Lead iodide (PbI2) is a layered material with unique optical and electrical properties. Despite being extensively studied in its bulk form and being used as a precursor for perovskite materials, the recently developed PbI2 nanosheets have shown a great promise for high-performance optoelectronic devices, such as flexible photodetectors, heterostructure photodetectors and nanolasers. These novel applications of PbI2 nanosheets, however, require careful characterisation of their crystalline structure. Here we experimentally and numerically demonstrate the nonlinear microscopy of PbI2 nanosheets. It is a non-invasive optical technique to precisely determine the thickness, crystalline orientation and strain.
Lead halide perovskites are widely applied in not only photovoltaics, but also on-chip light source, nanolaser, and photon detection. In order to promote the incorporation of perovskite into integrated devices, microscale color patterning flexibility is a very important step. Femtosecond (fs) laser fabrication has shown significant advantages of high spatial resolution, low surround damage, and high processing efficiency over the other laser fabrication. Compared to the state-of-art techniques, the straightforward fs-direct laser writing (fs-DLW) also has advantages of mask-free, simple one step, and contactless. Here, a specially designed formamidinium lead mixed-halide nanoplatelet (FAPb(BrxI1-x)3 NP) with gradient bandgap is fabricated by chemical vapor deposition method. Then, spatially resolved modulation of the fluorescence by fs-DLW is demonstrated on the as-grown NP. The fluorescence color is modulated from red to green under a controlled laser pulse, due to the replacement of iodide ions by bromide ions. Specifically, the as-grown NP (thickness≈800 nm) is with a gradual bromide-iodide composition along the depth, mainly exhibits an emission of 710-nm from the bottom iodine rich phase. After halide substitution induced by fs-DLW, new fluorescence peaks appear in the wavelength range of 540 to 700 nm, which is controlled by the fs-DLW conditions. The fluorescent color is spatially modulated from red to green, enabling microscale resolved multicolor emission, implying the potential applications in micro-encryption, sensors, multicolor displays, lasers, and light-emitting devices.
Black phosphorus (BP) is a very promising two-dimensional material as a saturable absorber for ultrashort pulse generation especially in telecommunication bands due to its ultrafast dynamic response and strong resonant absorption in the near-infrared wavelength range. However, the current fabrication methods of BP saturable absorbers are very complex and not suitable for practical large-scale production. We have successfully deposited BP with a thickness of ∼25 nm onto the fiber end facet as a saturable absorber by a simple optically driven deposition method. The BP saturable absorber shows excellent mode-locking performance with a stable pulse train repetition of 1.843 MHz and pulse duration of 117.6 ns.
We have investigated the broadband saturable absorption property of graphene–Bi2Te3 heterostructures and demonstrated their applications for stable harmonic mode-locking operation in a Yb-doped fiber laser and wavelength-tunable Q-switching operation in an Er-doped fiber laser. The modulation depth of a graphene–Bi2Te3 heterostructure saturable absorber (G-Bi2Te3-SA) is dependent on the coverage of Bi2Te3 on the graphene. By using 15%-Bi2Te3-covered G-Bi2Te3-SA with a modulation depth of 23.28% and saturable intensity of 3.32 MW/cm2, the harmonic mode-locked Yb-doped fiber laser outputs the mode-locked pulses with a pulse duration down to 189.94 ps, spectral bandwidth of 3.5 nm, and repetition rate of 79.13 MHz (21st order of the fundamental frequency). After inserting the G-Bi2Te3-SA with 85% coverage of Bi2Te3 on graphene into Er-doped fiber laser cavity, whose modulation depth and saturable intensity are about 40.79% and 12.48 MW/cm2, respectively, the wavelength-tunable Q-switched pulse with tunable wavelength range over 13.2 nm has been obtained by adjusting the intracavity fiber filter. These results suggest that the graphene–Bi2Te3 heterostructure could serve as a high nonlinear photonic device for practical applications.
The optical response and plasmon coupling between graphene sheets for graphene/polymer multilayer heterostructures with controlled separation were systematically investigated. Anomalous transmission of light was experimentally observed in mid-infrared range. The position of the broad passband in the transmission spectra was observed to red-shift with the increase of the number of layers.
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