Metasurfaces provide a compact, flexible, and efficient platform to manipulate the electromagnetic waves. However, chromatic aberration imposes severe restrictions on their applications in broadband polarization control. Here, we propose a broadband achromatic methodology to implement polarization-controlled multifunctional metadevices in mid-wavelength infrared with birefringent meta-atoms. We demonstrate the generation of polarization-controlled and achromatically on-axis focused optical vortex beams with diffraction-limited focal spots and switchable topological charge (L^∥ = 0 and L^⊥ = 2). Besides, we further implement broadband achromatic polarization beamsplitter with high polarization isolation (extinction ratio up to 21). The adoption of all-silicon configuration not only facilitates the integration with CMOS technology but also endows the polarization multiplexing meta-atoms with broad phase dispersion coverage, ensuring the large size and high performance of the metadevices. Compared with the state-of-the-art chromatic aberration-restricted polarization-controlled metadevices, our work represents a substantial advance and a step toward practical applications.

In semiconductor manufacturing, PN junction is formed by introducing dopants to activate neighboring electron and hole conductance. To avoid structural distortion and failure, it generally requires the foreign dopants localize in the designated micro-areas. This, however, is challenging due to an inevitable interdiffusion process. Here we report a brand-new junction architecture, called “layer PN junction”, that might break through such limit and help redefine the semiconductor device architecture. Different from all existing semiconductors, we find that a variety of van der Waals materials are doping themselves from n- to p-type conductance with an increasing/decreasing layer-number. It means the capability of constructing homogeneous PN junctions in monolayers’ dimension/precision, with record high rectification-ratio (>10⁵) and low cut-off current (<1 pA). More importantly, it spawns intriguing functionalities, like gate-switchable-rectification and noise-signal decoupled avalanching. Findings disclosed here might open up a path to develop novel nanodevice applications, where the geometrical size becomes the only critical factor in tuning charge-carrier distribution and thus functionality.

Avalanche or carrier-multiplication effect, based on impact ionization processes in semiconductors, has a great potential for enhancing the performance of photodetector and solar cells. However, in practical applications, it suffers from high threshold energy, reducing the advantages of carrier multiplication. Here, we report on a low-threshold avalanche effect in a stepwise WSe₂ structure, in which the combination of weak electron-phonon scattering and high electric fields leads to a low-loss carrier acceleration and multiplication. Owing to this effect, the room-temperature threshold energy approaches the fundamental limit, E_thre ≈ E_g, where E_g is the bandgap of the semiconductor. Our findings offer an alternative perspective on the design and fabrication of future avalanche and hot-carrier photovoltaic devices.

Titanium nitride (TiN), with its tunable work function, serves as an electrode metal in the scaling fin-type field-effect transistor and plays the key role for low threshold operation. Measuring the effective work function of thin TiN films is desirable for rapid evaluation before device fabrication. In this work, Kelvin probe force microscopy is applied to study the impact of various factors on the surface potential of TiN films with an uncertainty below 30 mV. By scraping and gauging the potential evolvement of TiN in different circumstances, it is revealed that the surface effect is the major obstacle to determine the work function of the as-deposited TiN. For thick films, the potential drops over 530 mV for a fresh TiN surface relative to that of an aged one. For TiN films thinner than 5 nm, the potential changes by 290 mV due to surface oxidation. This enables a quantitative assessment on the effective work function as well as the surface charge density of TiN films.