Exciton resonance tuning of an atomically thin lens

Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X(+)), neutral (X(o)) and negatively charged (X(-)) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5 meV) at temperatures <55 K. This is greater spectral contrast than in any known quasi-two-dimensional system. We also find the charging energies for X(+) and X(-) to be nearly identical implying the same effective mass for electrons and holes.

Recent progress in metasurface designs fueled by advanced-fabrication techniques has led to the realization of ultrathin, lightweight, and flat lenses (metalenses) with unprecedented functionalities. Due to straightforward fabrication, generally requiring a single-step lithography, and possibility of vertical integration, these planar lenses can potentially replace or complement their conventional refractive and diffractive counterparts leading to further miniaturization of high performance optical devices and systems. Here, we give a brief overview of the evolution of metalenses with an emphasis on the visible and near-infrared spectrum and summarize their important features: diffraction-limited focusing, high quality imaging and multifunctionalities. Future challenges including aberrations’ corrections, as well as current issues and solutions are discussed. We conclude by providing an outlook of this technology platform and by identifying future promising directions.