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Abstract
In this paper, a dual interface trapezium liquid prism with beam steering function is implemented and analyzed. The electrowetting-on-dielectric method is used to perform the desired beam steering function without mechanical moving parts. This work examines deflection angles at different applied voltages to determine the beam steering range. The deflection angle can be experimentally measured from 0° to 3.43°. The proposed liquid prism can be applied in the field of optical manipulation, solar collecting system and so on.
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1. Introduction
An electrowetting liquid device is a device capable of refracting light rays when subjected to electrical forward bias. Nowadays, many liquid devices or systems, such as solar collecting systems, laser scanners, and optical switches, are applied in the field of optical manipulation [1–6] and solar collecting system [7]. Beam steering in optoelectronic systems can be managed using optical devices such as mirrors, prisms, lenses, and the like, which have different materials that enable the beam to travel. The conventional beam steering devices (BSDs) can be replaced with the optoelectronic beam steering devices [8–11]. In the modern era, BSD has become more versatile because the mechanical adjustment is no longer required. Beam steering is a way to direct light beams to a customized target. Light rays can be generated more efficiently and reliably because of beam steering function [12–16]. Liquid crystals are versatile materials used in many different technologies because of their ability to respond to external fields such as electric, light, thermal, etc [17–23]. Especially as beam deflection devices, liquid crystal devices possess the advantages of being lightweight and compact in structure. However, when there are specific requirements for beam polarization, liquid crystal devices may not fully meet the criteria. Electrowetting-on-dielectric (EWOD) is a promising actuation method for optoelectronic devices [24–30]. This paper proposes a dual interface trapezium liquid prism with two different liquids and focuses on the deflection angle at different applied voltages. One important application of liquid prisms is in optical switching and modulation, where they are used to control the flow of light through a device. The dual-interface trapezium liquid prism has a good chance of getting a wider beam steering performance than the conventional single-interface liquid prism because the light rays can be deflected twice in the dual-interface prism. Dual-interface liquid prisms enable more efficient beam steering by having a high refractive index change at the interface between the two liquids. Moreover, the trapezium side wall needed less applied voltage than the conventional side wall to achieve the desired tilt angle. The low applied voltage is also thought to reduce environmental impact because of moving towards a green environment nowadays. The improvement of the device design is typically done by using computational fluid dynamics simulation software, which must pass through the physical processes occurring in the devices. Then, the proposed dual-interface trapezium liquid prism is manufactured and experimentally validated for its capability to steer light beams. This device features an innovative trapezium structure, effectively enhancing its light-beam steering capability. Our research holds significant potential for applications in infrared detection, laser guidance, energy harvesting, and so on.
2. Structure and principle of the trapezium liquid prism
The proposed liquid prism is shown in Fig. 1. The proposed trapezium-shaped liquid prism, has a new structure by adding two immiscible liquids. The isosceles trapezium has two equal angles α formed between its two legs and the bottom edge and two equal angles π-α formed between its two legs and the top edge. The structure of the lateral boundary of the proposed design mainly comprises three layers: the inner wall is a hydrophobic layer, the middle layer is a dielectric layer, and the outermost layer is an electrode.
The trapezium liquid prism with dual interfaces and operational concepts are shown in Fig. 2. When the voltage on the left side Vl is greater than the voltage on the right side Vr, the insulating layer moves towards the right sidewall, as shown in Fig. 2(a). Conversely, the middle layer moves towards the left, as shown in Fig. 2(b). By manipulating the applied voltage, the inclination of the liquid-liquid interface can be controlled. Thus, the incoming light can be steered. To manipulate the interface, it is necessary to control the contact angle. The Young- Lippmann equation represents the ideal relationship between the equilibrium contact angle ${\theta}$ and the three interfacial tensions in the system: sidewall contact line, conductive liquid, and insulating liquid [9].
Fig. 2. Dual-interfaces trapezium liquid prism when applying voltage to the device. (a) Vl > Vr, (b) Vl < Vr
As shown in Fig. 1, the relationship between the tilt angle and contact angle for the trapezium liquid prism can be expressed below:
As shown in Fig. 2, the basic equation from the theory of Snell’s law of refraction that mathematically describes the beam steering characteristic of the liquid prism is
The relationship between the tilt angle of the liquid-liquid interface and the beam deflection angle based on the conductive liquid and insulating liquid is illustrated in Fig. 3. Through calculations, the theoretical range for the beam deflection angle of the proposed liquid prism is estimated to be -5.3° to 5.3°.
The schematic diagram illustrating the principle of calculating the beam steering angle is shown in Fig. 4. We can use the following equation to experimentally measure the liquid prism's beam deflection angle $\delta .$
where $\delta $ is the beam steering angle, a is the displacement of the light spot, and b is the distance between the device and the screen.3. Fabrication
The specially trapezium-shaped design is selected to get specific beam steering function and to improve the way of light across the prism. In view of the fact that the trapezium prism allows for more precise control of beam steering due to their asymmetrical geometry. The liquid prism comprises glass substrates, dielectric layers, hydrophobic layers, electrodes, and two immiscible liquids. The front, back, and top walls use untreated clean glasses, and the left, right, and bottom walls use indium tin oxide (ITO) glasses to transmit the electric field. Teflon film adheres as the hydrophobic layer onto the two inner sidewalls using UV glue, with the UV glue also serving as the dielectric layer. We have set α to 80°, which means that the two sidewalls are inclined at an angle of 80° relative to the bottom wall. The dual-interface liquid prism with a trapezium shape inserted two immiscible liquids: conductive and insulating. 1w% tetrabutylammonium chloride (TBAC) propane-1, 3-diol solution as the conductive liquid, and silicon oil as the insulating liquid was selected for our design. The uppermost and lowest layers are conductive liquids, and the middle is insulating liquid because the ground electrode should be put at the conductive liquid layer side. The glass walls are bonded together by UV glue to keep them insulated. In addition, copper wires are connected to the left, right, and bottom walls, enabling alternating current (AC) voltage to the left and right walls, with the bottom wall grounded. As depicted in Fig. 5, the length and height of four sidewalls are 1 cm × 2 cm, the bottom wall is 1 cm × 1 cm, and the top wall is 0.75 cm × 1 cm. Table 1 shows the properties of the liquid.
Table 1. Properties of the liquid
4. Experimental results
The behaviors of the beam steering range are investigated, and the left and right sidewalls are applied to the two different voltages. The driving voltage signal is a sinusoidal AC with a frequency of 1.5 kHz. The laser light source employs a 632.8 nm helium-neon laser. After being reflected by the beam splitter, the beam enters the center of the bottom wall of the liquid prism in a vertical direction and, after refraction by the liquid prism, is directed onto the screen. In the subsequent experiments, we designed the applied voltages on the left and right sidewalls of the liquid prism based on the actual experimental results.
Figure 6 demonstrates the position of the light spot on the screen. Figure 6(a) shows the light spot's position when zero voltage is applied to both sidewalls. The center of the light spot is positioned at 5 cm. The shape of the light spot is malformation because the light spot traveled through the curved liquid interface. Figure 6(b) describes the light spot position after applying 180 V to the left sidewall and 120 V to the right sidewall. The center of the light spot is positioned at 4.85 cm, moving to the right at 0.15 cm. After applying 120 V to the left sidewall, and the right sidewall is 180 V, the center of the light spot is positioned at 6.15 cm, moving to the right at 0.15 cm, as shown in Fig. 6(c). As shown in Figs. 6(d) and (e), the center of the light spot is positioned at 4.7 cm and 5.3 cm. The light spot deviated by 0.3 cm to the left when the left voltage was set to 180 V, and 0.3 cm to the right when the right voltage was set to 180 V while the voltage on the opposite side was set to 0 V.
Fig. 6. Position of the light spot on the screen. (a) No applied voltage, (b) Vl = 180 V, Vr = 120 V, (c) Vl = 120 V, Vr = 180 V, (d) Vl = 180 V, Vr = 0 V, and (e) Vl = 0 V, Vr = 180V
The relationship between the beam deflection angle and applied voltage is shown in Fig. 7. 120 V and 180 V are set up as the right applied voltage for this experiment, as shown in Fig. 7(a). The range of the beam deflection angle is from 1.718° to 3.148° when the right applied voltage is fixed at 120 V and from 0° to 3.434° for the appropriate left applied voltage when the right applied voltage is fixed at 180 V. In contrast, Fig. 7(b) presents the left applied voltage set up as 120 V and 180 V, and the right applied voltage is changed. The range of the beam deflection angle is from -1.718° to -3.148° for the left applied voltage fixed at 120 V and from 0° to -3.434° for 180 V. The rapidly increasing or decreasing part indicates the highly sensitive response of the dual interface trapezium liquid prism because of the selected prism geometry design, liquid characteristics, and selected voltage value. So, the capability of dual interface liquid prism for rapid and reliable optical manipulation was discovered.
Fig. 7. Relationship between the beam deflection angle and the applied voltage. (a) Left applied voltage and (b) Right applied voltage
5. Discussion
In the previous section, the behaviors of the beam steering range for dual interface trapezium liquid prism are demonstrated. Here, AC voltage is selected in place of direct current (DC) voltage because AC voltage can reduce the hysteresis of contact angle, and DC voltage will persistently polarize the dielectric film in the long term. We conducted the tests using the applied voltage, which is 0 V to 180 V on one side and 120 V or 180 V on the other because the liquid prism can be harmed by voltage levels that are too high or low. When the electric potential is applied to the sidewall while varying the contact angle, the shape and curvature of the liquid droplet can be changed. Consequently, the modification of the droplet impacts the tilt angle. In the present study, the deflection angle for optical manipulation as a dual interface trapezium liquid prism is based on the conductive liquid, and insulating liquid has been examined [31]. Our device can deviate from -5.3° to 5.3° theoretically and from -3.43° to 3.43° experimentally. The results demonstrated that the two theoretical and experimental are almost the same. So, the experimental results can be proved with the theoretical-based results. The effect of the driving or applied voltage was also observed by investigating the beam steering range. Our work chose two electrodes placed on opposite sides of the prism sidewall for the applied voltage because the two-electrode prisms are less complicated and more affordable. However, the two-electrode prism impacts the manipulation of the upper interface. To solve this issue, we will test the four-electrode prism; two pairs of electrodes can be put at different orientations instead of the two-electrode prisms in future work.
6. Conclusion
The design and structure of a dual interface trapezium liquid prism with two immiscible liquids for beam steering function is demonstrated. According to the results, the performance of the beam steering range depends on the modification in the contact angle, and the applied voltage can be confirmed. Additionally, the trapezium liquid prism can deviate the deflection angle range from -5.3° to 5.3° theoretically and from -3.43° to 3.43° experimentally. The result also reveals that the dual interface trapezium liquid prism is an appropriate application for beam steering devices and optical manipulations.
Funding
National Natural Science Foundation of China under Grant No. (61927809,62175006); Science and Technology Innovation Commission of Shenzhen Municipality under Grant No.( JCYJ20220818100413030).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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