Optically Brilliant Metamaterials

Artwork illustration for 'Optically Brilliant Metamaterials', depicting a human eye looking at the nanoscopic world with the help of metamaterial molecules. Source: Canada Stock Journal

Smart Materials

They are made from assemblies of multiple elements fashioned from composite materials, like metals or plastics.  And they promise to revolutionise the way we look at things.

Metamaterials are smart materials engineered with properties not found in Nature.  The materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence.

Three diagrams showing the way carbon atoms are structured in fullerene, nanotubes and graphene metamaterials.Carbon NanoTubes (CNTs) are just one form of those revolutionary new meta-materials.  The diameter of a nanotube is on the order of a few nanometres – about 50,000 times smaller than the width of a human hair.  They take the form of cylindrical carbon molecules and have novel properties, making them potentially useful in a wide variety of applications.

Metamaterials derive their properties not from the properties of the base materials, but rather from their structures.  Their precise shape, geometry, size, orientation and arrangement gives them smart properties, going beyond what is possible with conventional materials.

Metamaterial research involves fields such as

  • Electrical Engineering

  • Electromagnetics

  • Solid State Physics

  • Microwave and Antennae Engineering

  • Optoelectronics

  • Material Sciences

  • Nano-Science

  • Semi-Conductor Engineering and

  • Optics.


Re-Inventing Optics

A photograph showing how a simple glass prism catches and splits a ray of light into its colour components.
Newton’s Prism  Source: Deepdotweb

Optics is the branch of physics, which describes the behaviour and properties of light, including its interactions with matter and instruments.

Geometric optics treats light as a collection of rays – made of visible, ultraviolet (UV) and infrared (IR) light – that travel in straight lines and bend as they pass through or reflect from surfaces.  Physical optics is more comprehensive, and the theoretical model of light, includes wave effects – diffraction and interference – that cannot be accounted for in geometric optics.

An animation illustrating the wave-particle duality of light - the interference pattern from Young's Double Slit Experiment.
Particles? Waves? Or… Wav-icles?

Historically, the ray-based model of light came first, followed by the wave model of light.  In the 19th century, advances in the electromagnetic theory led to the discovery that light waves were in fact electromagnetic radiation.

Diffraction and interference effects depend on the fact that light has both wave-like and particle-like properties.

The explanation for these requires Quantum Mechanics.


The Power of Metamaterials

The latest example of the power of metamaterials, whose novel properties arise from their very structure, comes from Capasso et al. 2016.  The researchers designed an optical lens, made of paint whitener on a sliver of glass.

Unlike the curved disks of glass we are familiar with from our cameras, binoculars and reading glass spectacles, the lens is made of a thin layer of transparent quartz coated in millions of tiny pillars – each just tens of nanometres across and hundreds high.

The lens is just 2 millimetres across, and finer than a human hair.  And it is flat.

Each pillar interacts strongly with light.  Their combined effect is to slice up a light beam and remould it as the rays pass through the array.

A frame from Capasso's metalens experiment, showing how light gets focused by the peaks.
Light passing through the “metalens” is focussed by the array of nanostructures on its surface. Source: Capasso Lab/Harvard

The tiny device has the potential to magnify nanoscale objects and gives a sharper focus than even the top-end microscope lenses. 

Additionally, these “meta-lenses” avoid the usual shortfall of aberrations, inherent in traditional glass optics.

As a result, the quality of the images is actually better than with a state-of-the-art objective lens.  That is potentially revolutionary.


Light Pillars

A close-up photograph of Capasso's metalens structure.
The meta-lens consists of titanium dioxide nanofins on a glass substrate. Scale bar: 2 mm Source: Capasso Lab

Computer calculations are needed to find the exact pattern which will replicate the focusing effect of a conventional lens.  The minuscule pillars have a powerful effect on light passing through.

Compared to the top-end lenses used in research microscopes, which are designed to achieve absolute maximum magnification, the focal spot of the planar lens was typically 30% sharper, meaning that in a lab setting, finer details can be revealed.  Another advantage is that planar lenses can be fabricated easily in the same foundries that make computer chips, since electronics manufacturers routinely craft components far smaller than the pillars.

The team has previously worked with silicon, which functions well in the infrared.  Other materials might be used to make ultraviolet lenses.  To obtain a different focus, engineers can change the size, spacing and orientation of the pillars.  It simply means doing the computer calculations and dialling the results into the new design.


Brilliant Optical Applications

The potential applications are diverse.  Metamaterials are relevant to many disciplines, such as astronomy, engineering, photography and medicine.

From uses in mass-produced cameras for quality control in factories to light-weight optics for virtual-reality headsets, and even contact lenses.  They include optical filters, medical devices, remote aerospace applications, sensor detection and infrastructure monitoring, smart solar power management, crowd control, radomes, high-frequency battlefield communication and lenses for high-gain antennas, improving ultrasonic sensors, and even shielding structures from earthquakes.

A photograph showing a real-life cloak of invisibility in action.
Being invisible… Impossible? What’s impossible?

And if the idea of an “invisibility cloak” sounds like a fantasy from a Harry Potter movie, and a feat of the imagination too far, look no further.  Scientists are already on it.

Are you ready to see the World differently?