Passively Q-switched ytterbium-doped fiber laser
using the evanescent field interaction with bulk-like
WTe
2
particles
Seunghwan Ko (高承煥), Jinho Lee (李珍昊), and Ju Han Lee (李周翰)*
School of Electrical and Computer Engineering, University of Seoul, Seoul 02504, Republic of Korea
*Corresponding author: j.h.lee@ ieee.org
Received October 23, 2017; accepted November 10, 2017; posted online January 26, 2018
The potential of bulk-like WTe
2
particles for the realization of a passive Q-switch operating at the 1 μm wave-
length was investigated. The WTe
2
particles were prepared using a simple mechanical exfoliation method to-
gether with Scotch tape. By attaching bulk-like WTe
2
particles, which remained on the top of the sticky surface
of a small segment of the Scotch tape, to the flat side of a side-polished fiber, a saturable absorber (SA) was
readily implemented. A strong saturable absorption was then readily obtained through an evanescent field in-
teraction with the WTe
2
particles. The modulation depth of the prepared SA was measured as ∼2.18% at
1.03 μm. By incorporating the proposed SA into an all-fiberized ytterbium-doped fiber ring cavity, stable Q-
switched pulses were readily achieved.
OCIS codes: 140.3510, 140.3540, 160.4330.
doi: 10.3788/COL201816.020017.
Q-switched fiber lasers have been useful light sources
for many applications, such as material processing, light
detection and ranging (LIDAR), laser surgery, and super-
continuum generation
[1–4]
. Since Q-switched pulses are
generated by either a passive or an active switching of
the laser oscillator Q-factor, the temporal width of their
output pulses usually ranges from nanoseconds to micro-
seconds. Compared with mode-locking, Q-switching can
be easily induced in a fiberized resonator, since it does
not require a careful design of the cavity–fiber parameters
for the achievement of a balance between the dispersion
and nonlinearity.
For the implementation of passively Q-switched fiber
lasers, saturable absorbers (SAs) are commonly used to
induce the Q-factor modulation within their cavities.
Although semiconductor-based SAs have been widely
used in practice as a passive Q-switch
[5]
, a range of their
drawbacks, such as the need of complicated fabrication-
facility processes and a limited operational wavelength,
motivated a number of researchers throughout the world
to investigate low-cost and effective-performance al terna-
tive SA materials, including carbo n nanotubes (CNTs)
[6,7]
,
graphene
[8,9]
, black phosphorus
[10–13]
, topological insula-
tors
[14–17]
, transition-metal dichalcogenides (TMDs)
[18–31]
,
filled skutterudites
[32]
, and MXene
[33]
.
Recently, TMDs have attracted great technical atten-
tion in the photonic and electronic fields, since their
bandgap structures can be engineered by controlling the
number of the bandgap layers
[34]
. TMDs are composed
of a hexagonal layer of metal atoms (M) that are sand-
wiched between two chalcogen-atom (X) layers with a
common formula of MX
2
. The binding of the TMD mono-
layers by weak van der Waals forces produces a bulk
structure. These materials possess excellent optoelectronic
properties, such as a large saturable absorption, strong
Kerr nonlinearity, and possible valley polarization, which
are useful for the device implementation in the fields of
plasmonics, quantum electrodynamics, and ultrafast
photonics
[35–38]
.
Among moly bdenum disulfide (MoS
2
), tungsten disul-
fide (WS
2
), molybdenum diselenide (MoSe
2
), molybde-
num ditelluride (MoTe
2
), tungsten diselenide (WSe
2
),
and tungsten ditelluride (WTe
2
), MoS
2
and WS
2
are
the TMDs that have been most widely investigated as al-
ternatives to graphene
[39,40]
. It is well-known that the TMD
bandgap varies depending on the number of layers due to
the hybridization between the d orbitals of the transition-
metal atoms and the p
z
orbitals of the chalcogen atoms
[41]
,
unlike the zero bandgap of graphene. Such a bandgap-
engineering property allows for the implementation of a
variety of photonic devices, such as optical switches, photo
detectors, and quantum-well modulators
[38,42]
.
Recently, the saturable-absorption properties of TMDs
have been intensively investigated, and a number of
TMD-based SAs have been successfully demonstrated
for the implementation of pulsed lasers
[18–31]
. One of the in-
teresting findings of those investigations is that the TMDs
exhibited saturable-absorption properties at the near- and
mid-infrared wavelengths even though the upper-cutoff
wavelengths that are determined by their bandgap ener-
gies are usually below the wavelength of 1 μm. The reason
for this abnormal saturable-absorption phenomenon is
attributable to a sub-bandgap absorption, which is caused
by defects and edge states
[43–46]
. Furthermore, the authors
recently theoretically and experimentally verified that the
structural dimensionality of TMDs is not critical regard-
ing their saturable-absorption applications
[31]
. It was dem-
onstrated that the bulk-like WTe
2
microflake is a low-cost
saturable-absorption material that can be readily used for
the implementation of broadband SAs.
COL 16(2), 020017(2018) CHINESE OPTICS LETTERS February 10, 2018
1671-7694/2018/020017(5) 020017-1 © 2018 Chinese Optics Letters