80
K. Huang
amplitude of scattering light with linear co-polarization, and completely releases the
ability of subwavelength structures in shaping the polarized light arbitrarily. The
successive works by Capasso group refresh the concept of traditional optics [20,
27–32], which leads to the persistent burst of optical metasurfaces from infrared and
visible to ultraviolet regions.
After the consequent development around 10 years, optical metasurfaces, nowadays, have the clear physics that shapes light through the electromagnetic interaction with subwavelength structure in terms of the interference of the confined
nanomodes. In theory, we can categorize optical metasurfaces into gradient metasurfaces with sized-varied geometry [11], geometric metasurfaces [16, 33], Huygens
metasurfaces [34–36], nanosieve metasurfaces with amplitude modulation [37–40].
The working principle of gradient and geometric metasurfaces have been introduced
above. Huygens metasurfaces support the simultaneous resonances of two transversely located electric and magnetic dipoles having the crossed directions and the
equal amplitudes, so that every meta-atom or nanostructure in Huygens metasurfaces
can be taken as an ideal Huygens point source that has the forward transmission of
1 but the backward scattering of 0, thus offering another strategy of high-efficiency
metasurfaces. Photon nanosieves are one kind of amplitude metasurfaces that shapes
the binary transmission of light with the etched holes on an opaque thin film. They
become the only choice of meta-devices at the extremely short wavelengths such as
extreme ultra-violet light and soft-X ray, where all the materials are absorbing [41]
so that the electromagnetic resonances of nanomodes cannot exist.
From the viewpoint of structure, optical metasurfaces could utilize the unit cell of
any geometric shape for controlling light. The size-varied circular, V-shape and rectangle nano-structures are frequently used in gradient metasurfaces [11, 42], while
the orientation-rotated rectangle structures are much preferred in geometric metasurfaces [16, 33] due to the limitation of its mechanism. Optical Huygens metasurfaces
frequently adopt the circular dielectric nanodisks to excite the electric and magnetic
dipoles [34–36]. Nanosieve metasurfaces generally use the circular or rectangle
apertures to modulate the amplitude or geometric phase of light [37–40].
Efficiency of optical metasurfaces is a key parameter for their practical applications in industry. Due to the intrinsic properties of electromagnetic resonances,
optical metasurfaces have the efficiency determined mainly by the material platform. The reflective metasurfaces could exhibit the total efficiency of ~90% by using
metals or a combination of metallic and dielectric materials with low absorption at
the wavelengths of interest [15, 42, 43]. However, the metals are not preferred in an
optical transmissive metasurface because of their strong ohmic loss and low polarization conversion [44]. High-efficiency optical metasurfaces in the transmission
mode usually uses the refraction-index and large-bandgap dieletric nano-structures
sitting on a low-index and transparent substrate, creating a well-confined dielectric
nano-cavity that has much higher refractive index than the surrounding environment. This conclusion is valid for all gradient, geometric and Huygens metasurfaces
in transmission. Since most of optical equipment and systems work in a transmission
mode, optical dielectric metasurfaces with high efficiency are the hotspot among the
K. Huang
amplitude of scattering light with linear co-polarization, and completely releases the
ability of subwavelength structures in shaping the polarized light arbitrarily. The
successive works by Capasso group refresh the concept of traditional optics [20,
27–32], which leads to the persistent burst of optical metasurfaces from infrared and
visible to ultraviolet regions.
After the consequent development around 10 years, optical metasurfaces, nowadays, have the clear physics that shapes light through the electromagnetic interaction with subwavelength structure in terms of the interference of the confined
nanomodes. In theory, we can categorize optical metasurfaces into gradient metasurfaces with sized-varied geometry [11], geometric metasurfaces [16, 33], Huygens
metasurfaces [34–36], nanosieve metasurfaces with amplitude modulation [37–40].
The working principle of gradient and geometric metasurfaces have been introduced
above. Huygens metasurfaces support the simultaneous resonances of two transversely located electric and magnetic dipoles having the crossed directions and the
equal amplitudes, so that every meta-atom or nanostructure in Huygens metasurfaces
can be taken as an ideal Huygens point source that has the forward transmission of
1 but the backward scattering of 0, thus offering another strategy of high-efficiency
metasurfaces. Photon nanosieves are one kind of amplitude metasurfaces that shapes
the binary transmission of light with the etched holes on an opaque thin film. They
become the only choice of meta-devices at the extremely short wavelengths such as
extreme ultra-violet light and soft-X ray, where all the materials are absorbing [41]
so that the electromagnetic resonances of nanomodes cannot exist.
From the viewpoint of structure, optical metasurfaces could utilize the unit cell of
any geometric shape for controlling light. The size-varied circular, V-shape and rectangle nano-structures are frequently used in gradient metasurfaces [11, 42], while
the orientation-rotated rectangle structures are much preferred in geometric metasurfaces [16, 33] due to the limitation of its mechanism. Optical Huygens metasurfaces
frequently adopt the circular dielectric nanodisks to excite the electric and magnetic
dipoles [34–36]. Nanosieve metasurfaces generally use the circular or rectangle
apertures to modulate the amplitude or geometric phase of light [37–40].
Efficiency of optical metasurfaces is a key parameter for their practical applications in industry. Due to the intrinsic properties of electromagnetic resonances,
optical metasurfaces have the efficiency determined mainly by the material platform. The reflective metasurfaces could exhibit the total efficiency of ~90% by using
metals or a combination of metallic and dielectric materials with low absorption at
the wavelengths of interest [15, 42, 43]. However, the metals are not preferred in an
optical transmissive metasurface because of their strong ohmic loss and low polarization conversion [44]. High-efficiency optical metasurfaces in the transmission
mode usually uses the refraction-index and large-bandgap dieletric nano-structures
sitting on a low-index and transparent substrate, creating a well-confined dielectric
nano-cavity that has much higher refractive index than the surrounding environment. This conclusion is valid for all gradient, geometric and Huygens metasurfaces
in transmission. Since most of optical equipment and systems work in a transmission
mode, optical dielectric metasurfaces with high efficiency are the hotspot among the