Generation of an Ultra-Long Transverse Optical Needle Focus Using a Monolayer MoS₂ Based Metalens

Line-scan mode enables rapid and high-throughput imaging through the development of an appropriate optical transverse needle focus. Diffraction gratings allow the generation of a line focus, but they face the challenge of low light power utilization due to multiple high-order diffractions. In addition, designing the focus requires the selection of functional materials. Atomically thin transition metal dichalcogenides with high dielectric constants provide significant phase shifts to incident light by utilizing phase singularity at zero reflection. However, at zero reflection, no light power is available for utilization, necessitating a balance between phase and amplitude modulation. In this work, the aforementioned issues are addressed by designing a monolayer MoS₂ based Fresnel strip metalens. An optical needle primary focus with a transverse length of 40 µm (≈80 λ, the longest value reported to date), a sub-diffraction-limited lateral spot, and a broad range of working wavelengths are achieved using the metalens. The metalens not only concentrate light power in the primary diffraction order by overcoming the constraint of momentum conservation but also maintains a constant phase modulation between distinct strips. The novel method for optical manipulation present here holds great promise for applications in biology, oncology, nanofabrication, energy harvesting, and optical information processing.