Guobao Jiang

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

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.

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

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 potentially give some new insights into two-dimensional layered materials related photonics.

Large Energy, Wavelength Widely Tunable, Topological Insulator Q-Switched Erbium-Doped Fiber Laser

We report on the generation of large energy, widely wavelength tunable pulses in an erbium-doped fiber laser passively Q-switched by a topological insulator saturable absorber (TI-SA). The TI-SA is prepared through an optical deposition method. Its saturating intensity and modulation depth are measured to be about 57 MW/cm2 and 22%, respectively. We show that the high modulation depth of TI-SA allows the generation of stable Q-switched pulses with per-pulse energy up to 1.5 μJ and its broadband saturable absorption favors the tunable Q-switching operation from 1510.9 nm to 1589.1 nm. Our study suggests that TI: Bi2Te3 could be a promising saturable absorber for both the high energy and broadband optical applications.

The formation of various multi-soliton patterns and noise-like pulse in a fiber laser passively mode-locked by a topological insulator based saturable absorber

We experimentally investigated the formation of various multi-soliton patterns and noise-like (NL) pulses in an erbium-doped fiber laser passively mode-locked by a new type of saturable absorber: topological insulator. With the increase of pump power, various multi-soliton operation states—ordered, chaotic and bunched multiple-soliton—were subsequently obtained. Once the pump power exceeds 401 mW, an NL pulse state emerged, with a maximum 3 dB bandwidth of about 9.3 nm. This systematic study clearly demonstrated that a topological insulator could be an effective saturable absorber for the formation of various soliton operation states in a fiber laser cavity.

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

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 and enhanced nonlinear optical response of MoS2/graphene nanocomposites for ultrafast photonics applications.

Due to their relatively high compatibility with specific photonic structures, strong light-matter interactions and unique nonlinear optical response, two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, are attractive for ultrafast photonics applications. Here, we fabricate MoS2/graphene nanocomposites by a typical hydrothermal method. In addition, we systematically investigate their nonlinear optical responses. Our experiments indicate that the combined advantages of ultrafast relaxation, a broadband response from graphene, and the strong light-matter interaction from MoS2, can be integrated together by composition. The optical properties in terms of carrier relaxation dynamics, saturation intensity and modulation depth suggest great potential for the MoS2/graphene nanocomposites in photonics applications. We have further fabricated 2D nanocomposites based optical saturable absorbers and integrated them into a 1.5 μm Erbium-doped fiber laser to demonstrate Q-switched and mode-locked pulse generation. The fabrication of 2D nanocomposites assembled from different types of 2D materials, via this simple and scalable growth approach, paves the way for the formation and tuning of new 2D materials with desirable photonic properties and applications.

Drop-Casted Self-Assembled Topological Insulator Membrane as an Effective Saturable Absorber for Ultrafast Laser Photonics

Through employing a cost-effective solvothermal method, ultrathin topological insulator (TI) bismuth telluride (Bi2Te3) nanosheets with uniform hexagonal nanostructures had been synthesized. Thanks to the uniformity of few-layer TI dispersion, we are able to adopt the drop-casting approach in order to directly transfer few-layer TI and, therefore, form a self-assembled uniform volatile TI membrane that is suitably deposited onto the end facet of an optical fiber as an effective optical saturable absorber. Its saturable absorption parameters could be deliberately tailored by thinning its thickness by mechanical exfoliation. The incorporation of the as-fabricated saturable absorber inside the fiber laser cavity allows for the operation of either a microsecond or a femtosecond pulse because different saturable absorption parameters can decide whether the fiber laser operates in the mode-locking or Q-switching state. Our work provides a convenient way of fabricating a high-quality TI membrane-based saturable absorber with promising applications for laser operation.

Graphene Q-Switched Vectorial Fiber Laser With Switchable Polarized Output

We report a graphene Q-switched ytterbium-doped fiber laser with switchable cylindrical vector beam output. The 6-8 layers CVD-grown graphene films were transferred to the target substrate by an ultrasonic processing method, and its saturation intensity and modulation depth are measured to be about 0.61 MW/cm2 and 13.2% at 1072 nm by the Z-scan technique. We used a nanograting spatially variant waveplate as an intracavity polarization controlling element to convert pulsed Gaussian beam to radially or azimuthally polarized beam in the fiber laser cavity. Stable Q-switching operation can be achieved with the output power up to 253 mW and pulse energy 7.73 μJ at the maximum incident pump power. The polarization extinction ratio of radially and azimuthally polarized beam is 97.4% and 96.9%, respectively. The experimental results suggest that the graphene can act as a potential nonlinear optical material to modulate pulsed fiber lasers with spatially inhomogeneous polarizations.

A Broadband Optical Modulator Based on a Graphene Hybrid Plasmonic Waveguide

In this paper, a graphene hybrid plasmonic waveguide (HPW) modulator, in which a single layer of graphene-hexagonal-boron-nitride-graphene (graphene-hBN-graphene) has been embedded to enhance the absorption of the graphene, is numerically investigated based on a three-dimensional (3D) finite-difference time domain. The influences of geometric parameters, chemical potential, and dispersion on the fundamental mode of this modulator were determined. The height and width of the low index material results in significant effects to the effective mode index, which can determine the performance of the optical modulator. Using appropriate geometric parameter settings, this modulator could simultaneously offer a large extinction rate (up to 39.75 dB), broadband modulation bandwidth (up to 190.5 GHz), low power consumption (as low as 7.68 fJ/bit), and also provide subwavelength field confinement and long propagation distances. Wide-range wavelength response studies show that this optical modulator has good wavelength tolerance from 1200 to 1800 nm, indicating that it may be employed as an optical device exhibiting the desired performance. Furthermore, this optical modulator is not only suitable for optical fiber communications but also for free-space optical communications Our simulation results may provide experimental guidelines for designing future high-performance graphene optical modulators.

Wavelength-locked vectorial fiber laser manipulated by Pancharatnam-Berry phase.

We report a wavelength-locked cladding-pumped ytterbium-doped fiber laser that can simultaneously emit radially and azimuthally polarized beams based on Pancharatnam-Berry phase optical elements. Multi-wavelength free running operation of the radially and azimuthally polarized laser beams can be switched to a single-wavelength one assisted by volume Bragg grating, with wavelength locked at around 1053.4 nm and spectral linewidth of 0.06 nm (FWHW). By rotating the glan-taylor polarizer, we can obtain switchable radially and azimuthally polarized beams output. The radially and azimuthally polarized beams mode purity can maintain 97.3% and 96.3% at maximum output power, and the polarization extinction ratio (PER) can reach 97.8% and 95.9% for the radially and azimuthally polarized laser, respectively.