Abstract:
Electrowetting-actuated optical shutters based on total internal reflection or beam steering. An electrowetting cell contains a conducting liquid and a non-conducting liquid configured to form a liquid-liquid interface extending to the inner walls of the cell. A beam of light is directed to the liquid-liquid interface at an angle near the total internal reflection angle of the interface. Voltage changes the shape of the liquid-liquid interface, without separating it from the inner walls of the cell. Thus, when depending on the voltage applied, the beam is either transmitted in part or substantially totally internal reflected.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    The present invention relates to optical shutters. In particular, the present invention relates to electrowetting optical shutters based on total internal reflection (TIR) or angular beam steering. 
         [0003]    Discussion of Related Art 
         [0004]    Optical shutters are used in many applications such as atomic clocks, communications, lab-on-a-chip devices, and optical displays. Existing technologies typically utilize integrated lithium-niobate electro-optic modulators, acousto-optic modulators, or mechanical methods based on blades or diaphragms. However, integrated modulators suffer from limited aperture and contrast ratio, acousto-optic modulators are constrained in their extinction ratio by scattering, and mechanical methods are prone to friction issues and long term wear. Liquid-based electrowetting optical devices provide an attractive alternative for applications requiring large extinction ratios and apertures with no moving mechanical parts. 
         [0005]    Although there are previous demonstrations of electrowetting-based displays, tunable irises, and switches, high extinction ratio (beyond 30 dB) shutters have not been demonstrated. There are many implementations of these functionalities, using opaque ink or oil droplets, tunable irises, and liquid interfaces operating around total internal reflection (TIR). The device using TIR uses a droplet in an enclosed box which spreads and contracts over a floor. This device is slow and achieves only modest extinction ratios. 
         [0006]    A need remains in the art for electrowetting-actuated optical shutters with improved extinction ratios. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide methods and apparatus for electrowetting-actuated optical shutters with improved extinction ratios. Embodiments of the present invention include large extinction ratio optical shutters using electrowetting liquids based on switching between a liquid-liquid interface curvature that produces total internal reflection and one that does not. Other embodiments angularly steer a beam transmitted through the shutter so that the output beam is transmitted or rejected based upon the angle of the output beam. 
         [0008]    The present invention achieves greater than 60 dB extinction ratios with electrowetting shutters owing to its minimal interface roughness and unique geometry. The liquid-liquid interfaces of the present invention are particularly useful thanks to their well-defined interfaces, angstrom level surface roughness, optical isotropy, and low optical loss. The electrowetting effect enables transmissive, compact devices requiring minimal voltages and no moving mechanical parts. 
         [0009]    An embodiment of the present invention comprises a device utilizing the electrowetting effect to control the shape of a conducting liquid droplet, or interface between a polar and non-polar liquid. The shutter is based on an electrowetting liquid interface switching between total internal reflection and transmission, or angular steering. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic block diagram of an optical shutter system according to the present invention. 
           [0011]      FIG. 2  is a side isometric view of an embodiment of an optical shutter according to the present invention. 
           [0012]      FIGS. 3A and 3B  are schematic side views of the optical shutter of  FIG. 2  illustrating cell structure and the effect of applied voltage on droplet shape. 
           [0013]      FIG. 4A-F  are schematic side views illustrating the fabrication process for optical shutters according to the present invention. 
           [0014]      FIGS. 5A and 5B  are plots showing experimental results of the system of  FIG. 1 . 
           [0015]      FIG. 6  is a front cutaway schematic diagram of a prism-based liquid shutter according to the present invention. 
           [0016]      FIG. 7  is a plot illustrating the performance of the shutter of  FIG. 6 . 
           [0017]      FIGS. 8A and 8B  are side cutaway views showing a third shutter embodiment according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  is a schematic block diagram of an embodiment of an optical shutter system  100  according to the present invention. This embodiment is an experimental setup for measuring extinction ratio and response time of shutter  120 . The 780 nm laser diode  102  generates beam  104 . Beam  104  is spatially filtered and focused by optics  106  to optimize spot size incident on the liquid-liquid interface  220  of shutter  120  (see  FIG. 2 ). The correct angle of incidence in this embodiment is achieved using a pair of mirrors  108 ,  110 . Transmitted beam  124  and totally internally reflected beam  122  are then passed through respective spatial filters  126 ,  128  to photodetectors  130 ,  132 . 
         [0019]    In this embodiment, laser diode beam  104  is spatially filtered and focused to a 250 μm diameter spot on liquid interface  220 , offset from the center of the device by 2-3 mm to optimize the incident light position for switching between transmission and total internal reflection. In the case of transmitted beam  124 , the output light is collimated and focused through optics  128  including two spatial filters of pinhole diameters of 100 and 75 mm. For totally internally reflected beam  122 , one 200 μm pinhole  126  is used. Spatial filters were selected to provide maximum extinction ratio for both the transmitted and internally reflected states. 
         [0020]      FIG. 2  is a side isometric view of an embodiment of optical shutter  120 . Optical shutter  120  is based on the principle of electrowetting, in which the contact angle and shape of a liquid droplet or interface  220  is controlled with an applied voltage (see  FIG. 3 ). In this design, the curvature of liquid-liquid interface  220  allows the device to switch between total internal reflection  122  and transmission  124 . Shutter  120  comprises a sealed glass cylinder  212  that is filled with two liquids, dodecane oil  210  and water  208 , and a prism  202  to couple in the incident light  112  at the correct angle. This combination of liquids is fairly well density matched and operates well at visible to 1000 nm wavelengths. Using propylene glycol and oil extend the range to 1550 nm. Beyond that would generally require ionic liquids or reflective surfaces. Those skilled in the art will appreciate that many combinations of fluids could be used depending upon what results are desired. Some examples are given in Appendix A. 
         [0021]    The sidewall of cylinder  212  has a thin film electrode followed by a dielectric layer and a hydrophobic coating (see  FIG. 4 ). At 0 V, incoming light strikes liquid interface  220  at approximately 72°, 4 degrees beyond the water-oil TIR critical angle of 68°, relative to the surface normal. Electrically tuning the interface curvature causes incident light at an appropriate angle and offset from the center to cross this critical angle and switch between total internal reflection and transmission states. 
         [0022]    This embodiment implements the electrowetting shutter  120  using a design that enables tuning of the curvature of an interface  220  between a polar liquid  208  (here water with 1% sodium dodecyl sulfate) and a non-polar liquid  210  (dodecane oil). By changing the applied voltage, the interface radius can switch from 9 mm to −45 mm. 
         [0023]      FIGS. 3A and 3B  are schematic side cutaway views of a shutter  120 , illustrating the process of applying voltage and the effect on liquid-liquid interface shape. In  FIG. 3A  with no voltage applied, liquid-liquid interface  220  is at its most convex. In  FIG. 3B  with voltage applied, liquid-liquid interface  220  flattens out. Electrodes apply voltage to the liquids and dielectric coatings insulate the fluids. Generally diameters would be in the range of 100 microns to 10 mm. Heights would be about 1-10 mm. 
         [0024]    The fabrication process for liquid shutter  120  is illustrated in  FIGS. 4A-F .  FIG. 4A  shows a glass capillary tube  402 . In  FIG. 4B , a glass frit solution  404  is placed on the rim of glass tube  402  and baked at 400° C. In  FIG. 4C , a platinum wire is annealed and shaped in the form of a loop and serves as base electrode  406 . Base electrode  406  is aligned between glass frit  404  and glass substrate  408  underneath glass tube  402 , with glass frit  404  facing down toward substrate  408 . In  FIG. 4D  tube  402  is clamped down to substrate  408  and the fixture is cured at 550° C. to allow the frit to bond the glass pieces together, resulting in a hermetic seal between tube  402  and base  408 . The inset shows a top view of the device at this point. 
         [0025]    In  FIG. 4E , glass tube  402  is cleaned and substrate  408  is masked with Kapton tape  410  outside the tube and a Teflon plug inside the tube (not shown). The sides of tube  404  are sputter-coated with indium zinc oxide (IZO)  412  to form a continuous film from the inside to the outside of tube  402  as shown in the inset. IZO, an optically transparent conductor, is sputtered onto the sample at an argon pressure of 5 mTorr and power of 120 W to achieve a deposition thickness of 300 nm. The mean free path of the sputtered IZO in the argon at this pressure is an order of magnitude shorter than the distance between target and substrate, enabling diffuse sputtering and a conformal coating from inside to outside of the tube. 
         [0026]    In  FIG. 4F , Parylene C  414  is deposited in a vapor-phase, low-vacuum system by VSI Parylene to a thickness of 0.93 μm. The samples are then dip-coated in a 1:20 solution  416  of Dupont&#39;s Teflon AF 1600: Fluorinert FC-40 for the 100 nm hydrophobic coating. To cure the Teflon  416  above its glass transition temperature of 165° C., the samples are heated in an oven at 120° C. for 10 min and then 170° C. for 20 min. The inset shows the thin film layers on the device. 
         [0027]      FIGS. 5A and 5B  are plots showing experimental results from shutter system  100  of  FIG. 1 .  FIG. 5A  illustrates the contact angle  206  vs. applied voltage. Contact angle  206  of liquid interface  220  was characterized by analyzing photographs of the device as a function of applied voltage. The resulting curvature of the liquid interface (dotted line) can be fit to the Lippmann-Young equation (solid line). Total internal reflection occurs at angles θ above about 105°, meaning an angle of incidence of the incident beam  112  (relative to the liquid normal) that exceeds 68 degrees, the critical angle. Both reflected beam  122  and transmitted beam  124  exhibit relatively small changes in optical power until the applied voltage exceeds ˜18 V. As the device enters the non-TIR state, the reflected beam power drops sharply, followed by a rise in the transmitted power as the beam becomes aligned to the spatial filter system. The reflected beam power remains high until the ˜18 V transition as some of the incident beam is internally reflected at this interval. However, the two spatial filter systems in the transmitted branch require very specific alignment to receive significant power, and it is not until the curvature has approached −45 mm radius that significant power is detected. 
         [0028]      FIG. 6  is a front cutaway schematic diagram of a prism-based liquid shutter  600  according to the present invention.  FIG. 7  is a plot illustrating the performance of the shutter of  FIG. 6 . 
         [0029]    Prism-based liquid optical shutter  600  uses two pressure-driven lenses  602  that are offset from each other horizontally. The first lens  602 A focuses light  620  on the edge of the second lens  602 B, which acts like a prism. The first lens can be used to control the output  622  spot size and divergence. As an alternative, electrowetting cells  120  such as those described above can be used. 
         [0030]    While the total internal reflection shutter of  FIG. 1  works well, it presents unique challenges for integration into a system, due to the angles of the input and output beams. For some setups, there are advantages to this alternate geometry, based on an adaptive prism. Input light  604  is guided into first lens  602 A, for example by mirror  606 . First lens  602 A focuses light on the edge of second lens  602 B, which acts like a prism. In addition, first lens  602 A can be used to control the output spot size and divergence. After second lens  602 B the beam  622  is transmitted or rejected, for example by a micron-size pinhole  608 , followed by a photodiode  610 . Using a 50-micron pinhole and a 650 nm laser, we achieved a 60 dB rejection ratio. The membrane in the pressure-driven lens limits the rejection ratio. With a tighter 20-micron pinhole, we achieved a 65 dB rejection ratio (see  FIG. 7 ). By changing the wavelength to 780 nm, relevant for atomic clocks, and using an electrowetting-based prism rather than the two offset pressure-driven lenses, the rejection ratio can be increased to above 70 dB. The results represent three orders of magnitude improvement over state-of-the-art. 
         [0031]      FIGS. 8A and 8B  are side cutaway views showing a third shutter embodiment  800 . In  FIG. 8A  the shutter is on and in  FIG. 8B  the shutter is off. This embodiment of shutter  800  is based on an electrowetting prism  820 , filled with water  208  and oil  210 , 2 mm in diameter and 5 mm high. Light  804  is incident from the bottom. Prism  820  is followed by lens  802  that amplifies the steering, a spatial filter  804  which allows light through only when the “on” angle is achieved, and a collimating lens  810 . When the water-oil interface  220  is flat as in  FIG. 8A , light passes through device  820  and aperture  804  as output beam  824 , and the switch is in transmission mode (“on”). When switch  800  is off, prism  820  is actuated and deflects light  804  away from aperture  804 . Those skilled in the art will appreciate that a number of different geometries would work to form shutter  800 . 
         [0032]    While the exemplary preferred embodiments of the present invention are described herein with particularity, those skilled in the art will appreciate various changes, additions, and applications other than those specifically mentioned, which are within the spirit of this invention. For example, the ultrasmooth, angstrom-level surface roughness at the liquid-liquid interface gives the device design the potential for higher extinction ratios if the secondary reflections and sidewall interactions can be avoided. Future designs could mitigate the effects of secondary reflections and scattering by using anti-reflection coatings. Scattering effects can be mitigated with new geometries to avoid interaction with solid interfaces such as the coated sidewalls. Furthermore, existing electrowetting prism designs can further reduce the effects of secondary reflections and scattering by altogether avoiding interaction with curved liquid-liquid interfaces, instead biasing the TIR state on a flat, angled liquid interface. This has the potential to further increase the extinction ratio. Future system response time may also benefit from biasing the device operation around the transition point of total internal reflection and transmission, taking advantage of the full extinction ratio while reducing necessary changes in contact angle.