butterfly or caterpillar? The holography system shown here can project both or only one of these animals – the hologram changes depending on the incidence of light and environmental conditions
Holograms are also referred to as three-dimensional photography because, unlike normal images, they also contain information about spatial depth. To generate them, one makes use of the fact that light rays are waves. In the simplest form of holography, a laser beam is split so that part hits the object to be projected before entering the detector. The second partial beam, on the other hand, reaches the detector unaffected. Because the laser beam was diffracted at the surface of the 3D object, the two partial beams now have different phases, which can be translated into a pattern and converted into the holographic representation.
Scientists at the University of St. Andrews in Scotland have been looking for a technique that not only generates holograms statically, but also allows several different holograms to be generated depending on the environmental conditions. “This could be applied for encryption, but also as a humidity sensor or part of a more complex, light-based biomedical system,” says project leader Andrea Di Falco.
To do this, the researchers use a metasurface that has different types of nanostructures. These manipulate the incident light rays and thus generate the phase shifts required for the hologram. A special feature of the new system, however, is that the different types of nanostructures only reflect light under certain conditions, such as a specific wavelength or humidity. “This is an effective way to use the refractive index of the surrounding medium as an additional degree of freedom,” explains Di Falco.
In a first test of the holography method, the system images a butterfly at 705 nanometers light wavelength and dry conditions and a caterpillar at 750 nanometers wavelength and high humidity. On the other hand, when the device is used in 660 nanometer light and dry air, both images are visible, as shown in the photo here.