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Abstract
In this paper, we demonstrate a multifunctional optofluidic (MO) lens with beam steering, which is actuated by electrowetting effect. A liquid lens chamber and a liquid prism chamber are stacked to form the MO lens. When the liquid lens chamber is actuated with voltage, the curvature of liquid-liquid interface changes accordingly and the focal length of the liquid lens can be varied. In the liquid prism chamber, a navigation sheet is just placed on the position of the liquid-liquid interface. When the liquid prism chamber is applied with voltage, the navigation sheet can be tilted to different angles in order to adjust the beam steering angle and keep high beam quality. Thereby, the MO lens has the zoom lens and the beam steering functions. The experiments show that the focal length can be tuned from -180 mm to -∞ and +∞ to 161 mm and the maximum beam tilt angle can be adjusted from 0° to 22.8° when the voltage is applied on one side of the electrode. The proposed MO lens can be applied in zoom imaging system, laser detecting system, and lighting system.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optofluidics fundamentally aims at manipulating fluids and light at the microscale and exploiting their interaction to create highly versatile systems. A varieties of optofluidic devices such as lenses [1–4], irises [5–8], prisms [9–11], microcavities [12–13], optical attenuators [14–15] and microresonators [16–17] have been studied intensively during recent decades due to the unique merits of low consumption, broadband, and high transmittance. Among them, adaptive optofluidic lenses are the most basic elements in the miniaturized systems. Different types of optofluidic lenses based on microfluidic technology have been demonstrated including refractive lenses [18], diffraction lenses [19], and gradient index lenses [20], which can be used for sample imaging and detection. Although the optofluidic lens has been studied intensively by many researchers, there are still some issues unsolved. One issue is that the effective imaging aperture of the optofluidic lens is limited, generally within a range of ∼5.0 mm [21–22], which is difficult to be applied to the large aperture imaging systems such as telescope systems. Most of the optofluidic lens with the capability of large aperture imaging is designed with elastic films [23]. As is well-known, the elastic film based optofluidic lens is affected by the gravity effect. Scholars also developed the so-called mechanical wetting whose effective aperture is 4.0 mm [24]. Even if high resolution and wide dynamic range of optical power were achieved in the experiments, the imaging area is still limited. Another main issue of the optofluidic lens is that the field of view (FOV) is within ∼60° [25] and cannot be adjusted to the optical systems.
Most of the optofluidic lenses only contain one function and the optical performance can be further improved. Thereby, the designs with multiple functions are highly desirable for the sake of suiting complex usage scenario in practice and might provide new solutions to the above issues. In 2018, the scholars proposed a novel multifunctional optofluidic (MO) lens based on electrowetting actuation for high-performance miniature cameras. It can achieve both liquid lens function and adaptive iris function [26]. Some researchers developed a new type of lens actuated by both electrostatic force and hydraulic pressure. Thereby the proposed liquid lens can not only adjust the focal length, but also dynamically correct wavefront aberration by forming an aspheric surface actuated by electrostatic force [27–28]. In 2019, our group proposed a variable optical attenuator controlled by hydraulic control [29]. The proposed attenuator can achieve both the variable attenuator function with dynamic attenuation ranges from 33.01 dB to 0.71 dB and the variable-focus lens function with 2.9× magnification. Inspired by the researchers, our group also designed some new types of MO lenses and applied them to improve the quality of computer-generated holography especially for eliminating the undesirable light [30–31].
In this paper, we focus on solving the two issues discussed above and propose a new type of MO lens which contains a liquid lens (LL) part for varying the focal length and a liquid prism (LP) part for beam steering which can be applied to adjust the FOV in the optical systems. Compared with our previous works and other works about MO lens [26,29–31], the liquid lens part has a relatively large effective imaging aperture about 9.0 mm by employing a high viscosity liquid. The liquid prism part can keep a relative high beam quality and adjust the FOV dynamically by using a transparent sheet suspended in the liquid-liquid interface. Although some researchers have designed several types of liquid prisms [32], micro electromechanical systems (MEMS) micromirrors [33], liquid crystal prims [34] and liquid-actuated reflectors [35] for beam detecting and adjusting the FOV, the shapes of the liquid-liquid interfaces of the traditional liquid prisms are curved shape. So, the light beam will diverge, and these liquid prisms could not be applied in the imaging systems. As for the MEMS-based micromirrors, we should make a trade-off cautiously between the precision control and the high cost. The proposed MO lens has the advantages of wide focal length ranging, accurate FOV tuning with high imaging quality, reasonable mechanical stability and low cost. The MO lens can be applied to varieties of optical systems and can be integrated in optofluidic chips.
2. Structure and operating principle
2.1 Calculation
Figure 1 shows the structure and mechanism of the proposed MO lens. It consists an LP part and an LL part. An LP dielectric layer, an LP electrode, and an LP sidewall electrode are fabricated in the LP chamber which is filled with a conductive liquid (liquid-1) and an immiscible liquid (liquid-2). A navigation sheet is just placed on the liquid-liquid interface in order to steer the tilt angle of the interface. Six sheet electrodes are used to constitute the LP sidewall electrode. The LL part mainly consists of a cylindrical chamber and a substrate, which are fabricated with the LL electrode and the LL dielectric layer respectively. The LL chamber is also filled with a conductive liquid and an immiscible liquid, as shown in Fig. 1(a). When a voltage is applied on one side of the LP sidewall electrode, liquid-1 rushes upward to the same side of the sidewall due to electrowetting effect, which makes the navigation sheet to be tilted to a certain degree in the meantime, as shown in Fig. 1(b). In a similar way, when the voltage is applied on the other side of the LP sidewall electrode, liquid-1 rushes upward to the opposite side of the sidewall, which make the navigation sheet to be tilted, as shown in Fig. 1(c). The tilt angles of the navigation sheet are determined by the values of the driving voltages. Thus, the MO lens can achieve the function of beam steering and FOV adjustment. When the voltage is applied on the LL electrode, liquid-1 rushes upward to the sidewall of the LL chamber, which leads to the changes of the liquid-liquid interface curvature to form a liquid lens, as shown in Fig. 1(d). Thus, the MO lens can also be used for focal length tuning.
Fig. 1. Structure and mechanism of the MO lens. (a) Cross-section of the MO lens and the pattern of the LP electrode; (b) Applying voltage on one side of the LP electrode; (c) Applying voltage on the opposite side of the LP electrode; (d) Applying voltages on both the LP and LL parts.
The actuation mechanism of the LP part is shown in Fig. 2. Figure 2(a) shows the contact angle changes of the LP during the states of voltage-on and voltage-off. Due to the electrowetting effect, the actuation mechanism is determined by the Young-Lippmann equation. The balance of the interfaces between liquid-1, liquid-2, and the dielectric layer tri-junction line meet the following equations [36]:
where γ12, γ1D, and γD2 are the surface tension between liquid-1 and liquid-2, liquid-1 and dielectric layer, dielectric layer and liquid-2, respectively. θ0 is the initial contact angle without voltage, θY is the contact angle when the electrodes are applied with voltages, d is the thickness of the dielectric insulator, ε = ε0εr is the dielectric constant of the dielectric insulator and U is the external voltage. When all the external forces reach to balance, as shown in Fig. 2(a), the liquids satisfy the following equation: where F represents the electric force of per meter. When different sides of the electrodes are actuated with voltages of U1, U2, U3, U4, U5, and U6, the navigation sheet will rotate to different directions, as shown in Fig. 2(b). In theory, the liquid prism part can steer the beam with a 360° rotation in latitude.Fig. 2. Actitation mechanism of the LP part. (a) Actitation mechanism of electrowetting in the sidewall of the LP chamber; (b) Mechanism of the beam steering by the navigation sheet when different voltages are applied.
2.2 Fabrication of the MO lens
The fabrication procedure of the MO lens is shown in Fig. 3. A cylindrical aluminum cavity fabricated with an indium tin oxide (ITO) glass is designed to be the LL part. The height and diameter of the LL chamber are 8.0 mm and 15.0 mm, respectively. The ITO glass and the sidewall of the LL chamber are coated with Parylene-C layer (∼1 µm) as an insulator followed by a thin Teflon layer (thickness ∼5 µm, surface tension ∼18 mN/m at 20°C, from DuPont with the type of AF-1600), as depicted in Figs. 3(a)–3(b). The thickness and diameter of the ITO glass are 1.1 mm and 15.0 mm, respectively. Then a transparent glass coated with a Parylene-C layer and a Teflon layer is covered on the top of the LL chamber as the bottom substrate, as shown in Figs. 3(c)–3(d). A cylindrical polymethylmethacrylate (PMMA) cavity is used as the LP chamber. The height and diameter of the LP chamber are 9.0 mm and 15.0 mm, respectively. The thicknesses of the liquid lens chamber and the liquid prism chamber are 3.0 mm and 1.5 mm, respectively. Six ITO films coated with the same two layers are fabricated on the sidewall of the LP chamber, as shown in Fig. 3(e). The size of each ITO film is 0.2 mm × 5.5 mm. At last, the navigation sheet made from polyamide (PA) is just placed on the liquid-liquid interface, and the top PMMA substrate is covered on the LP chamber, as shown in Fig. 3(f). The height and diameter of the navigation sheet are 0.2 mm and 10.0 mm, respectively. The elements of the MO lens are shown in Fig. 3(g). The NaCl solution is used as liquid-1 and the phenylmethyl silicone oil is used as liquid-2. The characteristics of the materials in the MO lens are also listed in Tab. 1. The process of liquids packaging in LL part and LP part is summarized as follows: the NaCl solution volume is dispensed followed by the dispensing of the phenylmethyl silicone oil volume. After this step, with the help of the filling marks, the volume of the NaCl solution can either be increased or decreased. In the last step, the volume of the phenylmethyl silicone oil is increased until a meniscus forms over the top edge of the tube, preventing the formation of air bubbles when the tube is sealed with the substrate [21,37].
Fig. 3. Fabrication procedure of the MO lens. (a) Coating the dielectric layers on the bottom substrate; (b) Coating the dielectric layers on the sidewall of the LL chamber; (c) Coating the dielectric layers on the top substrate; (d) Fixing the LP chamber; (e) Fabricating the electrodes and coating the dielectric layers on the sidewall of the LP chamber; (f) Fixing the navigation sheet and filling the liquids; (g) Elements of the MO lens.
Table 1. Characteristics of the materials in the MO lens
3. Experiments and optical properties
3.1 Experiments and results
In the experiment, a CCD camera (YVSion Co. Ltd, type of YS-HU800C: 2/3” COMS, China) is used to record the images and evaluate the performance of the liquid lens function during actuation. A printed color ‘Liquid lens’ is placed 5 mm away below the LL part. An upright virtual image can be observed as the object is always within the focal length of the liquid lens. When the voltage U (< 35 V) is applied on the sidewall of the electrode of the LL chamber, the liquid-liquid interface cannot change. As the voltage increases gradually, the images captured by the CCD begin to change. Thereby, the threshold voltage of the liquid lens part is ∼35 V. In this state, the liquid lens has the shortest focal length, as shown in Fig. 4(a). When the voltage varies from 35 V-80 V, the focal lengths change accordingly, as shown in Figs. 4(b)–4(f). When the voltage U >80 V, the focal length can be tuned little due to the contact angle saturation of the conducted liquid. We add a dotted box to label the difference more directly in the sub-figures. During the actuating process, no voltages are applied on the LP chamber. The LP part just works as a parallel plate. The dynamic response video of the liquid lens is included in Visualization 1.
Fig. 4. Captured images changing during driven procedure. (a) Initial state; (b) State when U = 40 V; (c) State when U = 50 V; (d) State when U = 60 V; (e) State when U = 70 V; (f) State when U = 80 V.
The focal length is changed when different voltages are applied. The focal length changes are measured five times and collated into the error bars graph, as shown in Fig. 5. The experiment demonstrates that the focal length can be varied from -180 mm to -∞ and +∞ to 161 mm when the voltage is changed from 0 to 80 V. As can be seen from Fig. 5, when the LL part works as a positive lens, the shortest focal power can reach to ∼6.2 D, while when the liquid lens works a negative lens, the shortest focal power can reach to ∼-5.6 D. When the driving voltage exceeds to 80 V, the focal length stays the same.
The experimental setup consists of a beam splitter, a He-Ne laser (λ = 632.8 nm) and a CCD camera, as shown in Fig. 6. The LP part is only used in order to avoid the effect by the LL part. The main reason is that the focal length of the liquid lens can be tuned from -180 mm to -∞ and +∞ to 161 mm. When the MO lens is irradiated by the laser beam, the beam spot will be diverged by the liquid lens. The beam spot cannot be observed clearly during the actuation process. Thus, the liquid prism part is only used in order to avoid the effect by the liquid lens part in the principle experiment. In the experiment, U3 and U6 sidewall electrode are actuated with 110 V, the light beam irradiates the MO lens vertically. The liquid-liquid interface would be tilted and drive the navigation sheet tilting to different angles, accordingly. The tilt angles of the liquid-liquid interface are captured by the CCD camera, as shown in Fig. 7. The measured liquid-liquid interface tilt angles are ∼14.0° and ∼16.0°. In the beam steering experiment, the distance (L) between the MO lens and screen is set to be ∼65 mm. The beam spot can be deflected to six directions when voltages U = 110 V are applied on each of the six ITO electrodes. The moves of the beam spot are shown in Fig. 8. As can be seen from Fig. 8, thanks to the navigation sheet, the beam spot can be kept high quality. As we know, the spherical aberration, coma and distortion will increase with the increase of the FOV. Therefore, the spots in Fig. 8(d) and Fig. 8(g) are not as ideal as the rest. That is to say, the MO lens can achieve the function of 360° beam deflection and steering. The principle dynamic video of the beam steering function is included in Visualization 2.
Fig. 8. Beam steering experiments. (a) Initial state; (b) Applied voltage of U1; (c) Applied voltage of U2; (d) Applied voltage of U3; (e) Applied voltage of U4; (f) Applied voltage of U5; (g) Applied voltage of U6.
The beam tilt angle is a key parameter of the liquid prism. The moving distance on the screen is marked with d. thereby, the beam tilt angle θ can be calculated by θ = arctan(d/D). As can be seen from Fig. 9, when the voltages change from the 35 V-110 V, the beam tilt angle can be tuned from 0°-16.5°@U1, 0°-18.2° @U2, 0°-21.1° @U3, 0°-22.3° @U4, 0°-20.0° @U5, and 0°-22.8° @U6.
3.2 Optical properties of the MO lens
As the MO lens is filled with two liquids about 16 mm height, thus it is necessary to measure the transmittance of the MO lens. We use a fiber optic spectrometer to measure the transmittance. The transmission of the MO lens is flat and varies linearly from 88% to 91% at the wavelength of 500-700 nm.
Response time is another key parameter to measure the performance of the MO lens. In the measuring experiments, a synchronous controller is used to link the power supply and the digital timer (Shanghai Instruments Co. LTD, Type of 411B). The digital timer can be started or stopped only when the voltage is applied or removed to the MO lens. To measure the response time of the LL part, we define the response time which is the time that the focal length changes from -180 mm to 161 mm. The measured response time is ∼180 ms, as shown in Fig. 10. For the LP part, we define the response time which is the time when the beam tilt angles change from 0° to 22.8°. The measured actuating time is ∼225 ms. As the time difference between the six electrodes is only within ∼15 ms, we take the maximum beam tilt angle as an example.
4. Application and discussion
4.1 Application of adjusting the FOV in the optical system
The proposed MO lens can be applied in optical systems for adjusting the FOV. In the experiment, a picture is place 100 mm behind the MO lens. No voltage is applied on the LL part and the LP part is actuated with the voltages of 110 V on the six electrodes, alternately. The experiment results are shown in Fig. 11. This experiment proves that the MO lens can not only tune the FOV, it can also enlarge the viewing angle in an optical system.
Fig. 11. Application of adjusting the FOV. (a) Initial state; (b) Applied voltage U1; (c) Applied voltage U2; (d) Applied voltage U3; (e) Applied voltage U4; (f) Applied voltage U5; (g) Applied voltage U6.
4.2 Application of lighting system
The MO lens can also be applied in lighting systems. During this experiment, a screen keeps in the position of 100 mm behind the MO lens. The LL part is applied with voltages changing from 30 V, 40 V, and 50 V. The LP part is applied with voltage of 70 V on one side of the electrode. The experiment results are shown in Fig. 12. From this experiment, we can claim that the MO lens can be used as a lighting control device in the exhibition, museum, and education. The dynamic response video of the lighting application is included in Visualization 3.
Fig. 12. Application of lighting. (a) LL part driven with 30 V-50 V and LP part driven with 0 V; (b) LL part driven with 30 V-50 V and LP part driven with 70 V on one electrode.
4.3 Discussion
Table 1 shows that the densities of the two liquids and navigation sheet are not matched, which means that the MO lens cannot be used in vertical position. While, the MO lens can maintain the reasonable mechanical stability and the sheet cannot be stuck to the side walls when the device is placed in the horizontal orientation. The main reason is that the densities of the filled liquids and the navigation sheet are 1.08 g/cm3, 1.14 g/cm3, and 1.20 g/cm3 respectively, as shown in Table 1. So, the sheet can be suspended between the liquid-liquid interface relatively stable. When the voltages are applied on the liquid prism part, the maximum steering angle is ∼16° and the response time is 200 ms. Thus, the sheet will not stick to the side wall even if it is driven for a long time. Although we can select other materials to make the navigation sheet and the liquids to be with the same densities. The navigation can be suspended between the liquid-liquid interface in this state. When the MO lens is placed vertically and actuated with voltages, the navigation sheet will still float at the surface or sink at the base. Thus, how to make the MO lens have a reasonable mechanical stability is our further work.
It is well-known that non-aqueous conductive fluids usually provide a much better long-term performance and stability. As described in the previous work [38], exemplary nonaqueous conducting fluids were found, such as propylene glycol and ethylene glycol, both are readily available, and propylene glycol is nontoxic as well. In our experiments, the densities of the filled liquids and the navigation sheet should meet the relationship of ρ1 > ρs > ρ2, where ρ1 is the density of liquid-1, ρn is the density of navigation sheet, and ρ2 is the density of liquid-2. While, the densities of propylene glycol and ethylene glycol are 1.036 g/cm3 and 1.l14 g/cm3, respectively. Though these parameters do not meet the experimental requirements, these liquids materials provide a good choice for future research on the optofluidic devices. Furthermore, the optical property of the MO lens can also be improved. In 2014 and 2017, F. Mugele’ group developed a new method which can tune various optical aberrations by electrically manipulating the shape of liquid lenses using one hundred individually addressable electrodes [27,39]. In our further work, we will consider using this approach to improve the image quality of the MO lens.
5. Conclusion
This paper reports an MO lens which has a zoom lens function and a beam steering function. When the voltages are applied on the LL chamber, the focal length can be tuned due to the changes of the liquid-liquid interface. When different voltages are applied on the LP chamber, the navigation sheet can be tilted to different angles for beam steering. The experiments show that the focal length can be tuned from -180 mm to -∞ and +∞ to 161 mm, and the maximum beam tilt angle can be adjusted from 0° to 22.8°. The proposed MO can be applied in zoom imaging systems for adjusting the FOV and it can also be used for lighting in variety of circumstances.
Funding
National Natural Science Foundation of China (61805130, 61805169, 61927809); China Postdoctoral Science Foundation (2019M650421,2019M650422).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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