Exciton-Enhanced Spontaneous Parametric Down-Conversion in Two-Dimensional Crystals.

We show that excitonic resonances and interexciton transitions can enhance the probability of spontaneous parametric down-conversion, a second-order optical response that generates entangled photon pairs. We benchmark our ab initio many-body calculations using experimental polar plots of second harmonic generation in NbOI_{2}, clearly demonstrating the relevance of excitons in the nonlinear response. A strong double-exciton resonance in 2D NbOCl_{2} leads to giant enhancement in the second order susceptibility.

Linear Electro-Optic Effect in 2D Ferroelectric for Electrically Tunable Metalens.

The advent of 2D ferroelectrics, characterized by their spontaneous polarization states in layer-by-layer domains without the limitation of a finite size effect, brings enormous promise for applications in integrated optoelectronic devices. Comparing with semiconductor/insulator devices, ferroelectric devices show natural advantages such as non-volatility, low energy consumption and high response speed. Several 2D ferroelectric materials have been reported, however, the device implementation particularly for optoelectronic application remains largely hypothetical.

Revamping Triboelectric Output by Deep Trap Construction.

High output performance is critical for building triboelectric nanogenerators (TENGs) for future multifunctional applications. Unfortunately, the high triboelectric charge dissipation rate has a significant negative impact on its electrical output performance. Herein, a new tribolayer is designed through introducing self-assembled molecules with large energy gaps on commercial PET fibric to form carrier deep traps, which improve charge retention while decreasing dissipation rates.

Sliding induced multiple polarization states in two-dimensional ferroelectrics.

When the atomic layers in a non-centrosymmetric van der Waals structure slide against each other, the interfacial charge transfer results in a reversal of the structure's spontaneous polarization. This phenomenon is known as sliding ferroelectricity and it is markedly different from conventional ferroelectric switching mechanisms relying on ion displacement. Here, we present layer dependence as a new dimension to control sliding ferroelectricity.

Data-driven discovery of high performance layered van der Waals piezoelectric NbOI.

Using high-throughput first-principles calculations to search for layered van der Waals materials with the largest piezoelectric stress coefficients, we discover NbOI to be the one among 2940 monolayers screened. The piezoelectric performance of NbOI is independent of thickness, and its electromechanical coupling factor of near unity is a hallmark of optimal interconversion between electrical and mechanical energy. Laser scanning vibrometer studies on bulk and few-layer NbOI crystals verify their huge piezoelectric responses, which exceed internal references such as InSe and CuInPS.