4 Chirality and Antiferromagnetism in Optical Metasurfaces
91
4.3.3 Dielectric Chiral Metasurfaces and Anti-ferromagnetic
Resonances
Both transmissive chiral metasurfaces mentioned above cannot obtain high conversion efficiency. According to (4.8), high-efficiency chiral metasurfaces should maximize s x and s y , which is determined by the simultaneous oscillation of electric and
magnetic nanomodes. For the plasmonic chiral metasurfaces and chiral nanosieves,
both resonances cannot be satisfied due to either the single-layer induced electric
responses or the strong ohmic loss of metallic materials [44]. In addition, the phase
delay of π between s x and s y should also be met to realize the polarization conversion, and any deviation from π will lead to the existence of co-polarized light even if
both s x and s y approaches 1. Although the reflective plasmonic chiral metasurfaces
[43] offers high conversion efficiency that can only be checked by numerical simulations, it is difficult to explain the fundamental reason of generating the phase delay
of π, which is usually ignored in most literatures. In addition to the reflective plasmonic chiral metasurfaces, dielectric chiral metasurfaces could also generate high
conversion efficiency by employing the low-loss and high-refractive-index highaspect-ratio dielectric nanostructures, which support the simultaneous electric and
magnetic dipoles [35]. The magnetic dipoles are generated by the induced circleshape electric displacements with alternating handedness. Thus, both resonances of
electric and magnetic dipoles promise the large s x and s y that approach 1. Due to the
high refractive index, the effective wavelength in dielectric nanostructures is much
smaller than the vacuum wavelength, thus allowing multiple electric and magnetic
dipoles in nanostructures. It leads to the existence of antiferromagnetic modes that
contain a series of vertically locate antiparallel magnetic dipoles [15], which holds
the fundamental physics of generating the phase delay of π.
By following our previous work about ultra-violet dielectric chiral metasurfaces,
we will discuss the roles of antiferromagnetic nanomodes [15]. Figure 4.7 plots the
sketch of the chiral metasurfaces and their optical performance with the simulated
efficiency as high as 80% at the designed wavelength of 355 nm. The material of
niobium pentoxide (Nb 2 O 5 ) offer high refractive index of 2.2 and low absorption
at the wavelengths ranging from 350 nm to 400 nm, hereby enabling the ultraviolet
metasurfaces. To reveal the underlying physics, we investigate the detailed electromagnetic responses of dielectric chiral metasurfaces with the maximum conversion
efficiency of 80% when the nanostructure has the dimension of L = 150 nm, W =
70 nm and H = 430 nm.
Figures 4.7d, e show the simulated electric and magnetic fields induced by E x and
E y components of the incident polarized light. For E x component in Fig. 4.7d, the
induced electric displacements contain four alternative circular currents in clockwise
and anti-clockwise directions, which could generate the relative antiparallel magnetic
dipoles (AMPs) sitting vertically along z axis. This staggered magnetization has the
similar behavior with one-dimensional antiferromagnetic chain, which has recently
been reported in plasmonic nano-disks and hybrid metamaterials. The antiferromagnetic modes have the even electric circle-shape currents with alternative directions,
91
4.3.3 Dielectric Chiral Metasurfaces and Anti-ferromagnetic
Resonances
Both transmissive chiral metasurfaces mentioned above cannot obtain high conversion efficiency. According to (4.8), high-efficiency chiral metasurfaces should maximize s x and s y , which is determined by the simultaneous oscillation of electric and
magnetic nanomodes. For the plasmonic chiral metasurfaces and chiral nanosieves,
both resonances cannot be satisfied due to either the single-layer induced electric
responses or the strong ohmic loss of metallic materials [44]. In addition, the phase
delay of π between s x and s y should also be met to realize the polarization conversion, and any deviation from π will lead to the existence of co-polarized light even if
both s x and s y approaches 1. Although the reflective plasmonic chiral metasurfaces
[43] offers high conversion efficiency that can only be checked by numerical simulations, it is difficult to explain the fundamental reason of generating the phase delay
of π, which is usually ignored in most literatures. In addition to the reflective plasmonic chiral metasurfaces, dielectric chiral metasurfaces could also generate high
conversion efficiency by employing the low-loss and high-refractive-index highaspect-ratio dielectric nanostructures, which support the simultaneous electric and
magnetic dipoles [35]. The magnetic dipoles are generated by the induced circleshape electric displacements with alternating handedness. Thus, both resonances of
electric and magnetic dipoles promise the large s x and s y that approach 1. Due to the
high refractive index, the effective wavelength in dielectric nanostructures is much
smaller than the vacuum wavelength, thus allowing multiple electric and magnetic
dipoles in nanostructures. It leads to the existence of antiferromagnetic modes that
contain a series of vertically locate antiparallel magnetic dipoles [15], which holds
the fundamental physics of generating the phase delay of π.
By following our previous work about ultra-violet dielectric chiral metasurfaces,
we will discuss the roles of antiferromagnetic nanomodes [15]. Figure 4.7 plots the
sketch of the chiral metasurfaces and their optical performance with the simulated
efficiency as high as 80% at the designed wavelength of 355 nm. The material of
niobium pentoxide (Nb 2 O 5 ) offer high refractive index of 2.2 and low absorption
at the wavelengths ranging from 350 nm to 400 nm, hereby enabling the ultraviolet
metasurfaces. To reveal the underlying physics, we investigate the detailed electromagnetic responses of dielectric chiral metasurfaces with the maximum conversion
efficiency of 80% when the nanostructure has the dimension of L = 150 nm, W =
70 nm and H = 430 nm.
Figures 4.7d, e show the simulated electric and magnetic fields induced by E x and
E y components of the incident polarized light. For E x component in Fig. 4.7d, the
induced electric displacements contain four alternative circular currents in clockwise
and anti-clockwise directions, which could generate the relative antiparallel magnetic
dipoles (AMPs) sitting vertically along z axis. This staggered magnetization has the
similar behavior with one-dimensional antiferromagnetic chain, which has recently
been reported in plasmonic nano-disks and hybrid metamaterials. The antiferromagnetic modes have the even electric circle-shape currents with alternative directions,