A comparison of how light travels through a conventional lens (left) and a scattering lens (right), coupled with the resulting images of gold particles. Photo courtesy of the MESA+ Institute at the University of TwenteElbert van Putten is a scientist and PhD candidate at the University of Twente in Enschede, Netherlands. The focus of his research is imaging with scattered light, which is a fancy way of saying makin’ pictures.
Normally if you want to image things, like if you want to look at cells through a microscope, you use clear lenses, which focus light on a point to create an image. However, any defect in such a lens—any surface roughness, and in practice these lenses will always have surface defects—will cause the image’s resolution to deviate from the theoretical perfect limit.
My team takes the opposite approach in our research and make lenses that are not clear but opaque, which causes light waves to scatter. Normally you cannot image with something like this. However, we found that if you can control every individual light wave with which you illuminate these lenses, it is possible to prepare for the scattering, allowing you to focus the light through opaque materials. So you can control scattered light by changing how it arrives at the lens.
To control the light, we use small liquid crystal devices that work like computer monitors but are much smaller. When light reflects from such a device, the phase of the light can be changed—you can basically move it forward or backward in time with respect to the rest of the wave. And because the output is a vast amount of pixels, which you can control individually, you can direct exactly where your reflected light goes.
You can predict how the light will potentially scatter through a certain material by measuring the transmission matrix, which determines the relation between the incident and the scattered light. Using a normal lens, the relation between light that comes in and out is very simple, because it begins as a normal wave that becomes increasingly focused. If a material scatters the light, however, this relationship becomes much more complex. You send light in at a certain angle and it comes out at all angles with different, random phases. A very large matrix with a lot of numbers is needed to describe this relationship between incoming and outgoing light, but once that information is obtained you can predict exactly how light will exit the sample.
With conventional optics, 200 nanometers is the approximate limit of the smallest thing you can image. This is because light is a wave, and the diffraction limit dictates that you can’t focus a wave on a point smaller than about half its wavelength, and visible light’s wavelength is between about 400 and 650 nanometers. In our recent study, however, we demonstrated that we could visualize things that were slightly smaller than 100 nanometers by exploiting materials with a significantly lower diffraction limit.
Generally speaking, it really doesn’t matter how a certain material scatters light—as long as the light is scrambled and you have the correct transmission matrix, it works. However, if the material absorbs a lot of light rather than scattering it, things become a bit more difficult. We once executed a small experiment in which we focused light through some chicken flesh bought from the supermarket and essentially had no problems.
The real-world applications for our scattering lens are mainly related to microscopy, although it also could be used in medicine, where very often scanners can only read above the skin, which, of course, scatters light. Imagine all the amazing medical applications and terrible art that would result from the ability to use your own skin as a lens to image what lies underneath.
