Ultrafast fiber laser based on HfSe2 saturable absorber

2D Semi-Metallic Hafnium Ditelluride: A Novel Nonlinear Optical Material for Ultrafast and Ultranarrow Photonics Applications

2D semi-metallic hafnium ditelluride material is used in several applications such as solar steam generation, gas sensing, and catalysis owing to its strong near-infrared absorbance, high sensitivity, and distinctive electronic structure. The zero-bandgap characteristics, along with the thermal and dynamic stability of 2D-HfTe2, make it a desirable choice for developing long-wavelength-range photonics devices. Herein, the HfTe2 -nanosheets are prepared using the liquid-phase exfoliation method, and their superior nonlinear optical properties are demonstrated by the obtained modulation depth of 11.9% (800 nm) and 6.35% (1560 nm), respectively. In addition, the observed transition from saturable to reverse saturable absorption indicates adaptability of the prepared material in nonlinear optics. By utilizing a side polished fiber-based HfTe2 -saturable absorber (SA) inside an Er-doped fiber laser cavity, a mode-locked laser with 724 fs pulse width and 56.63 dB signal-to-noise ratio (SNR) is realized for the first time. The generated laser with this SA has the second lowest mode-locking pump threshold (18.35 mW), among the other 2D material based-SAs, thus paving the way for future laser development with improved efficiency and reduced thermal impact. Finally, employing this HfTe2 -SA, a highly stable single-frequency fiber laser (SNR ≈ 74.56 dB; linewidth ≈ 1.268 kHz) is generated for the first time, indicating its promising ultranarrow photonic application.

2D layered MSe2 (M = Hf, Ti and Zr) for compact lasers: nonlinear optical properties and GHz lasing

Two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) are of great research interest because their superior layer-dependent optical modulation properties. In this work, three different kinds of TMDC nanosheets, including hafnium diselenide (HfSe2), titanium diselenide (TiSe2) and zirconium diselenide (ZrSe2), are prepared by liquid phase exfoliation (LPE) technique. The high-quality material properties of these TMDC nanosheets are confirmed by Raman spectroscopy and X-ray diffraction analysis. Furthermore, the bandgap information of five-layer MSe2 has been investigated via utilizing density functional theory. The calculation results exhibit ultra-narrow bandgap structure (lower than 1.1 eV) for all these three materials, indicating that MSe2 is suitable for broadband photonic applications. By applying the fabricated MSe2 as saturable absorbers, high-performance Q-switched mode-locked laser operation has been realized. The laser gain media are Nd:GdVO4 cladding waveguides fabricated by femtosecond laser direct writing. As a result, the pulsed waveguide lasers are able to deliver approximately 6-GHz laser pulses with a signal-to-noise ratio of over 45 dB. The minimum pulse width is determined to be as short as 26 ps. The results demonstrated in this work exhibit the great potential of TMDCs and waveguide structures in applications of pulsed lasers with compact footprints.

Low-Power Memristive Logic Device Enabled by Controllable Oxidation of 2D HfSe2 for In-Memory Computing

Memristive logic device is a promising unit for beyond von Neumann computing systems and 2D materials are widely used because of their controllable interfacial properties. Most of these 2D memristive devices, however, are made from semiconducting chalcogenides which fail to gate the off-state current. To this end, a crossbar device using 2D HfSe2 is fabricated, and then the top layers are oxidized into "high-k" dielectric HfSex Oy via oxygen plasma treatment, so that the cell resistance can be remarkably increased. This two-terminal Ti/HfSex Oy /HfSe2 /Au device exhibits excellent forming-free resistive switching performance with high switching speed (<50 ns), low operation voltage (<3 V), large switching window (103 ), and good data retention. Most importantly, the operation current and the power consumption reach 100 pA and 0.1 fJ to 0.1 pJ, much lower than other HfO based memristors. A functionally complete low-power Boolean logic is experimentally demonstrated using the memristive device, allowing it in the application of energy-efficient in-memory computing.

Ultrafast Photonics of Ternary RexNb(1-x)S2 in Fiber Lasers

Two-dimensional (2D) transition metal chalcogenides (TMCs) become more attractive upon addition of a third element owing to their unique structure and remarkable physical and chemical properties, which endow these materials with considerable potential for applications in nanoscale devices. In this work, a Re_xNb_(1-x)S₂-based saturable absorber (SA) device for ultrafast photonics applications is studied. The device is assembled by placing Re_xNb_(1-x)S₂ nanosheets with a thickness of 1-3 nm onto a microfiber to increase their compatibility with an all-fiber laser cavity. The prepared Re_xNb_(1-x)S₂-based device exhibits a modulation depth of 24.3%, a saturation intensity of 10.1 MW/cm², and a nonsaturable loss of 28.5%. Furthermore, the Re_xNb_(1-x)S₂-based device is used to generate ultrashort pulses in an erbium-doped fiber (EDF) laser cavity. At a pump power of 260 mW, the EDF laser operates in a conventional soliton mode-locked region. The pulse width is 285 fs, and the repetition frequency is 61.993 MHz. In particular, the bound-state soliton mode-locking operation is successfully obtained in a pump power range of 300-900 mW. The bound-state pulses are formed by doubling identical solitons with a temporal interval of 0.8 ps. The output power is as high as 47.9 mW, and the repetition frequency is 123.61 MHz. These results indicate that the proposed Re_xNb_(1-x)S₂-based SAs have comparable properties to currently used 2D SAs and provide a basis for their application in the field of ultrafast photonics.

WxNb(1-x)Se2 nanosheets for ultrafast photonics

Ternary transition metal chalcogenides (TTMDCs), a novel type of two-dimensional (2D) three-element materials, possess multiple physical and chemical properties and have promising potentials in basic physics and devices. Herein, the usage of W_xNb_(1-x)Se₂ nanosheets as a rising ultrafast photonic device to generate high power mode-locked and Q-switched pulses in a fiber laser is demonstrated. The W_xNb_(1-x)Se₂ nanosheets were successfully prepared by the liquid exfoliation method with thickness less than 3 nm. The nonlinear optical absorption of the W_xNb_(1-x)Se₂-based device was investigated with the saturable intensity of 40.93 MW cm^-2 and modulation depth of 5.43%. After integrating the W_xNb_(1-x)Se₂-based device into an Er-doped fiber (EDF) laser cavity, mode-locking and Q-switching laser pulses were formed. In the mode-locked mechanism output, the pulse width is as narrow as 131 fs and the output power is 52.93 mW. In Q-switched operation, the shortest pulse duration is 1.47 μs with the largest pulse energy of 257 nJ. Compared to recent studies, our results showed some improvements. This study suggests that 2D TTMDC-based devices could be developed as efficient ultrafast photonics candidates and widely used in nonlinear optical applications.