Metasurfaces are 2D artificial materials consisting of
arrays of
metamolecules, which are exquisitely designed to manipulate light
in terms of amplitude, phase, and polarization state with spatial
resolutions at the subwavelength scale. Traditional micro/nano-optical
sensors (MNOSs) pursue high sensitivity through strongly localized
optical fields based on diffractive and refractive optics, microcavities,
and interferometers. Although detections of ultra-low concentrations
of analytes have already been demonstrated, the label-free sensing
and recognition of complex and unknown samples remain challenging,
requiring multiple readouts from sensors, e.g., refractive
index, absorption/emission spectrum, chirality, etc. Additionally, the reliability of detecting large, inhomogeneous
biosamples may be compromised by the limited near-field sensing area
from the localization of light. Here, we review recent advances in
metasurface-based MNOSs and compare them with counterparts using micro-optics
from aspects of physics, working principles, and applications. By
virtue of underlying the physics and design flexibilities of metasurfaces,
MNOSs have now been endowed with superb performances and advanced
functionalities, leading toward highly integrated smart sensing platforms.