In order to change focus with traditional solid lenses, which have fixed optical properties, two or more lenses have to be jointly moved mechanically. In contrast, a variable focus lens can dynamically control its focal length by only using a single lens element. Liquid-filled variable focus lenses are based on the physical deformation of refractive surfaces, which changes their curvature. Examples include liquid-air lenses, which are highly responsive and have excellent optical performance. However, if this kind of lens is placed vertically and its aperture is large, the lens profile might be asymmetrically deformed due to gravity. On the other hand, the liquid-liquid interface formed by two immiscible liquids can act as a refractive surface, the shape of which can be controlled by fluid pressure, electro-wetting, or dielectrophoretic effect.
Nevertheless, in order to maintain high optical performance, the size of the aperture should be small compared to the capillary length, due to physical limitations. Hence the size of the apertures of liquid-liquid lenses is always in the order of millimeters, and a liquid-infused lens with a large optical aperture is still an unsolved problem.
We proposed a novel variable focus lens with a large optical aperture. The lens consists of two chambers separated by a membrane. The chambers were infused with two different liquids characterized by their similar density but different refractive indices. The membrane was prepared by applying a homogeneous in-plane pretension force. The membrane was stretched over a circular hole in the wall that separates the two chambers, effectively making it subject to a circular boundary condition. Thus its deformation was in the interface between the two liquids, and it acted as a refractive surface due to the difference in refractive index of these liquids. If one fluid was made to flow into and out of its chamber, while the other was locked, the lens could shift its power dynamically by means of a syringe pump.
Based on the above concept a prototype of the variable focus lens with a 26 mm aperture was produced. Note however that pursuing larger apertures does not create additional technical challenges. The range of available focal lengths was experimentally verified to be in the range [-150 150] mm, with a minimal of F/5.
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