Ting Mei

WS2 mode-locked ultrafast fiber laser

Graphene-like two dimensional materials, such as WS2 and MoS2, are highly anisotropic layered compounds that have attracted growing interest from basic research to practical applications. Similar with MoS2, few-layer WS2 has remarkable physical properties. Here, we demonstrate for the first time that WS2 nanosheets exhibit ultrafast nonlinear saturable absorption property and high optical damage threshold. Soliton mode-locking operations are achieved separately in an erbium-doped fiber laser using two types of WS2-based saturable absorbers, one of which is fabricated by depositing WS2 nanosheets on a D-shaped fiber, while the other is synthesized by mixing WS2 solution with polyvinyl alcohol, and then evaporating them on a substrate. At the maximum pump power of 600 mW, two saturable absorbers can work stably at mode-locking state without damage, indicating that few-layer WS2 is a promising high-power flexible saturable absorber for ultrafast optics. Numerous applications may benefit from the ultrafast nonlinear features of WS2 nanosheets, such as high-power pulsed laser, materials processing, and frequency comb spectroscopy.

WS 2 saturable absorber for dissipative soliton mode locking at 1.06 and 1.55 µm

Transition-metal dichalcogenides, such as tungsten disulfide (WS2) and molybdenium disulfide (MoS2), are highly anisotropic layered materials and have attracted growing interest from basic research to practical applications due to their exotic physical property that may complement graphene and other semiconductor materials. WS2 nanosheets are found to exhibit broadband nonlinear saturable absorption property, and saturable absorbers (SAs) are fabricated by depositing WS2 nanosheets on side-polished fibers. Attributing to the weak evanescent field and long interaction length, the WS2 nanosheets are not exposed to large optical intensity, which allows the SA to work at the high-power regime. The SAs are used to mode lock erbium- and ytterbium-doped fiber lasers with normal dispersion, producing trains of dissipative soliton at 1.55 and 1.06 µm respectively. Simulations show that the bandgap of WS2 nanosheets decreases from 1.18 to 0.02 and 0.65 eV by introducing W and S defects respectively, which may contribute to the broadband saturable absorption property of the WS2.

Cylindrical vector beam generation in fiber with mode selectivity and wavelength tunability over broadband by acoustic flexural wave.

Theoretical analysis and experimental demonstration are presented for the generation of cylindrical vector beams (CVBs) via mode conversion in fiber from HE11 mode to TM01 and TE01 modes, which have radial and azimuthal polarizations, respectively. Intermodal coupling is caused by an acoustic flexural wave applied on the fiber, whereas polarization control is necessary for the mode conversion, i.e. HE11x→TM01 and HE11y→TE01 for acoustic vibration along the x-axis. The frequency of the RF driving signal for actuating the acoustic wave is determined by the phase matching condition that the period of acoustic wave equals the beatlength of two coupled modes. With phase matching condition tunability, this approach can be used to generate different types of CVBs at the same wavelength over a broadband. Experimental demonstration was done in the visible and communication bands.

Electrical control of second harmonic generation in a graphene-based plasmonic Fano structure

We propose a strategy for active control of second harmonic generation (SHG) in a plasmonic Fano structure by electrically doping its underlying monolayer graphene. A detailed theoretical model for the proposed scheme is developed and numerical simulations are carried out to demonstrate the operation. Specifically, we show that a merely 30 meV change in graphene Fermi level can result in 45 times increase in SHG peak intensity, accompanied by a resonance wavelength shift spanning 220 nm. Further analysis uncovers that such tunability in SHG arises from the Fermi-level-modulated graphene permittivity, the real and imaginary parts of which dominate the resonance wavelength and the intensity of SHG, respectively.

Optical vortex generation with wavelength tunability based on an acoustically-induced fiber grating

We presented a method to actualize the optical vortex generation with wavelength tunability via an acoustically-induced fiber grating (AIFG) driven by a radio frequency source. The circular polarization fundamental mode could be converted to the first-order optical vortex through the AIFG, and its topological charges were verified by the spiral pattern of coaxial interference between the first-order optical vortex and a Gaussian-reference beam. A spectral tuning range from 1540 nm to 1560 nm was demonstrated with a wavelength tunability slope of 4.65 nm/kHz. The mode conversion efficiency was 95% within the whole tuning spectral range.

Ultra-compact LED lens with double freeform surfaces for uniform illumination.

An ultra-compact rotational symmetric lens with double freeform surfaces based on the edge-ray principle is designed in this paper. The lens redistributes light emitting from a Lambertian LED light source to achieve uniform illumination within the target area. The initial design is optimized for optics compactness under structural constraints and illumination requirement using the genetic algorithm. A design for the double-freeform-surface lens with a height of the optics system h = 12.56 mm for a circular LED source with a diameter D = 10 mm is demonstrated for uniform illumination within 45° and thus achieves optics compactness h/D = 1.256, which is half of that achieved by the single-freeform-surface lens. The Monte-Carlo ray-tracing result shows an illumination circular area with a clear cut-off at R = 1000 mm at the target plane in a distance H = 1000 mm. The uniformity within the target illumination area is greater than 0.9 and the light output efficiency is as high as 0.9865.

High-order optical vortex generation in a few-mode fiber via cascaded acoustically driven vector mode conversion

We propose a method to generate the high-order optical vortex in a few-mode fiber via cascaded acoustically driven vector mode conversion. Theoretical analysis showed that the vector mode conversion induced by the acoustically induced fiber grating (AIFG) could occur between two HE modes with adjacent azimuthal numbers. In the experiment conducted at 532 nm, two AIFGs were simultaneously induced in the same segment of the fiber by a radio frequency source containing two different frequency components. One AIFG was used to convert the left- and right-handed circular polarization fundamental modes to the ±1-order vortex modes, which were then further converted to the ±2-order vortex modes by the other AIFG. The topological charges of the vortex modes were verified using both coaxial and off-axial interference methods, showing typical signature patterns of spiral forms and forklike fringes, respectively.

Wide band dispersionless slow light in hetero-MIM plasmonic waveguide

A flat slow-light band over a wide frequency range is obtained in the hetero-MIM (metal-insulator-metal) waveguide with zero group velocity dispersion (GVD). The zero GVD originates from dispersion compensation by the photonic mode and the plasmonic mode, the mechanism of which does not exist in the homo-MIM structure. By changing dielectric permittivity of the insulator or the difference of two different metallic plasma frequencies, the group index and the bandwidth can be tuned. The dispersionless slow light characteristic in the hetero-MIM waveguide may be useful in the new design of plasmonic devices.

Effect of dielectric cladding on active plasmonic device based on InGaAsP multiple quantum wells

The Surface Plasmon Polariton (SPP) planar waveguide with amorphous silicon (α-Si) cladding is studied, for empowering the device modulation response. The device is fabricated with multiple quantum wells (MQWs) as the gain media electrically pumped for compensating SPP propagation loss on Au film waveguide. The SPP propagation greatly benefits from the modal gain for the long-range hybrid mode, which is optimized by adopting an α-Si cladding layer accompanied with minimal degradation of mode confinement. The proposed structure presented more sensitive response to electrical manipulation than the one without cladding in experiment.

Generation of femtosecond optical vortex pulse in fiber based on an acoustically induced fiber grating

We proposed a method for generation of a femtosecond optical vortex pulse in a two-mode fiber based on an acoustically induced fiber grating (AIFG) driven by a radio frequency source. Theoretical analysis and experimental results demonstrated that the left- and right-handed circular polarization fundamental modes of the femtosecond optical pulse could be converted to the linearly polarized ±1-order optical vortex modes through the AIFG with the mode conversion efficiency of ∼95%. The off-axial interference experiment and the polarization angle-dependent intensity examination were performed to verify the topological charge and the polarization state of the femtosecond optical vortex, respectively.