Electrowetting-actuated optical switch based on total internal reflection Chao Liu, Di Wang, Li-Xiao Yao, Lei Li, and Qiong-Hua Wang* School of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China *Corresponding author: [email protected]
Received 7 January 2015; revised 7 February 2015; accepted 23 February 2015; posted 25 February 2015 (Doc. ID 232059); published 25 March 2015
In this paper we demonstrate a liquid optical switch based on total internal reflection. Two indium tin oxide electrodes are fabricated on the bottom substrate. A conductive liquid (Liquid 1) is placed on one side of the chamber and surrounded by a density-matched silicone oil (Liquid 2). In initial state, when the light beam illuminates the interface of the two liquids, it just meets the conditions of total internal reflection. The light is totally reflected by Liquid 2, and the device shows light-off state. When we apply a voltage to the other side of the indium tin oxide electrode, Liquid 1 stretched towards this side of the substrate and the curvature of the liquid–liquid interface changes. The light beam is refracted by Liquid 1 and the device shows light-on state. So the device can achieve the functions of an optical switch. Because the light beam can be totally reflected by the liquid, the device can attain 100% light intensity attenuation. Our experiments show that the response time from light-on (off) to light-off (on) are 130 and 132 ms, respectively. The proposed optical switch has potential applications in variable optical attenuators, information displays, and light shutters. © 2015 Optical Society of America OCIS codes: (130.4815) Optical switching devices; (230.2090) Electro-optical devices. http://dx.doi.org/10.1364/AO.54.002672
Liquid optical switches have been intensively studied in recent years owing to the wide applications in spatial light modulators, microscopies, telecommunications, variable optical attenuators, and electronic displays [1–4]. According to the driving mechanisms, they can be roughly divided into mechanical driving, such as using an external mechanical motor, and nonmechanical driving [5– 7], such as electric force or magnetic force. Although the devices driven by mechanical systems can have a reasonable stability, they are lacking advantages of portability and low power consumption. According to the realization mechanisms, optical switches can be classified into three types: employing a dyed liquid to absorb the light [8–16], changing of the liquid–liquid interface to diffuse the light [17–20], and changing the shapes of the microchannels or 1559-128X/15/102672-05$15.00/0 © 2015 Optical Society of America 2672
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the refractive indices of injected liquids [21–24]. The device which employs a dyed liquid to block the light passing through has the advantages of fast response time and maintaining a favorable aperture shape. However, the ability of the light attenuation greatly depends on the concentration of the dyed liquids. So the contrast ratio is limited. A liquidlens-based optical switch is also reported . It requires a liquid lens to change its curvature of the liquid–interface to focus or diverge the light. When the liquid lens is in positive state, the incident light focuses on the pin hole and passes through the device without any loss, whereas when the liquid lens is in negative state, there is still a little light which can pass through the pin hole. So the ability of the light-off state still needs to improve. Some researchers have proposed a tunable planar optofluidic switch by three laminar flow streams introduced into a focusing chamber . The hydrodynamic tunability of the core-cladding interfaces is the key point to realize the function of switching via total internal reflection. The optical switch can achieve a switching
speed of 1.56 Hz. However, it needs to make several microchannels which increase the complication of the fabrication process and the cost of the device. Therefore, it is desirable to design a liquid optical switch which is affordable, easy to fabricate, and operates with less-expensive external equipment. In this paper, we demonstrate a liquid optical switch based on total internal reflection. Compared with the work in Refs. [10,11], the contrast ratio is 10:1 and 95:1, respectively. And the response time is 300 and 200 ms, respectively. Ren’s group also proposed an optical switch based on variable aperture . The device shows a fast response time, ∼10 ms. But the aperture of the light hole is relatively small, ∼230 μm. If we scale down the size of our device, the switching time is expected to decrease as predicted by τ ∝ ρ × ν∕γ1∕2, where ρ × ν is the liquid density–volume product, and γ is the liquid surface tension . The device can attain 100% light intensity attenuation. It has the advantages of reasonable fast response time, easy fabrication, and low power consumption. Our experiments show that the response time from light-on (off) to light-off (on) are 130 and 132 ms, respectively. 2. Device Mechanism and Fabrication
Figure 1 shows the schematic side view of the proposed liquid optical switch and the operation mechanism. Two indium tin oxide (ITO) electrodes are fabricated on the substrate and coated with a dielectric layer. A conductive liquid (Liquid 1) is placed on one side of the chamber and surrounded with an immiscible liquid (Liquid 2), as shown in Fig. 1(a). In the initial state, the light beam is adjusted to meet the condition of total internal reflection when illuminated at the interface of the two liquids. The light is totally reflected by the liquid and then the detector cannot detect any light. The device shows light-off state. When we apply voltage U to the right-side ITO electrode, as depicted in Fig. 1(b), Liquid 1
stretches towards to this side of the substrate because of electrowetting effect. The incident angle changes as the curvature of the liquid–liquid interface changes. The detector can detect the refracted light in this state. The device shows light-on state. So the device can achieve the functions of an optical switch. The balance of the interface between Liquid 1, Liquid 2, and the dielectric layer tri-junction line is governed by the following : cos θ1 cos θ0
U2ε ; 2dγ 12
where γ 12 is the surface tension between Liquid 1 and Liquid 2, θ0 is the initial contact angle without applied voltage, θ1 is the contact angle when applied voltage to the device, d is the thickness of the dielectric insulator, ε ε0 εr is the dielectric constant of the dielectric insulator, and U is the external voltage to the ITO electrode. The fabrication of the proposed device is described as follows: the chamber is made of a polymethyl methacrylate (PMMA) tube and is stuck on a PMMA substrate. The height and diameter of the chamber are 10 and 12 mm, respectively. The size of the PMMA substrate is 15 mm × 15 mm. Two ITO electrodes are inserted on the substrate and coated with a parylene-C layer (∼1 μm) as an insulator, followed by a thin Teflon layer (AF-1600, from DuPont, ∼1 μm) as a hydrophobic layer. The top view of the electrode is shown in Fig. 1(c). The surface tension of the Teflon layer is ∼18 mN∕m at 20°C. The sizes of the left-side ITO electrode and the right-side ITO are 12 mm × 4 mm and 12 mm × 8 mm, respectively. The gap between the two electrodes is ∼0.5 mm. A diaphragm with a light hole (the diameter is 2 mm) is placed below the device. NaCl solution is used as Liquid 1 (the density of the solution is 1.23 g∕cm3 , the refractive index is 1.34, the amount is ∼0.06 mL, and the surface tension is ∼56 mN∕m) and silicone oil (a density of 1.21 g∕cm3, the refractive index is 1.45, the viscosities is 10 mPa · s, the refractive index is 1.34, the amount is ∼0.7 mL, and the surface tension is ∼23 mN∕m) is used as Liquid 2. If we choose two density-matched liquids, the device can have a reasonable mechanical stability. 3. Experiment and Discussion
Fig. 1. Schematic structure of the proposed device and the operation mechanism: (a) side view of the device without any voltage; (b) applying an external voltage; (c) top view of the electrode.
First, we fabricate the proposed device as described previously to illustrate the principle. In the first experiment, Liquid 1 is placed on the right-side of the substrate. When we applied voltage U to the rightside ITO electrode, Liquid 1 stretched towards to the left side. We can see that when we apply different voltages, the curvature of the liquid–liquid interface changed because of electrowetting effect. The movement is shown in Fig. 2. So when the light illuminates the device, the incident angle could be changed. In the initial state, the light meets the condition of total internal reflection, as shown in 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
Fig. 2. Results of the liquid moving when applied different voltages; (a) State 1; (b) State 2; (c) State 3; (4) State 4 (Media 1).
Fig. 2(a), whereas in other states, the curvature of interface changes and the total internal reflection does not occur. So the light can be refracted by the liquid, as shown in Figs. 2(b)–2(d). The dynamic response of the liquid during the actuated process was also included in Media 1. Such an experiment shows that our device can achieve the function of an optical switch. In the experiment, when the external voltage U < 30 V, Liquid 1 cannot move because the driven voltage of the device is 30 V. Then we increase the voltage (30 V ≤ U ≤ 65 V) the droplet can move and stretch to the left-side substrate. When the voltage U > 65 V, it has driven the droplet to contact angle saturation. The droplet cannot stretch further. To drive the droplet moving to its original position, we only need to remove the applied voltage. This is because the surface tension between the PMMA tube and Liquid 1 is relatively high to enable the liquid to return to its original position freely. We simulate the geometric model when the light illuminates the device, as shown in Fig. 3(a). In the initial state, the light is adjusted to meet the condition of total internal reflection. We define the angle between the incident light and the side wall as α, the incident angle as i, the refracted angle as i0, and the angle between the emission light and the substrate as θ. We can calculate θ from a simple geometric relation: θ
π − α i − i0 : 2
In the initial state, when ∼36° < α < ∼42°, the light can meet the conditions of total reflection. In this state, the angle of the incident light between the side wall can be tuned within ∼14° to ∼30°, as shown in the dash area in Fig. 3(a). If we choose the two liquids with a large refractive index difference, the direction of the incident light can be tuned within a large angle. In this manner, the device can have a low sensitivity to the incident angle which would broaden applications of the device. It is also our further work to search for two suitable liquids. We also measured the change of the contact angle between Liquid 1 and the dielectric layer during the actuation process. The results are shown in Fig. 3(b). When the applied voltage U 30 V, the device just meets the condition of total internal reflection. So there is no data in this state. In the second experiment, we used a laser beam (λ 632.8 nm) to illuminate the device in order to check the abilities of light-on and light-off. The light power of the laser is attenuated to 0.05 mW. Fig. 4 shows the experimental setup for measuring the optical switching of the device. We placed a chargecoupled device (CCD) to record the light beam passing through the device. The results are shown in Fig. 5(a). In initial state, when the light illuminated the device, total internal reflection accrued. So the detector cannot detect any light. In other states, we can observe the light spot. We also measured the normalized light intensity under different applied voltages, as shown in Fig. 5(b). Response time is another key parameter to measure the performance of the optical switch. To measure the response time, we define the rise (decay) time as the time that it takes to increase (decrease) the light intensity from 0% (100%) to 100% (0%) of the total intensity. Figure 6 shows the normalized light intensity versus time evolution. The measured response times are ∼130 and ∼132 ms under the voltage of 65 V, respectively. From the results, the response time of our device is not as fast as those devices reported before. The main reason may be related to the size of our device as described in the Introduction and the fact that the viscosity of the silicone oil is relatively large. But if we choose an
Fig. 3. (a) Geometric model when the light illuminates the device; (b) contact angle and the refraction angle under different voltages. 2674
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Fig. 4. Experimental setup for measuring the optical switching of the device.
Fig. 5. Performance of the optical switch illuminated by a laser beam; (a) performance of light-on and light-off; (b) normalized light intensity versus applied voltage.
oil with low viscosity, Liquid 1 may not return to its original position automatically. So we can decrease the whole size of the device and choose a proper silicone oil to shorten the response time. In our device, the densities of the two liquids are very close and the surface tension of Liquid 1 (∼56 mN∕m) is relatively high. So we claim that the device has a reasonably stability. We did another two experiments when the device is placed in the vertical position. In the first experiment, we rotate the device as per Fig. 1(a) clockwise 90°. Liquid 1 is placed on the bottom of the device. We should change the angle of the incident light in order to meet the conditions of total
Fig. 6. Normalized light intensity versus time evolution.
reflection. The rise time and decay time are measured to be ∼210 and ∼70 ms, respectively. And Liquid 1 cannot travel the same distance along the substrate as placed in the horizontal position. We also put the device inside the vibrator by applying 200 rpm vibration and the device can keep a good shape of liquid–liquid interface. In the second experiment, we rotate the device as per Fig. 1(a) counterclockwise 90°. Liquid 1 stretches to the bottom substrate naturally and when we put it inside the vibrator by applying 200 rpm, the two liquids cannot keep a good interface. In summary, although the device is still affected by gravity, it can also be used in the vertical position. If we choose two liquids with similar densities and decrease the whole size of the device, the device can have much more mechanical stability. To measure the power consumption, a resistor of 100 kΩ was connected in series to the device, and the maximum current of the device was measured to be ∼32 μA at 65 V. The maximum power consumption is ∼2 mW. In our device, NaCl solution is placed on the hydrophobic layer (parylene-C) and not attached on the ITO electrode directly, as shown in Fig. 1(a). So it would not bring an electrolytic reaction. In our experiment, we used DC voltage to drive our device. When a high DC electric field is applied over a long period to the device, it would irreversibly polarize and permanently damage the dielectric layer. As some research discussed , the squarewave signals can significantly alleviate the side effects of dielectric polarization and facilitate the interface maintenance. So in our further work, we will consider the electrical stability of the device and using a low-frequency square-wave AC voltage to drive the device. 4. Conclusion
In conclusion, we propose a liquid optical switch based on total internal reflection. In the initial state, when the light beam illuminates the interface of the two liquids, it meets the condition of total internal reflection. The light is reflected by the liquid, and the device shows light-off state. When we apply voltage to the other side of the ITO electrode, the conductive liquid stretches towards to this side of the substrate and the curvature of the liquid–liquid interface changes. The light beam is refracted by the liquid and the device shows light-on state. The device can attain 100% light intensity attenuation. Our experiments show that the response time from light-on (off) to light-off (on) are 130 and 132 ms, respectively. The proposed optical switch has potential applications in variable optical attenuators, electronic displays, and light shutters. The work is supported by the NSFC under Grant Nos. 61225022 and 61320106015, the “973” Program under Grant No. 2013CB328802, the “863” Program under Grant No. 2012AA011901, and the RPSPC under Grant No. 2013TD0002. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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