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Abstract
This paper proposes a mid-infrared chiral structure, which consists of L-shaped indium tin oxide (ITO) films formed on self-assembled monolayer polystyrene microspheres in two orthogonal directions by oblique angle deposition technique. Experimental results demonstrate that the structure exhibit circular dichroism (CD) responses in the range of 2.5 – 4 µm. As the thickness difference of the ITO films in the two orthogonal directions increases, the CD response enhances. The reason is that the ITO films produce cross dipoles and their bigger differences in thickness bring to bigger phase differences in optical chirality. The experimental results also demonstrate that the CD signals are evidently stronger than those of the structure consisting of silver in the mid-infrared band. This work provides a new idea for the fabrication of mid-infrared chiral structures, which have potential applications in the polarization state control of mid-infrared lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Chiral structures mean that they cannot be superposed with their mirror images [1]. When a left-handed circularly polarized wave (LCP) and a right-handed circularly polarized wave (RCP) are incident on the structures, they will often exhibit different optical responses with circular dichroism (CD) and optical activity, which are capable of controlling polarization states [2–5]. For this reason, artificial chiral structures become hot areas due to their stronger responses. The proven responses of the artificial chiral structures mainly distributed in the microwave [6], terahertz [7], visible or near-infrared regions [8–14]. Only a small number of researchers reported the responses distributing in the mid-infrared band [15–17]. In these works, the used plasmonic materials were traditional metal [18]. Many applications, such as transformation-optics devices, require that the imaginary parts of metal's dielectric functions are small. However, in the mid-infrared band, they are large [18]. Additionally, the metal materials have poor chemical stability and are not compatible with the standard silicon manufacturing processes.
Mid-infrared lasers have important applications in medical treatments and communications. The polarization control during their transmission is indispensable for their better applications. The traditional control methods of polarization states mainly adopt mid-infrared quarters or half wave plates, or adjust the voltage applied to mid-infrared liquid crystals. The disadvantages of these methods are that the control elements used are bulky, costly and difficult to integrate. Artificial chiral structures can overcome the shortcomings of traditional mid-infrared laser polarization control components.
In this paper, indium tin oxide (ITO) [18], a good mid-infrared plasmonic material, is selected to fabricate a mid-infrared chiral structure by an oblique angle deposition (OAD) method [19–24]. The OAD is a physical vapor deposition technique. Under vacuum conditions, a material is evaporated into gas and the gas is deposited on the surface of a substrate. Its main mechanism is that the evaporated material does not deposit into geometrically shadowed areas. The method has the advantages of simple operation, low cost and suitable for mass production. The structure consists of L-shaped ITO films with different thicknesses formed on self-assembled monolayer polystyrene microspheres in two orthogonal directions. Zhang’s group [25] ever proposed a similar L-shaped chiral nanostructure. However, the L-shaped nanostructure is made up of silver. Its CD responses occur in the visible and near-infrared ranges. In our work, experimental results show that the structure exhibit CD responses in the range of 2.5–4 µm. The results also show that the CD signals are evidently stronger than those of the structure consisting of silver in this mid-infrared band. Such a chiral structure has potential applications in the polarization state control of mid-infrared lasers.
2. Experiment
2.1. Materials
ITO (m (In2O3): m (SnO2) = 95:5) for vapor deposition is purchased from Dingwei New Materials Co., Ltd. Polystyrene (PS, 10wt%) spheres with about 2100 nm diameter (See Supporting Information) were purchased from Huge Biotechnology Co., Ltd. Ultrapure water (18.25M cm−1) is from ultrapure water machines in our laboratory. Hydrogen peroxide, concentrated sulfuric acid, acetone, ethanol and ammonium hydroxide are purchased from China Pharmaceutical Group Chemical Reagents Co., Ltd.
2.2. Fabrication of samples
Monolayer PS sphere films were fabricated by utilizing a self-assembly method. The method [26–27] is as follows: We first prepare silicon wafers with 2 × 2 cm sizes and placed them in a mixed solution of deionized water, hydrogen peroxide and ammonium hydroxide with a ratio of 5:1:1 for at least 30 minutes of cleaning. The cleaned silicon wafers were then placed in deionized water, ultrasonically cleaned for at least 5 minutes, and finally blown dry with nitrogen for later use. The prepared PS sphere solution was dropped into a glass petri dish filled with deionized water by a peristaltic pump at a rate of 0.009 ml/min through a syringe. After a monolayer film had formed, a Teflon loop was put on it. The silicon wafers were then placed in the petri dish from the outside of the Teflon loop, and the water in it was drained off. Finally, monolayer PS sphere films on the silicon wafers were achieved after they had been dried in the air. The distribution of PS spheres is relatively uniform overall, which were processed by Image J. The distribution is shown Fig. 1. The result illustrates that the number of spheres is 240. Their minimum and maximum diameters are 1.979 µm and 2.412µm, respectively. And the average size of spheres is 2.144 µm.
ITO layers were deposited respectively on the monolayer PS sphere films using an electron beam evaporation system (DE500, DE Technology Inc.) in two orthogonal directions, as shown in Fig. 2. Here, φ [11,25] is defined as the azimuthal angle. The variation of the azimuthal angle is expressed as Δ φ = φ0 +φ, where φ0 is the initial azimuthal angle. During the deposition, the tilt angle was fixed at θ = 86°. The electron beam evaporation temperature was kept at 500 °C, the vacuum was maintained at about 3×10−5 torr, and the evaporation rate was monitored and maintained at about 0.8 A/S using a 6 MHz quartz crystal microbalance (QCM). The thicknesses of the two depositions were denoted as ta and tb. The QCM value of ta was set to 500 nm, while the QCM value of tb was 750 nm, 875 nm, 1000 nm, 1125 nm, 1250 nm, respectively. The resulting samples for five cases were obtained.
Fig. 2. Schematic diagram of the deposition process of L-shaped ITO films on the self-assembled PS spheres in two orthogonal directions. In process ①, the initial azimuthal angle was set to a certain value. In process ②, the variation of the azimuthal angle is set to Δφ = 90° by counterclockwise rotation. The tilt angle was fixed at θ = 86°.
The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the samples were obtained with SU8010, Hitachi and HT7700, Hitachi, respectively. Figure 3 shows SEM and TEM images of the sample with ta = 500 nm and tb = 1000 nm. From Fig. 3(a), we can see that the L-shaped ITO films are not completely uniform due to defects in the self-assembled PS spheres. Three different domains are selected and labeled by D1, D2, and D3. Their zoomed images are shown in Figs. 3(c), 3(d), and 3(e), respectively. We additionally simulated a large number of deposition structures by our programed bead software. It is shown that the initial azimuthal angles (the azimuthal angles of deposition process ①) corresponding to the three domains are equal to φ = 0°, 35°, and 20°, respectively, as shown in Figs. 3(f), 3(g), and 3(h). To more clearly see the evaporation of our samples, we prepared a certain PS sphere with L-shaped ITO films. Its TEM image is shown in Fig. 3(b). Evidently, there exist ITO films with uniform thicknesses on the PS sphere.
Fig. 3. (a) Top-view SEM image of the sample with ta = 500 nm and tb = 1000 nm. (b) TEM image of a certain PS sphere with L-shaped ITO films in the sample. Zoomed images corresponding to three different domains (c) D1, (d) D2, and (e) D3. Simulated deposition structures obtained using a programmed bead software with the initial azimuthal angles of (f) φ = 0°, (g) 35°, and (h) 20° correspond to three domains D1, D2, and D3, respectively.
3. Crystal structure analyses
An ITO film with a thickness of 400 nm was deposited on a silicon substrate the same as ones of the samples, and used to analyze its crystal properties, which were characterized by an x-ray diffractometer (XRD) [28] (Model D2, Bruker). The x-ray scan of the thin film was recorded with a Cu Kα1 radiation (λ = 1.5406 Å) in the 2θ range from 20° to 65° with a step of 0.010°. For more evident crystallinity, we annealed the ITO film. Temperature was set to 520 °C, time was 15 min, and rising temperature was 10 ° C/min. As shown in Fig. 4, the XRD spectrum shows that the annealed ITO film is polycrystalline, which is consistent with the data of the standard PDF card JCPDS 06-0416. The SnO2 (JCPDS Ref. No. 50-1429) is completely soluble in the In2O3 lattice. The strong diffraction peaks appear at 2θ = 21.601°, 30.681°, 35.590°, 51.165°, and 60.755°, which correspond to the (211), (222), (400), (440), and (622) planes of In2O3, respectively. It is understandable that the In2O3 is absolutely dominant in quality.
Fig. 4. XRD diffraction spectrum of an ITO film with a thickness of 400 nm deposited on a silicon substrate.
4. Chiral properties
CD responses of the samples were experimentally investigated by a Fourier transform infrared spectrometer (Bruker Tensor 27). The RCP and LCP waves were produced by adding and rotating a super-achromatic quarter-wave plate with a wavelength of 2.5 – 4 µm between a CaF2 polarizer with a wavelength of 2 – 8 µm and a CaF2 analyzer with its polarization direction perpendicular to one of the polarizer in the light path in the instrument. The measured corresponding transmittances T(RCP) and T(LCP) were substituted in ΔT = T(RCP) - T(LCP) [29], where ΔT is used to characterize the CD responses of the samples. Since the radii of the RCP and LCP waves are about 3 mm (estimated by the instrument setting), with much larger than the radius of PS spheres. Therefore, their corresponding transmittances T(RCP) and T(LCP) are averaged. Figure 5 shows the resulting CD spectra of the five samples. Each CD spectrum came from averaging measured one of five different domains on any sample. From this figure, we can see that there exist evident CD responses and ΔT is greater than zero, which indicates that the transmittances of RCP waves for all the samples are greater than ones of LCP waves. Under the excitation of incident light, the L-shaped structure forms electric dipoles, and there is a certain angle between two electric dipoles. At the same time, the electric dipole moments between the two electric dipoles are not in the same plane because of the curved surface structure on the spherical shell and the thickness difference. When the left-handed/right-handed circularly polarized light are separately incident on the structure, the difference of the electric dipole moments occurs. The difference causes a difference of resonance intensity, which leads to CD responses. There also exist two enhancement CD peaks at about wavelengths of 2.9 µm and 3.4 µm due to local surface plasmon resonances of the L-shaped ITO films. As tb increases, the CD responses become stronger. The reason lies in that increasing differences between tb and ta bring to stronger asymmetry in the transmission direction. The figure also shows that although the thickness is different, the line shape does not evidently change. This reason is in that the line shapes mainly come from the plasmon resonances decided by the lengths of the L-shaped ITO structure.
Fig. 5. Average CD spectra of the five different samples with ta = 500 nm and tb = 750 nm, 875 nm, 1000 nm, 1125 nm, and 1250 nm, respectively.
To investigate the difference of CD responses caused by ITO and traditional metal materials, we chose Ag for comparisons, with the three different deposition thicknesses, namely, ta = 100 nm and tb = 150 nm, ta = 200 nm and tb = 300 nm, and ta = 500 nm and tb = 750 nm respectively. The other preparation conditions are the same as those of ITO. The measured CD results are shown in Fig. 6, in which the CD curve of the ITO sample with ta = 500 nm and tb = 750 nm is involved. From the figure, we can see that the three Ag samples exhibit CD responses in the 2.5–3.1 µm band, and the CD value is almost fluctuating below −0.015 in the 3.1–4 µm band. However, in the whole band, the ITO sample exhibits strong and stable CD responses. Therefore, the chiral sample consisting of ITO has obvious advantages compared with the traditional Ag metal in the mid-infrared band of 2.5–4 µm. The reason is as following. In conventional plasmonic materials such as Ag, the carrier concentration is very large (∼1023 cm−3), and this therefore significantly increases the imaginary part of its dielectric function in the mid-infrared region [18]. Such a large imaginary part limits plasmon resonances of carriers. For ITO, however, the imaginary part of dielectric function is small due to its greatly low carrier concentration, which causes easy plasmon resonances of carriers.
Since electromagnetic waves are incident on the chiral structure, they will cause asymmetric transmission [30] due to the difference of incident light directions. We then chose the sample with ta = 500 nm and tb = 750 nm and put it in the forward and reverse directions, and measured the differences Δ cir of respective transmittance of RCP and LCP waves, as shown in Fig. 7. Evidently, the differences appear due to the different interaction of the chiral sample with electromagnetic waves with different incident directions. The difference for the RCP wave is more obvious than one for the LCP light. When exploring the vertical passage of circularly polarized waves through a planar chiral dielectric structure, the relationship between incident and transmitted waves is usually described by means of Jones matrix [31], which is expressed as follows:
Fig. 7. Asymmetric transmission spectra of the sample with ta = 500 nm and tb = 750 nm for RCP and LCP incidences.
Therefore, asymmetric transmission can be described as
5. Conclusion
In summary, we propose a mid-infrared chiral structure, consisting of L-shaped ITO films deposited on self-assembled PS microspheres in two orthogonal directions by an OAD method. Experimental results show that the structure exhibit strong CD responses in the range of 2.5 – 4 µm. As the thickness difference of the ITO films in the two orthogonal directions increases, the CD responses become stronger. The reason is that the ITO films produce cross dipoles and their bigger differences in thickness bring to bigger phase differences in optical chirality. It is also demonstrated that the CD signals are stronger than those of the similar structure made up of Ag. Such a mid-infrared chiral structure designed has more advantages than ones based on traditional metal, which means more potential applications in the polarization state control of mid-infrared lasers.
Funding
Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (17KJB480005); National Natural Science Foundation of China (1704162, 61575087, 61771227); Priority Academic Program Development of Jiangsu Higher Education Institutions.
Acknowledgments
The authors thank Prof. Yiping Zhao from University of Georgia for the suggestions in experiment and Prof. Zhiyong Yang from Jiangsu Normal University for his help in use of Fourier transform infrared spectrometer.
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