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Abstract
Modern imaging and spectroscopy systems require to implement diverse functionalities with thin thickness and wide wavelength ranges. In order to meet this demand, polarization-resolved imaging has been widely investigated with integrated circular polarizers. However, the circular polarizers which operate at the entire visible wavelengths and have a thickness of several tens of nanometers have not been developed yet. Here, a circular polarizer, operating at the entire visible wavelength range, is demonstrated using helically stacked aluminum nano-grating layers. High extinction ratio and broad operation bandwidth are simultaneously achieved by using non-resonant anisotropic characteristics of the nano-grating. It is theoretically verified that the averaged extinction ratio becomes up to 8 over the entire visible wavelength range while having a thickness of 390 nm. Also, the feasibility of the proposed structure and circular polarization selectivity at the visible wavelength range are experimentally verified. It is expected that the proposed structure will lead to extreme miniaturization of a circular polarizer and contribute greatly to the development of mobile/wearable imaging systems such as virtual reality and augmented reality displays.
© 2017 Optical Society of America
1. Introduction
A circular polarizer has played significant roles in optics. With the increasing importance of polarization resolved imaging system, circular polarizers have been applied to enhanced contrast [1], circular polarization microscopy [2,3], remote sensing of chiral signatures [4], and detection of biomolecules such as amino acid, DNA and glucose which have inherent chiral structures [2,4–6]. Especially, circular polarizers operating at visible wavelength have attracted much attention because they are exploited as crucial optical components to control the polarization state of light in the complex display systems such as three-dimensional (3D) displays for virtual reality and augmented reality (VR/AR) [7]. Conventionally, a circular polarizer is achieved by a combination of several polarizers. However, this polarization system has limitation in various applications due to bulky configuration and narrow operating bandwidths.
Recently, metamaterials with 3D helix structures have been demonstrated as potential solutions for broadband circular polarizers because of their thin thickness and broad bandwidth. The 3D helix metamaterials can be fabricated by using various fabrication processes such as direct laser writing (DLW) [8,9], DNA based self-assembly [10], focused ion beam induced deposition (FIBID) [11], focused electron beam induced deposition (FEBID) [12], glancing–angle deposition (GLAD) [13,14], and tomographic rotatory growth (TRG) [15]. With the 3D helix structures, broad bandwidth and high circular polarization selectivity are achieved at infrared frequencies. However, the aforementioned fabrication techniques are incompatible with the other optical components, and thus they have limitation in fabricating the circular polarizer integrated nanophotonics systems.
As an alternative approach, stacking a few layers of planar metasurfaces within the subwavelength distance has been proposed [16–22]. As twisting and stacking multiple metasurfaces layers, strong resonance in a single layer of metal inclusions is converted into broadband bianisotropic effect by magneto-electric coupling among the stacked layers [18,21]. This approach is a more attractive solution than the aforementioned 3D helix structures because it can be fabricated by the multiple conventional lithography techniques such as focused ion beam and electron beam lithography. The stacked metasurface based circular polarizers have been implemented in various wavelength ranges of microwave [16,17], infrared [19,21], and near-infrared [18,20]. However, on these resonance-based structures, the trade-off between bandwidth and resonance strength is inevitably accompanied, and thus it is hard to achieve high circular polarization selectivity and broad bandwidth simultaneously. Also, it is difficult to realize visible circular polarizer made of gold or silver inclusions because these noble metals have low plasma frequencies [23,24]. Therefore, broadband circular polarizers operating in the entire visible wavelength range have remained as a key challenge. In addition, precise alignment between the stacked unit cell structures is required, that increases complexity in the fabrication process. As a result of these limitations, the helically stacked metasurfaces with noble metals are far away from application to the practical polarizer integrated imaging systems even though they have great advantages in ultra-thin thickness and broad bandwidth.
Here, we present an ultrathin broadband circular polarizer operating over the entire visible and near-infrared frequencies using helically stacked aluminum (Al) nano-gratings as shown in Fig. 1. Specifically, each layer consists of Al nano-gratings, and they are stacked in gradually twisted manner with subwavelength separation d and twisted angle θ. By exploiting non-resonant and broadband anisotropic characteristics in the nano-gratings, the proposed structure provides distinct 3D chiral current loops depending on the polarization state of light. This gives a detour route for avoiding the trade-off relation between circular polarization selectivity and bandwidth, so that it is possible to achieve both properties simultaneously. Thus, it can be indicated that our proposed structures are different from the double-layered gold crossed-gratings with symmetry breaking structures and resonant characteristics proposed by Gao et al. [22]. Also, the fabrication process is simplified by alignment-free multiple processes of lithography as our design is based on twists of whole nano-grating layers rather than those of unit cell structures. The induced current distributions impinged by the circularly polarized light are investigated to demonstrate circular polarization selectivity in our design. Finally, in order to investigate the fabrication feasibility and circular polarization selectivity of the proposed structure, experimental demonstration is presented by fabricating a prototype of the proposed structure using successive ion-beam millings and multi-wavelength optical measurements.
Fig. 1 (a) Schematic illustration of the proposed helically stacked Al nano-grating structure for visible wavelength ranges. (b) The magnified schematic illustration of the double-layered Al nano-grating structure. The nano-gratings have period (p), width (w) and thickness (t), and they are helically stacked with twisted angle (θ) and separation (d).
2. Results and discussions
2.1 Helically stacked aluminum nano-gratings
For comparison, transmission properties of nanorod and nano-grating based structures are examined first by numerical analysis. In this paper, all numerical simulations are performed by commercial finite-difference time-domain tool (FDTD, Lumerical Solutions, Inc.). Dielectric constants of Al are referred to the textbook [25] and the refractive indexes of substrate and dielectric spacer between stacked layers are assumed as 1.5. For calculation of transmission matrices for circularly polarized light, the simulations are conducted under the linearly polarized incident light (x and y polarized incident light). Hence, the following equation is used:
where the subscripts r and l represent the right circularly polarized (RCP) light and left circularly polarized (LCP) light, respectively.As shown in Figs. 2(a) and 2(b), we calculate transmission spectra of the Al nanorods for linearly polarized light [Fig. 2(a)] and the double-layered one for circularly polarized light [Fig. 2(b)]. The Al nanorods with anisotropic structure make plasmonic resonances at different wavelengths depending on the polarization state of incident light [26]. As shown in Fig. 2(a), the Al nanorods, for y-polarized light which is parallel to the nanorod axis, exhibit the strong resonant transmission dip at the wavelength of 558 nm. Also, the weak transmission dip occurs around 880 nm due to the intrinsic optical loss of Al [25]. On the other hand, for x-polarized light which is vertical to the nanorod axis, the incident light easily passes through the nanorods without strong resonance over the wavelength range. Also, it is worth noticing that the anisotropic transmission in the entire visible range is obtained, which is difficult for gold and silver nanostructures since Al provides plasmonic resonance at shorter wavelength than that of the noble metals. When the unit cell of the nanorod array is twisted and closely stacked within subwavelength distance of d = 50 nm and twisted angle θ = 45° [inset of Fig. 2(b)], the linearly polarized eigenmodes on each single nanorod can be converted into circularly polarized ones by magneto-electric coupling between the cascaded nanorod layers. As shown in Fig. 2(b), the helically stacked two-layered nanorods show resonant transmission dips at 508 nm and 653 nm for RCP and LCP incident light, respectively. These distinct transmission dips depending on the polarization state of incident light lead to moderate circular polarization selectivity: LCP light selectively passes through the structure between 320 nm and 575 nm, while RCP light mainly passes through it between 575 nm and 780 nm. However, since this bianisotropic effect is based on the strong resonance in the nanorods, high extinction ratio (ER = TLCP/TRCP) and broad bandwidth cannot be achieved simultaneously by this resonance based structure.
Fig. 2 Transmission curves of (a) Al nanorod array for the linearly polarized light and (b) helically stacked nanorod array for the circularly polarized light. The nanorods have p = 150 nm, l = 100 nm, w = t = 30 nm. The bottom nanorods are parallel to y-axis, and the upper nanorods are helically stacked with d = 50 nm and θ = 45°. (c) Transmission curves of Al nano-gratings for the linearly polarized light and (d) helically stacked nano-gratings for the circularly polarized light (d) with p = 150 nm, w = t = 30 nm, d = 50 nm and θ = 45°. (e) ER and operating bandwidth of the stacked nanorod (black) and nano-grating (red).
Next, non-resonant metamaterial is considered, which provides low optical loss and broad bandwidth [27,28]. Al nano-gratings are exploited as shown in Fig. 2(c). The nano-gratings allow current to flow in a nearly infinite length on y-axis, but not on x-axis. Therefore, the nano-gratings exhibit non-resonant characteristics over the wide wavelength range of y-polarized light, but do not interact much with the x-polarized light. As shown in Fig. 2(c), the Al nano-grating layer transmits x-polarized light and reflects y-polarized light over broad wavelength range with a small transmission dip around 800 nm due to intrinsic loss of Al. It is easy to confirm that the nano-grating layer results in non-resonant and broadband anisotropic transmission from visible to telecommunication wavelength ranges. Then, the nano-gratings are stacked helically with twist of the entire nano-grating layer, rather than twisting the unit cell structure [Fig. 2(d)]. As shown in Fig. 2(d), it is apparently shown that the stacked nano-gratings, over the extremely broad wavelengths ranging from visible to telecommunication wavelengths, selectively transmit LCP light quite well, but the most of RCP incident light is reflected. ER and figure of merit (FOM) of the helically stacked nano-gratings and nanorods are compared over operating bandwidth as shown in Fig. 2(e). Here, we define the operating bandwidth as a wavelength span where ER is larger than, and FOM is defined by a product of the operating bandwidth and the averaged ER. The proposed nano-grating structure can provide the operating bandwidth of 922 nm and the FOM of 1816, which are about 8 times larger than those of the helically stacked nanorod structure.
To understand the underlying mechanism of the stacked nano-grating structure, electric current density in the stacked nanorod and the stacked nano-grating structures are compared as shown in Fig. 3. In this paper, the effective material parameters are not exploited because the proposed multiple layered structure comes from complicate interaction among the nano-grating layers.
Fig. 3 Electric current distribution in the helically stacked Al nanorods (a) for RCP incident light at 508 nm and (b) LCP incident light at 653 nm. Electric current distribution in the helically stacked Al nano-grating for (c) RCP and (d) LCP incident light at 609 nm. The white arrows indicate the direction of the induced current.
When the nanorod array is excited by the light polarized along the nanorod axis, free electrons are accumulated on both ends of the nanorod, and strong oscillation of current flows through the nanorod. In the case of the helically stacked nanorods, the strong oscillation of current in the bottom nanorods is transferred to the next upper nanorod by the magneto-electric coupling between the nanorods. Hence, 3D chiral current loop is formed. For RCP incident light, the helically stacked nanorods with left handed manner show resonantly induced current on the bottom and the upper nanorods at the resonance of 508 nm [Fig. 3(a)] with large reflection. For LCP incident light on the left handed configuration, similar resonant transmission dip occurs but it resonates at the longer wavelength of 652 nm [Fig. 3(b)], because it forms the opposite handedness of 3D current loop. On the other hand, in the case of the nano-gratings, when the linearly polarized light parallel to the nano-grating is incident on the structure, the induced current path becomes infinitely long. When the nano-gratings are helically stacked, the stacked structure forms quite different current distribution depending on the handedness of the incident light. For RCP incident light, the stacked nano-gratings with left handed manner induced current flows to the upper layer and exhibit relatively strong magneto-electric coupling. This coupling leads to 3D chiral current flows and provides high reflectance [Fig. 3(c)]. On the other hand, for LCP light, the induced current at the bottom layer cannot strongly interact with the upper layer [Fig. 3(d)]. Therefore, the proposed structure exhibits weak 3D chiral current loop and acts like nearly transparent material for the same handedness of light with a small amount of reflection at the bottom layer and the intrinsic Ohmic loss of Al. As a result, the proposed stacked nano-grating structure can provide ultra-broadband circular polarization selectivity.
The bianisotropic effect at the visible wavelength ranges of interest is investigated with the helically stacked multiple layers as shown in Fig. 4. The more the nano-grating layers are helically stacked with the left handed configuration, the more the structures block RCP light over the wide wavelength range because they exhibit the longer 3D chiral current loops along with the stacked layers. In the case of LCP incident light, it remains nearly transparent. But the overall transmittance decreases due to the intrinsic loss of Al. This result is in a good agreement with the aforementioned results that the helically stacked nano-grating can pass the same handedness of light and reflect most of the opposite handedness one. As increasing the number of the nano-grating layers, the averaged ER of the seven-layered structure reaches up to ~8 over the entire visible wavelength regime. Note that our design remains with subwavelength thickness of 390 nm even with seven layers of stacking, which corresponds to about one-tenth of the thickness required for conventional polymer based micro-circular polarizers. Therefore, the proposed helically stacked nano-grating structure can be a promising candidate as an ultrathin circular polarizer for visible wavelength range.
Fig. 4 Transmission curves and ER for circularly polarized light as changing the number of the helically stacked nano-grating layers. The structures are constructed with left handed manner, and structure parameters are p = 150 nm, w = t = 30 nm, d = 30 nm and θ = 45°. The number of helically stacked nano-gratings for the above six plots are (a) two, (b) three, (c) four, (d) five, (e) six, and (f) seven, respectively.
The four-layered nano-grating structure is optimized for experimental demonstration [Fig. 5]. For achieving high performance circular polarizer, it is needed to consider not only averaged ER but also averaged transmission of the selected polarization over the entire visible wavelength range. Also, the bianisotropic effect in the helically stacked structures may be determined by twisted angle θ and the separation distance d. Therefore, we calculate averaged ER and averaged transmission for LCP incident light as changing θ and d [Figs. 5(a) and 5(b)]. The averaged values are calculated over wavelength ranges from 400 nm to 700 nm. The nano-grating structures with small θ lead to low averaged ER because they form weak 3D chirality. In addition, the transmitted light decreases as the increase of θ since the nano-grating layer can be considered as a linear polarizer [cf. Figure 2(c)]. This transmission reduction appears strongly in LCP light which has high transmittance, which causes a decrease in ER at large θ. Thus, d = 30 nm and θ = 45° are set to optimized values with the highest averaged ER and moderate LCP transmission. Next, we investigate the averaged ER and averaged transmission for LCP incident light as changing t and fill factor with fixed d = 30 nm and θ = 45° [Figs. 5(c) and 5(d)]. The highest averaged ER can be obtained at fill factor of 0.2-0.3 and t of 20-30 nm, and the averaged transmission dramatically decreases when fill factor increases. Therefore, fill factor and t are set to 0.2 and 30 nm, respectively, for the high transmittance and fabrication feasibility of the nano-grating layer.
Fig. 5 (a) Averaged ER and (b) averaged transmission for LCP light of the four-layered Al nano-grating structure over the wavelength ranges from 400 nm to 700 nm as a function of d and θ. (c) The averaged ER and (d) averaged transmission for LCP light of the four-layered Al nano-grating structure with fixed d = 30 nm and θ = 45° as a function of t and fill factor.
Based on this optimization process, we fabricate a prototype of the proposed structure with p = 150 nm, w = t = 30 nm, d = 30 nm and θ = 45° in an area of 15 μm by 15 μm using successive focused ion beam (FIB) milling method. For the fabrication of four-layered nano-grating structure, at first, 30-nm-thick Al layer is deposited on a glass substrate using electron beam evaporator (MUHAN, MHS-1800). Next, the nano-grating patterns are milled on the Al layer using FIB milling machine (FEI, Quanta 200 3D), and then 30-nm-thickness polymethyl methacrylate (PMMA) is spin coated for planarization process. This process for the single nano-grating layer is conducted sequentially with twisted angle of 45° for the four-layered nano-grating based circular polarizer. For the optical measurement, three different monochromatic continuous wave lasers with center wavelength of 532 nm, 660 nm and 980 nm were exploited as the input light. The input light sequentially passes through a quarter-wave plate, a half-wave plate, the four-layered nano-grating sample, an objective lens, a second quarter-wave plate and a linear polarizer, and then the output light is captured by CCD cameras.
Figure 6(a) shows a cross-sectional SEM image of the four-layered nano-grating structure. The structure is fabricated with the left handed manner (θ = 45°), and the uppermost layer forms an angle of 45° with respect to y-axis. For polarization sensitive optical measurement, we capture CCD images of the transmitted light from the selected polarized light, LCP, and the unselected polarized light, RCP, at 532 nm, 660 nm and 980 nm [Figs. 6(b)-6(d)]. For ER calculation, the intensities of the CCD images are compared as shown in Figs. 6(e)-6(g). The ERs of 1.651, 1.479, and 1.821 are calculated at 532nm, 660nm and 980nm, respectively. It can be seen that the LCP incident light passes well through the fabricated structure while the RCP incident light passes through it with relatively low intensity. However, transmission for RCP incident light is not suppressed enough compared to the simulation result [Fig. 4(c)]. This is because the fabricated Al nano-gratings have blunt edges under the FIB milling process of nano-scale width and thickness less than 50 nm. Despite these imperfections in the fabricated structure, it is worth noticing that the proposed device and working principle present circular polarization selectivity in the full range of visible wavelengths for the first time.
Fig. 6 (a) A cross-sectional SEM image of the fabricated four-layered helically stacked nano-grating structure and its magnification. The captured CCD images from the fabricated sample at (b) 532 nm, (c) 660 nm and (d) 980 nm. The left figure is for LCP incident light and the right one for RCP incident light. (e)-(g) The output intensities of the LCP light (black) and RCP light (red) along the red line of (b)-(d) are demonstrated.
2.2 Helically stacked nano-gratings with other metals
In addition, our design can be extended to wider wavelength ranges when we replace Al to other noble metals. For example, helically stacked gold or silver nano-gratings can be good ultra-broadband circular polarizers for near infrared wavelength ranges. Our helically stacked nano-grating design can be applied to micro circular polarizers operating in various wavelength range through scaling up the structure with different metals having low intrinsic loss. Here, we calculate transmission curves of the gold (Au) and silver (Ag) based cases with four-layered structure [Fig. 7]. In the proposed structure, cross-polarization components of transmission matrices for circularly polarized incident light are successfully suppressed, and thus the transmitted wave is circularly polarized light. Unlike Al, Au and Ag do not have strong intrinsic losses in the near infrared wavelength range, so we can obtain an ultra-broadband circular polarization selectivity. Therefore, we believe that our design is strong incentive for developing broadband and ultra-thin circular polarizers operating at various wavelength ranging from visible to infrared.
Fig. 7 Transmission curves for (a) the helically stacked gold nano-grating and (b) the helically stacked silver nano-grating. The structures are helically stacked with the four layers of the nano-gratings as shown in the inset of Fig. 4(c), and the structure parameters are p = 150 nm, w = t = 30 nm, d = 30 nm and θ = 45°.
3. Conclusions
In this paper, we propose the concept of the helically stacked Al nano-grating structure which provides non-resonant characteristics of circular polarizer for the visible light with ultra-thin thickness. The bianisotropic effect is obtained in a broad range of visible wavelengths and experimentally demonstrated. For the visible wavelength range, our non-resonant structure provides broader bandwidth and higher ER than resonance-based structure by overcoming the trade-off between bandwidth and ER. Also, the helically stacked Al nano-grating structure is highly suitable for practical applications in terms of cost and mass production, especially considering abundancy of Al in the earth crust and simple fabrication process. Furthermore, due to the simple structure of the nano-grating, nano-imprinting lithography method [29] can be introduced to reduce cost with high throughput and resolution. Also, through scaling up the structures or using other materials having low intrinsic absorption loss, it is easy to apply our design to ultra-broadband strong biansiotropy effects in various wavelength ranges. Based on the proposed structure and methodology, it is possible to achieve extreme miniaturization of optical systems integrated with a circular polarizer such as polarization-resolved spectroscopies and mobile/wearable 3D displays.
Funding
Basic Science Research Program through the National Research Foundation of Korea (NRF) (2017R1A2B2006676).
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