Holograms have fascinated onlookers for over half a century. But the devices for producing these holographic images have been relatively bulky contraptions, forced into their large size in part by the wavelengths of light that are necessary to generate them.
Emerging technologies such as plasmonics and metamaterials have offered a way to manipulate light in such a way that these wavelengths can be shrunk down. This makes it possible to use light for devices such as integrated photonic circuits. And just this week, we’ve seen metasurfaces enable an elastic hologram that can switch images when stretched.
Now, a team of researchers at RMIT University in Melbourne Australia and the Beijing Institute of Technology has developed what is being described as the “world’s thinnest hologram.” It is only 60 nanometers thick; they produced it not by using either plasmonics or metamaterials, but with topological insulators. The resulting technology could enable future devices capable of producing holograms that can be seen by the naked eye, and are small enough to be integrated into our mobile devices.
Simply put, topological insulators are materials that behave like conductors near their surfaces but act as insulators throughout the bulk of their interiors. The question is, how do these materials enable the shrinking the wavelength of light so that a device for producing holograms can potentially be embedded into our mobile devices?
In an e-mail interview with IEEE Spectrum, Zengji Yue, a research fellow at RMIT University and co-author of the research paper published in Nature Communications, explained that the metallic surface’s low refractive index and the the insulating bulk’s high-refractive index together act as an intrinsic optical cavity, generating multiple reflections of light inside the thin film. This enhances the light phase shift, which is when the peaks and valleys of identical light waves don’t quite match up. This enhanced phase shift creates the holographic images.
Just as a quick primer, holograpy essentially operates based on the principle of interference. A hologram is the product of the interference between two or more beams of laser light. So in a typical holographic device, a reference beam is focused directly on a recording medium, while an object beam is focused on the object that then hits the reference beam on the recording medium, creating an interference pattern.
In the device produced by the international team of researchers, a light source shines on the material, and the output light from the material and from the substrate has a phase difference. The phase contains the information on the contours of the original object. Human eyes and a CCD camera can capture the information and images.
“Integrating holography into everyday electronics would make screen size irrelevant—a pop-up 3D hologram can display a wealth of data that doesn’t neatly fit on a phone or watch,” said Min Gu, a professor at RMIT and co-author of the research, in a press release. "From medical diagnostics to education, data storage, defense and cyber security, 3D holography has the potential to transform a range of industries and this research brings that revolution one critical step closer.”
In a video below you can see the potential for such a technology.
The RMIT researchers have been highlighting the idea that the material is relatively easy to make and scalable. The fabrication method is direct laser writing, a 3D printing technique, according to Yue. “A femtosecond laser manages to ablate the thin film material on a substrate quickly, producing centimeter-scale holograms for practical applications,” he added.
While all of this may conjure up sci-fi images of holograms popping up from our mobile phones, there remain some pretty significant engineering challenges that need to be overcome. Yue acknowledges that a way would need to be developed for creating the light source in the smartphone. In addition, a suitable film coating for the mobile device needs to be engineered and realized.
The challenge right now for Yue and his colleagues is to find a method for improving the efficiency and quality of the device they have. In the future, Yue says they will start looking at flexible holograms for wider applications.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.
