76
K. Huang
at the different scales ranging from macroscopic bulky crystals, to man-made nanostructures, to microscopic molecules and atoms. Recently, optical metasurfaces made
of artificial subwavelength structures has drawn great attention due to its capacity of
tailoring the amplitude, phase and polarization of light in a subwavelength thickness.
It therefore offers the opportunity to integrate optical elements (Sect. 4.1.1) within a
compact volume. The rapid development of optical metasurfaces (Sect. 4.1.2) benefits from matured nanotechnology, which enables the fabrication of optical subwavelength structures. These nanostructures could confine nanoscale resonating modes
that behave like electric and magnetic dipoles or multipoles. The electromagnetic
resonances of the confined nanomodes are modified through changing the dimension or geometry of the nanostructures, hereby realizing the required modulation
of light. Based on these fundamental origins, metasurfaces can be mainly categorized into shape-varied metasurfaces, geometric metasurfaces, Huygens metasurfaces. In this chapter, we only focus on geometric metasurfaces composed of sizefixed orientation-rotated nanostructures that control the phase of circularly polarized
light, which carries the spin angular momentum (SAM) of light and exhibits the
polarization chirality of light (Sect. 4.2.1). In addition, the orbital angular momentum
(OAM) of light carried by optical vortices beams with a helical wavefront (Sect. 4.2.2)
characterizes the phase chirality of light. Due to the strong response to the polarization chirality of light, the geometric metasurfaces can also be called as chiral metasurfaces (Sect. 4.3). According to their material platforms, chiral metasurfaces are
discussed in terms of plasmonic chiral metasurfaces (Sect. 4.3.1), chiral nanosieves
(Sect. 4.3.2), and dielectric chiral metasurfaces (Sect. 4.3.3). In the transmission
mode, the dielectric chiral metasurfaces have much higher efficiency than plasmonic
metasurfacea and chiral nanosieves, due to the low absorption of dielectrics. In the
configuration of dielectric metasurfaces, the nanostructures could support the induced
antiparallel magnetic dipoles, implying the antiferromagnetic resonances. Such a
concept of antiferromagnetism is used to explain the physics governing dielectric
nano-halfwaveplates, which is the functionality of nano-structures in chiral metasurfaces. Finally, the applications of chiral light and chiral metasurfaces (Sect. 4.4) are
introduced by addressing the exciting topics such as circular dichroism and helical
dichroism (Sect. 4.4.1), and chiral meta-optics (Sect. 4.4.2), followed by a brief
conclusion in Sect. 4.5.
4.1.1 Optical Elements
Before 1900, the optical elements such as lenses, glasses, gratings, birefringent crystals, polarizers, mirrors and filters have been well developed with the help of traditional technology of cold machining, thus forming the cornerstone of geometry optics
where the dimension is much larger than the operating wavelengths. Light in geometry optics is simplified into a ray of light so that the intensity of an optical beam can
be taken as the density of the rays at its cross section. The manipulation of light originates mainly from the refraction and refraction of the ray at the interface between
K. Huang
at the different scales ranging from macroscopic bulky crystals, to man-made nanostructures, to microscopic molecules and atoms. Recently, optical metasurfaces made
of artificial subwavelength structures has drawn great attention due to its capacity of
tailoring the amplitude, phase and polarization of light in a subwavelength thickness.
It therefore offers the opportunity to integrate optical elements (Sect. 4.1.1) within a
compact volume. The rapid development of optical metasurfaces (Sect. 4.1.2) benefits from matured nanotechnology, which enables the fabrication of optical subwavelength structures. These nanostructures could confine nanoscale resonating modes
that behave like electric and magnetic dipoles or multipoles. The electromagnetic
resonances of the confined nanomodes are modified through changing the dimension or geometry of the nanostructures, hereby realizing the required modulation
of light. Based on these fundamental origins, metasurfaces can be mainly categorized into shape-varied metasurfaces, geometric metasurfaces, Huygens metasurfaces. In this chapter, we only focus on geometric metasurfaces composed of sizefixed orientation-rotated nanostructures that control the phase of circularly polarized
light, which carries the spin angular momentum (SAM) of light and exhibits the
polarization chirality of light (Sect. 4.2.1). In addition, the orbital angular momentum
(OAM) of light carried by optical vortices beams with a helical wavefront (Sect. 4.2.2)
characterizes the phase chirality of light. Due to the strong response to the polarization chirality of light, the geometric metasurfaces can also be called as chiral metasurfaces (Sect. 4.3). According to their material platforms, chiral metasurfaces are
discussed in terms of plasmonic chiral metasurfaces (Sect. 4.3.1), chiral nanosieves
(Sect. 4.3.2), and dielectric chiral metasurfaces (Sect. 4.3.3). In the transmission
mode, the dielectric chiral metasurfaces have much higher efficiency than plasmonic
metasurfacea and chiral nanosieves, due to the low absorption of dielectrics. In the
configuration of dielectric metasurfaces, the nanostructures could support the induced
antiparallel magnetic dipoles, implying the antiferromagnetic resonances. Such a
concept of antiferromagnetism is used to explain the physics governing dielectric
nano-halfwaveplates, which is the functionality of nano-structures in chiral metasurfaces. Finally, the applications of chiral light and chiral metasurfaces (Sect. 4.4) are
introduced by addressing the exciting topics such as circular dichroism and helical
dichroism (Sect. 4.4.1), and chiral meta-optics (Sect. 4.4.2), followed by a brief
conclusion in Sect. 4.5.
4.1.1 Optical Elements
Before 1900, the optical elements such as lenses, glasses, gratings, birefringent crystals, polarizers, mirrors and filters have been well developed with the help of traditional technology of cold machining, thus forming the cornerstone of geometry optics
where the dimension is much larger than the operating wavelengths. Light in geometry optics is simplified into a ray of light so that the intensity of an optical beam can
be taken as the density of the rays at its cross section. The manipulation of light originates mainly from the refraction and refraction of the ray at the interface between