The Internet-of-Things (IoT) will significantly change both industrial manufacturing and our daily lives. Data collection and three-dimensional (3D) positioning of IoT devices are two indispensable services of such networks. However, in conventional networks, only terrestrial base stations (BSs) are used to provide these two services. On the one hand, this leads to high energy consumption for devices transmitting at cell edges. On the other hand, terrestrial BSs are relatively close in height, resulting in poor performance of device positioning in elevation. Due to their high maneuverability and flexible deployment, unmanned aerial vehicles (UAVs) could be a promising technology to overcome the above shortcomings. In this paper, we propose a novel UAV-assisted IoT network, in which a low-altitude UAV platform is employed as both a mobile data collector and an aerial anchor node to assist terrestrial BSs in data collection and device positioning. We aim to minimize the maximum energy consumption of all devices by jointly optimizing the UAV trajectory and devices' transmission schedule over time, while ensuring the reliability of data collection and required 3D positioning performance. This formulation is a mixed-integer non-convex optimization problem, and an efficient differential evolution (DE) based method is proposed for solving it. Numerical results demonstrate that the proposed network and optimization method achieve significant performance gains in both energy efficient data collection and 3D device positioning, as compared with a conventional terrestrial IoT network.

Van der Waals (vdWs) heterostructures based on in-plane isotropic/anisotropic 2D-layered semiconducting materials have recently received wide attention because of their unique interlayer coupling properties and hold a bright future as building blocks for advanced photodetectors. However, a fundamental understanding of charge behavior inside this kind of heterostructure in the photoexcited state remains elusive. In this work, we carry out a systematic investigation into the photoinduced interfacial charge behavior in type-II WS2/ReS2 vertical heterostructures via polarization-dependent pump–probe microscopy. Benefiting from the distinctive (ultrafast and anisotropic) charge-transfer mechanisms, the photodetector based on the WS2/ReS2 heterojunction displays more superior optoelectronic properties compared to its constituents with diverse functionalities including moderate photoresponsivity, polarization sensitivity, and fast photoresponse speed. Additionally, this device can function as a self-driven photodetector without the external bias. These results of our work tangibly corroborate the intriguing interlayer interaction in in-plane isotropic/anisotropic heterostructures and are expected to shed light on designing balanced-performance multifunctional optoelectrical devices.

Searching for ideal materials with strong effective optical nonlinear responses is a long-term task enabling remarkable breakthroughs in contemporary quantum and nonlinear optics. Polaritons, hybridized light-matter quasiparticles, are an appealing candidate to realize such nonlinearities. Here, we explore a class of peculiar polaritons, named plasmon–exciton polaritons (plexcitons), in a hybrid system composed of silver nanodisk arrays and monolayer tungsten-disulfide (WS2), which shows giant room-temperature nonlinearity due to their deep-subwavelength localized nature. Specifically, comprehensive ultrafast pump–probe measurements reveal that plexciton nonlinearity is dominated by the saturation and higher-order excitation-induced dephasing interactions, rather than the well-known exchange interaction in traditional microcavity polaritons. Furthermore, we demonstrate this giant nonlinearity can be exploited to manipulate the ultrafast nonlinear absorption properties of the solid-state system. Our findings suggest that plexcitons are intrinsically strongly interacting, thereby pioneering new horizons for practical implementations such as energy-efficient ultrafast all-optical switching and information processing.

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Due to the unique anisotropic chemical and physical properties, two-dimensional (2D) layered materials, such as IV-VI monochalcogenides with puckered honeycomb structure, have received considerable interest recently. Among the IV-VI layered MX (M = Ge, Sn; X = Se, S) compounds, germanium sulfide (GeS) stands out for its strongest anisotropic thermal conductivities and figure-of-merit values. Additionally, the layer-independent direct energy bands (E g~1 .6 eV, E 1~2 .1 eV) of GeS flake provide excellent insights into further applications as visible photodetectors. Herein, the polarization-tunable nonlinear absorption (NA) patterns of GeS flake have been systematically investigated. Specifically, both the polarization-dependent Raman spectroscopy and the linear absorption (LA) spectroscopy were employed to characterize the lattice orientation and absorption edges of the 251-nm GeS flake. Considering the low damage threshold of GeS flake, the GeS/graphene heterostructure was fabricated to increase the threshold without changing the nonlinear properties of GeS. Our NA results demonstrated that a 600-nm femtosecond laser with different polarizations would excite the saturated-absorption (SA) effect along armchair and reversesaturated-absorption (RSA) effect along zigzag in the GeS/graphene heterostructure. Moreover, the function of the polarization-based GeS/graphene heterostructure all-optical switch was experimentally verified. Notably, thanks to the polarization-dependent NA patterns (SA/RSA) of GeS, the "ON" and "OFF" states of the all-optical switch can be accomplished by high and low transmittance states of continuous-wave laser (532 nm, 80 nW), whose state can be controlled by the polarization of femtosecond switching laser (600 nm, 35 fs, 500 Hz, 12 GW cm −2). The ON/OFF ratio can achieve up to 17% by changing polarization, compared with the ratios of 3.0% by increasing the incident power of switching light in our experiment. The polarization-tunable absorption patterns introduced in this work open up real perspectives for the next-generation optoelectronic devices based on GeS/graphene heterostructure.

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The competition between different spatiotemporal carrier relaxation determines the carrier harvesting in optoelectronic semiconductors, which can be greatly optimized by utilizing the ultrafast spatial expansion of highly energetic carriers before their energy dissipation via carrier-phonon interactions. Here, the excited-state dynamics in layered tungsten disulfide (WS 2 ) are primarily imaged in the temporal, spatial, and spectral domains by transient absorption microscopy. Ultrafast hot carrier expansion is captured in the first 1.4 ps immediately after photoexcitation, with a mean diffusivity up to 980 cm 2 s −1 . This carrier diffusivity then rapidly weakens, reaching a conventional linear spread of 10.5 cm 2 s −1 after 2 ps after the hot carriers cool down to the band edge and form bound excitons. The novel carrier diffusion can be well characterized by a cascaded transport model including 3D thermal transport and thermo-optical conversion, in which the carrier temperature gradient and lattice thermal transport govern the initial hot carrier expansion and long-term exciton diffusion rates, respectively. The ultrafast hot carrier expansion breaks the limit of slow exciton diffusion in 2D transition metal dichalcogenides, providing potential guidance for high-performance applications and thermal management of optoelectronic technology.

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