Extremely long transverse optical needle focus for reflective metalens enabled by monolayer MoS$_2$

TL;DR

This paper presents a monolayer MoS2-based Fresnel structure that achieves an unprecedented 40 μm transverse optical needle focus, enabling high-efficiency, broad-wavelength flat optics for advanced imaging and optical applications.

Contribution

The work introduces a novel monolayer MoS2-based design that overcomes low light utilization and phase control challenges to produce the longest optical needle focus recorded.

Findings

Achieved a 40 μm transverse focus length, the longest recorded.

Produced a sub-diffraction-limited lateral spot.

Demonstrated broad wavelength range operation.

Abstract

Line-scan mode facilitates fast-speed and high-throughput imaging with developing a suitable optical transverse needle focus. Metasurface with periodic structures such as diffractive rings, ellipses, and gratings could enable discrete focus evolving into line focus under momentum conservation, but still face the challenge of extremely low light power utilization brought by inevitably multiple high-order diffractions. In addition, the designed focus requires the selection of particular optical functional materials. High dielectric constants in atomic transition metal dichalcogenides make significant phase modulation by bringing phase singularity at zero-reflection possible. However, no light power is available for use at zero-reflection and a balance between phase and amplitude modulation is needed. In this work, above issues are simultaneously solved by designing a monolayer MoS2 based Fresnel strip structure. An optical needle primary focus with a transverse length of 40 {\mu}m (~80 {\lambda}) is obtained, which is the longest value recorded so far, together with a sub-diffraction-limited lateral spot and a broad working wavelength range. This specially developed structure not only concentrates light power in primary diffraction by breaking restriction of momentum conservation, but also guarantees a consistent phase across different strips. The novel optical manipulation way provided here together with the longer focus length for flat optics will show promising applications in biology, oncology, nanofabrication, energy harvesting, and optical information processing.