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We report a plasmonic structure for switchable reflection and transmission by polarization. The structure is composed of a hexagonal-packed polystyrene sphere array with silver patches on them. Simulations and experiments demonstrated that the conversions between reflected beams and transmitted ones can be performed when the polarization directions of incident beams vary from 0° to 90°. A switchable reflection and transmission at a given wavelength can be obtained, as long as sizes of PS spheres and azimuthal angles are properly chosen. Such a patchy plasmonic structure serving as a switch between reflection and transmission have potential applications in photoelectric control devices.
© 2017 Optical Society of America
1. Introduction
At present, polarization controls have attracted the interest of researchers and been widely employed to investigate optical properties of metals [1, 2]. The resulting surface plasmonic-based polarization controls have permeated into many fields of science and their combinations generate a great number of new effects and applications, such as enhanced light transmission [3–5], nonlinear enhancement caused by localized surface plasmon resonances [6–9], bio-sensors [10–13], negative-refractive-index materials [14, 15], super-resolution imaging [16], surface plasmonic invisibility [17–20], and surface plasmonic waveguides [21–24]. There mainly exist three surface plasmonic-based polarization controls. The first one is to alter components of structures including dielectric, metal, or semiconductor. The second one is to change shapes of structures. Finally, they can also be combined to realize the polarization controls. Following the three methods, different plasmonic structures were proposed in succession for the polarization controls [1, 25–27]. As far as fabrication methods are concerned, there are electron-beam lithography [28], direct laser writing [29], focused ion beam lithography [30], and nanosphere lithography [31]. These fabrication methods mentioned above are often complicated, expensive, and cumbersome, as well as need greatly specific and high-end tools. Meanwhile, the samples fabricated are generally small. Glance angle deposition (GLAD) [32] is a physical vapor deposition technique in which a substrate is rotated in the polar and azimuthal directions by two stepper motors programmed by a computer. The fabrication process is simple and inexpensive. The fabricated samples have large areas. Utilizing the method, a kind of plasmonics structures based on self-assembly nanospheres were fabricated [2, 33–37]. Patchy-shaped plasmonic structures as representatives of the kind were proposed [2, 34–37]. More attention was paid to the properties of chirality, quantitative surface-enhanced Raman, and refractometric sensing of the structures. However, their optical properties for different incident polarization waves are not reported.
Here we investigate a patchy-shaped plasmonic structure under different polarization waves. Simulations and experiments show the conversion between reflection and transmission of the structure can occur by changing the polarization directions of the incident waves.
2. Structure
Figure 1(a) shows that self-assembly polystyrene (PS) spheres locate on a glass substrate. Different kinds of silver patches on the PS spheres can be obtained by a GLAD method, in which the incident angle θ of silver vapor is set to a certain large value and the azimuthal angle ɸ is changed with a range of 0° to 360°. Figure 1(b) shows five different shapes of silver patches on the PS spheres under the conditions of θ = 87° and ɸ = 0°, 15°, 30°, 45°, and 60° by simulations. The patch shapes will reemerge when ɸ is larger than 60° because the structure has a rotational symmetry of 60°.
Fig. 1 (a) Schematic for the deposition of the self-assembly PS spheres locating on a glass substrate with an incident angle of θ and an azimuthal angle of ɸ. (b) Five different shapes of silver patches formed on the PS spheres with θ = 87° and ɸ = 0°, 15°, 30°, 45°, and 60°, respectively.
3. Simulation
We first simulated the case of θ = 87° and ɸ = 0° shown in Fig. 1(b). The corresponding model with a single patchy PS sphere is shown in Fig. 2(a). A finite-difference time-domain (FDTD) software was employed to simulate responses for different polarization waves with normal incidence. The relative permittivity of silver was chosen from the dispersion data listed in [38], and the refractive index of the glass substrate was set to 1.55. Owing to the uniform and periodic feature of the structure, we therefore considered a single unit cell with periodic boundary conditions in simulations. A polarization angle of γ was changed from 0° to 90° with a 30° step. For accuracy of the simulations, the mesh size was set to 4 nm in the whole calculation domain.
Fig. 2 (a) Model of a single patchy PS sphere with θ = 87° and ɸ = 0°. The diameter of the PS sphere is D = 500 nm, and the thickness of silver is set to H = 50 nm. (b) Reflection and (c) transmission as functions of wavelength for different polarization angles, respectively.
The simulated reflection and transmission for different polarization angles are plotted in Figs. 2(b) and 2(c). From the figures, we can see that the apparent resonance occurs at the wavelength of 1310 nm. The resonance becomes weaker with increasing polarization angles. The transmission rises and the reflection decreases with the growth of the polarization angles. The peak value of the reflection is more than 0.95 while the corresponding transmission is less than 0.05 for the polarization angle of γ = 0°. When γ = 90°, the structure is almost transparent. As a result, by altering the polarization angles of incident waves, the reflection and the transmission can be switched.
To understand the mechanism of the switchable characteristic of the structure with different polarization directions, we plotted electric fields in one of its cross-sections at the resonance wavelength of 1310 nm for the polarization angles γ = 0° and γ = 90°, as shown in Fig. 3. It can be seen from Fig. 3(a) that localized surface plasmon resonances (LSPRs) are induced in the silver patches. The structure can thus be regarded as a plasmon resonance layer. The electromagnetic waves are completely reflected back due to their skin effect, which leads to greatly low transmission. However, for the case of γ = 90° shown in Fig. 3(b), there do not exist LSPRs in the silver patches. Therefore, the electromagnetic waves easily pass through the structure, giving rise to low reflection and high transmission.
Fig. 3 Electric field distributions in one of cross-sections of the structure at the resonance wavelength of 1310 nm for the polarization angles of (a) γ = 0° and (b) γ = 90°.
4. Discussion
To further understand the switchable characteristic for reflection and transmission of the structure, one needs to study the influences of sizes of PS spheres and azimuthal angles (or shapes of silver patches) on the characteristic.
Firstly, the diameter of PS spheres is changed from 500 nm to 300 nm, 400 nm, 600 nm, and 700 nm, respectively, while θ, ɸ, and boundary conditions are kept unchanged. The corresponding transmission was simulated and is illustrated in Fig. 4. From the transmission curves, we can see that resonances locate at 760 nm, 1000 nm, 1500 nm, and 1760 nm corresponding to the four different diameters, respectively. The variation tendency of the transmission curves for different diameters is similar to that of the 500 nm diameter with the increase of polarization angles. Besides, the resonance wavelengths grow with the diameters of PS spheres, which indicates a red shift. The reason is that the sizes of silver patches become larger with increasing diameters of PS spheres. That is to say, the resonance distances of the electrons in the silver patches increase, which causes increasing resonance wavelengths. To illustrate more clearly the relationship between the resonance wavelengths and the diameters of PS spheres, we plotted resonance wavelengths as functions of diameters, as shown in Fig. 5.
Fig. 4 Simulated transmission as a function of wavelength for different diameters of PS spheres with different polarization angles. The diameters are (a) D = 300 nm, (b) D = 400 nm, (c) D = 600, and (d) D = 700 nm, respectively.
By fitting several resonance wavelengths, a line is formed in Fig. 5 and its function relationship can be expressed as
where λ is the resonance wavelength. Following this relationship, resonance wavelengths for spheres with any size can be predicted.Next, the resonance characteristic of the structure will be investigated for different azimuthal angles with other conditions unchanged. Simulations demonstrated similar tendencies of reflection and transmission with those in Figs. 2(b) and 2(c) under different polarization angles. Here, the simulated results are not shown. However, their resonance wavelengths as functions of azimuthal angles are depicted in Fig. 6. Evidently, the figure has 60° periodic variation, which is determined by the rotational symmetry of 60°. It can be seen that there exist two maximum wavelengths of about 760 nm and 1250 nm in one period, respectively. They can be expressed as
where λe is the maximum wavelength, and n the integers from 0 to 5.By fitting the data from 0° to 60°, the relationship between the resonance wavelengths and the azimuthal angles can be expressed as
where ɸ is between 0° and 60°. Due to the resonance wavelengths have a 60° periodic change with ɸ, the relationship both them for the azimuthal angles more than 60° can also be calculated with Function (3).5. Experiment
We first used a self-assembly method to fabricate a colloid monolayer of D = 500 nm PS spheres on a glass substrate. Three main steps were needed for the fabrication. The first step was to clean a glass substrate and a petri dish. They were put in a mixed solution of sulfuric acid and hydrogen peroxide with a proportion of 4:1 and boiled. Subsequently, they were cleaned ultrasonically for 5 minutes successively with acetone, ethyl alcohol, and deionized water, and then dried with nitrogen. The second step was the preparation of PS spheres. An 150μl solution of PS spheres was diluted and centrifuged for three times, and then mixed with 325μl ethyl alcohol. The final step was the fabrication of a PS sphere monolayer. 10.5 ml pure water was first injected in the petri dish, and then the solution of PS spheres was dropped in the pure water with a speed of 0.011 ml/min. After the monolayer had formed, a Teflon loop was put on it, and additional pure water was injected in the petri dish for a higher water level with a speed of 1 ml/min. After that, a peristaltic pump was used for water injection and drainage at the same time till clear water was obtained in the petri dish. Finally, a glass substrate was put in the petri dish and the water in it was drained off. A PS sphere monolayer on the glass substrate was achieved after it had been dried in the air.
Silver was deposited on the PS sphere monolayer template by using a double-source electron beam evaporation coating system (DE500, DE Technology Inc). The incident angle of silver vapor is θ = 87° along with a fixed azimuthal angle. The vapor was in a 4 × 10−7 Torr environment, and was deposited at a thickness speed of 0.3 A/s for 72 minutes. A resulting patchy-shaped plasmonic structure was realized. Its images were taken by a scanning electron microscope (SU8010, Hitachi). Four of the images at different domains are shown in Fig. 7(a). From the figure, it can be seen that shapes of the silver patches have slight differences, which is caused by the non-uniformity of the self-assembly PS sphere monolayer.
Fig. 7 (a) SEM images of self-assembly polystyrene spheres with silver patches at four different domains. (b) Measured transmission curves of the sample for four different polarization angles. (c) Simulated transmission curves with ɸ = 1° for four different polarization angles.
We measured transmission for different polarization waves utilizing a spectrometer (Lambda950, PerkinElmer) with a beam spot of 1.8 mm in length and 0.6 mm in width. A same glass substrate was put on the sample station in the spectrometer. A polaroid ranging from 650 nm to 1700 nm at different rotational directions was employed to measure the transmitted beams of the glass substrate as reference lights. The transmitted beams of the fabricated sample replacing the glass substrate was measured at its five different domains again. The average transmission of the sample for four different polarization angles was calculated and is plotted in Fig. 7(b). In the measurement, a minimum of transmission at the resonance wavelength needs to be firstly decided by rotating the polaroid. The corresponding transmission is the case of γ = 0 according to the results shown in Fig. 2(c). Then, the transmission for other polarization angles can be achieved by further rotating the polaroid relative to the position. As is depicted in Fig. 7(b), the transmission can be manipulated by incident polarization waves, and the resonance wavelength lies in 1172 nm. The corresponding azimuthal angle of ɸ ≈1° can be inferred from Fig. 6 based on this resonance wavelength. Therefore, we simulated the transmission of the structure for ɸ = 1°, as shown in Fig. 7(c). Comparing the results shown in Figs. 7(b) and 7(c), we find that the simulations are in accord with experiments and their transmission values have similar trends for different polarization angles. Namely, the transmission becomes higher with the increase of polarization angles. Although the silver patches are not completely uniform due to defects in the self-assembly PS sphere monolayer, the structure can be averaged due to the greatly large beam spot compared with the PS spheres. As a result, the structure is viewed as the case of ɸ = 1°.
6. Conclusion
In summary, we designed a plasmonic structure consisting of self-assembly PS spheres with silver patches. Simulation and experimental results show that reflection and transmission can be converted by changing incident polarization angles from 0° to 90°. Additionally, when the sphere diameters and the azimuthal angles of deposition are changed, the resonance wavelengths of the structure can also be tuned. Therefore, switchable reflection and transmission at a given wavelength can be obtained, as long as sizes of PS spheres and azimuthal angles are properly chosen. Such a structure of polarization control for reflection and transmission could have potential applications in optical switches and also offers a design reference for a polarization sensitive structure.
Funding
National Natural Science Foundation of China (NSFC)(61401182,61575087);Natural Science Foundation (NSF)(BK20151164) ; Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) of Jiangsu Province; Opening Foundation (KLALMD-2015-04) of Jiangsu Key Laboratory of Advanced Laser Materials and Devices.
Acknowledgments
The authors thank Prof Yi-Ping Zhao and Prof Qiu-Ju Zhang from University of Georgia and Shandong Normal University for discussions on simulations and experiments, respectively.
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