We propose and experimentally demonstrate a broadband and high efficient circularly polarizing dichroism using a simple single-cycle and single-helical plasmonic surface array arranged in square lattice. Two types of helical surface structures (partially or completely covered with a gold film) are investigated. It is shown that the circular polarization dichroism in the mid-IR range (3µm - 5µm) can reach 80% (when the surface is partially covered with gold) or 65% (when the surface is completely covered with gold) with a single-cycle and single-helical surface. Experimental fabrications of the proposed helical plasmonic surface are implemented with direct 3D laser writing followed by electron beam evaporation deposition of gold. The experimental evaluations of the circular polarization dichroism are in excellent agreement with the simulation. The proposed helical surface structure is of advantages of easy-fabrication, high-dichroism and scalable to other frequencies as a high efficient broadband circular polarizer.
© 2016 Optical Society of America
Circularly polarized light (CPL) generation and detection has attracted increasingly attentions recently due to its wide applications in various optical techniques and devices, ranging from quantum computation [1–3], spin optical communication , to circular polarization dichroism (CPD) spectroscopy  and magnetic recording . In CPL, the electric field vector travels along a helical trajectory, either clockwise or counterclockwise (i.e., right-handed circular polarization or left-handed circular polarization) , which can be decomposed into two linearly polarized components with perpendicular electric field vectors that are oscillating with a 90° phase difference. The most common method of generating and detecting circularly polarized light is the use of combination of a linear polarizer and a quarter-wave plate , which faces increasingly challenges in terms of feasibility of integration and complexity of the devices and systems.
Over the past few years, advances in surface plasmonic polaritons have opened new avenues for designing integrated functional circular polarizers on subwavelength scale. In 2009, Gansel et al. proposed and demonstrated a 3-dimensioanl helical nano-wire circular polarizer in the IR range , in which a circular polarization dichroism of as high as 80% (CPD = TRCP-TLCP with TRCP and TLCP being the transmission of right-handed and left-handed polarization) was achieved. The physical mechanism of CPD behind the 3D helical nano-wire structure is the phase matching between the polarized electric vector along the propagating direction and surface plasmonics along the spatial spiral direction of the 3D helical nano-wires. Detailed numerical analysis show that the optical performance, especially CPD of the 3D helical wires are sensitively dominated by multi-geometrical dimensions of the individual helices, such as helix pitch, helix radius, wire radius, and number of helix pitches [10, 11], from which multiple number of the helix pitches (i.e., multi-cycle helix) and precisely controlled wire radius must be employed in order to achieve a high CPD over a bandwidth. Following the concept of the 3D helical wires, Yang et al. further investigated theoretically the optical performance of a multi-cycle double-helical intertwined  or multi-cycle multiple-helices intertwined  3D wire structures in the visible and near-IR regions, which shows that the working bandwidth can be further improved with the increased multi-helical wires while the CPD remains more or less the same as that of multi-cycle single-helical wire due to the increased optical absorption through the multi-helical structure. These helical-wire based 3D structures exhibit great potential and excellent behavior in circular polarization manipulation, the fabrication of these metal-wire structures with either multi-cycle single-helical or multi-cycle multi-helical is, however, very complicated [9, 11, 14]. In order to make the gold metal helices with desired radius and height, Gansel et al. used multiple fabrication techniques including direct laser writing or stimulated emission depletion-inspired 3D laser lithography for a resolved multi-helical thin-wire structure followed by multi-step and well-controlled electro-chemical metal deposition processes [9,14]. In 2015, Esposito et al. reported a 3D triple-helical nanowires engineered by tomographic rotatory growth based on focused ion beam-induced deposition. This triple-helical wire nanostructure generates a 37% of circular dichroism in a wavelength range of 500-1000 nm . Other types of 3D structures are also proposed for the circularly polarized dichroism. Zhao et al. proposed a stacked nanorod metamaterials arrays with a tailored rotational twist, from which circular dichroism can reach 70% theoretically, but 30% only was achieved experimentally due to optical losses through multilayer adhesive metals . Two dimensional (2D) Archimedean spirals have also shown capabilities for circular polarization analyzing [17, 18], in which different transmission patterns with either a bright spot or a donut shape corresponding to left or right circular polarized incidence can be obtained. The energy of the total transmission of left or right circular polarized incidence is, however, indistinguishable. In 2012, Bachman et al. reported that a structure of nested Archimedean spiral gratings in a dielectric-coated metal film with a metal cap fabricated over a central subwavelength aperture exhibits high circular polarization dichroism, in which one circular polarization preferentially transmits through the aperture while the other is trapped in an optical vortex in the structure due to surface plasmons . Recently, planar chiral plasmonic metasurfaces exhibit capabilities in circular polarization dichroism. Fedotov et al. reported that asymmetric transmission of a circularly polarized light through a planar metal nanostructure consisting of continuous chiral “fish-scale” elements can be observed and a maximum circular dichroism of 25% in the visible to near-IR spectral region can be achieved . In 2015, Li et al. also demonstrated a chiral circular dichroism metamaterial consisting of a periodic array of ‘Z’-shaped silver (Ag) chiral metamolecules, from which a circular dichroism of 90% in the reflection mode was achieved in a very narrow bandwidth due to chiral metal-atom anisotropic absorption . While these metasurface-based 2D chiral structures show promising in circular dichroism, the narrow bandwidth and low circular dichroism of this type of structure still remain challenging.
In this work, we propose and demonstrate a broadband and high efficient circular polarizer using a single-cycle and single-helical surface metamaterial array. The employment of a helical surface enhances significantly the interaction between the polarized incident light and the effective helical area such that high circular dichroism and large bandwidth can be efficiently obtained with just a single-cycle and single-helical surface, which significantly reduces complexity of the fabrication. Two types of the helical surface structures (partially and completely covered with gold film) are investigated, from which a circular dichroism of up to 80% or 65% can be achieved, respectively, with the structure partially or completely covered by a thin gold film (100nm) in the mid-IR range (3µm - 5µm). Experimental fabrications of the proposed helical plasmonic surfaces are implemented with direct 3D laser writing followed by electron beam evaporation deposition of gold. The experimental measurements of the circular dichroism validates the theoretical design and simulations.
2. Design and characterizing of a plasmonic helical surface
The proposed plasmonic circular polarization dichroism based on chiral helical surface is shown in Fig. 1. A poly (methyl methacrylate) (PMMA) cylinder single-cycle helical surface (radius r and height L), covered by a gold (Au) film (thickness H) is arranged in a periodic array of square lattice (lattice period P) sitting on a silicon dioxide substrate. The handedness of the helical structure shown in Fig. 1 is left-handed. The performance of the circular polarized dichroism of the proposed structure can be characterized using finite difference time domain method (FDTD) (Lumerical FDTD solutions, Canada). The perfectly matched layers (PMLs) along Z direction and the periodic boundary conditions along X and Y directions owing to the periodicity of the helical cylinder are assumed in the simulation. The dielectric properties of Au given by Johnson & Christy are adopted . Two types of circularly polarized light, i.e., left-handed circular polarization (LCP) and right-handed circular polarization (RCP), are assumed to be incident from the substrate side along Z direction, and the transmission of the circularly polarized incident lights are calculated.
The effects of different parameters of the structure on the performance of different circular polarizations in the wavelength range of 2μm-6μm are firstly investigated. Figure 2(a) shows the effect of the cylinder radius of the helical surface on the transmission of RCP and LCP incidences. The parameters used in the simulation are: P = 1.5μm, L = 2.0μm, and H = 0.1μm. It is seen that the transmission of LCP (the handedness of the incident polarization is the same as that of the helical structure) is very sensitive to the radius of the helical surface with two surface plasmonic resonances at wavelengths of 3.1μm and 4.5μm. With the increase of the helical radius, the interaction between the LCP light and the plasmonic helical surface increases resulting in quickly decrease of the LCP transmission while the transmission of RCP (i.e., handedness of the polarization is opposite to that of the helical structure) changes slowly with the radius due to the weak interaction between the opposite handedness of structure and polarization. When the diameter (2r) of the helical surface is equal to the lattice period P, the transmission of the LCP becomes relatively flat in the designated waveband of 2μm-6μm. The difference in transmission between the RCP and LCP incidence renders a pronounced behavior of circular polarization dichroism (CPD) with the helical surface structure. Figure 2 (b) shows the effect of the lattice period P on the transmission of LCP and RCP incidences. The parameters used in simulation are: r = 0.75 μm, L = 2.0μm and H = 0.1μm. It is seen that the plasmonic resonant wavelengths shift sensitively towards longer wavelengths with the increased lattice period P in the case of LCP incidence. This is in contrast to that in Fig. 2 (a) in which the resonant wavelengths remain unchanged when the helical radius changes only. To maximize the interaction between the helical surface and the polarized incidence, the diameter of the helical surface would be the same as the lattice period P, as witnessed in Figs. 2(a) and 2(b). Figure 2(c) shows the effect of the lattice constant P on the transmission of RCP and LCP incidences when the diameter of the helical surface is assumed to be the same as P. Other parameters are: L = 2.0μm and H = 0.1μm. As seen in Fig. 2(c), while the interaction between the helical surface and the LCP incidence can be maximized in condition of P = 2r, the plasmonic resonant wavelengths shift towards longer wavelengths with the increased lattice period P( = 2r), which suggests that the working waveband of the circular dichroism can be tuned into different ranges by scaling the helical diameter. Figures 2 (d) and 2 (e) illustrate the dependence of the LCP and RCP transmission on the height of the helical surface and the thickness of the Au film, respectively. The parameters used in the simulation are r = 0.75 μm, P = 1.5 μm, and H = 0.1 μm in Fig. 2(d) and r = 0.75 μm, P = 1.5 μm, and L = 2.0 μm in Fig. 2(e). It is seen that both transmission of the LCP and RCP show a slow response to the height of the helical surface in Fig. 2(d). While the interaction between the helical structure and the circular polarized light can be enhanced with the increasing of the thickness of the Au film, the working bandwidth of high CPD is narrowed as seen in Fig. 2 (e). For the designated working wavelength band in mid-IR 3µm - 5µm, it is seen that, Fig. 2(f), a CPD of up to 80% in 3µm - 5µm wavelength range can be obtained with parameters: P = 1.5µm, r = 0.75 μm, L = 2.0μm and H = 0.1μm.
The 3D helical surface exhibits strong CPD with only a single-cycle surface in contrast to those multi-cycle and multi-helix wire structures. The physical mechanism behind the strong CPD of the helical surface can be well understood based on the surface plasmonic resonance. Strong interactions occur between the circular polarized incidence and the helical surface of the same handedness, in which the component of the incident circularly polarized light of the same handedness as the helical structure is reflected, while no such interactions is expected between the circular polarized incidence and the helical surface of opposite handedness, resulting in high transmission of the opposite handedness component, as seen in Fig. 2(a). Giant circular polarized dichroism can thus be generated in the helical surface structure.
To gain more insights into the underlying physics of the plasmonic resonant effect in the helical surface, Figs. 3(a)-3(h) shows the excited electric current on a left-handed helical surface under different circular polarized incidences at two resonant wavelengths 3.1μm and 4.5μm in Fig. 2(a) (i.e., two dips in the spectra), respectively. Figures 3(a), 3(c), 3(e) and 3(g) (first row) are the 3D view of the electric current on the helical surface and Figs. 3(b), 3(d), 3(f) and 3(h) (second row) are the top-view of the electric current on the helical surface. It is seen that strong plasmonic resonances occur at both wavelengths (i.e., 3.1μm and 4.5μm) when the incident circularly polarized light is of the same handedness as the helical structure, in which the direction of electric current exhibits symmetric and antisymmetric characteristics at 3.1µm [Figs. 3(a), 3(b)] and 4.5µm [Figs. 3(e), 3(f)]. In contrast, the electric current on the left-handed helical surface under RCP incidence at the wavelengths of 3.1μm [Figs. 3(c), 3(d)] and 4.5μm [Figs. 3(g), (h)] exhibits very low and random distribution, implying that no strong interaction occurs in the case of opposite handedness of the helical surface and polarized incidence. The plasmonic resonances shown in Figs. 3(a) and 3(e) represents two different resonant modes, i.e., the bonding mode and the antibonding mode, respectively, which is similar to that of the helix wire structures [9, 23]. In the case of helical surface, half-cycle helical surface can be considered as a basic element, and a one-cycle helical surface can be constructed by two half-cycle helices stacked vertically along z-direction. The two stacked elements are thus bonded and the resonance modes are coupled, resulting in hybridization and the splitting of resonance, from which two hybridized modes, i.e., symmetric [Fig. 3(a)] or antisymmetric [Fig. 3(e)] mode is generated.
Figure 4 shows the simulated polarization state of the transmission of a LCP incidence on a left-handed helical surface with structural parameters: r = 0.75 μm, P = 1.5 μm, L = 2.0μm and H = 0.1μm at wavelength of 4.5µm. Same as that observed in Fig. 2, high transmission of the RCP incidence can be obtained after the left-handed helical surface [Fig. 4(a)] while LCP incidence on the same handedness structure experiences an opposite case [Fig. 4(b)] in which very low transmission is observed. It should be noted that the polarization state in the transmission experiences a certain degree of the changes after the transmission through the helical surface, which may result in change of the degree of polarization ellipse. This will be discussed in details separately.
The helical surface structure proposed above shows strong circular polarization dichroism because of the Au film covered helical surface. To facilitate the fabrication difficulties of depositing Au film selectively onto the designated helical area, a more convenient plasmonic helical structure can be employed in which the whole surface including both the helical and the bottom flat substrate surface are fully covered by an Au film. Figure 5(a) shows the schematic diagram of an Au-film-fully-covered left-handed helical struture in which the Au film can be directly deposited onto the fabricated helical structure without the need of complicated etching process or complicated selective deposition technique. Figure 5(b) shows the optimized CPD performance of the fully covered structure in the same mid-infrared region of 3µm-5µm as that in Fig. 2(f). The parameters of the structure are r = 0.95µm, L = 2.0µm, H = 0.1µm and P = 1.90µm. When the result is compared with that of the helical-surface-covered only structure in Fig. 2(f), it is seen that the fully-covered helical structure also shows strong CPD (70% at wavelength of 4µm) with only a small degeneration (~10% decrease in CPD when compared with that in Fig. 2(f)).
It is noticed that the radius of the helical surface in the case of fully-covered structure becomes larger than that of the partially covered strcuture [Fig. 1] if the same operating wavelength range of 3µm −5µm is desired (from r = 0.75µm to 0.95µm). This can be explained by the difference of the current distribution between the two cases shown in Figs. 6(a) and 6(b), which shows the current distribution at the resonant wavelengths 3.1µm [Fig. 2(a)] in the Au film partially covered structure and 3.3µm [Fig. 5(b)] in the Au film fully covered structure, respectively. Additional currents with different directions are generated in the Au film on the four corners of the substrate surface in the case of Au film fully covered structure [Fig. 6(b)], which is in high contrast to the case of Au film partially covered structure shown in Fig. 6(a). The effect of the different current distributions shown in Figs. 6(a) and 6(b) can be further illustrated by the electric field vector distributions in Figs. 6(c) and 6(d), which are the electric field in the plane of 50nm above the substrate. From Fig. 6(c), it is clearly seen that the electric dipoles p induced by the left-handed circularly polarized incidence are aligned in the same direction in the partially covered helical structure, which results in an attractive interaction between dipoles. In contrast, from Fig. 6(d), it is seen that the electric dipoles are anti-aligned between helical surface units in the case of fully covered helical structure. The repulsive interaction occurred between the anti-aligned dipoles results in a shrinked effective helical diameter and hence the effective period, which will lead to a blue shift in the resonant wavelength, as shown in Fig. 2(c). To obtain an optimal performance in the desired working waveband between 3 and 5µm, the diameter of the helical surface and hence the period of the fully covered structure must thus be increased to compensate the shrinkage of the effective helical diameter due to the repulsive dipoles, which results in an enlarged helical diameter. The enlarged diameter of the helical surface in the case of fully covered structure is benefical to the fabrication.
3. Experimental fabrication and characterization
Experimental verifications of the proposed helical surface are performed with a left-handed Au film fully covered plasmonic helical structure. The plasmonic micro-helical surface (scale of array is 60 × 60, i.e., an area ~120 × 120μm2) are fabricated by a direct laser writing lithography in a negative-tone photoresist (Nanoscribe GmbH, Photonic Professional) followed by electron beam evaporation of gold directly onto the whole surface. Figures 7(a)-7(c) shows the scanning electron micrographs (SEM) of top-view (uncoated), oblique-view (uncoated) and oblique-view (Au coated) images of the fabricated helical cylinder surface structures, respectively. From the SEMs, the dimensions of the fabricated helical surface are measured as r = 0.97µm, L = 2.06µm and P = 1.94µm, which are very close to the designed parameters as that in Fig. 5(b).
The polarization measurement setup is shown in Fig. 8(a). A broadband IR light source (Zolix, LSH-SiN40) was modulated by a chopper with a constant frequency (135Hz was used in our measurement). The modulated light was then directed to a monochromator and a collimator. A linear polarizer (Thorlabs, LPMIR050-MP2) and a mid-infrared tunable quarter-wave plate (Alphalas GmbH, PO-TWP-L4-12-UVIR) were used to generate the desired circular polarization light. The quality of the generated circular polarization was checked with an additional polarizer, and the helicity of the generated circular polarization was evaluated and confirmed by another combination of a quarter-wave plate and a linear polarizer (not shown in Fig. 8(a)). The light was then focused onto the sample by an objective. During the process, an infrared CCD camera was used to assist to adjust and calibrate optical system. The weak IR signal was detected by a HgCdTe detector (EG&G Judson). The signal from the detector was preamplified and then fed into a lock-in amplifier (SRS, SR830) interfaced with a personal computer. Figure 8(b) shows the measured transmission spectra with two different circular polarization incidences (i.e., left-handed and right-handed circular polarization). Theoretical transmissions of the two different circular polarization incidences are also shown in Fig. 8(b) for comparison. It is clear that a large transmittance dichroism between left-handed and right-handed circular polarization appears in the desired wavelength range from 3µm to 5 µm, as theoretically predicted. The discrepancies between the simulated and measured results can be attributed to fabrication tolerance as well as the roughness of the helical surface, which can be further improved by increasing the resolution of direct laser writing. In addition, the small difference of materials permittivity between that used in the simulations and that in experiments may contribute to the discrepancies as well.
We have proposed and experimentally demonstrated a broadband and high efficient circular polarizer using a single-cycle and single-helical surface metamaterial array. The employment of a helical surface enhances significantly the interaction between the circularly polarized incident light and the effective helical area such that high circular polarization dichroism and large bandwidth can be efficiently obtained with only a single-cycle and single-helical surface, which reduces significantly the complexity of fabrication. Two types of helical surface structures (partially and completely covered with gold film) are investigated, from which a CPD of up to 80% or 65% can be achieved, respectively, with the partially or completely covered structure by a gold film in the mid-IR range of 3µm - 5µm. Experimental fabrications of the proposed helical plasmonic surface are implemented with direct 3D laser writing. Experimental measurement results of the CPD are in excellent agreement with the theoretical simulation. The proposed helical surface structure is of advantages of easy fabrication, high dichroism and scalable to other frequencies, which may be useful in chiral sensing and possible applications in optoelectronic devices, integrated quantum optics, analytical chemistry, light-splitting systems, CPD spectroscopy for biological detection and displays.
The work was supported by the National Natural Science Foundation of China (No.61378057), the National High Technology Research and Development Program of China (No. 2013AA031901), and the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. The work was supported by the graduate research and innovation plans of colleges and universities in Jiangsu (KYZZ15_0329). The work was supported by Natural Science Foundation of Jiangsu Province of China (BK20130392) and the Open Project of Key Laboratory of Universities and Colleges in Jiangsu Province (KJS1204).
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