Metamaterials

Materials are lefties too

Southpaw: Barack Obama, surely the world’s most famous left-hander. 

Leonardo Da Vinci, Isaac Newton, Marie Curie and Alan Turing – there’s a trait that all four trailblazers share apart from their remarkable influence on science, and today we celebrate it. Southpaw throw your hand up with pride – it’s Left Handers Day.

10% of the human population are left-handed – a club that boasts four of the last five US Presidents. The American inventor Christopher Latham Sholes made life a little easier for his fellow lefties when he invented the typewriter – a way to write that eliminated the left-hander’s pen-and-paper fist smudge dilemma. And Fidel Castro is a man of the left in more ways than one.

But it’s not only people that can be left-handed. In fact, you don’t even need hands to be left-handed. Stay with me… I’m talking about left-handed materials, otherwise known as negative index metamaterials (NIMs). This is Materials World, after all.

Refraction: The usual, positive refractive index – show with a ray of light refracted by a plastic block

The Russian physicist Victor Veselago (writing hand unknown) coined the term ‘left-handed material’ in 1967. Most of us will know a little about refraction – the bending of light as it crosses the interface between two materials. It’s a fundamental principle behind optical devices such as camera and microscope lenses – complex optical equipment is designed with materials carefully shaped to refract light in ways that focus and manipulate it as desired.

Every material, including air, has a refractive index – the measurement of how light or any other radiation propagates through it. When any electromagnetic wave (not only visual light) traverses an interface to another material with a differing refractive index, the angle of its trajectory changes.

Negative refractive index: The basic principle of negative refraction.

Glass, air, and all other natural transparent media that we know, have a positive refractive index, which means that light bends, to a different degree, in the same direction across all of them – the same direction as the flow of energy.

Veselago imagined materials with both negative electric permittivity and negative magnetic permeability – the parameters that describe how materials polarise in the presence of electric and magnetic fields. He theorised that a material with such properties would have a negative index, and 33 years later David R. Smith and a team of researchers at the University of California proved his prediction correct, creating the world’s first left-handed material.

What’s the use a left-handed material? For a start, as Eoin Redahan recently wrote, there’s the prospect of invisibility cloaks, which would work by using NIMs to redirect light.

Sir John Pendry: Pioneer of superlens theory. Wikipedia Commons.

Another key strand of research is the superlens  – a lens that can capture detail beyond those possible with materials that have a positive refractive index. This ‘perfect’ lens was first theorised by Sir John Pendry in 1999, and while engineering obstacles still need to be overcome to make it a reality, research is ongoing to develop a lens with substantially higher resolution capabilities than the microscope.

For more on the superlens, watch this video from the Institute of Physics.

So, lefties, enjoy the day in the knowledge that you can claim not only Eminem, Pele, Morgan Freeman, Robert De Niro, Angelina Jolie, Lady Gaga, Bill Gates, Whoopi Goldberg and Ross Kemp as your own, but a whole class of wave-bending-invisibility-enabling-microscopy-revolutionising wonder materials too. I was feeling a bit ‘left’ out, until I read that us right-handers can join in by shifting our allegiance for the day. I don’t think it’s going to stick.   

Metaflex (also written “Meta-flex“), which is a flexible metamaterial (flexible MM) developed by a research team out of the University of St. Andrews School of Physics and Astronomy, Scotland, with some help from the Complex Light ERC Project (Department of Physics, University Sapienza, Italy) and the Bielefeld University School of Physics. The University of St. Andrews team is headed by Dr. Andrea Di Falco, who wrote the following about the Metaflex project:

“My interest in Metaflex arises from diverse theoretical and experimental projects in photonic structures and nanofabrication and from the knowledge gained throughout these projects, including the physics and applications of MMs. This project contains many exciting scientific challenges, which offer the possibility of developing the extraordinary properties of MMs for every-day life applications that were unimaginable only a few years ago. Recently we managed to demonstrate that it is possible to fabricate Metaflex with a fishnet meta-atom, with a resonance in the visible. This could be the first step towards three-dimensional flexible metamaterials at visible frequencies.”

Dr. Di Falco and the St. Andrews team placed a single layer of Metaflex material on a commercial disposable contact lens in order to “show the potential of the approach”.

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The Metaflex adaptive camo/visual cloaking tech is interesting, but in order to be effective for military infantry  and/or Special Operations (SPECOPS) applications, an active electro-optical camo system must work effectively/successfully in all three (3) light spectrums (visible light, near-infrared (near-IR) a.k.a. night vision spectrum, and thermal/IR spectrum), must be lightweight, must be sufficiently flexible, must be comfortable to wear, must work reliably, must be durable, and must be cost-effective. If it’s too expensive, heavy, unreliable, fragile, uncomfortable, too stiff, or any combination of those things, it can’t be employed. In other words, the University of St. Andrews team has a pretty large hill to climb, and the climb will most likely be a long one, if it’s successful at all. Starting with non-flexible applications, like rendering main battle tanks and fighter/attack jet aircraft invisible may be easier. Then again, it may not be.

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How Metamaterials Work
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How Metamaterials Work:

Metamaterials are stuff that we humans have made – you won’t find them in nature. And these materials are made in such a way that they can have properties that are either really uncommon or don’t occur at all in the natural universe as we know it.

See, conventional materials have properties that are determined by the individual atoms and molecules they are made of. In other words, a gold brick’s properties are due to all those wacky gold atoms. The chemical composition of the material gives it its properties.

But metamaterials get their properties not from the kind of atoms inside them but from their physical structure. It’s not the chemical composition that matters – it’s how the atoms are physically stacked together and patterned within the metamaterial.

They’re built to interact with electromagnetic radiation and physical force in weird and wonderful ways, and may eventually make sci-fi stuff like cloaking devices (and impossible-sounding things like a substance that expands when you squish it and contracts when you stretch it) possible.

The delivery guy from Pat’s Pizza knows me by name and when he came by for a delivery tonight he asked me, “so you’re an engineering major of some sort, right?”  And I said, “well, I’m actually a physicist” and then, instead of the “ah, ok” reaction I was expecting he let out this big, enthusiastic “AHH, man, that’s awesome!  You know about metamaterials?  Like, negative indexes of refraction, man… you should read about them.  My buddy and I, we’ve got a couple of ideas ourselves… man, there’s something going on out there.”  It was amazing and baffling at the same time– this guy is always so modest and nice (think of a younger Arthur Weasley - you’re thinking of this guy, exactly), and he never says much– but then out of nowhere, he starts talking to me about physics.

It made my night, to say the least.