Qingkai Wang

Bio: Qingkai Wang is an academic researcher from Shenzhen University. The author has contributed to research in topics: Topological insulator & Photonics. The author has an hindex of 13, co-authored 20 publications receiving 1069 citations. Previous affiliations of Qingkai Wang include Hunan University & Jiujiang University.

Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction

Abstract: By coupling few-layer Molybdenum Disulfide (MoS2) with fiber-taper evanescent light field, a new type of MoS2 based nonlinear optical modulating element had been successfully fabricated as a two-dimensional layered saturable absorber with strong light-matter interaction. This MoS2-taper-fiber device is not only capable of passively mode-locking an all-normal-dispersion ytterbium-doped fiber laser and enduring high power laser excitation (up to 1 W), but also functions as a polarization sensitive optical modulating component (that is, different polarized light can induce different nonlinear optical response). Thanks to the combined advantages from the strong nonlinear optical response in MoS2 together with the sufficiently-long-range interaction between light and MoS2, this device allows for the generation of high power stable dissipative solitons at 1042.6 nm with pulse duration of 656 ps and a repetition rate of 6.74 MHz at a pump power of 210 mW. Our work may also constitute the first example of MoS2-enabled wave-guiding photonic device, and potential y give some new insights into two-dimensional layered materials related photonics.

Sensitivity enhancement by using few-layer black phosphorus-graphene/TMDCs heterostructure in surface plasmon resonance biochemical sensor

Abstract: The heterostructure of two-dimensional (2D) materials are promising and useful in the field of surface plasmon resonance (SPR) biochemical sensors. To enhance the sensitivity, we design a novel SPR biochemical sensor by using heterostructures of few-layer black phosphorus (BP) and graphene/transition metal dichalcogenides (TMDCs). The SPR biochemical sensor based on the different heterostructures of BP and graphene/TMDCs are analyzed, and the highest sensitivity with 279°/RIU for the heterostructure of BP and bilayer WSe2 is obtained. Moreover, the proposed biochemical sensor can be used to detect the analyte with different refractive index. The most prominent advantage of the proposed structure is its high sensitivity. The maximum sensitivity of our proposed SPR biochemical sensor is about 2.4 times of the conventional biochemical sensor. We believe that this biochemical sensor could find potential applications in chemical examination, medical diagnosis and biological detection.

Wide spectral and wavelength-tunable dissipative soliton fiber laser with topological insulator nano-sheets self-assembly films sandwiched by PMMA polymer.

Abstract: Topological insulators have been theoretically predicted as promising candidates for broadband photonics devices due to its large bulk band gap states in association with the spin-momentum-locked mass-less Dirac edge/surface states. Unlike the bulk counterpart, few-layer topological insulators possess some intrinsic optical advantages, such as low optical loss, low saturation intensity and high concentration of surface state. Herein, we use a solvothermal method to prepare few-layer Bi2Te3 flakes. By sandwiching few-layer Bi2Te3 flakes with polymethyl methacrylate (PMMA) polymer, a novel light modulation device had been successfully fabricated with high chemical and thermal stabilities as well as excellent mechanical durability, originating from the contribution of PMMA acting as buffer layers that counteract excessive mechanical bending within the fragile Bi2Te3 flakes. The incorporation of the as-fabricated PMMA-TI-PMMA as saturable absorber, which could bear long-term mechanical loadings, into the fiber laser cavity generated the stable dissipative soliton mode-locking with a 3-dB spectral bandwidth up to 51.62 nm and tunable wavelength range of 22 nm. Our work provides a new way of fabricating PMMA-TI-PMMA sandwiched composite structure as saturable absorber with promising applications for laser operation.

Broadband ultrafast spatial self-phase modulation for topological insulator Bi2Te3 dispersions

Abstract: Ultrathin topological insulator bismuth telluride (Bi2Te3) nanosheets with uniform hexagonal nanostructure have been synthesized by cost-effective solvothermal method. Broadband spatial self-phase modulation phenomena of these topological insulator nanosheets have been observed with 400 nm, 800 nm, and 1070 nm ultrafast lasers. The experimental results suggest that this coherent light scattering is due to the broadband, ultrafast, and large third-order optical nonlinearity of Bi2Te3. With the pulsed laser excitation, the nonlinear refractive index (n2) of Bi2Te3 dispersion solution was measured to be ∼10−12 m2/W, and the third-order nonlinear susceptibility ∼10−7 esu. Our work may provide an inroad for developing the nonlinear optical applications based on topological insulators.

Electrically Tunable Goos–Hänchen Shift of Light Beam Reflected From a Graphene-on-Dielectric Surface

Abstract: We have theoretically investigated the Goos-Hanchen (GH) shift of the TM-polarized beam reflected from a graphene-on-dielectric surface near the Brewster angle. It is shown that even a single-layer graphene allows for notable variation of the GH shift. The GH shift can be enlarged and switched from positive to negative or vice versa. Importantly, the GH shift depends on the Fermi energy, and thus, it can be electrically controlled through electrical or chemical modification of the charge carrier density of the graphene. Furthermore, the relationship between the GH shift and the electron-phonon relaxation time and the number of graphene layers is clarified.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation

Abstract: Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

Broadband nonlinear optical response in multi-layer black phosphorus: an emerging infrared and mid-infrared optical material

Abstract: Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, is a high-mobility layered semiconductor with direct band-gap determined by the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer). Therefore, BP is considered as a natural candidate for broadband optical applications, particularly in the infrared (IR) and mid-IR part of the spectrum. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for infrared and mid-infrared opto-electronics. Herein, we experimentally verified its broadband and enhanced saturable absorption of multi-layer BP (with a thickness of ~10 nm) by wide-band Z-scan measurement technique, and anticipated that multi-layer BPs could be developed as another new type of two-dimensional saturable absorber with operation bandwidth ranging from the visible (400 nm) towards mid-IR (at least 1930 nm). Our results might suggest that ultra-thin multi-layer BP films could be potentially developed as broadband ultra-fast photonics devices, such as passive Q-switcher, mode-locker, optical switcher etc.

Metallic MXene Saturable Absorber for Femtosecond Mode-Locked Lasers.

Abstract: 2D transition metal carbides, nitrides, and carbonitides called MXenes have attracted much attention due to their outstanding properties. However, MXene's potential in laser technology is not explored. It is demonstrated here that Ti3 CN, one of MXene compounds, can serve as an excellent mode-locker that can produce femtosecond laser pulses from fiber cavities. Stable laser pulses with a duration as short as 660 fs are readily obtained at a repetition rate of 15.4 MHz and a wavelength of 1557 nm. Density functional theory calculations show that Ti3 CN is metallic, in contrast to other 2D saturable absorber materials reported so far to be operative for mode-locking. 2D structural and electronic characteristics are well conserved in their stacked form, possibly due to the unique interlayer coupling formed by MXene surface termination groups. Noticeably, the calculations suggest a promise of MXenes in broadband saturable absorber applications due to metallic characteristics, which agrees well with the experiments of passively Q-switched lasers using Ti3 CN at wavelengths of 1558 and 1875 nm. This study provides a valuable strategy and intuition for the development of nanomaterial-based saturable absorbers opening new avenues toward advanced photonic devices based on MXenes.

Ultrasensitive detection of miRNA with an antimonene-based surface plasmon resonance sensor.

Abstract: MicroRNA exhibits differential expression levels in cancer and can affect cellular transformation, carcinogenesis and metastasis. Although fluorescence techniques using dye molecule labels have been studied, label-free molecular-level quantification of miRNA is extremely challenging. We developed a surface plasmon resonance sensor based on two-dimensional nanomaterial of antimonene for the specific label-free detection of clinically relevant biomarkers such as miRNA-21 and miRNA-155. First-principles energetic calculations reveal that antimonene has substantially stronger interaction with ssDNA than the graphene that has been previously used in DNA molecule sensing, due to thanking for more delocalized 5s/5p orbitals in antimonene. The detection limit can reach 10 aM, which is 2.3-10,000 times higher than those of existing miRNA sensors. The combination of not-attempted-before exotic sensing material and SPR architecture represents an approach to unlocking the ultrasensitive detection of miRNA and DNA and provides a promising avenue for the early diagnosis, staging, and monitoring of cancer.

2D Materials for Optical Modulation: Challenges and Opportunities.

Abstract: 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.