4 Chirality and Antiferromagnetism in Optical Metasurfaces
95
where R is the optical reflectivity, l denotes the topological charge of optical vortices,
the spin s of the incident beam is set to be identical for avoiding the CD effect. The
definition of vortical differential scattering (VDS) emphasizes the importance of
optical vortices by evaluating the OAM-induced dichroism between the topologicalcharge-opposite vortices. We have to emphasize that, the VDS has the fundamental
difference from CD, because the VDS refers to the scattering difference between two
chirality-opposite vortices while the CD comes from the difference of absorption to
both circularly polarized light [83].
Based on the definition of VDS, the experimental setup is proposed to characterize the reflectivity difference induced by the chirality-opposite vortices [83]. A
femtosecond laser with linear polarization is used here to generate the chiralityopposite optical vortices with a spatial light modulator. The optical vortices are
focused by a high numerical aperture objective lens and then illuminate the chiral
microstructures. The scattering signals that reflect from the chiral microstructures
are collected by the focusing objective lens and later recorded by a CCD camera.
The achiral structure of circular disk is used as a control of experiment. The recorded
signals at the different topological charges of |l| clearly shows the difference between
the optical vortices with the opposite wavefronts (RHW and LHW) for the chiral
micro-structures. In comparison, the controlled experiment gives nearly the same
reflectivity from both optical vortices. All these experimental observations have
suggested that the optical vortices with opposite topological charges can be used
to distinguish the chirality of micro-structures due to the difference of the scattering
light. To check its VDS further, we carried out the numerical calculation by using
the experimental data. It exhibits that the VDS is positive for left-handed structures,
negative for right-handed structures and zero for achiral structures. It gives the proof
that the chirality of microstructures has been discriminated by using VDS, which
therefore provides another method of detecting the chirality of objects in addition to
the traditional CD spectroscopy [83].
Compared with CD method, the VDS has the following properties. Firstly, VDS
operates at a single wavelength, and therefore releases the requirement of broadband
sources in CD spectroscopy. Secondly, the strong VDS happens when the microstructures have the dimension larger than the operating wavelength. Such a feature is
determined by the large interacting area of optical vortices due to the doughnut-shape
intensity profiles. Thirdly, the topological charges of optical vortices are unlimited
so that it provides the infinite dimension of chiral OAMs, implying that the VDS has
much larger degree of freedom than the CD with two spins of photons in a circularly
polarized light [83].
4.4.2 Chiral Meta-Optics
By using (4.8), one can find that the arbitrary manipulation of chiral light could
be realized by customizing the spatially varied phase profiles to meet the special
requirement in practical applications. Here, we introduce three kinds of phase profiles
95
where R is the optical reflectivity, l denotes the topological charge of optical vortices,
the spin s of the incident beam is set to be identical for avoiding the CD effect. The
definition of vortical differential scattering (VDS) emphasizes the importance of
optical vortices by evaluating the OAM-induced dichroism between the topologicalcharge-opposite vortices. We have to emphasize that, the VDS has the fundamental
difference from CD, because the VDS refers to the scattering difference between two
chirality-opposite vortices while the CD comes from the difference of absorption to
both circularly polarized light [83].
Based on the definition of VDS, the experimental setup is proposed to characterize the reflectivity difference induced by the chirality-opposite vortices [83]. A
femtosecond laser with linear polarization is used here to generate the chiralityopposite optical vortices with a spatial light modulator. The optical vortices are
focused by a high numerical aperture objective lens and then illuminate the chiral
microstructures. The scattering signals that reflect from the chiral microstructures
are collected by the focusing objective lens and later recorded by a CCD camera.
The achiral structure of circular disk is used as a control of experiment. The recorded
signals at the different topological charges of |l| clearly shows the difference between
the optical vortices with the opposite wavefronts (RHW and LHW) for the chiral
micro-structures. In comparison, the controlled experiment gives nearly the same
reflectivity from both optical vortices. All these experimental observations have
suggested that the optical vortices with opposite topological charges can be used
to distinguish the chirality of micro-structures due to the difference of the scattering
light. To check its VDS further, we carried out the numerical calculation by using
the experimental data. It exhibits that the VDS is positive for left-handed structures,
negative for right-handed structures and zero for achiral structures. It gives the proof
that the chirality of microstructures has been discriminated by using VDS, which
therefore provides another method of detecting the chirality of objects in addition to
the traditional CD spectroscopy [83].
Compared with CD method, the VDS has the following properties. Firstly, VDS
operates at a single wavelength, and therefore releases the requirement of broadband
sources in CD spectroscopy. Secondly, the strong VDS happens when the microstructures have the dimension larger than the operating wavelength. Such a feature is
determined by the large interacting area of optical vortices due to the doughnut-shape
intensity profiles. Thirdly, the topological charges of optical vortices are unlimited
so that it provides the infinite dimension of chiral OAMs, implying that the VDS has
much larger degree of freedom than the CD with two spins of photons in a circularly
polarized light [83].
4.4.2 Chiral Meta-Optics
By using (4.8), one can find that the arbitrary manipulation of chiral light could
be realized by customizing the spatially varied phase profiles to meet the special
requirement in practical applications. Here, we introduce three kinds of phase profiles