Metamaterials are artificial materials made of subwavelength elementary cells that give rise to unexpected wave properties that do not exist naturally. However, these properties are generally achieved due to 3D patterning, which is hardly feasible at short wavelengths in the visible and near-infrared regions targeted by most photonic applications. To overcome this limitation, metasurfaces, which are the 2D counterparts of metamaterials, have emerged as promising platforms that are compatible with planar nanotechnologies and thus mass production, which platforms the properties of a metamaterial into a 2D sheet. In the linear regime, wavefront manipulation for lensing, holography, and polarization control has been achieved recently. Interest in metasurfaces operating in the nonlinear regime has also increased due to the ability of metasurfaces to efficiently convert incident light into harmonic frequencies with unusual polarization properties. However, to date, the nonlinear absorption of metasurfaces has been mostly ignored. Here, we demonstrate that plasmonic metasurfaces behave as saturable absorbers with modulation performances superior to the modulation performance of other 2D materials and exhibit unusual polarimetric nonlinear transfer functions. We quantify the link between saturable absorption, the plasmonic resonances of the unit cell and their distribution in a 2D metasurface, and finally provide a practical implementation by integrating the metasurfaces into a fiber laser cavity operating in pulsed regimes driven by the metasurface properties. As such, this work provides new perspectives on ultrathin nonlinear saturable absorbers for applications where tunable nonlinear transfer functions are needed, such as in ultrafast lasers or neuromorphic circuits.
FeVO4 is a potentially promising n-type multimetal oxide semiconductor for photoelectrochemical water splitting based on its favorable optical band gap of ca. 2.06 eV that allows for the absorption of visible light up to around 600 nm. However, the presently demonstrated photocurrent values on FeVO4 photoanodes are yet considerably low when comparing with α-Fe2O3, although FeVO4 can absorb comparable wavelengths of sunlight as α-Fe2O3. Donor-type doping and constructing nanoporous film morphology have afforded desirable (but far from satisfactory) improvements in FeVO4 photoanodes, whereas the fundamental properties, such as absorption coefficients and the nature of optical transition, and a quantitative analysis of the efficiency losses for FeVO4 photoanodes remain elusive. In the present study, we conduct a thorough experimental analysis of structural, optical, charge transport, and surface catalysis properties of FeVO4 thin films to investigate and clarify how and where the efficiency losses occur. Based on the results, the charge recombination pathways and light-harvesting loss in FeVO4 thin-film photoanodes are identified and quantitatively determined. Our study will deepen the understanding on the photoelectrochemical behaviors of FeVO4 photoanodes and will also shed light on the optimization routes to engineer this material to approach its theoretical maximum.
We systematically investigate the passive harmonic mode locking (HML) of the bound states of two solitons in a fiber laser that has been mode locked by nonlinear polarization rotation (NPR). The experiment shows that the stable HML state of the bound solitons (BSs) with a fixed and discrete separation is obtained. The repetition rate changes by increasing the pump power and slightly altering the polarization state in the cavity. The dynamic is similar to the HML of a single-pulse operation. In our experiment, the repetition rate can be turned from the fundamental mode locking up to ninth-order HML when the pump power is increased from 168 to 476.1 mW. Once the BSs are obtained, their separation is fixed at 1.5 ps, regardless of the HML order. Under the direct BS interaction, the BS trains are very stable and easily reproducible. This HML behavior of BSs confirms that the BS is another intrinsic feature of the laser, except for the single soliton.
Two-dimensional (2D) materials have attracted extensive attention for use in fiber lasers for pulse generation due to their unique nonlinear optical properties. While 2D materials with tunable band gaps hold promise as versatile saturable absorber materials, their L-band (longband) pulse generation capability remains challenging. Metal phosphorus trichalcogenides (MPX 3 ) have recently attracted the attention of researchers and shown potential for sub-band gap saturable absorption in the L-band due to their high diversity of chemical components and band structural complexity. Herein, high-quality MnPSe 3 is synthesized and exhibits broad-band linear and nonlinear absorption with the modulation depth and saturation intensity of 5.4% and 0.295 MW/cm 2 , respectively. Moreover, a stable passive pulse generation in the L-band is demonstrated in a fiber laser. The wavelengths of the passively pulsed laser at different pump powers are recorded, featuring a fixed central wavelength located at around 1602 nm with a maximum output power of 19.54 mW. This research promotes the realization of L-band pulsed lasers based on 2D materials, inspiring further exploration of the unique properties of the MPX 3 family.
We report a novel saturable absorber (SA) based on anhydrous alcohol for mode-locked fiber lasers (MLFLs). The SA is an optical ferrule with one alcoholic end-facet sealed by a polyethylene (PE) film. Its modulation depth is measured to be 5.9%. Also, a self-starting MLFL using such an alcohol-SA has been demonstrated to generate 972-fs pulses at 1594.6 nm. The single pulse energy is up to 1.8 nJ with the repetition rate of 20.97 MHz, and the signal-to-noise ratio (SNR) is higher than 50 dB. The MLFL exhibits the performance of self-starting, good stability, and high pulse energy. Such a cost-effective and easily-prepared SA with high damage threshold may find wide applications for ultrafast lasers. Besides, it may arouse wide considerations of the mode-locking function of organic liquids for ultrafast lasers.
Laser induced damage of fused silica is a serious problem for high power laser systems, and damage precursors are mainly induced by manufacturing processes. In this work, fused silica samples were prepared from a manufacturing process including grinding, polishing and etching procedures. The chemical disorder of the prepared samples was inspected by using fluorescence microscopy and ultra-violet fluorescence spectrometer. The physical disorder was characterized by using Infrared and Raman spectrometer. Laser induced damage thresholds (LIDTs) were measured in R-on-1 mode by 355 nm 6.4 ns laser pulse. Results showed that with the manufacturing processes transforming from grinding to etching, the magnitude of fluorescence point defects reduced while their types did not change, the Si-O-Si bonds of prepared samples were strained and the strained bonds were mitigated. The LIDTs increased with the reducing of fluorescence defects and strained Si-O-Si bonds. However, these structural defects can not be eliminated by the current manufacturing process. Improvements may be needed to eliminate the structural defects for a higher LIDT of fused silica.
An alcohol not full-filled high-birefringence photonic crystal fiber (HiBi-PCF) temperature sensor based on an optical fiber Sagnac interferometer (OFSI) is demonstrated and investigated in detail. A new phenomenon that the resonant dip wavelengths of the temperature sensor blueshift with temperature increasing is observed, which is contrary to that of the previously reported alcohol filled HiBi-PCF OFSI temperature sensor. By considering the influences of the group birefringence and the thermo expansion of alcohol, this phenomenon is explained very well. The temperature sensitivity of the proposed sensor is about -1.17 nm/°C and is only one-sixth of that of the alcohol full-filled HiBi-PCF OSFI.
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