In this paper, a sensitive chirality selective metamaterial absorber (CSMA) is constructed by using 'I-shaped' resonator with asymmetric twisted metallic wires. Absorption of 95.18% and 91.77% at two resonant frequencies can be achieved for left-handed circularly polarized (LCP) incident wave, with little loss of right-handed circularly polarized (RCP) incident wave, which results in significant absorptive circular dichroism. Not only can the CSMA intensely absorb LCP illumination with dual bands, but also circularly polarized (CP) conversion for RCP wave is achieved over a broad bandwidth. The spin-dependent absorption, closely linked to chiral symmetry breaking, is investigated through oblique incidence, power loss distribution and scanning parameters optimization. The proposed strategy is further demonstrated in mid-infrared band which could advance the applications in polarization manipulation to circularly polarized detectors/lasers, chiral sensing/bolometers, and molecular spectroscopy.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Since the first experimental demonstration of a near-unity metamaterial absorber (MMA) proposed by N. I. Landy et al. in 2008 , MMAs have attracted great attention during the past decade. MMA has many advantages over traditional absorbers (e.g. ferromagnetic materials, carbon) including thin thickness, light weight, dynamically tunable resonance, and definitely sub-wavelength scale suitable with microwave/optical devices integration. Most MMAs are based on linearly polarized (single-polarized, dual-polarized and polarization insensitive) waves, which can realize multi-frequency, broadband, tunable absorption from microwave to optical regions [2–4]. MMAs for circularly polarized (CP) waves have not been extensively investigated. Circular dichroism (CD), i.e., differential absorption of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) waves, existing in nature like DNA and protein molecules , has a wide range of applications in practical fields. The reconstruction and research of chiral structures based on metamaterials provide a tunable method to realize spin-dependent absorptive molecules .
The most common structures utilized in yielding CD are the helices [7,8]. In addition, remarkable CD can be realized by complementary Y-shaped resonator , split rings , twisted U-shaped structure and other asymmetry structures . Strong optical activities are also widely demonstrated in the researches of chiral metamaterials with negative refractive index [11–14]. Besides, a novel form of CD was further observed by the resonant excitation of toroidal dipole and electric quadrupole [15–17]. These works mostly concentrate on the CD performance in transmission, with little attention paid on reflection, both of which should be taken into account in the designs of perfect absorbers. In 2015, a new type of chiral mirror based on split rings was proposed by reflecting one circular polarization wave without changing its handedness , while absorbing the other one. Then, tunable optically active reflectors and perfect absorbers  were achieved based on extrinsic chirality [20,21]. The circular polarization state and the magnetic circular dichroism (MCD) can also be tuned by varying the external magnetic field . Perfect ultra-compact CP wave detector was also developed by using chiral plasmonic metamaterials with hot electron injection . Meanwhile, a kind of chirality dependent absorber based on twisted L-shaped folded metallic wires was achieved around 8.72 GHz . A similar structure was also numerically demonstrated in terahertz region . Moreover, highly efficient absorption of RCP wave was also realized through the capacitive loading of metal circular split rings . These models, although the thickness is much thinner compared with the operating wavelength, are not easy to be transplanted to higher frequencies (THz, IR and optical regions), due to the fabrication difficulties of metal via-holes and lumped elements realization. In 2016, a mid-infrared metamirror was achieved by comprising two layers of anisotropic metamaterials based on multi-layer twisted metallic wires . Later, a similar metamirror possessing a reflection CD of ∼0.5 was demonstrated in a near-infrared wavelength band . Besides, by combining two chiral resonant modes, a chiral metamirror was experimentally realized, which only absorbs LCP wave with a bandwidth of 5.1% .
However, many of these solutions including bulk three-dimensional structures and multi-layer constructions suffer from large thickness and fabrication difficulties especially for optical region . Furthermore, chirality selective absorption resonance is limited to single narrow band and the absorptive CD is not strong enough, all of which are crucial to practical applications such as thermal bolometers and chiral sensing  where maximum CD of light is desired.
In this paper, we aim to achieve high-efficiency chirality dependent absorption of CP waves with dual bands. An ultrathin, bilayer metamaterial absorber with remarkable absorptive circular dichroism is then designed and optimized in the microwave region. The proposed efficient structure can be scaled to other wavelengths, which is also demonstrated in mid-infrared region.
2. Simulated and experimental method
The proposed chirality selective metamaterial absorber (CSMA) is shown in Fig. 1(a), which consists of top resonator, bottom metallic shielding and middle dielectric spacing. The parameters of top periodic 'I-shaped' resonator are displayed in Fig. 1(b). Asymmetric twisted metal wires are introduced and the rotation angle between L2 and L3 is . The thickness of the middle dielectric (FR-4) is 2.5 mm, which has a relative dielectric constant of 4.3 and loss tangent of 0.025. The bottom of the unit cell is an optically thick copper film with the thickness of 0.035 mm and the conductivity of copper is . Optimized geometric dimensions of the unit cell are as follows: L1 = 5.4 mm, L2 = 6.7 mm, L3 = 5.7 mm, w = 1.25 mm, a = 2.9 mm, b = 1.25 mm, c = 1.0 mm, d = 3.3 mm, e = 1.6 mm, f = 1.0 mm, P = 11.5 mm. The O (0,0) point represents the central original position of one unit cell. Strong chirality can be obtained due to the lacking of mirror symmetries.
We perform full-wave simulations using CST Microwave Studio. The periodic boundary conditions of the unit cell are along the x and y directions, and the wave propagation is along the z direction. Because of the bottom metallic shielding, the absorptive performance is only associated with reflection. The relationship between the incident electric field and the reflective electric field can be defined based on Jones matrix as:32]:25,33]. There is no transmission because of the bottom consecutive metallic shielding. In consideration of cross polarization of chiral structure, the total absorption of CSMA can then be simplified as :34]:
Here, can be equipped with positive or negative value indicating the domination of LCP or RCP illumination absorption, respectively. In experiments, the CSMA was fabricated into a 40 × 40 unit cell sample (460 × 460 × 2.5 mm3) based on printed circuit board (PCB) technology, as shown in Fig. 1(c). The sample is ultra-thin with the thickness of (for the first absorptive resonance of LCP). Because of our simple construction, this chirality selective concept can be easily transplanted to higher frequencies (with certain limitations), which will be demonstrated later. Reflection of CP waves can be achieved by transmitting and receiving antennas together with an Agilent E8362B network analyzer.
3. Results and discussions
Both the simulated and experimental results of the reflectance/absorption for the LCP and RCP illuminations are presented in Fig. 2. The cross-polarization reflection and curves are completely coincident since the impedance for LCP and RCP illuminations are the same. The average value is low with maximum under 0.23 indicating the nearly idealized impedance matching with the free space. For the LCP incident wave, the reflective waves decompose into and , both of which possess minimum values at two resonant frequencies as shown in Fig. 2(a). The corresponding absorption and circular dichroism () spectra curves of the CSMA structure can be calculated in Fig. 2(b) ( corresponds to the right y-coordinate with blue values). Two significant absorption resonances for the LCP wave have been yielded with peak values of 95.18% and 91.77% at 12.04 GHz and 14.22 GHz, respectively. Nevertheless, for RCP incident wave, the magnitudes are only 17.2% and 23.0% at the corresponding two resonance frequencies in simulation. The simulated at two selective resonances are both above 69% (approximately 77.8% and 69.0%, respectively). The necessary condition to realize the absorptive circular dichroism is simultaneously breaking of the n-fold rotational (n > 2) and mirror symmetries . So in the principle design of our model, L1 and L3 are introduced with varied extending lengths of L2 along + x and -x directions (a, b, c, d). Additionally, L3 has been twisted counterclockwise with an intersection angle of between L2 and L3, which has sensitive influence on the dual-band spin-dependent performance. The experimental results match well with the simulated results, and the slight tolerance can be tolerated due to imperfect PCB fabrication precision.
Interestingly, the handedness of CP waves will be changed because of the reverse direction of propagation vector, i.e., the RCP wave illuminating on an ideal metal mirror will be reflected to the LCP wave, and vice versa [18,36]. That is, and represent the polarization conversion capability for LCP and RCP waves, also known as circular polarization converter/transformer. has three peak values with 0.86, 0.81 and 0.69 at the resonant frequencies of 11.02 GHz, 12.04 GHz and 14.22 GHz, respectively ( is greater than 0.6 spanning 9.49-14.52 GHz), with the latter two resonances are the chirality selective absorption for LCP illumination. Correspondingly, the values of are only 0.008, 0.04 and 0.07, respectively. The conversion efficiency for RCP incident wave can be expressed by polarization conversion ratio (PCR) which is defined as :
Furthermore, the angle sensitivity with oblique incidence is discussed, since it is important for practical applications. The feature is investigated for two different cases with the wave vector confined in the plane of x-z and y-z, respectively (see right panels in Fig. 3) [19,27]. Oblique incidence with wide angles may make the Eq. (2) unavailable . Here, LCP/RCP ports serving as transmitter and receiver can be set up in CST. Then the CP reflection coefficients can be obtained with varied oblique incident angles utilizing the build-in tools. One can see from Fig. 3(a) that when the wave vector is located in the x-z plane, the absorption of LCP wave can maintain two peak values (over 81.5% and 77.4% for lower and higher resonance frequencies, respectively) from to . The tiny decrease of absorption attributes to the weakness of magnetic coupling between the top and bottom metallic structures, where the dielectric loss dominates the absorption in the Fabry-Perot (F-P) cavity (details in Fig. 4). On the contrary, in Fig. 3(b), there are no significant changes in the absorption performance of RCP wave, with magnitudes always lower than 39.6%. So, the absorption of two polarization states still maintains considerable contrast with dual bands for oblique incident angle up to 45 degree. At the same time, the PCR curves for RCP illumination at different incident angles are described in Fig. 3(e). Obviously, for normal incidence, though the efficiency decreases around 13 GHz, it still has strong values more than 0.7 from 9.55 GHz to 14.75 GHz, with a broadband bandwidth of 5.2 GHz. Therefore, not only can the CSMA intensely absorb LCP illumination with dual bands, but also CP conversion for RCP wave is achieved with broad bandwidth. Although the absorptive remains relatively high with the increase of angle, the ratio between and drops significantly, which results in intense fluctuations of PCR curves. In other words, the capability of our structure in CP conversion for RCP wave is weakened under oblique incidence.
The other situation is with the wave vector confined in the y-z plane, as shown in Figs. 3(c) and 3(d). Unfortunately, both the absorption amplitude and operating wavelength of LCP and RCP waves change sharply as the angle increases. Moreover, it can be seen from Fig. 3(f) that the PCR deteriorates seriously with oblique incidence of RCP illumination, making CP conversion unavailable. Compared with the first situation (wave vector confined in the x-z plane), more serious reduction in the degree of asymmetry has been introduced with the wave vector confined in the y-z plane. This mirror asymmetry is important in yielding different phase shifts for opposite handedness states, and further impacts the constructive/destructive interference within multi-layer reflections [19,38]. Therefore, the chiral selective absorption performance of the structure is not stable in this case, and this problem should be carefully considered in practical applications.
To further reveal the origin and physical mechanism, we calculate the power loss, electric field and current distributions in Fig. 4. Figures 4(a)-4(f) are calculated at an x-y plane located in the middle between the front and bottom metallic layers. Owing to the high efficient polarization conversion of RCP wave as discussed above, the power loss is very weak at both 12.04 GHz and 14.22 GHz for RCP illumination (Figs. 4(a) and 4(c)). In contrast, in Figs. 4(b) and 4(d), for LCP illumination, strong power loss distributions have been stimulated inside the CSMA structure. The stimulated currents on the top 'I-shaped' resonator, as the solid line shown in Figs. 4(g) and 4(h), flow anti-parallel with the corresponding currents (dashed line) on the bottom metal plate, yielding intense normal component electric field () in the middle dielectric (Figs. 4(e) and 4(f)) . , originating from the displacement currents flowing out (blue color, + z) or into (red color, -z) along propagation direction, results from the magnetic coupling between two metallic layers. Specifically, at 12.02 GHz, most power loss aggregates in FR-4 just below the left side of L1, right side of L3 and the middle of L2 (red color regions in Fig. 4(b)), which is consistent with distributions in Fig. 4(e). Situation is similar to the high resonance. So, the dielectric loss plays a dominate role in the strong absorption for LCP illumination.
As the power loss described above, it can be concluded that the left side of L1 and the right side of L3 determine the low frequency absorption peak, while the right side of L1 and the left side of L3 determine the high one. The influences of four crucial geometric parameters a, b, c and d on selective absorption performance have been investigated in Fig. 5. There is little differential absorption of CP waves without the left side of L1 (a = 0), and the spectra of at higher resonance frequency is almost unchanged with the increase of a as shown in Fig. 5(a). Simultaneously, the lower center resonance frequency of has significant red-shift with the parameter increasing from 2.3 mm to 2.9 mm, and the peak value reaches maximum around a = 2.6 mm. In particular, the CSMA has opposite differential absorption when parameter a = 3.5 mm, that is, RCP absorption dominates the power loss (with the peak value of 0.73 for RCP illumination). LCP/RCP wave can be decomposed into two orthogonally linear polarized fields, and , with the phase of is 90 ahead/behind of . Phase difference can be produced from the anisotropy of the asymmetric top 'I-shaped' resonator, resulting in certain polarization conversions. Thus, the converted scattered fields will destructively/constructively interfere with the unconverted one with phase shift . So the dielectric thickness, associated with the Fabry-Perot (F-P) cavity, linked with multiple reflections , plays a determinate role in the matching between the amplitude and phase. The change of a, also with other parameters, will definitely affect the phase difference along and , and the single-chiral dependent performance can be reversed by enough phase accumulation. Situation is similar with d = 2.7 mm. In Figs. 5(b) and 5(c), with the increase of b and c, there is no obvious change in the spectra at low frequency. Even when b reduced to 0, there is still strong circular dichroism with 0.6 at 12.5 GHz. On the contrary, the resonant frequency of spectra at high frequency is significantly red-shifted and its amplitude fluctuates.
Rotation angle between L2 and L3 is also an important factor affecting the degree of asymmetry of 'I-shaped' resonator. The differential absorption characteristics under various rotation angles are shown in Fig. 6(a). When the angle is (L3 is in parallel with x-axis), the value of is close to zero (small negligible spike around 11.75 GHz), indicating the asymmetry is almost broken up. Once L3 is no longer in a horizontal position, vertical component can be provided with the rotation angle from to . This strategy effectively introduces high asymmetry, together with certain polarization transformation, and eventually enhances the circular dichroism. With an optimized rotation angle , strongest can be achieved at both the lower and higher resonance frequencies.
Our proposed model has been experimentally demonstrated at microwave range. However, the design strategy is not limited to GHz, and it can be transplanted to higher frequencies. Furthermore, not only the 'I-shaped' metallic wires are highly tolerant to photoetching fabrication errors, but also the metal-dielectric-metal construction is easy to be fabricated. Generally, it works well down to mid-infrared. But as the wavelength further decreases, metals are not perfect, and the electron inertia will cause breakdown of the scaling law [40,41]. So the parameters should be made certain adaptive modifications. The metal is replaced by silver and described using the Drude model (the dielectric constant is 6.0, plasma frequency is, and collision frequency is) . The dielectric layer is silicon dioxide () with thickness of 480 nm and a relative dielectric constant of 2.25. Other optimized dimensions are: L1 = 1129.7 nm, L2 = 1161.7 nm, L3 = 1199.6 nm, w = 240.0 nm, a = 695.0 nm, b = 194.6 nm, c = 97.5 nm, d = 825.0 nm, , P = 2180 nm. As shown in Fig. 6(b), two differential absorption peaks for LCP and RCP illuminations are achieved in mid-infrared band, with the maximums of 98.3% at 65.9 THz and 92.0% at 97.3 THz, respectively. It is worth noting that, this absorption mechanism for mid-infrared model is similar with the references [20,43]. Strong absorption mainly originates from the excitation of surface plasmon resonance (SPR), which utilizes localized or un-localized surface plasmon (SP) coupling. Here, the electric and magnetic field are mainly bonded around the top 'I-shaped' resonator, with no power loss in the dielectric (loss free dielectric). The electromagnetic energy concentrates in the top metal, and almost all absorbed energy is consumed in metal (as the power loss inset in Fig. 6(b)). That is also the reason why enough thickness of the top metallic resonator should be taken into account. So, although the design strategy is demonstrated at mid-infrared band, the absorption mechanism is different with that of microwave model. In contrast to their low-frequency counterparts [1,44,45], for the mid-infrared absorber, the power carried by light is predominantly dissipated through ohmic losses in metals rather than in a dielectric spacer.
A high-efficiency chiral metamaterial absorber (CSMA) has been demonstrated both theoretically and experimentally. The absorptive peaks for LCP illumination are 95.18% at 12.04 GHz and 91.77% at 14.22 GHz, with little absorption of RCP illumination, resulting in significant absorptive circular dichroism. Not only can the CSMA intensely absorb LCP illumination with dual bands, but also CP conversion for RCP wave is achieved over a broad bandwidth (normal incidence). Besides, the proposed strategy is also demonstrated in mid-infrared band. This spin-dependent absorptive performance, together with certain polarization transformer, could boost the plasmonic devices with circular polarization control from microwave to optical region, especially in the applications of circularly polarized laser, absorber filter, chiral sensing/bolometers, molecular spectroscopy and satellite communication.
National Natural Science Foundation of China (61631012, 61701206), and K.C. Wong Magna Fund in Ningbo University.
The author M. H. Li acknowledges great concern and discussion with Vassili A. Fedotov in Optoelectoncis Research Centre, University of Southampton.
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