A group of scientists recently published a paper in the journal Optics & Technology that demonstrated the effectiveness of microfiber-based InSe-Au saturable absorber (SA) for ultrafast photonics in the mid- and near-infrared regions.
Study: Two-dimensional gold decorated indium selenide for near-infrared and mid-infrared ultrafast photonics. Image Credit: wacomka/Shutterstock.com
Mode-locked fiber lasers (MLFLs) have gained considerable prominence owing to their extensive application potential in telecommunications, biomedicine, and optical sensing.
Soliton dynamics were studied extensively to investigate different mode-locking mechanisms and optical phenomena.
Passive mode-locking was found to be more advantageous than active mode-locking owing to its simple design, excellent compactness, and low cost.
Since the first development of saturable absorber mirror (SESAM) and its application as a saturable absorber (SA), similar types of passive SAs were developed and commercialized.
Recently, two-dimensional (2D) materials and their applications have attracted significant attention due to their exclusive nonlinear optical properties (NOPs) such as broad absorption bandwidth and low cost.
InSe flakes, which are primarily layered metal-chalcogenide semiconductors, have gained attraction as promising 2D material for fabricating SAs owing to their exceptional optoelectronic properties.
For instance, they demonstrate higher environmental stability and electron mobility at room temperature compared to black phosphorus and dichalcogenides, respectively, and excellent plasticity and deformability compared to graphene.
However, the 1.26–2.20 eV bandgap in InSe nanosheets makes them unsuitable for application in mid- and near-infrared photonic areas.
In this study, researchers modified the bandgap in InSe by growing gold (Au) nanoparticles on the nanosheet surfaces to obtain an InSe-Au heterostructure.
Au nanoparticles were selected as modifying agents considering their high absorption capabilities in mid-and near-infrared regions.
A liquid-phase exfoliation approach was employed to synthesize the InSe nanosheets. In the preparation, 0.3 gm of bulk InSe was submerged into 10 mL of N-methyl-2-pyrrolidone and then processed by continuous sonication for 6 h at 300 W power.
Temperature-controlled equipment was used to keep the mixture temperature below 25 oC to prevent product degradation.
Subsequently, a centrifugal machine was used to perform centrifugation of the dispersion product for 2 min at 6000 rpm, followed by another centrifugation for 2 min at 7500 rpm, to obtain high-quality InSe nanosheets.
The synthesized InSe nanosheets were dispersed at a concentration of 1 mg/mL in deionized water and the concentration of Au in the chloroauric acid water solution was kept at 0.1 mg/mL.
1 mL of sodium borohydride aqueous solution was mixed with 1 mL InSe dispersant and 0.1 mL chloroauric acid, and processed by sonication. The mixture was then centrifuged for 2 min at 6000 rpm, and the resultant sediment was denoted as InSe-Au nanosheets.
Transmission electron microscopy (TEM) was used to characterize the structural information and morphologies of InSe-Au nanosheets, while atomic force microscopy (AFM) was employed to obtain the thickness information of nanosheets.
The Raman spectrum was obtained using the Witec-Alpha 300R Raman microscope.
The absorption characteristics of the sample were measured by dispersing the sheets in carbon disulfide for 10 min, and the absorption spectra were obtained using an ultraviolet–visible-near-infrared spectrophotometer.
The synthesized InSe-Au sheets were initially dissolved into an isopropyl alcohol solution and then fabricated into microfiber-based SAs through a common optical deposition method. The narrowest waist diameter of a single-mode microfiber was 10 µm.
A common power-dependent transmission method involving ultrafast fiber lasers as pulse sources was used to measure the microfiber-based InSe-Au SA NOPs.
A wavelength division multiplexer (WDM) was used to inject the output beam obtained from the pump sources comprising 1570 nm laser diode for thulium-doped fiber laser (TDFL) and 976 nm laser diode for erbium-doped fiber laser (EDFL) into the 5 m-thulium-doped fiber and 5 m-erbium-doped fiber.
A microfiber-based InSe-Au SA and two polarization controllers (PCs) were used for the generation of ultrashort pulses and mode-locking operation, while the ultrafast incidents were detected with a real-time horizon using a homemade dispersive Fourier transform (DFT) device.
The thickness of the InSe nanosheets was 7.78 nm, while the thickness of the Au nanoparticles attached to the InSe nanosheets was 11.90 nm and 12.48 nm.
The Raman spectrum demonstrated three Raman modes that were consistent with the Raman modes of InSe crystals.
The InSe-Au heterojunction demonstrated stronger absorption characteristics from the ultraviolet to the mid-infrared wavelength region compared to the pristine InSe.
Two sub-band gaps of 0.80 eV and 0.61 eV were observed in the InSe-Au due to the interface between Au particles and InSe nanosheets, which contributed to the exceptional NOPs of InSe-Au nanosheets at 2.0 and 1.5-μm respectively.
Stable mode-locking pulses were generated using the microfiber-based InSe-Au SA with pulse durations of ~2.51 ps at 2.0 μm and ~507 fs at 1.5 μm.
The spectral evolution of a usual unstable breathing mode-locking state was observed at 1.5 μm with a real-time horizon.
To summarize, the findings of this study demonstrated the effectiveness of 2D materials to investigate the nonlinear soliton dynamics in ultrafast fiber laser systems, and the suitability of 2D InSe-Au in applications related to ultrafast photonics in the mid- and near-infrared regions.
Zhang, C., Chen, T., Zhao, T. et al. (2022) Two-dimensional gold decorated indium selenide for near-infrared and mid-infrared ultrafast photonics. Optics & Laser Technology https://www.sciencedirect.com/science/article/pii/S0030399222000779.