3) the ends of the Tm,Ho:LaF3 sample are not coated. Therefore, the high-quality Tm,Ho:LaF3 crystals with a modified ratio of Tm3+ and Ho3+ ions needs to be grown in our next work. 4. Conclusion The Tm,Ho:LaF3 crystal was grown successfully by the Bridgman method and investigated by structural, thermal, spectroscopic, and laser characterizations for the first time. The XRD pattern showed that the cell parameters are a = 7.185 Å and c = 7.332 Å, respectively. The thermal conductivity is 1.99 W/(mK) at room temperature. The absorption cross-section at 792 nm and emission cross-section at 1987 nm are 0.18 × 10−20 cm2 and 0.32 × 10−20 cm2 , respectively. The FWHMs of the absorption peak at 792 nm and the emission peak at 2 μm are 38 nm and 140 nm, respectively. Furthermore, laser properties at 2 μm were also analyzed and the maximum output power of 574 mW was obtained with an absorbed pump power of 3.53 W pumped by 793 nm LD. Therefore, the Tm,Ho:LaF3 crystal is supposed to be a candidate for the tunable and ultra-short pulse laser medium around 2 μm pumped by the commercial AlGaAs LD. Conflict of interest The authors declared that there is no conflict of interest. Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC) (51472257,
51502321, 51872307); National Key R&D Program of China [2016YFB0701002, 2016YFB0402105, 2016YFB1102302]; Strategic Priority Research Program (B) [No. XDB16]; Major Project of Shanghai Science and Technology Research Foundation [ 16JC1420600]. References: [1]
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PII: DOI: Reference:
www.elsevier.com/locate/jlumin
S0022-2313(18)32346-9 https://doi.org/10.1016/j.jlumin.2019.02.003 LUMIN16261
To appear in: Journal of Luminescence Received date: 17 December 2018 Revised date: 1 February 2019 Accepted date: 2 February 2019 Cite this article as: Shanming Li, Lianhan Zhang, Chun Li, Yuxin Leng, Mingzhu He, Guangzhu Chen, Yilun Yang, Shulong Zhang, Peixiong Zhang, Zhenqiang Chen, Min Xu and Yin Hang, Growth, thermal conductivity, spectra, and 2 μm continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.02.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Growth, thermal conductivity, spectra, and 𝟐 𝛍𝐦 continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal Shanming Li1, 2, Lianhan Zhang1, Chun Li3,4, Yuxin Leng4, Mingzhu He1, Guangzhu Chen1, 2, Yilun Yang1, 2, Shulong Zhang1, 2, Peixiong Zhang5, Zhenqiang Chen5, Min Xu1, Yin Hang*1 1
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2 3
4
University of Chinese Academy of Sciences, Beijing 100039, China
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 5
Department of Optoelectronic Engineering, Jinan University, Guangzhou, Guangdong 510632, China *Corresponding author: [email protected]
Abstract The Tm3+ and Ho3+ ions co-doped LaF3 crystal was grown successfully by the Bridgman method for the first time. X-ray diffraction, thermal conductivity, absorption, and fluorescence properties were investigated systematically. The thermal conductivity is 1.99 W/(mK) at room temperature. The absorption cross-section at 792 nm and emission cross-section at 1987 nm are 0.18 × 10−20 cm2 and 0.32 × 10−20 cm2 , respectively. The Tm,Ho:LaF3 crystal exhibits wide absorption band (38 nm) and emission band (140 nm), as well as long fluorescence lifetime (17.56 ms). With a 10-mm-long sample, the output power of 574 mW at 2047 nm was obtained for the incident power of 3.53 W with a slope efficiency of 18.5 %. It can be proposed that Tm,Ho:LaF3 crystal can be a potential material for LD-pumped tunable and ultra-short pulse laser applications operating at around 2 μm. Keywords: Tm,Ho:LaF3 crystal; spectroscopic property; 2 μm laser 1. Introduction In recent years, Tm3+ and Ho3+ ions co-doped laser materials operating at around 2 μm have attracted much attention in lots of areas. Lasers in this range are eye-safe and match well with the absorption lines of NO2, CO2, and H2O, which makes it possible for the applications of light detection and ranging (LIDAR), free-space optical communication, atmospheric sensors, and the laser microsurgery [1-4]. In addition, 2 μm lasers can be pumping sources of optical parametric oscillators (OPOs) in mid- and far-infrared spectral regions [5]. It is the key point to find appropriate gain media in this wave band. The gain media require the following advantages: (1) low phonon energy to ensure the high fluorescence efficiency; (2) good infrared transmission characteristics; (3) large emission cross-sections and long fluorescence lifetime. Furthermore, wide and smooth emission bands are necessary to realize the ultra-short pulse generation. A lot of research has been done on the exploration of gain media [6-16]. At present, our research group focuses on the low-phonon-energy LaF3 crystal [17-20]. The maximum phonon energy of LaF3 crystal is about 360 cm-1 which is lower than most of the reported hosts [17]. No charge compensation exists when doping rare earth ions because the doping ions replace the site of La3+ ions. What’s more, there is high transmittance at the mid-infrared wavelength. Actually, several works have been done on the exploration of 2 μm emission in LaF3 host [19-20]. However, there is no relevant report on the Tm3+ and Ho3+ co-doped LaF3 crystal. For the first time, Tm3+ and Ho3+ ions were introduced into the LaF3 crystal to explore the properties of 2 μm emission in this paper. The Tm,Ho:LaF3 crystal was grown with the Bridgman method. Structural, thermal, and spectroscopic characteristics were measured and analyzed systematically. It exhibits wide emission band and long fluorescence lifetime, which are advantageous for generation of
the ultra-short pulse with high energy. Above all, laser output at around 2 μm was obtained from Tm,Ho:LaF3 crystal to indicate a potential use in the ultra-short infrared field. 2. Experimental sections 2.1 Crystal growth A novel Tm3+ and Ho3+ co-doped LaF3 crystal was grown by the Bridgman method in the medium-frequency induction furnace. The raw materials were fluoride powder of the LaF 3, TmF3, and HoF3 provided commercially with the high purity of 99.99%. The doping concentrations of Tm3+ and Ho3+ ions introduced in the raw materials are 5 at.% and 0.5 at.%, respectively. The crystal crystallized spontaneously in a graphite crucible under the atmosphere of high-purity Argon (80 %) and Carbon tetra-fluoride (20 %) during the whole procedure. The temperature zones consists of high-temperature zone (raw material melting) and low-temperature zone (crystal growth and annealing). The temperature of the high-temperature zone is higher than 1500 ℃. At first, the crucible was placed at the high-temperature zone with the heating rate of 80 ℃/h. Notably, the heating rate shouldn’t be slow when the temperature is high due to the sublimation of LaF3 at ~ 1350 ℃. Heat preservation time of the raw materials at the high-temperature zone stayed for 4 hours. Then the crucible was pulled down to the low-temperature zone at the speed of 1 mm/hour. After a 70 mm-long journey, the as-grown crystal was annealed for 20 hours at the same atmosphere to release the heat stress. Finally, it took 20 hours to cool down to room temperature. The as-grown crystal was obtained with the size of ∅20mm × 30mm, shown in the inset of Fig.1. 2.2 Measurement conditions The concentrations of the Tm3+ and Ho3+ were measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The crystal structure identification was undertaken on a D⁄max2550 X-ray Diffraction (XRD) using Cu Kα1 radiation. The crystal was cut into several samples with the size of ∅12.6 × 1.5 mm3 and polished to spectral quality for further thermal and spectral tests. It is worth mentioning that all of the samples are in the c-axial direction during the experimental procedure for thermal and spectroscopic investigation. The thermal diffusivity in the range of 300 - 600 K was detected by using a xenon flash apparatus (LFA 457 Netzsch). The absorption spectra of the grown crystals were recorded by a Lambda 1050 UV/VIS/NIR spectrophotometer (Perkin-Elmer). The fluorescence spectra of the crystal were measured with Edinburgh Instruments FLS920 under excitation of 808 nm. The fluorescence decay curves were measured at 2 μm under excitation of the 808 nm pulse laser. The pump source was a laser diode centered at 793 nm with the fiber core diameter of 200 um and the numerical aperture of 0.22. The pumped beam passes through a coupled system into the cavity. The cavity consisted of two mirrors: an input mirror (M 1) with a curvature of 100 mm and a flat output mirror (M2). M1 mirror had high transmission at 793 nm and high reflection at around 2 μm. M2 mirror was used with different transmittances (2 % and 5 %) at output wavelength to optimize the output laser efficiency. In addition, a third mirror (M3) was used to separate the pump laser (transmitted) and gained laser (reflected). The Tm,Ho:LaF3 crystal sample with the size of 3 × 3 × 10 mm3, polished end faces and uncoated, was mounted in a copper holder with an indium foil. A water-cooled heat sink permits the temperature of the crystal to be maintained at 20 ℃. 3. Results and discussion 3.1 Segregation and XRD analysis Table 1: The concentrations of Tm and Ho in the as-grown Tm,Ho:LaF3 crystal
Chemical element
Atomic ratio
Lattice concentration
Segregation coefficient
Tm
2.93 at. %
3.9 × 1020 cm−3
0.58
Ho
0.277 at. %
0.37 × 1020 cm−3
0.55
The inset of Fig.1 shows the as-grown Tm,Ho:LaF3 crystal and the polished sample. As can be seen from the polished sample, no bubbles are found inside the crystal. The crystal is well-crystallized and transparent, despite the matte surface. The matte surface results from incomplete crystallization of the surface. The process of crystal growth removes impurities when the segregation coefficient is smaller than 1. Therefore, there is a high concentration of the Tm3+ and Ho3+ ions at the end of the crystal growth in the Bridgman method. Table 1 shows the concentrations of Tm3+ and Ho3+ ions. The lattice concentration and segregation coefficient of Tm3+ ions are 3.9 × 1020 cm−3 and 0.58, respectively. The lattice concentration and segregation coefficient of Ho3+ ions are 0.37 × 1020 cm−3 and 0.55, respectively. Both of the segregation coefficients are smaller than 1, which is the main reason for the matte surface. Fig.1 shows the XRD pattern of powder of Tm,Ho:LaF3 crystal. After calculation by commercial software Jade, the cell parameters are a = 7.185 Å and c = 7.332 Å, respectively. Compared with the standard card (JCPDS #32-0483): a = 7.187 Å, c= 7.350 Å, it is relatively reduced, which is also caused by the fact that the radiuses of Tm3+ and Ho3+ ions are less than La3+. These doping doesn’t change the overall structure of LaF3 crystal as all peaks match perfectly.
(111)
Tm,Ho:LaF3 crystal
20
30
40
50
70
(413) (116) (332)
(411) (224) (412)
(223)
60
(304)
(222)
(311)
(114)
(115)
(113) (302) (221)
(202) (211)
(112)
(002) (110)
(300)
PDF #32-0483
80
90
2 ()
Fig.1 XRD patterns of the as-grown Tm,Ho:LaF3 crystal and the database JCPDS 32-0483; the inset shows the as-grown Tm,Ho:LaF3 crystal and sample. 3.2 Thermal conductivity
0.57
0.70
0.56
2
Specific heat (J/(gK))
0.55
Thermal diffucivity (mm /s)
0.65
0.60 0.54 0.53
0.55
0.52 0.50 0.51 0.45 0.50 0.49
0.40 300
350
400
450
500
550
600
Temperature (K)
Fig.2 Thermal diffusivity and specific heat capacity versus temperature of the as-grown Tm,Ho:LaF3 crystal
Thermal conductivity (W/(mK))
2.0
1.9
1.8
1.7
1.6
1.5 300
350
400
450
500
550
600
Temperature (K)
Fig.3 The calculated thermal conductivity versus temperature of the as-grown Tm,Ho:LaF3 crystal. Thermal conductivity plays an important part in laser crystals. To evaluate the thermal performance of Tm,Ho:LaF3 crystal, the thermal diffusivity and the specific heat capacity versus temperature in the range of 300 – 600 K were measured and shown in Fig.2. With the obtained thermal diffusivity and specific heat capacity, the thermal conductivity thermal diffusivity,
is the density and
can be calculated according to Eq. (1) (
is the
is the specific heat). × ×
As shown in Fig.3, the thermal conductivity is 1.99 3+
(1) ⁄ m
value of the pure LaF3 crystal [21]. The doped Tm and Ho
3+
at 300 K, which is smaller than the ions result in the lattice distortion. The
consequently increased phonon scattering centers bring down the mean free path of phonons. That’s the
main reason for the decrease in thermal conductivity. Besides, as the temperature rises, the thermal conductivity decreases. It is because that the intrinsic scattering of phonons decreases with increasing temperature. The thermal conductivity goes to a minimum value because of the limited mean free path by the lattice spacing. Therefore, the slope of the thermal conductivity becomes flat when the temperature rises [22]. 3.3 Absorption spectra Fig.4 shows the absorption spectra of Tm,Ho:LaF 3 crystal in the range of 300 – 2100 nm. There are five main absorption peaks centered at around 356 nm, 688 nm, 792 nm, 1212 nm, and 1696 nm, corresponding to the transitions from Tm3+: 3H6 to Tm3+: 1D2, 3F3, 3H4, 3H5, and 3F4, respectively. In addition, five main absorption peaks centers at 488 nm, 536 nm, 640 nm, 1150 nm, and 1926 nm, corresponding to the transitions from Ho 3+: 5I8 to Ho3+: 5F1+5G6, 5F4+5S2, 5F5, 5I6, and 5I7, respectively. The overlap of the peaks between 1696 nm and 1926 nm confirms the energy transfer from Tm 3+: 3F4 to Ho3+: 5I7 energy level. The peak at 792 nm is important as it matches well with the emission of the commercial AlGaAs LD. The absorption coefficient (α) at 792 nm is 0.71 cm-1. Calculated by Eq. (2), the absorption cross-section is 0.18 × 10−20 cm2, where NTm is the lattice concentration of Tm in Tm,Ho:LaF3 crystal: α⁄NTm
σabs
(2)
Notably, the full width at half maximum (FWHM) is about 38 nm. It is advantageous to possess such a broad absorption band for the crucial temperature stability of the output wavelength of the AlGaAs LD.
-1
Absorption coefficient (cm )
1.4
1.2
3
1
Tm: F3
Tm: D2
3
Tm: H5 1.0 5
5
F1+ G6
3
Tm: H4
0.8
3
5
Tm: F4
5
F4+ S2
0.6
5
I7
5
F5
0.4
5
I6
0.2
0.0 400
600
800
1000
1200
1400
1600
1800
2000
Wavelength (nm)
Fig.4 Room temperature absorption spectra of Tm,Ho:LaF3 crystal 3.4 Fluorescence properties
1.0
Fitted curve Exp. data
0.8
Intensity (a.u.)
Normalized intensity (a.u.)
1600
0.6
1200
3+ 5
Ho , I7: =17.56 ms Adj. R-Square: 0.9995
800
400
0 0
10
20
30
40
50
60
Time (ms)
0.4
0.2
0.0 1600
1700
1800
1900
2000
2100
2200
Wavelength (nm)
Fig.5 Fluorescence spectra of the Tm,Ho:LaF3 crystal under 808 nm excitation；The inset shows the fluorescence lifetime of 2 μm emission excited by 808 nm pulse laser. The fluorescence spectrum in the range of 1600 – 2200 nm under the 808 nm laser excitation was shown in Fig. 5. Five main peaks were recorded corresponding to the transitions of Tm3+: 3F4 - 3H6 (1843 nm) and Ho3+: 5I6 - 5I7 (1927 nm, 1949 nm, 1987 nm, and 2039 nm). The crystal exhibits a broad fluorescence band with the FWHM of near 140 nm which is advantageous for the wide turning range and short pulse generation. The emission cross-section was calculated with the help of the F-L equation: σem where I
5
I
8πcτ ∫ n2
I
d
(3)
is the fluorescence intensity, τ is the measured lifetime of the upper laser level, n is the
refractive index, c is the velocity of light, and cross-section at 1987 nm is approximately 0.32 × 10
is the emission wavelength. The emission −20
cm2 , smaller than the singly doped Ho:LaF3
crystal. This is due to the fact that the energy transfer up-conversion excites the 5I7 energy level to 5I5 energy level, which has been reported in several pieces of literature. The fluorescence lifetime of 2 μm emission was also measured under an 808 nm pulse laser and fitted by the single-exponential function. The powder sample was used in this measurement process to avoid the effect of radiation trapping and the re-absorption can be limited to a large extent. The measured fluorescence lifetime of 2 μm emission was 17.56 ms, which is longer than that of the laser crystals, such as 7 ms of Ho:YAG [23], 2.44 ms of Tm,Ho:YVO4 [24], and 16 ms of Ho:LLF [25]. This is mainly benefited from the lower phonon energy of the host (LaF3 crystal) which is reported to be 360 cm-1. As a consequence, the Tm,Ho:LaF3 crystal can be a potential for the high-power pulse laser material due to the perfect property of energy storage. 3.5 Laser performance
0.6
laser spectrum
Output power (W)
0.5
0.4
0.3
0.2
T=2% T=5%
0.1
0.0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
Absorbed pump power (W)
Fig.6 Output power versus incident power for different output couplers. Fig. 6 shows the output power as a function of the absorbed pump power for different transmissions (2% and 5 %) of M2. The laser spectrum is also shown in the inset of Fig.6. Obviously, the laser emission exhibits better performance when the mirror of 2 % transmission is used. The peak of the laser emission is 2047 nm when the transmission is 2%. The threshold value was 0.418 mW with the incident pump power of 1.585 W. The low threshold arises from the facts that the mechanism of Tm-Ho co-doping is advantageous for the population conversion. Moreover, the lower threshold also indicates the lower loss of the cavity. The maximum output power of 574 mW was obtained with an absorbed pump power of 3.53 W, corresponding to the slope efficiency of 18.5 %. This may be caused by the following reasons: (1) the emission cross-section is not very high (0.32 × 10−20 cm2 ) at the current ratio (5: 0.5) of Tm3+ and Ho3+ ions; (2) the absorbed pump power is calculated by the power difference before and after the Tm,Ho:LaF3 crystal, which also involves scattering power; (3) the ends of the Tm,Ho:LaF3 sample are not coated. Therefore, the high-quality Tm,Ho:LaF3 crystals with a modified ratio of Tm3+ and Ho3+ ions needs to be grown in our next work. 4. Conclusion The Tm,Ho:LaF3 crystal was grown successfully by the Bridgman method and investigated by structural, thermal, spectroscopic, and laser characterizations for the first time. The XRD pattern showed that the cell parameters are a = 7.185 Å and c = 7.332 Å, respectively. The thermal conductivity is 1.99 W/(mK) at room temperature. The absorption cross-section at 792 nm and emission cross-section at 1987 nm are 0.18 × 10−20 cm2 and 0.32 × 10−20 cm2 , respectively. The FWHMs of the absorption peak at 792 nm and the emission peak at 2 μm are 38 nm and 140 nm, respectively. Furthermore, laser properties at 2 μm were also analyzed and the maximum output power of 574 mW was obtained with an absorbed pump power of 3.53 W pumped by 793 nm LD. Therefore, the Tm,Ho:LaF3 crystal is supposed to be a candidate for the tunable and ultra-short pulse laser medium around 2 μm pumped by the commercial AlGaAs LD. Conflict of interest The authors declared that there is no conflict of interest. Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC) (51472257,
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