Yanqi Ge

Size-dependent nonlinear optical properties of black phosphorus nanosheets and their applications in ultrafast photonics

Multi-layer black phosphorus nanosheets (BPs) with different sizes are synthesized by using a basic solvent exfoliation method in association with controlled gradient centrifugation. Size-dependent nonlinear saturable absorption and Kerr nonlinearity as well as ultrafast carrier dynamics of BPs is systematically studied by using a Z-scan and pump–probe technique. Furthermore, an ultrashort pulse with a pulse duration of about 635 fs centered at a wavelength of 1562 nm is generated by using smaller sized BPs as a saturable absorber. These results directly reveal the physical processes of size-dependent nonlinear optical (NLO) properties of BPs and provide researchers with a viable approach in tailoring the NLO response of BPs through controlling the sizes, paving the way towards BP based electronics and optoelectronics applications such as ultrafast optical switches, modulators, fiber lasers, etc.

Ultrafast nonlinear absorption and nonlinear refraction in few-layer oxidized black phosphorus

We experimentally investigated the nonlinear optical response in few-layer oxidized black phosphorus (OBP) by the femtosecond Z-scan measurement technique, and found that OBP not only possesses strong ultrafast saturable absorption but also a nonlinear self-defocusing effect that is absent in black phosphorus (BP). The saturable absorption property originates mainly from the direct band structure, which is still maintained in OBP. The emergence of self-defocusing might originate from the combined consequences of the oxygen-induced defects in BP. Our experimental findings might constitute the first experimental evidence on how to dynamically tune its nonlinear property, offering an inroad in tailoring its optical properties through chemical modification (oxidation, introducing defects, etc.). The versatile ultrafast nonlinear optical properties (saturable absorption and self-defocusing) imply a significant potential of the layered OBP in the development of unprecedented optoelectronic devices, such as mode lockers, optical switches, laser beam shapers, and wavelength converters.

Few-layer selenium-doped black phosphorus: synthesis, nonlinear optical properties and ultrafast photonics applications

We reported on the synthesis of high quality bulk selenium (Se)-doped black phosphorus (BP) crystals using a mineralizer assisted gas-phase transformation in a mild growth condition and employed a liquid phase exfoliation approach to obtain few-layer (FL) Se-doped BP nano-sheets from the bulk counterpart. We experimentally found that FL Se-doped BP shows enhanced optical saturable absorption with long term stability, which may originate from selenium doping-induced changes of intrinsic carrier density. Taking advantage of its nonlinear optical property, we designed an optical saturable absorber (SA) device based on FL Se-doped BP, allowing the generation of a femto-second pulse in the fiber laser. Our results not only provide a new strategy in tailoring the nonlinear optical response of BP through element doping, but also suggest that FL Se-doped BP can be developed as a excellent candidate for ultrafast photonics.

Fundamental and harmonic mode-locking at 21 μm with black phosphorus saturable absorber

We report mode-locking in holmium-doped all-fiber laser based on black phosphorus saturable absorber. The generated solitons are centered at 2094 nm with bandwidth reaching 4.2 nm and pulse duration of 1.3 ps. In harmonic mode-locking, up to 10th harmonic (290 MHz) was obtained. Properties of black phosphorus saturable absorber are investigated. Our findings validate black phosphorus suitability for ultrafast applications in mid-infrared.

Sub-200 fs soliton mode-locked fiber laser based on bismuthene saturable absorber

Few-layer bismuthene is an emerging two-dimensional material in the fields of physics, chemistry, and material science. However, its nonlinear optical property and the related photonics device have been seldom studied so far. Here, we demonstrate a sub-200 fs soliton mode-locked erbium-doped fiber laser (EDFL) using a microfiber-based bismuthene saturable absorber for the first time, to the best of our knowledge. The bismuthene nanosheets are synthesized by the sonochemical exfoliation method and transferred onto the taper region of a microfiber by the optical deposition method. Stable soliton pulses centered at 1561 nm with the shortest pulse duration of about 193 fs were obtained. Our findings unambiguously imply that apart from its fantastic electric and thermal properties, few-layer bismuthene may also possess attractive optoelectronic properties for nonlinear photonics, such as mode-lockers, Q-switchers, optical modulators and so on.

Two-dimensional bismuth nanosheets as prospective photo-detector with tunable optoelectronic performance

Two dimensional Bi nanosheets have been employed to fabricate electrodes for broadband photo-detection. A series of characterization techniques including scanning electron microscopy and high-resolution transmission electron microscopy have verified that Bi nanosheets with intact lamellar structure have been obtained after facile liquid phase exfoliation. In the meanwhile, UV-vis and Raman spectra are also carried out and the inherent optical and physical properties of Bi nanosheets are confirmed. Inherited from the topological characteristics of Bi bulk counterpart, the resultant Bi nanosheet-based photo-detector exhibits preferable photo-response activity as well as environmental robustness. We then evaluate the photo-electrochemical (PEC) performance of the photodetector in 1 M NaOH and 0.5 M Na2SO4 electrolytes, and demonstrated that the as-prepared Bi nanosheets may possess a great potential as PEC-type photo-detector. Additional PEC measurements show that the current density of Bi nanosheets can reach up to 830 nA cm-2, while an enhanced responsivity (1.8 μA W-1) had been achieved. We anticipate that this contribution can provide feasibility towards the construction of high-performance elemental Bi nanosheets-based optoelectronic devices in the future.

Nonlinear optical absorption and ultrafast carrier dynamics of copper antimony sulfide semiconductor nanocrystals

Ternary copper antimony sulfide nanocrystals (CAS NCs), a promising solar cell candidate, have been proposed and investigated from the perspective of synthesis method, linear optical response, and size and band structure tunability. Herein, we present the intensity related nonlinear absorption using the Z-scan technique. Electron and hole relaxation dynamics for the as-prepared three phases, CuSbS2, Cu3SbS4 and Cu12Sb4S13 NCs, are studied at the visible spectrum band. With the assistance of fs-resolved transient absorption spectrum technology, photon induced charge carrier dynamics within the valence/conduction band and trap states are analyzed and attributed to three relaxation processes, the corresponding lifetimes of which are ∼400 fs, ∼5 ps and ∼60 ps, respectively. This analysis on electron and hole spatial separation and recombination is significant for the improvement of CAS NC based devices, and paves the way for the application of semiconductor NCs in photovoltaic devices, optical detection and other optoelectronic devices.

Black-phosphorus-analogue tin monosulfide: an emerging optoelectronic two-dimensional material for high-performance photodetection with improved stability under ambient/harsh conditions

It is widely known that excellent intrinsic electronic and optoelectronic advantages of black phosphorus (BP) make it attractive for applications in transistor, logic and optoelectronic devices. However, easy degradation upon exposure to ambient conditions turns out to be the most critical bottleneck in restricting its practical applications. In order to simultaneously achieve good stability and high device performance, a two-dimensional (2D) size-selected black-phosphorus-analogue (BPA), tin monosulfide nanosheets (SnS Ns), with enhanced chemical stability was successfully fabricated by a facile liquid phase exfoliation (LPE) method, followed by a liquid cascade centrifugation (LCC) technique. Similar to BP, SnS shows thickness modulated bandgap energy. The as-prepared SnS Ns were employed to fabricate working electrodes for photoelectrochemical (PEC)-type broadband photodetection for the first time, and they exhibit an appropriate capacity for a self-powered broadband photoresponse. More importantly, due to the weak polar character of Sn–S bonds, the SnS-based photodetector shows strong long-term temporal and cycling stability of ON/OFF switching behaviour without any external protection not only in regular basic electrolytes but also under strong polar conditions including harsh acidic and neutral electrolytes. We therefore anticipate that SnS as an emerging optoelectronic material might be developed towards versatile applications such as photodetection, sensing and modulation, etc.