CsPbBr3 nanocrystal saturable absorber for mode-locking ytterbium fiber laser

Article abstract of DOI:10.1063/1.4955037

Cesium lead halide perovskite nanocrystals (CsPbX₃, X = Cl, Br, I) have been reported as efficient light-harvesting and light-emitting semiconductor materials, but their nonlinear optical properties have been seldom touched upon. In this paper, we prepare layered CsPbBr₃ nanocrystal films and characterize their physical properties. Broadband linear absorption from ～0.8 to over 2.2 μm and nonlinear optical absorption at the 1-μm wavelength region are measured. The CsPbBr₃ saturable absorber (SA), manufactured by drop-casting of colloidal CsPbBr₃ liquid solution on a gold mirror, shows modulation depth and saturation intensity of 13.1% and 10.7 MW/cm², respectively. With this SA, mode-locking operation of a polarization-maintained ytterbium fiber laser produces single pulses with duration of ～216 ps, maximum average output power of 10.5 mW, and the laser spectrum is centered at ～1076 nm. This work shows that CsPbBr₃ films can be efficient SA candidates for fiber lasers and also have great potential to become broadband linear and nonlinear optical materials for photonics and optoelectronics.

Full text of DOI:10.1063/1.4955037

CsPbBr nanocrystal saturable absorber for mode-locking ytterbium fiber laser
3
,
Yan Zhou, Zhiping Hu, Yue Li, Jianqiu Xu, Xiaosheng Tang, and Yulong Tang
Citation: Appl. Phys. Lett. 108, 261108 (2016); doi: 10.1063/1.4955037
View online: http://dx.doi.org/10.1063/1.4955037
View Table of Contents: http://aip.scitation.org/toc/apl/108/26
Published by the American Institute of Physics

APPLIED PHYSICS LETTERS 108, 261108 (2016)
CsPbBr3 nanocrystal saturable absorber for mode-locking
ytterbium fiber laser
Yan Zhou,1 Zhiping Hu,2 Yue Li,1 Jianqiu Xu,1 Xiaosheng Tang,2 and Yulong Tang1,a)
1Key Laboratory for Laser Plasmas (MOE), Department of Physics and Astronomy, Collaborative Innovation
Center of IFSA, Shanghai Jiao Tong University, Shanghai 200240, China
2Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China,
College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China
(Received 23 May 2016; accepted 18 June 2016; published online 30 June 2016)
Cesium lead halide perovskite nanocrystals (CsPbX3, X ¼ Cl, Br, I) have been reported as efficient
light-harvesting and light-emitting semiconductor materials, but their nonlinear optical properties
have been seldom touched upon. In this paper, we prepare layered CsPbBr3 nanocrystal films and
characterize their physical properties. Broadband linear absorption from ꢀ0.8 to over 2.2 lm and
nonlinear optical absorption at the 1-lm wavelength region are measured. The CsPbBr3 saturable
absorber (SA), manufactured by drop-casting of colloidal CsPbBr3 liquid solution on a gold mirror,
shows modulation depth and saturation intensity of 13.1% and 10.7 MW/cm2, respectively. With
this SA, mode-locking operation of a polarization-maintained ytterbium fiber laser produces single
pulses with duration of ꢀ216 ps, maximum average output power of 10.5 mW, and the laser spec-
trum is centered at ꢀ1076 nm. This work shows that CsPbBr3 films can be efficient SA candidates
for fiber lasers and also have great potential to become broadband linear and nonlinear optical
materials for photonics and optoelectronics. Published by AIP Publishing.
[http://dx.doi.org/10.1063/1.4955037]
Passively mode-locked fiber lasers based on saturable
absorbers (SAs) have wide applications in medical care, opti-
cal communication, and material processing for their com-
pactness and deployment flexibility.1 The SA in the laser
cavity acts as a periodic loss modulator, regulating the cavity
gain temporally to produce ultrashort pulses with high peak
power. Many different kinds of SAs have been developed for
mode-locking fiber lasers, including semiconductor saturable
absorber mirrors (SESAMs),2 carbon nanotubes (CNTs),3
ion-doped crystals,4 graphene,5 topological insulators (TIs),6
transitional metallic dichalcogenides (TMDCs),7 and black
phosphorus (BP).8 Among them, SESAMs are most widely
used because of their stability and flexibility, but they usu-
ally have complex fabrication processes and relatively nar-
row optical bandwidth. CNTs have controllable saturable
absorption but their damage thresholds are usually low.
Graphene has ultrafast recovery time and broadband
response; however, the relatively low absorption of single
layer limits its application in the fields that require strong
light-matter interaction. Two dimensional (2D) TMDCs
(MoS2, WS2, etc.) have also shown broadband response
through introducing suitable atomic defects,9 and mode-
locking of fiber lasers with MoS2 has been reported in the 1-,
1.55-, and 2-lm wavelength regions.7,10,11 TIs (Bi2Se3,
Bi2Te3, etc.) can also function as SAs, but with complicated
preparation processes. Recently, 2D BP was widely used to
mode-lock solid-state lasers12 and fiber lasers13 with the
advantage of layer-number dependent bandgap, and the
bandgap can also be modulated with strain.14 One challenge
for BP is that it is susceptible to oxidation under ambient
conditions. These nonlinear materials steadily push the
development of ultrafast optics and optoelectronic devices.
In recent years, cesium lead halide perovskite nanocrys-
tals (NCs) (CsPbX3, X ¼ Cl, Br, I) have been shown as effi-
cient light-harvesting and photoluminescent materials.15–17
As light absorbers, CsPbX3 NCs have high power conversion
efficiency, while as laser gain media, they possess low pump
threshold with high net gain coefficient, exhibiting narrow
emission bandwidth and high photoluminescence quantum
yields. Compared with other light-emitting materials, the
main characteristic of CsPbX3 NCs is that their emission
wavelength can be tunable over the entire visible wavelength
region through composition manipulation and size control
based on their perovskite structure and quantum confinement
effect.18 The efficient and stable emission in the blue-green
wavelength region makes them a better choice compared
with traditional metal chalcogenide NCs such as CdSe NCs.
In addition, CsPbX3 NCs are dissolvable in various solvents
and can be deposited on arbitrary substrates, greatly facilitat-
ing their applications and integration into various optoelec-
tronic devices.15 The solution based processing is convenient
and can provide mass production with compact structures.
Despite the recent extensive researches of CsPbX3 NCs as
light-harvesting and light-emitting materials, their nonlinear
optical properties are seldom explored up to now.
In this paper, we prepare CsPbBr3 NCs films by the
solution-processing method and report their broadband linear
optical absorption (from ꢀ0.8 to over 2.2 lm) and nonlinear
saturable absorption property at the 1-lm wavelength region
(13.1% modulation depth). The CsPbBr3 NCs films based
SA is used to passively mode-lock a polarization-maintained
(PM) ytterbium fiber laser. The mode-locked pulses have
pulse duration of ꢀ216 ps at 14.8 MHz repetition rate and
a)Author to whom correspondence should be addressed. Electronic mail:
yulong@sjtu.edu.cn.
0003-6951/2016/108(26)/261108/4/$30.00
108, 261108-1
Published by AIP Publishing.

261108-2
Zhou et al.
Appl. Phys. Lett. 108, 261108 (2016)
maximum output power of ꢀ10.5 mW. The laser spectrum is
centered at ꢀ1076 nm with 2 nm bandwidth. Our work shows
that CsPbBr3 NCs can be efficient SAs for fiber lasers and
have potential to be intriguing linear and/or nonlinear optical
materials for photonics and optoelectronics.
To prepare the CsPbBr3 crude solution, a mixture of
100 mg Cs2CO3, 0.6 ml of oleic acid (OA), and 5 ml of octa-
decene (ODE) was degassed under nitrogen flow in a three-
neck flask at 120 ꢁC for 1 h until all the Cs2CO3 reacted with
OA. Five ml of ODE and 0.36 mmol of PbBr2 (0.367 g/mmol)
were mixed in a 100 ml three-necked flask which was placed
in a heating jacket with nitrogen. This three-necked flask was
heated to 120 ꢁC under nitrogen flow for 1 h. Then the temper-
ature was set up to 150ꢁC. A 0.5 ml of oleylamine (OLA) and
0.5 ml of OA were injected into the PbBr2 crude solution
quickly when the temperature reached 150 ꢁC. After complete
solubilisation of PbBr2 salts, 0.4 ml Cs-precursor solution was
swiftly injected. The reaction was kept for ꢀ10 s, and then the
mixture was cooled with an ice bath. For isolation and purifi-
cation of CsPbBr3 NCs, the NCs crude solution was centri-
fuged and precipitated by dissolving it in toluene. After 3–4
centrifugations, the supernatant was discarded and the rest
particles were dispersed in toluene to form long-term stable
solutions. This synthesis method is similar to Ref. 19.
FIG. 2. AFM image of CsPbBr3 NCs film on a silicon substrate indicating
the surface profile (a) and the marked height curve (b).
CsPbBr3 liquid solution on a silicon substrate, and an atomic
force microscopy (AFM, Nano Navi E-sweep, Japan) was
adopted. The surface profile of the CsPbBr3 NCs film is
shown in Fig. 2(a) and the thickness profile is shown in
Fig. 2(b). We can see the thickness of the film is ꢀ100 nm
with thickness deviation of tens of nanometers, implying
that ꢀ6 layers of NCs films are stacked. It should be noted
that the thickness of the film is controllable by changing the
concentration of CsPbBr3 liquid solution and the volume (or
times) of the liquid solution that dropped on the substrate.
To measure the linear optical absorption of the CsPbBr3
NCs, colloidal liquid solution was dropped on a quartz sub-
strate and dried at room temperature, and then it was meas-
ured with a Fourier Transform Infrared Spectrometer (FTIR,
Nicolet6700, Thermal Fisher Scientific Co.). Fig. 3(a) shows
the linear optical absorption spectrum of the CsPbBr3 NCs
film. It is clear that CsPbBr3 NCs film has moderate absorp-
tion in the near-infrared (NIR) wavelength region, the absorp-
tion coefficient at 1-lm wavelength region is 0.21lmꢂ1 and
the absorption steadily decreases with increased light wave-
length. It should be noted that the bandgap energy of CsPbBr3
NCs is larger than 2 eV and increases with decreased size of
CsPbBr3 NCs, so the absorption may be caused by edge states
and crystallographic defects of the film. The measured absorp-
tion coefficient in the NIR region is less than one tenth of the
band-gap-edge absorption reported in Ref. 15. Combined with
the absorption in the visible wavelength region,15 the absorp-
tion of CsPbBr3 NCs film can spread from ꢀ0.4 lm to over
2.2 lm. This wide band absorption is comparable to graphene,
TMDCs, and BP,4,8,9 which makes CsPbBr3 NCs have great
application potentials in optoelectronics where broadband tun-
ability is required.
Figures 1(a) and 1(b) are the transmission electron
microscopy (TEM) images of the CsPbBr3 NCs with differ-
ent optical resolutions, which were measured by ZEISS
LIBRA 200FE microscopy. We can see that the edge length
of most CsPbBr3 NCs is 15–20 nm with non-uniform size
distribution. Fig. 1(c) is the statistical estimation of CsPbBr3
NCs size distribution of Fig. 1(b), giving an average size of
ꢀ18 nm. The size range is 11–25 nm and the percentage of
size between 17 nm and 19 nm is 45.5%. Fig. 1(d) is the
X-ray diffraction (XRD) spectrum of this sample, which con-
firms the formation of CsPbBr3 NCs with a well-defined 3D
structure. It also shows that this sample crystallizes in pure
cubic phase, which may be due to the combined effect of
high synthesis temperature and surface energy. To estimate
the thickness of the CsPbBr3 NCs film, we dropped the
After fabrication, the CsPbBr3 NCs liquid solution was
dropped onto a gold mirror (with 2.54 cm diameter) and dried by
FIG. 1. (a) and (b) TEM images of CsPbBr3 NCs with different optical reso-
lution, (c) statistical size distribution of CsPbBr3 NCs, (d) XRD spectrum of
CsPbBr3 NCs.
FIG. 3. (a) Linear optical absorption spectrum of CsPbBr3 NCs film, (b) sat-
urable absorption curve of CsPbBr3 NCs at ꢀ1060 nm, inset: as-fabricated
CsPbBr3 NCs film on a gold mirror.

261108-3
Zhou et al.
Appl. Phys. Lett. 108, 261108 (2016)
air at room temperature to fabricate SA. As-fabricated SA is
shown in the inset of Fig. 3(b). With this SA, a reflection method
was designed to investigate its nonlinear optical absorption
characteristics, similar to Ref. 11. A self-constructed ꢀ500 ps
graphene-mode-locked fiber laser centered at ꢀ1060 nm with
repetition rate of ꢀ7.8 MHz was used as the probe source.
By gradually changing the probe power intensity, the power
reflection as a function of pump intensity was recorded and
shown in Fig. 3(b). To obtain the nonlinear optical parameters,
a simple saturable absorption equation of R(I) ¼ 1ꢂ a0
ꢃ exp(ꢂI/Isat)ꢂ ans20 was used to fit the measured data. R(I) is
the reflection, a0 is the modulation depth, I is the input intensity,
Isat is the saturation intensity, and ans is the non-saturable loss.
As shown, a0, ans, and Isat were obtained to be 13.1%, 17.2%,
and 10.7 MW/cm2, respectively. The modulation depth of the
CsPbBr3 NCs SA is higher than that of many previously pro-
posed 2D materials (for example, 9.3% for few-layered MoS2 at
1-lm (Ref. 7)), which facilitates solution based NCs acting as ef-
ficient light modulators in optoelectronic devices, especially for
mode-locking fiber lasers that need a comparatively large modu-
lation depth.
FIG. 5. Experimental results of the CsPbBr3 NCs mode-locked fiber laser.
(a) Recorded pulse train, (b) RF spectrum (1 kHz resolution), (c) single pulse
waveform, and (d) optical spectrum.
64 times average due to moderate supermode noise occurred in
the mode-locking state.
A ring cavity is adopted to fulfill passive mode-locking
of an Yb-doped fiber laser with the CsPbBr3 NCs SA, as
shown in Fig. 4. The pump source is an 8-W 976 nm laser
diode (Focuslight Co.). The ring cavity includes a fiber beam
combiner (Lightcomm Co.), ꢀ4 m Yb-doped fiber (Liekki
YB1200, 6/125 lm), ꢀ1 m polarization-maintained fiber (PM
980 fiber), a fiber circulator (PMCIR-05-1-N-B-Q, PM 980
fiber, AFR Co.), a CsPbBr3-based SA, and a 20:80 fiber cou-
pler (RoHS Co.). It should be noted that all the optical ele-
ments (beam combiner, Yb gain fiber, PM fiber, circulator, and
output coupler) are polarization maintained. For the beam
combiner, the pump pigtail is multimode fiber (105/125 lm),
while the signal pigtail is single-mode fiber (6/125 lm). The
total length of the laser cavity is ꢀ13 m. Laser beam travels
unidirectionally in the cavity with the aid of the circulator and
passes through the CsPbBr3 SA twice every trip. Laser output
power was measured with a power meter (Thorlabs PM320E)
and laser spectrum was detected with an Ocean Optics spec-
trometer (USB4000, 0.22 nm resolution). A real time 2.5 GHz
oscilloscope (Agilent DSO9254A) combined with a photode-
tector (10 GHz, Newport) was used to monitor temporal pulses.
Radio frequency (RF) spectrum was measured by combining
the photodetector with a spectrum analyzer (AV4037B, CETC
Co.). In the following, all the laser pulses were measured with
Figure 5 shows the mode-locking results with CsPbBr3
NCs. The mode-locked pulse train (Fig. 5(a)) has 14.8 MHz
repetition rate (68 ns pulse period), coinciding with the
ꢀ13 m total cavity length. Figure 5(b) indicates the RF spec-
trum of the laser, giving a signal-to-noise ratio (SNR) of
ꢀ37 dB for the fundamental frequency, which accounts for
the moderate intensity fluctuation of the pulse train (ꢀ5%).
The recorded single laser pulse (Fig. 5(c)) shows a Gaussian-
like shape with full width half maximum (FWHM) of
ꢀ216 ps. Comparatively, broad pulse width is due to that
all optical elements in the ring cavity are normal dispersive,
leading to great chirp and non-transform-limited laser pulses.
This is similar to the case in Ref. 7. The measured laser
spectrum is shown in Fig. 5(d), providing center wavelength
of ꢀ1076 nm and spectral FWHM of ꢀ2 nm. The time-
bandwidth product of the laser pulses is calculated to be
ꢀ112, confirming large chirp accumulated in the cavity.
The relationship of output power versus pump power of
the mode-locked fiber laser is shown in Fig. 6(a). Continuous
wave mode-locking starts when the pump power is over
0.38 W, and the mode-locking state can be sustained up to
the maximum output power. The maximum output power is
ꢀ10.5 mW under pump power of 1.2 W, corresponding to
0.7 nJ single pulse energy (ꢀ0.88% optical-to-optical con-
version efficiency). Under low pumping, the output power is
FIG. 4. Experimental setup of the CsPbBr3 NCs mode-locked fiber laser.
LD: laser diode, MMF: multi-mode fiber, PMF: polarization-maintained
fiber (PM 980 fiber), coupler: 20/80 splitting ratio, inset: picture of liquid
solution used for the film formation.
FIG. 6. (a) Relationship between output power and pump power of the
CsPbBr3 NCs mode-locked fiber laser, (b) normalized transmission versus
the rotation angle of the Glan-Taylor prism for the output laser pulses.

261108-4
Zhou et al.
Appl. Phys. Lett. 108, 261108 (2016)
near linearly dependent on the pump, giving a slope effi-
ciency of 1.6%. Under high pump power (>1 W), the output
power is saturated, which may be due to the relatively short
Yb gain fiber.
efficient nonlinear optical material for laser beam modulating
but also be potentially broadband and tunable nonlinear mate-
rials for photonics and optoelectronics.
A Glan-Taylor prism is used to measure the polarization
features of the output laser pulse (with fixed pump power of
ꢀ1 W). The transmitted laser power was measured by rotat-
ing the prism, and the normalized transmission is depicted in
Fig. 6(b). We can see that the experimental data (black dot)
coincide with the fitted sine function curve (red curve) very
well. The laser beam indicates a polarization degree larger
than 99%, implying the linear polarization is highly sus-
tained because of all the PM elements in the laser cavity.
To test the stability of the mode-locking state, we moni-
tored the output power and the pulsing characteristics (both
pulse shape and intensity) when the pump power was kept at
1.2 W. During 2 h of operation, the pulsing state did not show
any change; both the output power (ꢀ10.5 mW) and pulse in-
tensity only showed fluctuation of less than 5%. Stable mode-
locking with the same configuration can be recovered after
this NCs SA being placed for several months, showing good
environmental stability of this kind of CsPbBr3 NC SAs. To
explore the optical damage threshold of this CsPbBr3 SA, a
1064-nm Nd:YAG solid-state laser (500 ps pulse width, 1 kHz
repetition rate) was used as the probe source, and the laser
was focused onto the SA by a lens (50 mm focal length). The
CsPbBr3 SA was damaged when the laser intensity was
increased to ꢀ32 mJ/cm2.
This work is supported by National Natural Science
Foundation of China (NSFC) (61275136, 61138006,
11121504, and 61520106012), Research Fund for the Doctoral
Program of Higher Education of China (20120073120085),
the Advanced and Fundamental Research Funds of
Chongqing (cstc2015jcyjA1055, cstc2015jcyjA90007), and
the Fundamental Research Funds for the Central Universities
(106112015CDJZR125511, 106112015CDJXY120001).
1U. Keller, Nature 424(14), 831 (2003).
2
€
R. Bek, G. Kersteen, H. Kahle, T. Schwarzback, M. Jetter, and P. Michler,
Appl. Phys. Lett. 105(8), 082107 (2014).
3M. A. Solodyankin, E. D. Obraztsova, A. S. Lobach, A. I. Chernov, A. V.
Tausenev, V. I. Konov, and E. M. Dianov, Opt. Lett. 33(12), 1336 (2008).
4L. Pan, I. Utkin, and R. Fedosejevs, IEEE Photonics Technol. Lett. 19(24),
1979 (2007).
5Y. W. Song, S. Y. Jiang, W. S. Han, and M. K. Bae, Appl. Phys. Lett.
96(5), 051122 (2010).
6F. Bonaccorso and Z. P. Sun, Opt. Mater. Express 4(1), 63 (2014).
7H. Zhang, S. B. Lu, J. Zheng, J. Du, S. C. Wen, D. Y. Tang, and K. P.
Loh, Opt. Express 22(6), 7249 (2014).
8Y. Chen, G. B. Jiang, S. Q. Chen, Z. N. Guo, X. F. Yu, C. J. Zhao, H.
Zhang, Q. L. Bao, S. C. Wen, D. Y. Tang, and D. Y. Fan, Opt. Express
23(10), 12823 (2015).
9D. Mao, S. L. Zhang, Y. D. Wang, X. T. Gan, W. D. Zhang, T. Mei, Y. G.
Wang, Y. C. Wang, H. B. Zeng, and J. L. Zhao, Opt. Express 23(21),
27509 (2015).
10H. Liu, A. P. Luo, F. Z. Wang, R. Tang, M. Liu, Z. C. Luo, W. C. Xu, C.
J. Zhao, and H. Zhang, Opt. Lett. 39(15), 4591 (2014).
11Z. Tian, K. Wu, L. C. Kong, N. Yang, Y. Wang, R. Chen, W. S. Hu, J. Q.
Xu, and Y. L. Tang, Laser Phys. Lett. 12(6), 065104 (2015).
12B. T. Zhang, F. Lou, R. W. Zhao, J. L. He, J. Li, X. C. Su, J. Ning, and K.
J. Yang, Opt. Lett. 40(16), 3691 (2015).
In conclusion, we develop solution based CsPbBr3 NCs
films and measure their optical linear and nonlinear absorption
features. Such NCs films possess moderate linear absorption
over a wide spectral region spanning from ꢀ0.8 lm to over
2.2 lm, comparable to previously proposed 2D materials such
as graphene and TMDCs. Nonlinear absorption measurement
of the CsPbBr3 NCs film in the 1-lm wavelength region gives
modulation depth of 13.1% and saturation intensity of
10.7 MW/cm2, both of which imply that CsPbBr3 NCs have
prospective application potentials in future photonic and
optoelectronic devices. The large modulation depth of the
CsPbBr3 NCs film makes it a desirable material for light mod-
ulation and is also an advantage for mode-locking fiber lasers
(usually require large modulation depth). In the operation of
passive mode-locking a ring-cavity ytterbium fiber laser, the
output laser pulses have 14.8 MHz repetition rate, ꢀ216 ps
single pulse width, maximum output power of ꢀ10.5 mW,
and ꢀ37dB radio-frequency SNR. This kind of CsPbBr3 NCs
modulator is manufactured using the dropping coating method
based on solution processing, which can be easily integrated
into various optoelectronic devices with compact structures.
Our result shows that CsPbBr3 NCs films can not only be an
13J. Sotor, W. Macherzynski, P. Paletko, and K. M. Abramski, Appl. Phys.
Lett. 107(5), 051108 (2015).
14Q. Wei and X. H. Peng, Appl. Phys. Lett. 104(25), 251915 (2014).
15S. Yakunin, L. Protesescu, F. Krieg, M. I. Bodnarchuk, G. Nedelcu, M.
Humer, G. D. Luca, M. Fiebig, W. Heiss, and M. V. Kovalenko, Nat.
Commun. 6, 8056 (2015).
16I. Dursun, C. Shen, M. R. Parida, J. Pan, S. P. Sarmah, D. Priante, N.
Alyami, J. K. Liu, M. I. Saidaminov, M. S. Alias, A. L. Abdelhady, T. K.
Ng, O. F. Mohammed, B. S. Ooi, and O. M. Bakr, “Perovskite nanocrys-
tals as a color converter for visible light communication,” ACS Photonics
(published online).
17J. Pan, S. P. Sarmah, B. Murali, I. Dursun, W. Peng, M. R. Parida, J. K.
Liu, L. Sinatra, N. Alyami, C. Zhao, E. Alarousu, T. K. Ng, B. S. Ooi, O.
M. Bakr, and O. F. Mohammed, J. Phys. Chem. Lett. 6(24), 5027 (2015).
18Y. Wang, X. M. Li, J. Z. Song, L. Xiao, H. B. Zeng, and H. D. Sun, Adv.
Mater. 27(44), 7101 (2015).
19L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H.
Hendon, R. X. Yang, A. Walsh, and M. V. Kovalenko, Nano Lett. 15(6),
3692 (2015).
20Y. Zhou, M. Zhao, S. W. Wang, C. X. Hu, Y. Wang, S. Yan, Y. Li, J. Q.
Xu, Y. L. Tang, L. F. Gao, Q. Wang, and H. L. Zhang, Opt. Lett. 41(6),
1221 (2016).