Silicon Nanospheres Could Be Building Blocks of Optical Invisibility Cloaks

Given the headlines associated with invisibility cloaks, you could be forgiven for thinking that a Star Trek-style cloaking device will be available in stores before the holiday season. Sadly, no. 

It’s true that in recent years researchers have made great strides in their theoretical understanding of how these cloaks work and consequently built increasingly complex and impressive devices.

But these devices generally work in the microwave region of the electromagnetic spectrum, where wavelengths are measured in centimetres. A few teams have made devices that work in the optical realm but only in two dimensions and over extremely short distances.

The problem is simple. The building blocks of microwave cloaks are split ring resonators–c-shaped pieces of metal in which electric and magnetic fields resonate when they interact with a electromagnetic wave. This interaction is what steers microwaves in a way that cloaks objects

These split ring resonators need to be a little smaller than the wavelengths they are designed to interact with. For microwaves,  that’s the centimetre scale and so are relatively easy to make and assemble.

But that’s much harder at optical wavelengths, where the scale is measured in nanometres..What’s more, losses become much more significant on this scale because most metals tend to absorb visible light rather than transmit it.

Consequently, nobody has been able to make split ring resonators that work for visible light. The optical cloaks made so far all rely on other structures such as gold nanorods or holes drilled through silicon slabs.)

So anybody who finds a way to create the optical version of split ring resonators might reasonably be said to be sitting on an important breakthrough.

Enter Arseniy Kuznetsov at the Data Storage Institute in Singapore and a few pals. These guys say they’ve found an alternative to split ring resonators that work well at optical frequencies, with few losses. 

This alternative is silicon nanospheres between 100 and 200 nm in diameter. It turns out that these spheres behave just like split ring resonators in the sense that they allow for the same kind of magnetic resonances when they interact with light.

The trouble of course is making these spheres. Chemists have been making much smaller spheres–between 2nm and 50nm in diameter–for some time now using various self assembly techniques to grow tiny crystals or ‘quantum dots’ as they are called. (These are useful because they fluoresce at well defined and controllable wavelengths.)

To make larger spheres, Kuznetsov blast a slab of silicon with a short pulse of high-powered laser light. This evaporates the silicon which then condenses into liquid balls. These solidify into spheres as they fall back on to the surface as a kind of silicon rain. 

The end result is a surface covered with solid silicon nanospheres between 100 and 200nm in diameter.

Kuznetsov calculated how the magnetic fields inside these balls should resonate when they are zapped with light  and then used a hi-tec tweezer and magnifying set to see whether the behaviour matched the theory.

Turns out it does. This magnetic resonance can be tuned to match any part of the visible spectrum simply selecting spheres of a specific size. 

That’s important because it opens up an entirely new way make invisibility cloaks that operate in the visible region. “These optical systems open up new perspectives for fabrication of low-loss optical metamaterials and nanophotonic devices,” they say. 

That could have a significant impact on the way optical cloaks are designed and made.

There are plenty of challenges ahead, however. Not least of these will be finding a reliable way to make nanospheres of a specific size. 

Then there is the problem of assembling nanospheres into useful devices in a way that scales. That’s not something that a tweezer and magnifying set  can help with, no matter how hi-tec.

Having said that, there is huge interest and big money invested in cloaking research, which is one reason why progress has been so rapid in the last ten years.   So it wouldn’t be a total surprise if these problems were solved in double quick time.

Ref: arxiv.org/abs/1205.1610: Magnetic light