Zoom lens without motorized optical elements

Ruud Oldenbeuving
Several years ago, digital cameras started appearing in mobile phones. Nowadays they are so common that you can hardly buy a mobile phone without one. But these camera-phones are not very high quality, especially when compared to conventional digital cameras. This is partly due to the fact that most conventional digital cameras have an adjustable lens system to focus images from different distances. These lens systems are made with several lenses that have fixed focal lengths but are able to change their spacing. In a mobile phone this is undesirable, because these lenses have a diameter which is too wide for implementation in a phone. Additionally, the distance between the lenses is too large, making the phone too bulky to be desirable for consumers. Finally, moving parts are not as robust as the rest of the mobile phone, making it more fragile.

To cope with these problems, researchers at Philips proposed a novel method to produce lenses using a process called electrowetting. This process allows the manufacture of lenses with a variable focal length. However, that research had two big limitations-the image plane moved and the lens was easily distorted. This means that as the focal distance is changed (e.g., focusing the camera on an object that is further away), the resulting image remained blurred unless the lens was physically moved. The resulting lens system was more compact and rigid than a fixed focal length, motorized lens system but still failed to meet some of the requirements for a camera phone. Recently, Chinese researchers proposed some important adjustments to the design that prevents distortion and may lead to a lens system with no moving parts.

Electrowetting
The variable focal length lenses rely on the principle of electrowetting. Electrowetting is the process by which the wetability (the way a liquid sits on a surface) of a surface is modified by an applied voltage. Water is a bi-polar fluid, which means that it has an asymmetric molecular structure, with the two hydrogen atoms (H⁺) at one side and the oxygen atom (O^2-) at the other side. This means that one side of the molecule is positively charged and the other side negatively charged so water can be manipulated with an electric field, as illustrated in figure 1. By applying a voltage across a droplet, one can attract some fluid to a place it normally would not want to be, e.g. onto a water repellant surface. The only problem is that pure water does not conduct electricity very well, due to the lack of free electrons. This can easily be resolved by dissolving a little salt into the water. By applying a voltage, water can be attracted to, or repelled from a surface, which changes the curvature of the surface of the water droplet.

Figure 1: The principle of electrowetting.

Lens
As one can see from figure 1, the curvature of the surface changes, so this phenomenon can be used to make a very small lens, whose focal length can be varied by changing the applied voltage. Note, however, that it cannot be used when the droplet is in air, because, the lens is deformed by gravity when it is tilted. For camera phones, the lens system should be rigid, small, and have as little aberration as possible. Peng et al. showed that adding another fluid to the system prevents the lens from being distorted by gravity and keeps it properly aligned. This fluid is not affected by the applied electric field and is water repellent to prevent them from mixing. The insulating fluid has a larger refractive index than the salty water and it also has an equal density to prevent aberrations in the lens. The negative electrode, as seen in figure 1, is placed at the sides and the positive electrode on the bottom of the hydrophobic but conducting substrate. A cylindrical lens assembly is used to ensure that the lens remains centered when a voltage is applied. As the voltage is changed the lens will change its shape, resulting in a different focus (see figure 2). In practice this actually works, as can be seen from figure 3.

Figure 2: A different voltage applied to the system, results in a different lens. The figure is derived from the work performed at Philips.


Figure 3: Different applied voltages result in different radii of curvature for the lens system, i.e. result in a different focal distance for the liquid lens. These images are derived from the work performed at Philips.

Results
The presence of the stabilizing fluid reduces the refractive index difference experienced by the light as it enters the lens, which reduces the focal range of the lens. To counter this problem, the lens system has a solid lens for primary focusing, while the fluid lens is used for shifting the focal distance of the entire system. Measurements show that focusing occurs between "infinity" and about 2 cm. In other words, the focus of the liquid lens system has a rather large focal range. Two very nice pictures of their results are depicted in figure 4.

Figure 4: Two pictures taken with the liquid lens at two different foci. The pictures are derived from the work performed by Philips. Picture (A) is focused at an image 50 cm away, (B) is 2 cm away from the lens.

Focal plane
The whole point of these lens systems is, of course, the moving focal plane. But real camera systems have one moving focal plane to pick out an object, and one fixed focal plane to image the object on film or a sensor. One major drawback of the system described by Philips is that both planes move. To overcome this problem, Peng et al., from Shanghai, came up with the following idea: simply add another liquid lens to your system to maintain the image plane at the same location. In their theoretical paper, they show, through calculations that their system should work. However, they have not produced a real lens system yet.

Conclusion
If Philips can resolve the problem of the moving image plane, then I think we can expect this system in our mobile phones in the near future. Since there are no very recent publications on this topic, the chances are that the lens system is being prepared for mass-production. If this is the case, it will be less than a two years before it's on the market. Since the system uses relatively high voltages (<100V) but doesn't need a high current, this can be easily upconverted from the battery voltage, with only a very small additional drain on battery life. I think this system has a lot of potential to be implemented in the next generation of mobile phones, because it is small, cheap, easy to build, and rigid.

Beyond the diffraction limit

By Olivier Rekers
Due to the diffraction limit, it is hard to look at things that are smaller than the wavelength of the imaging light. But with the help of a 'hyperlens,' it is possible to produce magnified images of objects that are smaller than the wavelength of the imaging light. Hyperlenses and their close cousins, superlenses, have received a lot of attention recently because they have the potential to provide detailed information in living biological systems, unlike other high resolution imaging systems such as scanning tunneling microscopy.

How do hyperlenses avoid the diffraction limit? To understand that we need to understand what happens to the light when it strikes an object smaller than its wavelength. When light hits any object, information about the object is encoded in the light by changing the amplitude, phase, and direction that the light travels. When the object is smaller than the wavelength of light, the part of the light that carries the information doesn't propagate like normal light, instead it vanishes just a short distance from the object—after traveling one wavelength it is already half gone. These so-called evanescent waves are the key to breaking through the diffraction limit. One property of evanescent waves is that by controlling the refractive index it is possible to create a situation where they do not vanish, but rather propagate like normal light and can then be used to image the very small object. Two papers published in Science contain experimental details of hyperlenses that do just this.

A 'hyperlens' is made out of a cylindrical layered object that has dielectric constants of different signs across the layer (radial axis) and along the layers (tangential axis). It is both possible to use a half-cylindrical or a cylindrical lens. The papers summarized here report on one of each.

The two groups differ in their methods for achieving the necessary strong anisotropy in their 'hyperlens' medium. The group of Liu used a curved, periodic stack of silver and aluminum oxide. This stack is deposited on a concave quarts substrate. The object to be imageed is placed in contact with the lens—in this case a chromium layer inscribed with a pattern. With use of a conventional lens is it possible to make a projection of a sub-wavelength structure.

The group of Smolyaninov combine the idea of a hyperlens with the earlier concept of a superlens. A superlens is made of a single layer of a meta-material with a negative refractive index. In this case, the negative refractive index is created by depositing concentric rings of poly (methyl methacrylate) on a golden film surface. The evanescent wave impinging on the gold surface excites a surface plasmon polariton wave, which experiences the structure as a negative refractive index. Snell's law then ensures that the superlens magnifies a sub-wavelength sample in a ring—the magnification increases as you travel outwards from the center point. Near the edge of the superlens, the magnification is sufficient that it is possible to see use a microscope objective to image the sample.

Both lenses have significant advantages. Conventional microscopy is limited due to the diffraction limit. This limit makes it impossible to see things smaller than 200 nm. Thus, viruses, proteins, DNA molecules, and many other samples that are impossible to clearly visualize with a regular microscope may soon be accessible to visible light microscopy. Used in combination with labeling or spectroscopic techniques that enable the observer to identify different structures this could become a very useful tool in identifying molecular pathways.