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2D molybdenum disulfide: a promising new optical material for ultra-fast photonics
(Nanowerk Spotlight) Inspired by the unique optical and electronic property of graphene, two-dimensional layered materials have been intensively investigated in recent years, driven by their potential applications for future high speed and broadband electronic and optoelectronic devices. Layers of molybdenum disulfide (MoS2), one kind of transition metals chalcogenides, have been proven to be a very interesting material with the semiconducting property.
The basic infrastructure of molybdenum disulfide is a single-atomic layer of molybdenum sandwiched between two adjacent atomic layers of sulfide. This compound exists in nature as molybdenite, a crystal material found in rocks around the world, frequently taking the characteristic form of silver-colored hexagonal plates. For decades, molybdenite has been used in the manufacturing of lubricants and metal alloys. Like in the case of graphite, the properties of single-atom sheets of MoS2 long went unnoticed.
From the view point of applications in electronics, molybdenum disulfide sheets exhibit a significant advantage over graphene: they have an energy gap – an energy range within which no electron states can exist.
By applying an electric field, the sheets can be switched between a state that conducts electricity and one that behaves like an insulator.
Theoretically, a switched-off molybdenum disulfide transistor would consume even as little as several hundred thousand times less energy than a silicon transistor.
Graphene, on the other hand, has no energy gap and transistors made of graphene cannot be fully switched off. More importantly, the relatively weak absorption co-efficiency of graphene (2.3 % of incident light per layer) might significantly delimit its light modulation ability for optical communication devices such as light detector, modulator and absorber.
Molybdenum disulfide’s semiconducting ability, strong light-matter interaction and similarity to the carbon-based graphene makes it of interest to scientists as a viable alternative to graphene in the manufacture of electronics, particularly photoelectronics.
Scientists have found that the physical properties of two-dimensional (2D) MoS2 change markedly when it has nanoscale properties. A slab of MoS2 that is even a micron thick has an “indirect” bandgap while a two-dimensional sheet of molybdenum disulfide has a “direct” bandgap. It shows thickness dependent band-gap properties, allowing for the production of tunable optoelectronic devices with diversified spectral operation.
In pushing towards practical optical applications of 2D MoS2, an essential gap on understanding the nonlinear optical response of 2D MoS2 and how it interacts with light, must be filled.
Now, one research group on photonics based on 2D materials, from Shenzhen University, reports a breakthrough in the light-matter interaction of 2D MoS2 and fabricating a novel optical device using few layers of molybdenum disulfide (see paper in Optics Express: "Molybdenum disulfide (MoS2) as a broadband saturable absorber for ultra-fast photonics").
Thanks to the direct-band and ultrafast response in few layer MoS2, its optical absorbance can become saturated if under high power excitation, as a result of the band filling effect in conduction band. A saturable absorber is an important element for pulse operation in a laser cavity which absorb weaker energy of light modes while get across higher energy. After millions of circulation in laser cavity, ultra-short (ps or fs in temporal duration) pulses with a high concentration power could be generated.
MoS2 has indirect bandgap in bulk material with a band gap of ∼1.2 eV and direct band-gap in monolayer structure with a broader band gap of ∼1.9 eV. Although it seems that few-layer MoS2 might have limited operation bandwidth and fails to operate as a broadband saturable absorber.
However, according to their careful experimental studies, the team found that few-layer MoS2 could still possess wavelength insensitive saturable absorption responses, which is caused by the special molecular structures in few-layer MoS2.
It is worth commenting on the broadband performance of graphene and MoS2. The broadband performance of graphene is intrinsic, due to its gapless nature. However, it is more complex in the exfoliated MoS2 nanoparticle sample they used (see paper in Scientific Reports: "Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction") due to the mixture of 1T (metallic) and 2H (semiconducting) phases present. The 1T phases usually predominate in as-exfoliated samples due to doping by impurities, giving rise to similar broadband performance as graphene.
If the MoS2 can be rendered predominantly 2H, its absorption at resonance energy will be stronger.
This means that at specific wavelength that is in resonance with the band gap, we expect that MoS2 saturable absorber can potentially give stronger saturable absorption response than graphene in view of its strong bulk-like photon absorption and exciton generation owing to Van Hove singularities.
The broadband saturable absoption of few-layer MoS2
Fig. 1: The broadband saturable absorption of few-layer MoS2 and the performance of mode locked operation. (click on image to enlarge)
The enhanced, broadband and ultra-fast nonlinear optical response in 2D semiconducting transition metal disulfides (TMDs) indicates unprecedented potential for ultra-fast photonics, ranging from high speed light modulation, ultra-short pulse generation to ultra-fast optical switching. However, the stability and robustness issues of TMDs turns out to be a significant problem if exposed to high power laser illumination.
Unlike graphene that has extremely high thermal conductivity, flexibility and mechanical stability, TMDs may show much lower optical damage threshold than graphene because of their poorer thermal and mechanical property, although explorations on the photonic applications are being fueled by their advantages.
It is worth mentioning that polymethacrylate (PMMA) is indispensable for protecting few-layer MoS2 from vertical transmission if under strong optical power density.
In principle, MoS2 couldn’t afford even higher laser illumination than 100 mW (pure material) and 500 mW (with PMMA protection) adheres to a fiber tail with mode field diameter of several micrometers in our experiment, which might seriously limit its potential applications in practical optical devices.
Taper fibers inspired us to solve this challenge, schematically shown in Fig. 2. Few layer MoS2 was coupled on the waist of the taper fiber and interacted with an evanescent field of laser illumination. In this approach, the material doesn't need to bear high optical power.
This optical device could bear 1 W laser injection without damage and also could achieve mode locked operation in a fiber laser as a saturable absorber.
taper fiber and ytterbium-doped fiber laser
Fig. 2: Schematic diagram of the taper fiber and the ytterbium-doped fiber laser passively mode locked by the MoS2-taper-fiber-saturable absorber.
"By depositing few-layer MoS2 upon the tapered fiber, we can employ a ‘lateral interaction scheme’ of utilizing the strong optical response of 2D MoS2, through which not only the light-matter interaction can be significantly enhanced owing to the long interaction distance, but also the drawback of optical damage of MoS2 can be mitigated. This MoS2-taper-fiber device can withstand strong laser illumination up to 1 W. Considering that layered TMDs hold similar problems as MoS2, our findings may provide an effective approach to solve the optical damage problem on those layered semiconductor materials," Prof. Han Zhang from the Key Laboratory for Micro-Nano Optoelectronic Devices at Hunan University, concludes. "Beyond MoS2, we anticipated that a number of MoS2-like layered TMDs (such as, WSe2, MoSe2, TaS2 etc) can also be developed as promising optoelectronic devices with high power tolerance, offering inroads for more practical applications, such as large energy laser mode-locking, nonlinear optical modulation and signal processing etc."
This work provides a very convenient but practical way to overcome the disadvantages (very low optical damage threshold) of 2D semiconducting TMDs, simply by adopting a 'lateral interaction scheme'.
Stimulated by this technological innovation, we anticipate that researcher might propose new types of light interaction modes with 2D materials, particularly, the integration of 2D materials with various waveguide structures, such as Silicon waveguide. It will definitely not only solve the problems concerning easily optical damage, but also lead to new physics on how light propagates along and interacts with the 2D semiconducting surface, in the present of waveguides. Eventually, it might revolutionize our viewpoints on 2D optoelectronics, and open up a new test-bed with unprecedented chances for conceptually new optoelectronic devices.
By Dr. Feng Luan, Assistant Professor, Division of Communication Engineering, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore
 

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