Passively mode-locked Er-doped fiber laser based
on SnS
2
nanosheets as a saturable absorber
KANGDI NIU,
1
RUYI SUN,
1
QINGYUN CHEN,
1
BAOYUAN MAN,
1
AND HUANIAN ZHANG
1,2,
*
1
Shandong Provincial Key Laboratory of Optics and Photonic Devices, School of Physics and Electronics, Shandong Normal University,
Jinan 250014, China
2
Institute of Data Science and Technology, Shandong Normal University, Jinan 250014, China
*Corresponding author: huanian_zhang@163.com
Received 21 August 2017; revised 2 November 2017; accepted 5 December 2017; posted 6 December 2017 (Doc. ID 305245);
published 24 January 2018
In this paper, tin disulfide ( SnS
2
), a two-dimensional (2D) n-type direct bandgap layered metal dichalcogenide
with a gap value of 2.24 eV, was employed as a saturable absorber. Its appearance and nonlinear saturable ab-
sorption characteristics were also investigated experimentally. SnS
2
-PVA (polyvinyl alcohol) film was successfully
prepared and employed as a mode-locker for achieving a mode-locked Er-doped fiber laser with a pulse width of
623 fs at a pulse repetition rate of 29.33 MHz. The results prove that SnS
2
nanosheets will have wide potential
ultrafast photonic applications due to their suitable bandgap value and excellent nonlinear saturable absorption
characteristics.
© 2018 Chinese Laser Press
OCIS codes: (160.6000) Semiconductor materials; (140.4050) Mode-locked lasers; (140.3500) Lasers, erbium.
https://doi.org/10.1364/PRJ.6.000072
1. INTRODUCTION
Recentl y, two-dimensional ( 2D) mate rials, i ncluding graphene
[1–5], carbon nanotubes [6–9], topologi cal insulators [10–14],
transition metal dichalcogenides [15–20], and black phosphorus
[21–23], have been widely employed as saturable absorbers (SAs)
for demonstrating passively Q-switched or mode-locked fiber
lasers. Especially, various TMDs (MoS
2
[15,16], WS
2
[17,18],
MoSe
2
[19,20], WSe
2
[19,20]) have been extensively used as a
mode-locker for achieving mode-locked Yb, Er, and Tm-doped
fiber lasers due to their advantages of unusual electronic and struc-
tural properties, wide absorption range, and ultrafast recovery time.
Recently, IV–VI group TMDs, including SnSe
2
and ReS
2
,have
held certain attention in the fields of ultrafast optics due to their
suitable layer-independent bandgap value and anisotropic crystal
structure. Thereinto, by employing the SnSe
2
-coatedmirrorasa
saturable absorber , passively Q-switched lasing within a crystalline
waveguide platform at ∼1 μm was reported by Cheng et al. [24].
In 2017, M ao et al. reported an ReS
2
-polyvinyl alcohol (PVA) film
based passively Q-switched and mode-locked erbium-doped fiber
laser [25]. Obviously, studies on the application of IV–VI group
TMDs in achieving ultrafast pulse lasers are still in their infancy.
Therefore, it is of great significance and urgency to expand the
ultrafast nonlinear optical applications of IV–VI group TMDs.
Tin disulfide (SnS
2
) was an n-type direct bandgap semicon-
ductor with a value of 2.24 eV; it has a CdI
2
crystal structure,
and the sandwich structure consists of two layers of close-
packed sulfur anions and one-layer tin cations [26–29].
Previously, its low-cost, environmentally friendly, Earth-
abundant characteristics made it better to fulfill industrial
and scientific requirements in the fields of solar cells, photoca-
talysts, lithium-ion batteries, and so on. However, to our
knowledge, the nonlinear optical absorpt ion properties of SnS
2
have been rarely reported. Due to its suitable bandgap value
of 2.24 eV, which corresponds to the visible optical region,
SnS
2
is expected to have the same saturable absorption charac-
teristics as the reported TMDs (MoS
2
, WS
2
, MoSe
2
, WSe
2
).
In this paper, SnS
2
-PVA film was successfully prepared and
employed as a mode-locker in obtaining an Er-doped mode-
locked laser. Its appearance and nonlinear saturable absorption
characteristics were investigated; the saturation intensity and
modulation depth were about 125 MW∕cm
2
and 4.6%, re-
spectively. Based on the SnS
2
-PVA film as SA, a mode-locked
Er-doped fiber laser operating at the central wavelength of
1562.01 nm with a 3 dB bandwidth of 6.09 nm was demon-
strated; the pulse width was 623 fs with a pulse repetition rate
of 29.33 MHz. The experimental results prove that, in com-
parison with the reported TMDs, SnS
2
has analogous saturable
absorption characteristics and equal excellent performance in
the field of ultrafast optics.
2. PREPARATION AND CHARACTERIZATION
OF MATERIALS
Figure 1 shows the preparation process of the film-type
SnS
2
-PVA SA. First, an SnS
2
dispersion solution was prepared
72
Vol. 6, No. 2 / February 2018 / Photonics Research
Research Article
2327-9125/18/020072-05 Journal © 2018 Chinese Laser Press