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Abstract
This paper proposed ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones with broadband CD in the mid-infrared band. The experimental results show that when the deposition angle θ = 45°, the first ITO deposition of ta = 100 nm, the second Si deposition of tb = 200 nm with the azimuth angle unchanged, and the third ITO deposition of tc = 200 nm after rotating the azimuth angle of 60°, the prepared chiral structure has a broadband CD response in the mid-infrared band of 2.5-4 µm. The broadband CD effect is produced by the internal resonance of the three-dimensional open cavity. The cone structure can be regarded as a plurality of planar open resonant rings with different diameters, and these rings resonate at different wavelengths. The experimental results also show that the proposed chiral ITO structure exhibits a better broadband CD response than that of the structure composed of traditional metal Ag. Such a chiral structure provides a new method for the design of CD devices in the mid-infrared band.
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1. Introduction
Mid-infrared circular dichroic devices have been widely used in mid-infrared lasers [1–4], detectors [5–7], molecular spectroscopy [8,9], and thermal imaging [10,11]. The polarization state control of circular dichroism (CD) is a key function in its practical application. However, the traditional optical system used for mid-infrared polarization state control is large in size and weight, high in cost, and low in integration, which severely restricts the development of such devices. In recent years, chiral metamaterials [12], which have potential applications in polarization control [13,14], have been proposed to resolve these problems. From the structural point of view, mid-infrared chiral metamaterials can be divided into three-dimensional chiral metamaterials [15–23] and two-dimensional ones [24–27]. Compared with the two-dimensional chiral metamaterials, it is more difficult for the three-dimensional chiral metamaterials [28–32] to produce. However, they have greater spatial freedom, richer and more innovative functional characteristics. Their structures are generally helical [15–19] and laminated [20,21]. Gansel et al. [15] combined laser direct writing technology and an electrochemical deposition method to prepare a gold spiral chiral metamaterial. Its CD value in the 3.5-6.5 μm wavelength range reaches 70% on average. Ji et al. [16] proposed a hybrid spiral structure. The simulation results show that the structure achieves an average extinction ratio of 8.5 × 103 and a transmittance of 72% within a wide response band of 3.5-9.5 μm. He et al. [17] used 3D laser direct writing technology and electron beam evaporation to fabricate a circular dichroic chiral hyperlens which works in the 3-5 µm band. Our research group proposed a mid-infrared broadband chiral spiral structure, which was composed of four indium tin oxide (ITO) spiral subunits with different radii [18]. The simulation results show that a broadband CD spectrum can be achieved in the mid-infrared band. Zhou et al. [20] theoretically studied the CD of stretchable chiral metamaterials with laminated L-shaped and T-shaped gold patterns on a polydimethylsiloxane substrate. This structure can produce 53.6% of the CD in the mid-infrared region. Barnard et al. [21] designed a laminated structure. The single layer of the structure was an array composed of four U-shaped gold rings periodically distributed. The adjacent two layers were separated by a SiO2 dielectric layer. Due to the chiral nature of the stack and the magnetic induction coupling between the stacked ring resonators, the CD is produced. In addition to the above-mentioned common three-dimensional structures, there were also three-dimensional chiral nanoparticle oligomers [22], and zigzag metal and graphene ribbon tunable combined structures [23], and so on. In the chiral metamaterials, plasmonic materials used were often traditional metals such as gold and silver. These metal materials often have shortcomings such as non-adjustable properties, insufficient compatibility with existing silicon manufacturing processes, and large real part values of dielectric constants in the mid-infrared band. Additionally, for the preparation of the chiral metamaterials, methods such as direct laser writing or focused ion beams were mainly used. The fabricated samples were small, the processes were relatively complicated, time-consuming, and expensive, which was not suitable for large-scale production.
In this paper, we introduced the mid-infrared indium tin oxide (ITO) plasmonic material [33] into a chiral metamaterial structure, which is composed of ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones. The glancing angle deposition (GLAD) method, which is suitable for mass production and easy to produce large-area structural materials [34–37], and the inductively coupled plasma etching technology were combined to fabricate the structure. Experimental results show that the structure exhibits a broadband CD response in the mid-infrared band of 2.5-4 µm. It is also shown that under the same preparation conditions, the CD signal of the structure based on ITO in the mid-infrared band is significantly stronger than that of the structure composed of silver.
2. Experiment
2.1 Materials
The substrate was an n-type (100) silicon wafer (thickness 400 ± 10 μm and resistivity ρ = 4000 Ω·cm), purchased from Zhejiang Lijing Optoelectronics Technology Co., Ltd.. The silicon target, purchased from Beijing Jinyu Sunshine New Material Technology Co., Ltd., had a specification of 1-10 mm and a purity of 99.999%. ITO (m(In2O3):m(SnO2) = 90:10) was purchased from Dingwei New Material Co., Ltd.. 5 wt% polystyrene (PS) with a diameter of 2.1 μm was purchased from Huge Biotechnology Co., Ltd.. Ultrapure water (18.25 M·cm−1) was prepared by our laboratory's ultrapure water machine. Acetone, ethanol, hydrogen peroxide, concentrated sulfuric acid, and ammonium hydroxide were purchased from Nanjing Wanqing Instrument Co., Ltd., and their purity was analytical pure.
2.2 Fabrication of samples
The preparation process of ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones is shown in Fig. 1. First, a periodic and uniformly distributed hexagonal lattice PS microsphere film was fabricated on a 3-inch silicon wafer by molecular self-assembly method. This procedure is the same as the previously proposed L-shaped ITO chiral structure [36]. Subsequently, using the monolayer PS film formed on the silicon wafer as a mask, silicon nanocones were prepared. Place the silicon wafer with PS film in the chamber of an ICP-98A high-density plasma etcher. During the etching process, the silicon wafer undergoes a physical and chemical reaction with the gas to generate gaseous products. By adjusting the etching conditions, a periodic silicon nanocone with a bottom diameter of 1.8 μm and a height of 1.2 μm is finally obtained. In our previous work, it was experimentally demonstrated that an L-shaped ITO structure based on arrays of polystyrene microspheres spaced at 2.1 μm intervals exhibit a good circular dichroism in the wavelength range from 2.5 μm to 4 μm [36]. In this work, we thus choose such a period of 2.1 μm. The etched samples were placed in a chloroform solution for 5 min and ultrasonically cleaned with deionized water for several times. Finally, the chiral structure on the surface of the silicon nanocone was prepared. An electron beam evaporation system (DE500, Technology Inc.) was used to deposit ITO and a silicon target on the silicon nanocone, respectively. During the deposition process, the chamber temperature of the electron beam evaporation system was maintained at around 500 °C, the vacuum degree was 5 × 10−5 Torr, two independent 6 MHz quartz crystal microbalances (QCMs) were used to monitor the deposition rate, the ITO evaporation rate was set to 1.5 A/s, and the Si evaporation rate was set to 1 A/s. The deposition angle θ (the angle between the electron beam and the normal of the silicon nanocone substrate) was set to 45°. φ was the azimuth angle, φ0 = 0° was the initial azimuth angle, and the azimuth angle φ could be expressed as φ = φ0+φi (i = 1, 2, 3), where φi was the azimuth angle of the i-th deposition. The first deposition of ITO: The azimuth angle was φ1 = 0° and the QCM monitoring thickness was ta = 100 nm. The second deposition of Si: The azimuth angle was φ2 = 0° and the QCM monitoring thickness was tb = 200 nm. The third deposition of ITO: The azimuth angle was φ3 = 60° and the QCM monitoring thickness was tc = 200 nm.
Fig. 1. Flow charts of preparation of mid-infrared broadband chiral structure composed of ITO/Si/ITO semi-cone-shell complexes on silicon nanocones.
2.3 Characterization
A field emission scanning electron microscope (SEM) (SU8010, Hitachi) was used to observe the morphology of the sample during the preparation process. Figure 2(a) show single-layer self-assembly PS microspheres. Figure 2(b) is an electron micrograph of silicon nanocones. Figures 2(c)–2(e) show the top and side views of the chiral structure. It can be seen from these figures that the structure is arranged periodically and its uniformity is good. Multiple repeated experiments showed that the final prepared samples have good uniformities. In Fig. 2(d), the red and cyan areas represent ITO and Si, respectively. From Fig. 2(e), the distributions of the ITO and Si can also be displayed by the outlines plotted, and they are formed on the surfaces of the silicon nanocones.
Fig. 2. SEM images of (a) single-layer PS microspheres, (b) silicon nanocones with a height of 1.2 μm, (c) top view, (d) enlarged top view, and (e) side view of the chiral structure composed of ITO/Si/ITO semi-cone-shell complexes on silicon nanocones.
3. Results and discussions
A Fourier transform infrared spectrometer (Bruker Tensor 27) was used to measure the CD of the samples [36,38]. By combining a linear CaF2 high extinction ratio polarizer with a wavelength of 2-8 µm and a 2.5-4 µm superachromatic quarter-wave plate, left-circularly polarized (LCP) light and right-circularly polarized (RCP) light are obtained, and then the corresponding transmittance T(RCP) of the RCP light and the transmittance T(LCP) of the LCP light were measured, respectively. Therefore, the difference between the two transmittance values was expressed as ΔT = T(RCP)-T(LCP), where the ΔT could be used to characterize the CD response of the sample [22]. Since the spot radius of RCP and LCP light was about 3 mm (determined by the size of the diaphragm aperture in the instrument), it was much larger than the bottom radii of the silicon nanocones. Therefore, the measured transmittances T(RCP) and T(LCP) were average values. In order to further reduce the error, the final CD spectrum was the average value of five different regions of the sample.
Figure 3 shows the transmittance and the corresponding CD spectrum of the sample under RCP and LCP light irradiation. It can be seen from the figure that the sample shows a broadband CD response in the 2.7-4 µm band, and the RCP light transmittance of the sample is less than that of the LCP light in the 2.7-4 µm band, that is, ΔT is less than zero. The main reason for CD is that the three-time film deposition forms a three-dimensional laminated semi-cone-shell chiral structure, which enhances the chirality. Generally, the surfaces of the ITO films will generate plasmon resonances under the excitation of the incident electromagnetic field. Due to the chirality of structure, the resulting asymmetric plasmon resonances induce a current flowing in a rotating manner. In the structure design, the Si dielectric layer between the two ITO films is to hinder the directional movement of the current, highlighting the effective distortion effects of the geometric changes on the incident light field, and making the current oscillation and electric field present asymmetrical distributions [39]. These asymmetric behaviors play major roles in the generation of CD. In order to verify the role of the Si dielectric layer, ta = 100 nm (φ1 = 0°) ITO was firstly deposited on the surface of the 1.2 μm nano-cone substrate, Si vapor-deposition process was then cancelled, the azimuth angle φ3 = 60° was maintained directly by rotating and tc = 200 nm ITO was deposited. During the fabrication, other preparation conditions remained unchanged. The measured CD spectrum of the sample is shown in Fig. 4. The CD response of the structure without the Si dielectric layer is significantly reduced in the measurement waveband. The broadband CD effect is produced by internal resonances [15]. The space laminated structure forms a three-dimensional open cone resonant cavity. The cone structure can be regarded as a plurality of planar open resonant rings with different diameters, and these rings resonate at different wavelengths [40,41].
Fig. 3. Left-handed light transmittance, right-handed light transmittance, and the corresponding CD spectrum of the chiral structure shown in Fig. 2(c).
Fig. 4. CD spectra of the chiral structure shown in Fig. 2(c) and the ITO structure without the interlayer silicon.
3.1 Influences of different φ on CD
We studied influences of the relative positions of the two layers of ITO films on the chirality of the structure. We kept the conditions of the first deposition of ITO and the second deposition of Si unchanged, and only changed the azimuth angle φ3 of the third deposition of ITO. The relative positions between the two ITO films were used to optimize the CD characteristics. We prepared five sets of samples with different azimuth angles φ3 of 30°, 60°, 90°, 120°, and 150°, respectively. The thicknesses of their third deposition of ITO were tc = 200 nm. The CD spectra of the five sets of samples are shown in Fig. 5. When the azimuth angle φ3 is 60°, the CD is the most significant. Different azimuth angle deposition means that the three-layer superimposed areas are different. In other words, the sizes of the areas directly affect the values of CD. From the CD curves of samples deposited at different azimuth angles, it can be seen that there is the best azimuth angle, that is, there is a three-layer superimposed area with the largest CD value. When light waves propagate through the sample, this area produces the strongest asymmetric transmission, resulting in the largest CD.
3.2 Influences of different ITO thicknesses on CD
Influences of the thicknesses of the ITO films on the chirality of the structure were also investigated. The conditions of the first deposition of ITO and the second deposition of Si, and the azimuth angle φ3 of the third deposition of ITO were kept unchanged. Only the thickness tc of ITO during the third deposition was changed. Other three samples with thicknesses tc of 100 nm, 300 nm, and 400 nm were prepared, respectively. The experimentally measured CD spectra of these samples and the original samples are shown in Fig. 6. It can be seen from the figure that when tc increases, the responses of CD present a non-monotonic change. When tc is 200 nm, the CD response is strongest. When tc increases from 100 nm to 200 nm, the asymmetry of the structure will increases and the chirality of the structure thus becomes stronger. After tc further increases, the CD responses keep decreasing. The reason is that although the asymmetry of the structure increases with increasing tc, both T(RCP) and T(LCP) of the structure significantly decrease 42. The resulting transmission difference between them is also reduced.
Fig. 6. CD spectra for different ITO thicknesses tc of 100 nm, 200 nm, 300 nm, and 400 nm, respectively.
3.3 Influences of different plasmonic materials on CD
In order to compare the CD responses of different plasmonic materials, we chose the traditional metal Ag to replace the mid-infrared plasmonic material ITO. Except for different materials and evaporation powers, other preparation conditions were the same as those of the sample in Fig. 3(c). The CD spectra of the Ag sample as well as the original ITO sample are shown in Fig. 7. It can be seen from the figure that the Ag sample exhibits a weak CD response in the 2.5-4 µm band, and its CD value fluctuates around −0.01, which is significantly lower than the CD response of the ITO sample. Therefore, in the mid-infrared band of 2.5-4 µm, the chiral structure based on ITO has an obvious advantage over the same structure based on traditional metal Ag. The reason is as follows. In the plasmonic material Ag, the carrier concentration is greatly large (about 1023 cm−3), hence the imaginary part of the dielectric function in the mid-infrared region are significantly increased [18,33,36], limiting the plasmon resonances of the carriers. As for ITO, due to its low carrier concentration, the imaginary part of the dielectric function is quite small, which easily causes the plasmon resonances of the carriers.
3.4 Simulations on absorption of structures
In order to verify the CD measurement result of a chiral sample, the finite difference time domain (FDTD) software was used to simulate the CD response of the sample. The structure model used in this section was from the figure shown after the third step of the preparation process in Fig. 1. Since the structure presented a hexagonal lattice periodic arrangement, we only considered a single unit cell perpendicular to the direction of electromagnetic wave propagation. It was assumed that a RCP wave and a LCP wave were incident along the z direction, respectively. Additionally, perfect matched layer (PML) boundary conditions were set in the z direction. The permittivity of ITO was determined by the Drude-Lorentz model [33,37].
In order to clarify the reason for the broadband CD of chiral samples, the distributions of electric field intensities in them were simulated. The origin of coordinate was set at the center of the ground of the central silicon nanocone in unit cell. The z-axis direction was along the silicon nanocone, as shown in Fig. 1. The distributions of electric field intensities in several sections with different z of 100 nm, 400 nm, and 700 nm for RCP and LCP waves with a wavelength of 4 μm were mainly simulated, as shown in Fig. 9. It can be seen from the figure that under the action of the incident electromagnetic field, the surface of ITO produces obvious plasmon resonances, and there is a strong electric field on the surface of the structure, indicating that surface plasmon polaritons (SPPs) are induced. When the LCP and RCP waves are incident, the electric field distributions show obvious differences at the wavelength of λ=4000 nm due to the geometrical changes of the protruding structure. The Si dielectric layer between the ITO film layers hinders the current flow, which induces the electric field distributed asymmetrically. Meanwhile, the cross-sectional electric field intensity distributions perpendicular to the XY plane can also verify these differences, as shown in Fig. 10.The protruding geometric changes produce an effective twisting effect on the incident waves, making the electric fields distributed asymmetrically. Such distributions result in asymmetric SPPs, which play a major role in the generation of CD.
Fig. 9. Distributions of the electric field intensities in the XY planes with different z under different circular polarization wave incidence. The incident wavelength is 4 μm (a) z = 100 nm, (b) z = 400 nm, and (c) z = 700 nm for the LCP wave incidence. (d) z = 100 nm, (e) z = 400 nm, and (f) z = 700 nm for the RCP wave incidence.
Fig. 10. Distributions of the electric field intensities in the two different XZ planes under different circular polarization wave incidence with the wavelength of 4 μm. Distributions (a) in one plane and (b) in another plane for the LCP wave incidence. (c) and (d) Distributions (c) in one plane and (d) in another plane for the RCP wave incidence.
4. Conclusion
In short, this paper proposed ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones with broadband CD in the mid-infrared band using the GLAD method. The experimental results show that azimuth angles, interlayer Si between ITO layers, ITO thicknesses, and plasmonic materials have great impacts on the CD of the structure. When the deposition angle θ = 45°, the first ITO deposition of ta = 100 nm, the second Si deposition of tb = 200 nm with the azimuth angle unchanged, and the third ITO deposition of tc = 200 nm after rotating the azimuth angle of 60°, the prepared chiral structure has a broadband CD response in the mid-infrared band of 2.5-4 µm. The geometric changes produce an effective twisting effect on the incident waves, making the electric fields distibuted asymmetrically. Such distributions lead to asymmetric surface plasmon polaritons, which play a major role in the generation of CD. The broadband CD effect is produced by the internal resonance of the three-dimensional open cavity. The cone structure can be regarded as a plurality of planar open resonant rings with different diameters, and these rings resonate at different wavelengths. The experimental results also show that the proposed chiral ITO structure exhibits a better broadband CD response than those of the structure composed of traditional metal Ag. Such a chiral structure with a three-dimensional opening resonant cavity provides a new method for the design of CD devices in the mid-infrared band. It has potential applications in polarization state control and mid-infrared sensing. By increasing the size of the structure, the response wavelengths can be extended to the far-infrared or even the terahertz band, further expanding the range of possible applications.
Funding
National Natural Science Foundation of China (62071208, 61771227); Graduate Research and Innovation Projects of Jiangsu Province (KYCX20_2338); Priority Academic Program Development of Jiangsu Higher Education Institutions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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