Circular-polarization-sensitive absorption in refractory metamaterials composed of molybdenum zigzag arrays

Circularly polarized light (CPL) is utilized in various fields, including optical communication and biological imaging. To overcome the lack of circular-polarizationsensitive absorbers working at high temperature, a refractory and circular-polarizationsensitive absorber comprised of molybdenum zigzag arrays is proposed. At certain resonant wavelengths, one component of circular polarization is absorbed by confining electromagnetic field in the dielectric layer, while the other component is backscattered. The circular-polarization-sensitive absorber could be applied as a CPL thermal radiator as well as a reflective linear-to-circular polarizer. As a CPL thermal radiator, left-handed circular radiation and right-handed circular radiation are dominant at different temperatures, respectively. As a linear-to-circular polarizer, both perfect left-handed circularly polarized light and nearly perfect right-handed circularly polarized light are obtained. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (130.3060) Infrared; (130.5440) Polarization-selective devices; (160.3918) Metamaterials. References and links 1. C. Wagenknecht, C. Li, A. Reingruber, X. Bao, A. Goebel, Y. Chen, Q. Zhang, K. Chen, and J. Pan, “Experimental demonstration of a heralded entanglement source,” Nat. Photonics 4(8), 549–552 (2010). 2. E. Karimi, S. A. Schulz, I. De Leon, H. Qassim, J. Upham, and R. W. Boyd, “Generating optical orbital angular momentum at visible wavelengths using a plasmonic metasurface,” Light Sci. Appl. 3(5), e167 (2014). 3. C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and T. Rasing, “All-optical magnetic recording with circularly polarized light,” Phys. Rev. Lett. 99(4), 047601 (2007). 4. S. M. 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Aiming at the lack of a circular-polarization-sensitive absorber that can work at high temperatures, a MIM structure comprised of zigzag arrays is proposed. Molybdenum (Mo) and aluminum dioxide (Al 2 O 3 ) are utilized because both of them in nanoscale are reported to be thermally stable at 1000 °C [27]. At resonant wavelengths, one CPL is absorbed by confining electromagnetic field in the dielectric layer, while the other is reflected. The circular-polarization-sensitive absorber could be applied as a CPL thermal radiator as well as a reflective linear-to-circular polarizer. As a CPL thermal radiator, LCP radiation and RCP radiation are dominant at different temperatures, respectively. As a reflective linear-tocircular polarizer, perfect LCP and nearly perfect RCP are both obtained. The proposed circular-polarization-sensitive absorber may further be applied to biological imaging and sensing, quantum-based optical computing and communication, etc., using a highly integrated photonic platform.  Fig. 1(b), the chiral geometry brings about circular-polarizationsensitive absorptions at different wavelengths. In particular, LCP is strongly absorbed around the wavelength of 4.6 μm (the strength of the LCP absorption is about five times that of the RCP absorption) while RCP is strongly absorbed around the wavelength of 2.6 μm (the strength of the RCP absorption is about five times that of the LCP absorption). A weak circular polarization sensitivity also occurs around the wavelength of 3.4 μm, which nearly corresponds to the periodicity along the y axis of the grating. Circular dichroism (CD), defined as the difference between LCP and RCP response, is usually introduced to describe the circular polarization sensitivity [22,29]. As the result, the CD value is about −0.53 and 0.55 at the wavelengths of 2.6 and 4.6 μm, respectively.

Discussions
It is clear that the CPL absorption can be explained by the destructive and constructive interference between the unconverted and converted scattered fields [21]. To unveil the physics behind varied CPL absorption at different wavelengths, the corresponding electromagnetic fields are shown in Figs. 2(b) and 2(d).
Firstly, at the wavelength of 4.6 μm, where LCP absorption is dominant, the electromagnetic field is localized in the Al 2 O 3 layer and confined around the chiral structure. It is illustrated in Fig. 2(b) that an electric dipole is excited on the surface of the Mo zigzag pattern, where the notation "+" and "-" represents the positive and negative charges, respectively. Hence, electromagnetic field normal to the x axis is investigated. As illustrated in Fig. 2(d), the LCP absorption at 4.6 μm is generated from two antiparallel magnetic resonances (the second order magnetic resonance), excitation of which is rarely observed at normal incidence [30, 31] because of a destructive interference. However, the destructive interference fails in this structure because these two magnetic resonances are misaligned by the asymmetric geometry.
Secondly, at the wavelength of 2.6 μm, where RCP absorption dominates, the electromagnetic field is localized in the Al 2 O 3 layer but mostly confined around the Mo zigzag pattern. A couple of electric dipoles are excited and a weak magnetic resonance in high order is also observed. This resonance is a hybridization of a lattice resonance and a magnetic resonance.
Thirdly, at the wavelength of 3.4 μm, where the circular-polarization-sensitive absorption is not effective, the electromagnetic field is concentrated on the air side of the Mo zigzag pattern and diffuses to air, as shown in Figs. 2(b) and 2(d). The electromagnetic field profiles and the value of resonant frequency both imply that absorption at 3.4 μm is induced by a lattice mode resonance. The resonant modes could also be deduced from the dependence on incident angle, which is depicted in Fig. 3.
As shown in Fig. 3(a), the LCP resonance around 4.6 μm at normal incidence is almost insensitive to the incident angle θ for oblique incident in the x-z plane, indicating a magnetic resonance. On the other hand, the resonance displays a good linear relationship between the resonant wavelength and the incident angle φ for oblique incidence in the y-z plane [see Fig.

3(c)].
When θ increases, the RCP resonance around 2.6 μm at normal incidence splits [see Fig.  3(b)] for an oblique incidence in the x-z plane, where a lattice mode and a magnetic mode can be distinguished. This implies that the absorption at 2.6 μm at θ = 0° is due to a hybridization of a magnetic resonance and a lattice mode resonance.
In addition, the fact that the resonance around 3.4 μm at normal incidence is a lattice mode can be deduced from the drastic shift of the absorption peak with oblique incidence. It is a property of lattice mode resonance because the needed wave vector compensation is different for different angles of incidence.
Furthermore, it is worth noting that the good linear relationship between the resonant absorption wavelengths and the incident angle φ could be further applied in CPL spatial angle measurement. The difference between two dispersions of different polarizations only remains on intensity.  Fig. 1(a), which is not challenging for electron beam lithography technique. As far as the fabrication is concerned, accurate dimensions of the hybrid nanostructure are challenging to meet. In particular, the accurate sharp corner is very difficult to be achieved by electron beam lithography. Nevertheless, the simulated performance would be easy to be obtained in the experiment once the structure is fabricated because of the high intrinsic fabrication tolerance. As shown in Fig. 4, with round corner (with a radius of 200 nm) [ Fig. 4(a)] and other variations of geometric parameters including h d (thickness of the Al 2 O 3 layer), v 1 , v 2 , v 3 , the changes of CPL absorptions are quite slight. Furthermore, the periodicity along x axis and thickness of Mo patterns have little effect on the CPL absorption. That is to say, the performance of the proposed absorber is very robust to the fabrication errors on the geometry.  3 which determine the periodicity along y axis. Positive/negative value means that the corresponding parameter is larger/smaller than that of the optimized design, whose features are illustrated by the black curves. Responses to LCP are illustrated by solid lines while that to RCP are illustrated by dash lines. According to Kirchhoff's law of thermal radiation, at thermal equilibrium the emissivity of a subject is equal to its absorptivity. The absorbers can also be used as thermal emitters, which show great potential in a wide range of energy-harvesting applications [34][35][36][37][38][39]. Therefore, the circular-polarization-sensitive absorber radiates energy as described by their absorptivity, including the polarization and direction. The radiation intensity spectra are calculated by multiplying the blackbody radiation at the given temperature with the dispersive emissivity, i.e., I(λ,T) = ε(λ)I B (λ,T), where ε(λ) is the emissivity of the circular-polarization-sensitive absorber and I B (λ,T) is the blackbody radiation spectrum at temperature T. The calculated results are plotted in Fig. 5. As shown in Fig. 5(b), when the circular-polarization-sensitive absorber is at low temperatures (T = 100 °C and T = 360 °C), LCP radiation at the wavelength of around 4.6 μm dominates (for instance, at T = 100 °C, LCP radiation at 4.6 μm is 36 times higher than RCP radiation at 2.6 μm). With the increasing temperature from 100 °C to 844 °C (at which nanosized molybdenum and aluminum dioxide are both thermally stable [25, 27]), a dominant LCP radiation at 4.6 μm is replaced by a dominant RCP radiation at 2.6 μm (at T = 884 °C, RCP radiation at 2.6 μm is twice as high as LCP radiation at 4.6 μm). That is to say, the radiated circular polarization (although with the corresponding wavelength) is alternative by changing the heating temperature. In addition, for the wavelength range we interested, the total energy of LCP radiation always exceeds that of RCP radiation unless the temperature is higher than 1050 °C [ Fig. 5(b)]. The average emissivity within the wavelength range is also is the blackbody radiation intensity. As indicated in Fig. 5(c), the average emissivity of LCP radiation decreases while that of RCP radiation increases with increasing temperature. The circular-polarization-sensitive absorber presented in this report could also act as a reflective linear-to-circular polarizer. To investigate the performance of the proposed structure as a linear-to-circular polarizer, a criterion for the ellipticity of the reflected wave is defined as

A reflective linear-to-circular polarizer
where E x and E y components are extracted from the reflected wave, and φ is the phase difference. Another definition is χ = |a R | 2 -|a L | 2 [41], where a L and a R are the amplitudes of LCP and RCP, respectively. When χ = −1, the reflected wave is a LCP light and when χ = 1, the reflected wave is a RCP light. As shown in Fig. 6(c), the proposed circular-polarization-sensitive absorber shows a good performance as a linear-to-circular polarizer. For the resonant wavelengths (2.6 μm and 4.6 μm), the linearly polarized light can be regarded as a superposition of LCP and RCP, so the linear-to-circular polarization conversion is achieved by absorbing one CPL while reflecting the other. When the polarization angle (Ф) is about 135°, a nearly perfect RCP light (χ ≈1) at 2.6 μm can be obtained. When Ф ≈110°, a perfect LCP light (χ = −1) at 4.6 μm can be obtained. Note that at these two resonant wavelengths, around half of the light energy is absorbed by the linear-to-circular polarizer. Besides, the dependence of χ on the polarization angle Ф results from the interference of reversed and unreversed polarizations of the reflected light.

Conclusions
In summary, a refractory and circular-polarization-sensitive absorber based on chiral plasmonic metamaterials is proposed. At the resonant wavelengths, one circular polarized component of light is absorbed by confining electromagnetic field in the dielectric layer, while the other is reflected. The proposed circular-polarization-sensitive absorber shows a good tolerance to toughness and can be applied as a CPL thermal radiator as well as a reflective linear-to-circular polarizer. With the proposed structure acting as a CPL thermal radiator, LCP radiation and RCP radiation is alternative (at respectively corresponding wavelengths) by changing the heating temperature. With the proposed structure acting as a linear-to-circular polarizer, perfect LCP and nearly perfect RCP are both achieved.