Owing to their atomic layer thickness, strong light–material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

2D Materials for Optical Modulation: Challenges and Opportunities.

A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

Reduced graphene oxide has similar mechanical, optoelectronic or conductive properties to pristine graphene because it possesses a heterogeneous structure comprised of a graphene-like basal plane that is additionally decorated with structural defects and populated with areas containing oxidized chemical groups. The graphene-like properties make reduced graphene oxide a highly desirable material to be used in a plethora of sensorial, biological, environmental or catalytic applications as well as optoelectronic and storage devices. To further advance the development of the existent technologies and to design novel and better applications based on reduced graphene oxide, it is first necessary to understand which synthetic routes and processing strategies are suitable to significantly boost specific properties of this material alone or as a component in various composites. Therefore, in this work, we review the most important categories of recent applications based on reduced graphene oxide, with the emphasis on the relationship between the enhanced composite/device functionality and methods used to synthesize, to functionalize and/or to process and to structure reduced graphene oxide.

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Reduced graphene oxide today

2D Black Phosphorus Saturable Absorbers for Ultrafast Photonics

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Transition metal dichalcogenides based saturable absorbers for pulsed laser technology

We demonstrate a Q-switched erbium-doped fiber laser using tungsten disulfide (WS₂) as a saturable absorber. The WS₂ is deposited onto fiber ferrules using a drop-casting method. Passive Q-switched pulses operating in the C-band region with a central wavelength of 1560.7 nm are successfully generated by a tunable pulse repetition rate ranging from 27.2 to 84.8 kHz when pump power is increased from 40 to 220 mW. At the same time, the pulse width decreases from a maximum value of 3.84 μs to a minimum value of 1.44 μs. The signal-to-noise ratio gives a stable value of 43.7 dB. The modulation depth and saturation intensity are measured to be 0.99% and 36.2  MW/cm², respectively.

Passively Q-switched erbium-doped fiber laser at C-band region based on WS₂ saturable absorber

All fiber-optic ultra-sensitive temperature sensor using few-layer MoS 2 coated D-shaped fiber

Using finite-element method, we propose a photonic quasi-crystal fiber-based refractive index biosensor (PQF-RIBS), which works based on the surface plasmon polariton. We determine the loss spectra for two different variations of the refractive index of analyte, $n_{a}$ . From the detailed numerical analysis, we find that the PQF-RIBS exhibits a maximum refractive index sensitivity of 6000 nm/RIU and a resolution of $1.6\,\, \times \,\,10^{-6}$ RIU when $n_{a}$ is increased from 1.45 to 1.46. Besides, this sensor does exhibit the negative refractive index sensitivity of −4000 nm/RIU and a resolution of $2.5\,\, \times \,\,10^{-6}$ RIU for a sensing range from 1.52 to 1.53. Furthermore, we carry out selective filling of liquid in the selective holes of the proposed biosensor for a sensing wavelength range from 900 to 1200 nm. Finally, we also study the influence of the structural parameters, namely, diameter of the core and diameter of the air holes in the cladding over the loss spectra of a fundamental mode for a particular $n_{a}$ of 1.47.

Designing a Biosensor Using a Photonic Quasi-Crystal Fiber

We propose a fiber stretcher for stretching the high-energy ultrashort pulses which require high dispersion and large mode area to secure high stretching ratio and for ensuring distortionless pulses. In this line, we suggest a novel design of photonic quasi-crystal fiber (PQF) to achieve the desired high normal group velocity dispersion (GVD) of 1000 ps2/km at 1.06 μm and inner core effective mode area as 35 μm2 . These unique properties have been exploited to achieve a high stretching ratio (>; 105 ) as well as less nonlinearity, which, in turn, results in a distortionless fiber stretcher for an Yb-doped high-energy ultrafast fiber laser system based on chirped pulse amplification.

S. Sivabalan

Papers

Transition metal dichalcogenides based saturable absorbers for pulsed laser technology

Passively Q-switched erbium-doped fiber laser at C-band region based on WS₂ saturable absorber

All fiber-optic ultra-sensitive temperature sensor using few-layer MoS 2 coated D-shaped fiber

Designing a Biosensor Using a Photonic Quasi-Crystal Fiber

High Normal Dispersion and Large Mode Area Photonic Quasi-Crystal Fiber Stretcher