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OPTICS LETTERS / Vol. 39, No. 13 / July 1, 2014
Intense 2.8 μm emission of Ho3+ doped PbF2 single crystal
Peixiong Zhang,1,2 Jigang Yin,1 Baitao Zhang,3 Lianhan Zhang,1 Jiaqi Hong,1,2 Jingliang He,3 and Yin Hang1,*
1
Key Laboratory of High Power Laser Materials, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800, China
2
University of Chinese Academy of Sciences, Beijing 100039, China
3
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
*Corresponding author: [email protected]
Received April 21, 2014; revised May 23, 2014; accepted May 23, 2014;
posted May 27, 2014 (Doc. ID 210612); published June 26, 2014
A Ho3 -doped PbF2 mid-IR laser crystal was successfully grown using the vertical Bridgman method. An intense
2.8 μm emission in Ho:PbF2 crystal was observed for the first time. By analyzing the absorption and emission measurements of the Ho:PbF2 crystal with the Judd–Ofelt theory, the intensity parameters Ω2;4;6 , exited state lifetimes,
branching ratios, and emission cross-sections were calculated. It is found that the Ho:PbF2 crystal has high
fluorescence branching ratio (20.99%), large emission cross section (1.44 × 10−20 cm2 ), long fluorescence lifetime
(5.4 ms), and high quantum efficiency (88.4%) corresponding to the stimulated emission of Ho3 : 5 I6 → 5 I7 transition. The structure of Ho:PbF2 crystal was also analyzed by the Raman spectrum, and it was found that the Ho:PbF2
crystal possesses low phonon energy of 257 cm−1 . We propose that the Ho:PbF2 crystal may be a promising material
for 2.8 μm laser applications. © 2014 Optical Society of America
OCIS codes: (300.6340) Spectroscopy, infrared; (300.6280) Spectroscopy, fluorescence and luminescence; (160.5690)
Rare-earth-doped materials; (160.4760) Optical properties; (140.3380) Laser materials.
http://dx.doi.org/10.1364/OL.39.003942
3 μm mid-IR (MIR) lasers have attracted much attention
for numerous applications in medical and sensing technologies because of the vibrational absorption band of
water in this spectral region [1–3]. It can also be used in
efficient and high-quality pump sources for longer wavelength MIR lasers and optical parametric oscillators [4].
It is well known that two essential factors should be considered in developing more efficient optical devices
based on rare earth ions, including the active ions and
the host. Among various active ions, Ho3 is a natural
candidate for 3 μm lasers owing to the 5 I6 → 5 I7 transition. The host material for MIR lasers is expected to possess low phonon energy, minimal absorption coefficient
in the H2 O absorption band at 3 μm, and high radiative
emission rates.
More success has been achieved with laser operation
in the range of 3 μm with rare-earth-doped fluoride hosts,
having effective phonon frequency of 400–560 cm−1 ,
about two times lower than that in the oxygen containing
compounds [5–10]. Compared with oxide crystals, fluoride crystals have several advantages, for instance, low
phonon energy, which reduces nonradiative relaxation
between adjacent energy levels; lower refractive index,
which reduces the effect of nonlinear thermo optic effects arising in intense laser pump; and longer fluorescence lifetime, which improves energy storage [11–14].
Among the different fluoride single crystals, we are
now focusing our scientific program on a new potential
gain medium ∼β-PbF2 crystal for efficient laser operation, which has high thermal conductivity (28 W/m/K)
[15], low phonon energy, and potential interest for laser
applications [16]. Furthermore, the β-PbF2 has a fluoritetype structure belonging to the cubic space group
(Fm3̄m). This cubic crystalline structure has a main
advantage that it is possible for it to grow into the form
of large-size transparent single crystals [17]. To our
knowledge, there are still no reports on the emission
around 3 μm in Ho3 -doped PbF2 crystal up to now. In
this Letter, Ho3 -doped PbF2 laser crystal was
0146-9592/14/133942-04$15.00/0
successfully grown. The absorption and emission spectra
were measured while the radiative properties, emission,
and absorption cross sections in Ho: PbF2 crystal were
investigated to demonstrate the crystal’s feasibility for
a 3 μm solid state laser. Moreover, the Raman spectra
was used to interpret the physical and optical properties
of the Ho:PbF2 crystal.
Ho(2 mol. %):PbF2 crystal was grown using the Bridgman technique. The PbF2 (99.99%), and HoF3 (99.99%)
precursors have been used as starting material. The
mixtures were heated again at 150°C for 10 h. The melt
was homogenized in a platinum crucible in the hightemperature zone at 960°C for 8 h. The growth process
was driven by lowering the crucible at a rate of
0.5 mm∕h. The solid–liquid interface was located in the
gradient zone. The temperature gradient across the solid–
liquid interface was around 35°C/cm–40°C/cm. After the
growth has been finished, the furnace was cooled to
room temperature at the rate of 30°C/cm–40°C/h.
The concentration of Ho3 ions in the crystal was measured by the inductively coupled plasma-atomic emission
spectrometry (ICP-AES) method, and the result was
2.3 mol. % (2.2 × 1020 ions∕cm3 ). The distribution coefficient of Ho3 ions (kHo ) can be calculated from the following formula: kHo C s ∕C 0 , where C s is the dopant ion
concentration in the as-grown crystals and C o is the dopant ion concentration in the melt [18]. The kHo of Ho3
ions in the Ho:PbF2 crystal can be calculated to be 1.15.
In PbF2 crystal, Ho3 ions usually occupy the Pb2
sites, which brings about the excess of positive charge.
To maintain the electrical neutrality of the system, charge
compensation is attained by the presence of interstitial
fluorine ion (F−i ) [19,20]. The Ho3 ion symmetry centers
depend upon where the interstitial F−i is compensated.
First, the Ho3 ion symmetry centers are cubic local
symmetry (OH ) if the interstitial F−i is situated in another
unit cell than the one containing the Ho3 ion. Second,
when the interstitial F−i is situated in the same unit cell
as the Ho3 ion, there are two types of Ho3 ion symmetry
© 2014 Optical Society of America
July 1, 2014 / Vol. 39, No. 13 / OPTICS LETTERS
Fig. 1. Absorption spectrum of Ho:PbF2 crystal in the range of
300–2300 nm. The inset shows the mid-infrared transmittance
spectrum of Ho:PbF2 crystal.
centers: one is tetragonal (C 4v ) if the interstitial F−i is
situated in the h100i direction, and the other is trigonal
(C 3v ) if it is situated in the h111i direction [16].
The absorption spectrum of Ho:PbF2 crystal in the
range of 300–2300 nm was measured at room temperature by a JASCO V-570 UV/VIS spectrophotometer.
Figure 1 shows the absorption spectrum of the Ho:PbF2
crystal. There are six absorption bands centered at
around 416, 450, 536, 643, 1150, and 1914 nm, which correspond to the transitions starting from the 5 I8 ground
state to higher levels 5 G5 , 5 G6 5 F1 , 5 S2 5 F4 , 5 F5 , 5 I6 ,
and 5 I7 , respectively. In the range of 1100–1250 nm, the
highest absorption of 0.30 cm−1 has been observed at
1150 nm and with a full band width at half-maximum
(FWHM) about 50 nm and turns out to be particularly
suitable for 1150 nm laser diode (LD) pumping. The inset
of Fig. 1 shows the MIR transmittance spectrum of
Ho:PbF2 crystal. As can be seen, the maximum transmittance reaches as high as 85% around 2.8 μm and extends
up to 9 μm. The 15% loss contains the Fresnel reflections,
dispersion, and absorption of the crystal. It is well known
that the absorption band region from 2.8 to 4.0 μm is due
to the stretching vibration of free OH− groups [21].
The content of OH− groups in Ho:PbF2 crystal is evaluated by absorption coefficient of the OH− vibration band
at 3 μm, which can be calculated by using the equation:
αOH− lnT∕T 0 ∕l, where l is the thickness of sample at
4 mm and T and T 0 are the incident and transmitted
intensities, respectively. The calculated value of αOH− at
3 μm is as low as 0.03 cm−1 , indicating that the Ho:PbF2
crystal is a highly promising candidate for 3 μm lasing
operation.
The room-temperature absorption spectra were used
to calculate the Judd–Ofelt [22,23] intensity parameters
3943
Ω2;4;6 of Ho3 (shown in Table 1). The fluorescence
branching ratio (β) and the radiative lifetime (τR ) can be
calculated by using the Ω2 (0.41 × 10−20 cm2 ), Ω4
(1.50 × 10−20 cm2 ), and Ω6 (0.86 × 10−20 cm2 ) of Ho3
in our crystal and are shown in Table 1. It is evident that
the fluorescence branching ratio β of 5 I6 → 5 I7 transition
for 3 μm fluorescence emission in the Ho:PbF2 crystal is
20.99%, which is much higher than that of other crystals
[24–26]. This higher fluorescence branching ratio indicates that the PbF2 crystal more easily induces the
3 μm fluorescence emission and facilitates laser operation. The radiative lifetime τR of 5 I6 manifold in PbF2
crystal (6.11 ms) is longer than that of other crystals.
A longer radiative lifetime provides the PbF2 crystal with
a better opportunity for energy storage to obtain laser
actions. Therefore, the PbF2 crystal is a promising
material for 3 μm laser.
The fluorescence spectra and fluorescence decay
curve for the Ho:PbF2 crystal under excitation of 637 nm
were obtained with Edinburgh Instruments FLS920 and
FSP920 spectrophotometers, under excitation with an
8 ns pulse of an optical parametric oscillator. All measurements were done at room temperature. Figure 2(a)
shows the emission spectra of Ho3 doped PbF2 crystal
in the wavelength of 2700–3100 nm corresponding to
radiative transitions between 5 I6 and 5 I7 energy levels.
The emission cross section for the Ho:PbF2 crystal is
subsequently calculated by the Fuchtbauer–Ladenburg
equation [27]
σ em Aβλ5 Iλ
R
;
8πcn2 λIλdλ
(1)
R
where Iλ∕ λIλdλ is the normalized line shape function of the experimental emission spectrum, β is the fluorescence branching ratio, c is the speed of light, n is the
refractive index, and A is the spontaneous emission probability. The calculated emission cross section spectra of
Ho3 doped PbF2 crystal in the wavelength of 2700–
3100 nm is also shown in Fig. 2(a). Clearly, the Ho:PbF2
crystal shows strong emission bands around 2860 and
2895 nm, and the maximum emission cross section is
1.44 × 10−20 cm2 at 2860 nm. The 2860 nm emission wavelength overlaps precisely with the peak of the fundamental OH− absorption, thus providing the opportunity for
accurate cutting of many water-based materials, such
as biological tissues. Furthermore, the emission spectrum is broad and smooth with a FWHM about 96 around
2860 nm spectral region.
The fluorescence decay curve of the emission intensity
at 2860 nm of Ho3 in the PbF2 crystal was determined
and given in Fig. 2(b). The experiment curve can be
Table 1. Judd–Ofelt Parameters, Calculated Branching Ratio, and Lifetime of Ho3 Doped Materials
PbF2 (this work)
YLF ([24])
LLF ([25])
YAG ([26])
Ω2
Ω4
(10−20 cm2 )
Ω6
β[5 I6 → 5 I7 ]
(%)
τR [5 I6 ]
(ms)
τmeas [5 I6 ]
(ms)
η
(%)
Phonon Energy
(cm−1 )
0.41
1.16
1.24
0.04
1.50
2.22
2.21
2.67
0.86
2.08
2.11
1.89
20.99
9.02
9.02
8.99
6.11
6.01
5.93
1.36
5.4
2.5
1.8
0.03
88.4
41.6
30.4
2.2
257
442
400
800
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OPTICS LETTERS / Vol. 39, No. 13 / July 1, 2014
Fig. 2. (a) 2.8 μm emission and 2.8 μm emission cross-section
spectra of Ho3 -doped PbF2 crystal. (b) Fluorescence decay
curves of Ho:PbF2 for the 5 I6 mainfold.
well fitted to single exponential functions. The measured
lifetime of the 5 I6 manifold in the Ho:PbF2 crystal is
5.4 ms. The ratio of the measured (τmeas ) and calculated
(τR via Judd–Ofelt theory) radiative lifetime yields the
quantum efficiency η (η τmeas ∕τR ) of the fluorescent
level, where τR includes both the radiative and nonradiative lifetimes. The quantum efficiency is found to be relatively high (88.4%), which indicates the PbF2 crystal can
be used as a good host material. The high quantum efficiency of the material is a result of the low phonon energy of the PbF2 crystal host.
When the crystal is pumped by a 1150 nm LD, on one
hand, some ions in the 5 I6 level decay radiatively to 5 I7
with ∼3 μm emission. On the other hand, other ions in
the 5 I6 level decay nonradiatively to the 5 I7 level by
the multiphonon relaxation. Generally, it is hard to realize a ∼3 μm emission in Ho3 doped system because of
the multiphonon relaxation’s leading role between the 5 I6
and 5 I7 level [28]. In the work, the detected ∼3 μm emission indicates that the PbF2 crystal is suited to be used as
a good host material for MIR lasers. To the best of our
knowledge, it is the first time a successful observation
of emission around ∼3 μm in Ho:PbF2 crystal.
To further investigate the impact of configuration on
the ∼3 μm emission, the Raman spectrum was measured
and analyzed. Figure 3 shows the Raman spectrum of
the as-grown crystal. Only one strong peak is seen at approximately 257 cm−1 . The zone-center Raman-active
phonon in fluorites has F2g symmetry and occurs in
Fig. 3.
Raman spectrum of
Ho3 -doped
PbF2 crystal.
PbF2 at 253 cm−1 at 4 K [29]. The F2g representation
corresponds to a triply degenerate Raman active vibrate
against each other while the metallic ion lattice is at rest.
At room temperature, this vibrancy shifts slightly to
about 257 cm−1 in Ho:PbF2 crystal. According to the
analysis of the Raman spectrum, the maximum phonon
energy of the Ho:PbF2 crystal can be presumed as low
as 257 cm−1 , which might be one of the most important
reasons of the intense ∼3 μm emission in the Ho:PbF2
crystal.
The absorption cross-section σ abs can be derived from
the calculated σ em by using the McCumber equation [30]:
Zu
hc∕λ − E zl
;
(2)
exp
σ abs σ em
Zl
kB T
where Z u and Z l are the partition functions of the upper
and lower manifolds, and the value of Z u ∕Z l is 0.867. E zl
is the zero-line energy defined as the energy gap between
the lowest Stark levels of the 5 I6 and 5 I7 manifolds, equal
to 3367 cm−1 . kB is the Boltzmann constant. Based on the
above absorption and emission cross-section spectra, the
gain cross-section spectrum Gλ can be calculated by
the following equation:
Gλ Pσ em − 1 − Pσ abs ;
(3)
where population inversion P represents the ratio of the
excited state 5 I6 level to total Ho3 population. The calculated gain cross-sections as a function of wavelength
with different P values are shown in Fig. 4. Evidently,
the gain cross-section becomes positive once the population inversion level reaches 60%.
In conclusion, Ho3 -doped PbF2 crystal was successfully prepared, and an intense 2.8 μm emission was obtained in the present fluoride crystal. Compared with
other Ho3 doped hosts (shown in Table 1), it is found
that the Ho:PbF2 crystal has higher fluorescence branching ratio (20.99%), longer fluorescence lifetime (5.4 ms),
and higher quantum efficiency (88.4%) corresponding
to the stimulated emission of Ho3 : 5 I6 → 5 I7 transition.
It was also demonstrated that the Ho:PbF2 crystal possesses low phonon energy (257 cm−1 ), which is one of
the main reasons for the intense 2.8 μm emission. All
Fig. 4. Gain cross-sections spectra of Ho3 : 5 I6 → 5 I7 in the
PbF2 crystal.
July 1, 2014 / Vol. 39, No. 13 / OPTICS LETTERS
of the results indicate that the Ho:PbF2 crystal is a promising material for 2.8 μm laser applications.
We thank the State Key Program for Basic Research of
China (Grant No. 2010CB630703), the National Natural
Science Foundation of China (Grant No. 51302283),
and the Shanghai Natural Science Foundation under Projects (Grant No. 13ZR1463400) for their financial support.
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