Metamaterials get their name from the Greek word “meta” meaning “beyond”, which is truly apropos. They are a new class of engineered materials that can be imbued with strange and unnatural properties. These materials gain man-made attributes that are far beyond and utterly non-existent in the material they are created. For example, how about an optical lens with a negative refractive index? That certainly qualifies as a bit bizarre.
The scope and scale of metamaterials are astounding as they can be made to operate on any type of wave energy such as sound in air, liquids, solids, or magnetic and electromagnetic waves. But here we will focus on the application of metamaterials as applied to manipulating light and the science of holography.
Their attributes are derived by adding small meta-structures, smaller than the wavelength in question, that interact with the wavefront to alter its behavior. This is referred to as function through structure. These meta-structures give rise to emergent properties that are non-existent in the original material and can defy both logic and the laws of nature.
An emergent property is a wholly new characteristic that appears from interactions between the components of a system. An example would be consciousness developing from neurons. No amount of analysis of the individual neurons would reveal the emergent property of consciousness.
This is science fiction-level technology such as invisibility cloaks, Lidar on a chip, earthquake attenuators, flexible body armor that snaps to solid upon impact, superlenses that resolve sub-wavelength details, surfaces that reflect no light, microscopic sensors embedded in the body, and holograms projected into 3D space without any headgear.
The Electromagnetic Spectrum
Metamaterials can manipulate the entire length of the electromagnetic spectrum, from longwave radio waves to X-rays, as long as the meta-feature elements are smaller than the wavelength being manipulated. Referring to Figure 1, large meta-features for radio waves can be built using standard fabrication tech. For smaller features like microwaves, additive manufacturing might be needed. For light waves, the sub-micron photolithography etching technology commonly used for microchips is used. As long as the meta-features are smaller than the wavelength to be affected you are good to go.
Using metamaterials, electromagnetic (EM) radiation can be bent, amplified, absorbed, shifted, or blocked depending on the size and shape of the metamaterial features, far exceeding what can be done with conventional materials. Cloaking devices for microwaves have been demonstrated - not theorized or postulated, but demonstrated.
Light Waves
The path to holography first passes through light waves. Metamaterials manipulate light waves at optical scales by altering their photonic wavefronts in bizarre and unnatural ways to produce optical aberrations like the negative refractive index mentioned above. In the natural world, refractive indexes have only positive values obedient to Snell’s law. Figure 2 illustrates this with “Normal refraction” on the left and “Negative refraction” on the right.
For more useful applications of optical metamaterials than bending straws in the wrong direction, we can look to “superlenses”. Physicists have been bending light with glass lenses since the 13th century but they are still hobbled by lens diffraction limits. This is determined by the Rayleigh criterion to be around ¼ the wavelength of the light. A superlens can achieve perfect focus without the diffraction limits of conventional lenses.
Diffraction limited optics are discussed in more detail in the Holowire CEO Corner article, “Holographic Resolution: The Eye as a Camera”.
Conventional camera lenses only capture the intensity of the light while ignoring its polarization. MetaLenz in Boston, MA is developing superlenses that also capture the polarity of the incoming light. From linear polarized light, the appearance of an object in 3D space can be determined and from circular polarized light, the nature of the material in the image can be gleaned. The astonishing fact is that these superlenses are wafer-thin and replace entire conventional lens assemblies as illustrated in Figure 3.
These “metalenses” gain their special optical attributes with sub-wavelength meta-features spaced at sub-wavelength distances. Visible light wavelengths range from about 400 to 700 nanometers (nm) which is around 25 times smaller than a human hair. However, we routinely make features much smaller than that for microchips using photolithography so it is being adapted to create some very smart cameras in the near future.
Because the surface structures are synthesized they can be made any desired size and shape limited only by your imagination and the power of your computer simulations. They can be made to reflect light, refract it, absorb it, shift the frequency, shift the phase, change the refractive index, alter the direction, eliminate chromatic aberration, and add or subtract polarization. Because the meta-features can be made of any size, a metalens can operate on any wavelength of light from ultraviolet through visible light to infrared and beyond.
Holography
When referencing metamaterials in the context of holograms the industry prefers using the term “metasurface”. Most holographic technologies are monochromatic and suffer from low image resolution, information loss, and narrow viewing angles. Using metasurfaces 3D holograms can be constructed with much higher resolution and in full color. It is even possible to encode multiple images that are accessed by changing the polarity of the illuminating light source. Currently, these are static images, but getting them to move is next on the development list.
Figure 4 represents an academic breakthrough for 3-color holographic images developed by researchers at Duke University. Classic monochrome holograms are created by scanning a solid object with a laser beam to create the necessary interference patterns. The new process creates those same interference patterns computationally. Their secret sauce is to combine the colors in the interference pattern in such a way that they are still able to precisely separate them at projection time.
This technique encodes a 3-color image into a 300x300 micron hologram in a 2D waveguide structure using the same photolithography techniques used to make microchips. When illuminated by red, green, and blue light it projects a 3-color hologram without beam splitters or prisms.
The Duke University device is as small as a piece of glitter so it can fit into the slender frame of a pair of glasses to project into the pupil. At this stage, the device only projects one static image, so the next step is to create moving holograms by incorporating liquid crystal displays at higher resolutions.
Conclusion
In 1904, two British scientists, Horace Lamb and Arthur Schuster, first mused over the potential of metamaterials. But lacking the manufacturing technology to build them, it remained a theoretical ambition. The first metamaterial was not created until after World War II, which was an artificial dielectric for microwaves to produce a negative refractive index shown in Figure 5. As you can see, the metamaterial was large and simple enough to be manufactured with WWII technology.
Fast forward to today, where metamaterials and metasurfaces have advanced enormously on both the theoretical front (powered by computer simulations) and in manufacturing technology where we can manipulate materials down to atomic levels. By borrowing the photolithography technology used for manufacturing microchips we have seen an explosion in the application of microsurfaces to manipulate light, and from there to creating full-color 3D holograms with the promise of making them move one day.
[1] Illustration courtesy Ministry of Science and Technology, Govt. of India
[2] Diffractive Optical lens assembly courtesy of Canon
[3] courtesy Pearson PLC HoloPatient medical training hologram
[4] courtesy Pratt School of Engineering, Duke University