1. Introduction

With the rapid development of nano procession and characterization techniques, research on the surface plasmon properties of micro-nanostructured metal materials has attracted great interests in the fields of physics, chemistry, materials, information science and other cross-domains [1,2]. Numerous kinds of artificial metal nanostructures were fabricated by carefully designing suitable shapes and sizes to regulate light-substance interactions. With the continuous updating and advancement of nanotechnology, the penetration of the surface plasmon photonics into various disciplines has been continuously deepened and strengthened. It is now even known as the most promising information carrier of nanoscale integrated photonic device and shows great potential for applications in such areas as Surface-enhanced Raman Scattering (SERS) [3], sensors [4], photodetectors [5], optical antennas [6], solar photovoltaics [7], and nonlinear optics [8].

Recently, people are expecting to achieve a more convenient and effective method for tunable control of plasmons, making it possible to realize the integration of high density and fast modulation devices which can greatly expand the application of plasmons. By proposing a device structure which was fabricated by depositing a silver coating on the ordered gold nanoparticles, Chu et al. realized the tunable response of surface plasmons [9]. Specifically, the modulation is realized by changing the electrochemical voltage to control the amount of silver deposited on the gold nanoparticles. However, the poor reproducibility of silver coating deposition quality, the unstable nature of silver chemistry in water-oxygen environment, and what’s more, the irreversible modulation performance of such solid-based device all limited its practical application. Chatdanai et al. demonstrated a method for post-deposition control of local plasmon resonance of a single gold nanoparticle on an aluminum thin film [10]. By varying the electrical voltage applied to the aluminum film, they modulated the thickness of aluminum oxide (Al2O3) which was acting as a spacer layer, thereby the particle-substrate interaction was changed continuously. But, the issue is that the device function could not be tuned repeatedly as the anodizing reaction process of the metal is irreversible and time-cost. Currently, plasma materials have been expanding from traditional noble metals to cutting-edge functional materials, e.g. graphene. Fan et al. demonstrated that the photoexcited graphene, in which the quasi-Fermi energy changes corresponding to optical pumping, can boost the originally extremely weak magnetic resonance in a split-ring metastructure, showing remarkable modulations in the transmission [11]. However, the modulation performance is unsatisfactory because the losses are unavoidable for graphene. Besides the above mentioned, although there are some other regulating methods that have emerged, such as thermally control [12–14], electrically control [15–17], magnetically control [18–21], chemically control [22,23] and so on, scarcely any method was proposed to realize a dynamically and reversible control.

In our contribution, we are proposing a method of using fluid-control technique to regulate the environmental media surrounding silver particle. The advanced electrowetting technology is used to regulate the morphology of the water-oil interface in which the silver particle is suspended. Electrowetting refers to changing the wettability of a liquid on a insulator-coated electrode by varying the voltage between the liquid and the electrode, that is, changing the contact angle to cause deformation and displacement of the liquid. In recent years, some scholars have made progress in the development of electrowetting technology for practical applications, including new electronic paper display devices [24], lab-on-chip microfluidic chips [25] and new continuous zoom liquid lens [26].

By changing the applied electrowetting voltage, the morphology of water-oil interface and thereby the environmental media of silver particle could be tuned simultaneously and continuously. Therefore, the electric field around the particle can be selectively enhanced and the absorption of light spectrum can be tuned dynamically. Compared with solid-state devices which have invariant structural parameters, our fluid-based devices have superior features such as reusability, fast response, high efficiency, and a wide range of flexibility, thus providing new inspiration in the design and preparation of rapidly adjustable optical components. Such unit cells can be arbitrarily integrated to achieve various color regulation and combination, which is practically meaningful in many fields, including the development of novel information displays technology. In addition, the image quality of plasmonic display device is theoretically much more smoother and softer, without the appearance of graininess of conventional devices. More importantly, the unit cell size is only hundreds of nanometers, which is much smaller than that of current display technologies on the market, such as liquid crystal displays, LEDs, etc., so the resolution of the manufactured display device based on such novel technology could be increased very significantly.

2. Theory and model

The working principle of electrowetting is shown in Figs. 1(a) and 1(b). Two types of fluids, conducting fluid and insulating fluid are sandwiched between two paralleled electrodes, which are both coated with hydrophobic dielectric layer. In various electrowetting devices, some high molecular polymer, e.g. Teflon or PMMA, is typically selected as the hydrophobic dielectric layer material as it has low surface free energy, meanwhile, water and oil are used as the conducting fluid and insulating fluid respectively. In such configuration, the two-fluid interface is automatically formed as indicated in Fig. 1(a). The contact angle of the conducting fluid with the hydrophobic solid surface is determined by the balance of the surface tension forces at the contact point. The initial equilibrium contact angle,${\theta }_{\text{0}}$, is given by Young’s equation:

 

Fig. 1 (a) the initial state and (b) the situation after applying voltage of a classic electrowetting device in sandwich configuration. (c) is the sketch of the model we created. (d), (e) and (f) are the three cases of water/oil interface morphology after voltage application.

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Here ${\gamma }_{s1}$ is the surface energy per unit area between water and the hydrophobic surface,${\gamma }_{s2}$ is the surface energy per unit area between oil and the hydrophobic surface, and ${\sigma }_{\text{12}}$ is the surface tension at the interface between the two fluids. As ${\gamma }_{s1}$ is larger than ${\gamma }_{s2}$, to minimize the system energy, the oil-water interface spontaneously forms a shape as shown in Fig. 1(a). The balance of surface tension forces at the point of contact can be reversibly changed by varying the voltage applied between water and the driving electrode. As in this case, the effective surface energy is reduced with the storing energy in the capacitor formed by the hydrophobic dielectric layer, Young’s equation is therefore modified as follows:

Here $\epsilon$ is the permittivity of the dielectric, $V$ is the potential difference applied, and $d_f$ is the dielectric thickness. Combining Eq. (1) and Eq. (2) yields

Electrowetting can therefore be used to modify the contact angle dynamically by simply changing the voltage applied as indicated in Fig. 1(b).

Based on the above we have established the model as shown in Fig. 1(c). The cavity of the entire structure is surrounded by a layer of metal copper electrode, which is covered with a layer of hydrophobic PMMA inside so that the liquid does not directly contact with the electrode. The cavity is filled with oil and water, and the silver particle is suspended at the water-oil interface. Please note that the suspension of silver particle is steadily realizable by simply modifying the silver particle with hydrophilic and lipophilic groups simultaneously. The bottom of the cavity is made of silver metal plate acting as the ground electrode, and directly contacting with water.

When the voltage is applied between the bottom metal plate and the driving electrode on the sidewall, the morphology of the water-oil interface will be changed according to Eq. (3). Figures 1(d)-1(f) shows the change of the interface shape when the applied voltage is varying. The contact angle of the water is relatively large in the initial state, and the interface bulges in the direction of the oil. As the voltage increases, the contact angle gradually decreases, and the interface gradually sinks towards the direction of water, meanwhile, the distance between the particle and the metal plate keeps decreasing. If the voltage is reduced, the interface will rise back until to the previous height. The response time of electrowetting device is very short, so the interface height and the environmental media surrounding silver particle can be adjusted quickly as desired. More importantly, such designed device is mechanically stable even under violent disturbance, as the surface tension force will dominate over the force of gravity in the concerned physical dimensions.

Figure 2 is a graph of the contact angle values as a function of voltage. The curves are derived from COMSOL Multiphysics software in which we set the device parameters as described in Eq. (3). Here, we selected three materials from the COMSOL material library, Teflon (red lines), PMMA (green lines) and polycarbonate (blue lines), which are commonly used as hydrophobic dielectric layer materials. Since they have little difference in surface energy with oil and water, we approximate that the initial water contact angles${\theta }_{\text{0}}$ on the three materials in the structure are the same 140°according to Eq. (1). It can be seen from the Fig. 2 that the contact angle value varies more significantly on the material with larger dielectric constant over the same range of voltage variations. At the same time, we also studied the effect of film thickness ($d_f$) on the change of contact angle. It can be observed that for the same material, the smaller the thickness is, the larger the range of contact angle variation can achieve.

 

Fig. 2 The relationship between the contact angle and the voltage. The red line indicates Teflon, the green line indicates PMMA, and the blue line indicates polycarbonate.

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In order to make the device more compact and low power consumption, we generally choose the one with higher dielectric constant and smaller thickness for the hydrophobic dielectric material selection, so as to achieve the largest possible contact angle change at lower voltage variations while at the same time achieving more significant phenomenon in regulation of the surrounding medium. Therefore, according to the data indicated in the Fig. 2, polycarbonate and PMMA have a better performance at a thickness of 30 nm. Due to the low cost and good ductility of PMMA, our further simulation investigations are all based on it.

Now, we set dielectric permittivity of the hydrophobic dielectric layer as 2.65, ${\sigma }_{\text{12}}$as 5*10^(−2)N/m, and $d_f$as 30nm. In the FDTD simulation, we set the device diameter as 200 nm, the cavity height as 300 nm, and the bottom metal plate height as 100 nm. We chose oil with a refractive index of 1.7, water with a refractive index of 1.3, and silver for palik (0-2um) in the material library. The boundary condition is a symmetric condition, and the light source is a plane wave. The diameter of the silver nanoparticle in the cavity is firstly selected as 50 nm.

3. Results and discussion

In our designed model, when the voltage is 0, contact angle is equal to${\theta }_{\text{0}}$, the centre point of oil-water interface is at the highest position, the particle is initially mostly surrounded by oil as shown in Fig. 3(a). As the voltage increases, the contact angle obtained by the formula decreases as shown in Fig. 2, and the center point height decreases nonlinearly. The main surrounding medium of particle alters from oil to water, as shown in Figs. 3(b) and 3(c). As the interface morphology changes, the particle continues to drop down and significant changes occur in the environmental media surrounding the particles, resulting in a modulation of excitation intensity of the plasmons. When the surface plasmon is excited, an electric field is generated around the metal particles due to charge accumulation, causing polarization of the surrounding dielectric. The polarized dielectric cancels out some free charges on the surface of metal nanoparticles, thereby reducing the recovery force of the free charges in the particle. The generation of the polarization charge depends on the dielectric constant of the medium. The larger the dielectric constant is, the more polarization charge it produces, resulting in less recovery force of the free charge. It is well known that a reduction in recovery force causes a decrease in the resonant frequency. Therefore, increasing the dielectric constant of the surrounding medium will cause the resonance absorption peak to red-shift. Therefore, we could notice that when the applied voltage varying from 3.9 V, 6.6 V, to 13.8 V, the absorber has absorption peaks moved from 607 nm (red), to 550 nm (green) and 460 nm (blue) as shown in Fig. 3(d), respectively; meanwhile, the device accordingly reflects the complementary color.

 

Fig. 3 (a), (b) and (c) are schematic of interface morphology and particle height at three different voltages. (d) is the absorptivity of the device correspond to the absorption spectrum.

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Based on the model introduced above, an integrated dynamically tunable absorber consisted of an array of the designed unit cells can be realized as indicated in Fig. 4(a). Each single unit cell in this absorber can be controlled individually and reflects different primary colors in visible band as desired. Therefore, this integrated device can widely adjust its spectral absorption peak position during the whole visible band by spatial color mixing method. In another word, this proposed device has effective visible color sorting capability. To provide a quantitative measure of the reflected color, we calculated the chromaticity coordinates of the spectral reflection light and plotted it on the CIE 1931 XY color space diagram by gray dots, as shown in Fig. 4(b). The triangular region formed by these dots is the complementary color the device can realize.

 

Fig. 4 (a) is a schematic diagram of the proposed integrated spectrum absorber. (b) In the CIE 1931xy chromaticity coordinates, the range of color gamut that the absorber can generate.

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Obviously, such integrated device can also produce different reflection colors, by adjusting geometric parameters including cavity size and particle diameter. However, unlike the previous color filter devices, our absorber is tunable even after fixing the device geometric parameters. With the change of the externally applied voltage, the curvature of the interface and the height of the nanosphere inside the device cavity will change accordingly. Based on the previous analysis, the position of the absorption peak will shift, which significantly changes the color of the light reflected by the device.

In addition to the externally applied voltage, there are many other factors that affect the reflected spectral characteristics, particle size, for instance. Figure 5(a) plots the color blocks corresponding to the absorption spectrum, which was obtained from the simulation investigation by adopting different nanoparticle diameters and external field voltages. Obviously, the wavelength of the absorption peak will decrease when the voltage is increasing. Based on the electrowetting principle structure diagram, we can also conclude that the wavelength of absorption peak will decrease as the distance between particle and substrate decreases, and it will also decrease when there is more water around the particle. Actually, these three factors are interrelated in our designed electrowetting device. By increasing the voltage, the contact angle of water/oil interface will decrease, which makes the particle position sink toward the substrate. During this process, the continuously changing curvature of interface will increase the contact area of water with particle.

 

Fig. 5 (a) shows the different absorbance by the nanoparticles (R = 30 - 70nm, vertical axis) and external voltage (U = 0 - 14.5V, horizontal axis) in the designed optofluidic system. (b) generation and modulation of absorption peaks for different nanoparticle radius and external voltages. (c), (d), (e) and (f) are absorption peaks at different voltages for nanoparticle radius ranging from 30 to 40, 60, and 70 nm, respectively.

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Figure 5(b) shows the effect of different nanoparticle radius on the tunable range of the absorption peak. By selecting different sizes of the particles, the tunable ranges can be changed a lot. When the radius of the silver nanoparticles in the cavities is 50nm, it has a relatively larger tunable range of spectral absorption peaks (460-607nm). Benefiting from this, one single unit cell can selectively absorb one of the three primary colors of red, green and blue, by simply applying different external electric fields. For particles having a radius less than 50 nm, the hybrid excitation between the particles and the metal plate is weakened because the surface is far from the metal plate. Conversely, for particles having a radius greater than 50 nm, the change of liquid distribution around the particles become smaller because it is too close to the wall of the cavity. Therefore, a particle parameter that does not match the structure will narrow the absorption peak adjustment range of the device. This investigation fully demonstrates the color sorting capability and adjustability of the invented device under standard CIE-D65 white light illumination.

Figures 5(c)-5(f) show the absorption peaks at different voltages for nanoparticles with different radius ranging from 30nm to 40nm, 60nm, and 70 nm, respectively. Although some of the devices don’t have a large range of absorption peak modulation, they could be useful in obtaining some certain colors such as purple (30nm, 13.3V), orange (60nm, 6.3V), yellow (40nm, 4.4V) and so on. Therefore, some specific colors can be selectively absorbed by simply selecting the particles with specific radius in the proposed device.

Finally, we can also get the desired modulation range by changing the fluid type and volume in the cavity or the material thickness of the medium surrounding the cavity. This further increases the extensive flexibility of usability of the device.

4. Conclusion

In conclusion, we use electrowetting to regulate the contact angle of water on the hydrophobic dielectric coated metal surface. Meanwhile, the dielectric constant of the surrounding media around the metal particles suspended at water/oil interface also get changed. By adjusting a series of parameters, including the electrowetting voltage, we achieved the selective absorption of various colors, including the three primary colors in the visible spectrum in a single unit cell. The integrated device consisted of an array of the designed unit cells is reusable, easy to change parameters, and has a fast response time, with potential advantages in some tunable or high frequency response areas such as advanced displays, imaging and photovoltaics. Our devices have high reflectivity, wide modulation range, high combination flexibility, and the control effect is not affected by the polarization state of incident light, which can meet the needs of various working environments.