Plasmonic Hot Carriers-Controlled Second Harmonic Generation in WSe₂ Bilayers

Modulating second harmonic generation (SHG) by a static electric field through either electric-field-induced SHG or charge-induced SHG has been well documented. Nonetheless, it is essential to develop the ability to dynamically control and manipulate the nonlinear properties, preferably at high speed. Plasmonic hot carriers can be resonantly excited in metal nanoparticles and then injected into semiconductors within 10–100 fs, where they eventually decay on a comparable time scale. This allows one to rapidly manipulate all kinds of characteristics of semiconductors, including their nonlinear properties. Here we demonstrate that plasmonically generated hot electrons can be injected from plasmonic nanostructure into bilayer (2L) tungsten diselenide (WSe₂), breaking the material inversion symmetry and thus inducing an SHG. With a set of pump–probe experiments we confirm that it is the dynamic separation electric field resulting from the hot carrier injection (rather than a simple optical field enhancement) that is the cause of SHG. Transient absorption measurement further substantiate the plasmonic hot electrons injection and allow us to measure a rise time of ∼120 fs and a fall time of 1.9 ps. Our study creates opportunity for the ultrafast all-optical control of SHG in an all-optical manner that may enable a variety of applications

Chiral Structure Determination of Aligned Single-Walled Carbon Nanotubes on Graphite Surface

Chiral structure determination of single-walled carbon nanotube (SWNT), including its handedness and chiral index (<i>n</i>,<i>m</i>), has been regarded as an intractable issue for both fundamental research and practical application. For a given SWNT, the <i>n</i> and <i>m</i> values can be conveniently deduced if an arbitrary two of its three crucial structural parameters, that is, diameter <i>d</i>, chiral angle θ, and electron transition energy <i>E</i>_<i>ii</i>, are obtained. Here, we have demonstrated a novel approach to derive the (<i>n</i>,<i>m</i>) indices from the θ, <i>d</i>, and <i>E</i>_<i>ii</i> of SWNTs. Handedness and θ were quickly measured based on the chirality-dependent alignment of SWNTs on graphite surface. By combining their measured <i>d</i> and <i>E</i>_<i>ii</i>, (<i>n</i>,<i>m</i>) indices of SWNTs can be independently and uniquely identified from the (θ,<i>d</i>) or (θ,<i>E</i>_<i>ii</i>) plots, respectively. This approach offers intense practical merits of high-efficiency, low-cost, and simplicity

Room-Temperature Polariton Lasing in All-Inorganic Perovskite Nanoplatelets

Polariton lasing is the coherent emission arising from a macroscopic polariton condensate first proposed in 1996. Over the past two decades, polariton lasing has been demonstrated in a few inorganic and organic semiconductors in both low and room temperatures. Polariton lasing in inorganic materials significantly relies on sophisticated epitaxial growth of crystalline gain medium layers sandwiched by two distributed Bragg reflectors in which combating the built-in strain and mismatched thermal properties is nontrivial. On the other hand, organic active media usually suffer from large threshold density and weak nonlinearity due to the Frenkel exciton nature. Further development of polariton lasing toward technologically significant applications demand more accessible materials, ease of device fabrication, and broadly tunable emission at room temperature. Herein, we report the experimental realization of room-temperature polariton lasing based on an epitaxy-free all-inorganic cesium lead chloride perovskite nanoplatelet microcavity. Polariton lasing is unambiguously evidenced by a superlinear power dependence, macroscopic ground-state occupation, blueshift of the ground-state emission, narrowing of the line width and the buildup of long-range spatial coherence. Our work suggests considerable promise of lead halide perovskites toward large-area, low-cost, high-performance room-temperature polariton devices and coherent light sources extending from the ultraviolet to near-infrared range