Dispersion-engineered metasurfaces reaching broadband 90% relative diffraction efficiency.

Dispersion results from the variation of index of refraction as well as electric field confinement in sub-wavelength structures. It usually results in efficiency decrease in metasurface components leading to troublesome scattering into unwanted directions. In this letter, by dispersion engineering, we report a set of eight nanostructures whose dispersion properties are nearly identical to each other while being capable of providing 0 to 2π full-phase coverage. Our nanostructure set enables broadband and polarization-insensitive metasurface components reaching 90% relative diffraction efficiency (normalized to the power of transmitted light) from 450 nm to 700 nm in wavelength. Relative diffraction efficiency is important at a system level - in addition to diffraction efficiency (normalized to the power of incident light) - as it considers only the transmitted optical power that can affect the signal to noise ratio. We first illustrate our design principle by a chromatic dispersion-engineered metasurface grating, then show that other metasurface components such as chromatic metalenses can also be implemented by the same set of nanostructures with significantly improved relative diffraction efficiency.

Coherent Raman scattering imaging with a near-infrared achromatic metalens.

Miniature handheld imaging devices and endoscopes based on coherent Raman scattering are promising for label-free in vivo optical diagnosis. Toward the development of these small-scale systems, a challenge arises from the design and fabrication of achromatic and high-end miniature optical components for both pump and Stokes laser wavelengths. Here, we report a metasurface converting a low-cost plano-convex lens into a water-immersion, nearly diffraction-limited and achromatic lens. The metasurface comprising amorphous silicon nanopillars is designed in a way that all incident rays arrive at the focus with the same phase and group delay, leading to corrections of monochromatic and chromatic aberrations of the refractive lens, respectively. Compared to the case without the metasurface, the hybrid metasurface-refractive lens has higher Strehl ratios than the plano-convex lens and a tighter depth of focus. The hybrid metasurface-refractive lens is utilized in spectroscopic stimulated Raman scattering and coherent anti-Stokes Raman scattering imaging for the differentiation of two different polymer microbeads. Subsequently, the hybrid metalens is harnessed for volumetric coherent Raman scattering imaging of bead and tissue samples. Finally, we discuss possible approaches to integrate such hybrid metalens in a miniature scanning system for label-free coherent Raman scattering endoscopes.