Two-dimensional (2D) transition metal dichalcogenides (TMDCs) and graphene compose a new family of crystalline materials with atomic thicknesses and exotic mechanical, electronic, and optical properties. Due to their inherent exceptional mechanical flexibility and strength, these 2D materials provide an ideal platform for strain engineering, enabling versatile modulation and significant enhancement of their optical properties. For instance, recent theoretical and experimental investigations have demonstrated flexible control over their electronic states via application of external strains, such as uniaxial strain and biaxial strain. Meanwhile, many nondestructive optical measurement methods, typically including absorption, reflectance, photoluminescence, and Raman spectroscopies, can be readily exploited to quantitatively determine strain-engineered optical properties. This review begins with an introduction to the macroscopic theory of crystal elasticity and microscopic effective low-energy Hamiltonians coupled with strain fields, and then summarizes recent advances in strain-induced optical responses of 2D TMDCs and graphene, followed by the strain engineering techniques. It concludes with exciting applications associated with strained 2D materials, discussions on existing open questions, and an outlook on this intriguing emerging field.

Spin-forbidden excitons in monolayer transition metal dichalcogenides are optically inactive at room temperature. Probing and manipulating these dark excitons are essential for understanding exciton spin relaxation and valley coherence of these 2D materials. Here, we show that the coupling of dark excitons to a metal nanoparticle-on-mirror cavity leads to plasmon-induced resonant emission with the intensity comparable to that of the spin-allowed bright excitons. A three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly 6 orders of magnitude through the Purcell effect, therefore compensating its intrinsic nature of weak radiation. Our nanocavity approach provides a useful paradigm for understanding the room-temperature dynamics of dark excitons, potentially paving the road for employing dark exciton in quantum computing and nanoscale optoelectronics.

We use the semianalytical Cartesian multipole method to investigate the light transmission and reflection spectra of anisotropic dielectric metasurfaces, and extend the multipole decompositions method to account for cross-polarization conversion effects. We observe sharp high- Q resonances arising from a distortion of symmetry-protected bound states in the continuum in asymmetric dielectric metasurfaces, i.e., quasibound states in the continuum. In addition, by further introducing in-plane symmetric breaking perturbation, the polarization conversions of linearly polarized light can be achieved through quasibound states in the continuum. With the aid of temporal coupled-mode theory, we can obtain the limit of cross-conversion under single high- Q resonance, i.e., 0.25. Our work will help to design dielectric metasurfaces to control the polarization states of light.

The imaging capability of conventional lenses is mainly limited by the diffraction of light, and the so-called superlens has been developed allowing the recovery of evanescent waves in the focal plane. However, the remarkable focusing behavior of the superlens is greatly confined in the near-field regime due to the exponential decay of evanescent waves. To tackle this issue, we design a waveguide metasurface-based superlens with an extraordinary quasi-far-field focusing capability beyond the diffraction limit in the present work. Specifically, we analyze the underlying physical mechanism and provide experimental verification of the proposed superlens. The metasurface superlens is formed by an array of gradient nanoslits perforated in a gold slab, and supports transverse-electric (TE) waveguide modes under linearly polarized illumination along the long axis of the slits. Numerical results illustrate that exciting such TE waveguide modes can modulate not only optical phase but also evanescent waves. Consequently, some high-spatial-frequency waves can contribute to the focusing of the superlens, leading to the quasi-far-field super-resolution focusing of light. Under 405 nm illumination and oil immersion, the fabricated superlens shows a focus spot of 98 nm (i.e. λ/4.13) at a focal distance of 1.49 μm (i.e. 3.68λ) using an oil immersion objective, breaking the diffraction limit of λ/2.38 in the quasi-far field regime. The developed metasurface optical superlens with such extraordinary capabilities promises exciting avenues to nanolithography and ultra-small optoelectronic devices.

We investigate the scattering properties of coupled parity-time (PT) symmetric chiral nanospheres with scattering matrix formalism. The exceptional points, i.e., spectral singularities at which the eigenvalues and eigenvectors simultaneously coalesce in the parameter space, of scattering matrix can be tailored by the chirality of the nanospheres. We also calculate the scattering, absorption and extinction cross sections of the PT-symmetric chiral scatter under illumination by monochromatic left- and right-circularly polarized plane waves. We find that the scattering cross section of the nanostructures exhibits an asymmetry when the plane waves are incident from the loss and gain regions, respectively, especially in the broken phase, and the optical cross section exhibits circular dichroism, i.e., differential extinction when the PT-symmetric scatter is endowed with chirality. In particular, under illumination by linearly polarized monochromatic plane waves without intrinsic chirality, the ellipticity of scattered fields in the forward direction, denoting the chirality of light, becomes larger when the scatter is in the PT-symmetry-broken phase. Our findings demonstrate that the gain and loss can control the optical chirality and enhance the chiroptical interactions and pave the way for studying the resonant chiral light–matter interactions in non-Hermitian photonics.