A View from Lulu Liu
Nano Optical Antennas Could Have a Big Impact
A lab at Harvard is developing materials with potentially revolutionary optical characteristics.
The antenna, a century-old technology, is everywhere. Listening to the radio? Making a call on your cell phone? Surfing the web over Wi-Fi? The antenna made it all possible.
Now, the antenna is changing optics, too.
At Federico Capasso’s lab at Harvard University, researchers have devised a new way of manipulating light using nano-scale optical antennas. They effectively take a radio antenna, bend it into a V, and shrink it by a factor of about a million, to create what is called an “optical resonator.” By patterning a surface with a number of these resonators, bent at different angles—to create what’s known as a metasurface—they discovered they could get light to do just about anything they want.
Here’s what they’ve managed to make so far:
Mirrors that (seem to) Violate the Laws of Reflection
We learned it in grade school: the angle of incidence equals the angle of reflection. Send light straight at a mirror and it will come right back. Not anymore. The researchers’ antenna-based metasurface mirrors reflect light into arbitrary directions of their choosing. Besides giving us a more general understanding of the physics of reflection and refraction, these devices permit us to control and redirect light in ever more versatile ways.
A Flat Lens as Thin as a Hair
A lens, we’re told, is a round piece of glass with a bulge in the center, used to bring light to a point. To focus a camera, we have to move the lens, forward and back, until an image sharpens into view. But what if the focusing element can be made just 60 nanometers thick, the whole lens the width of a few human hairs? What if by changing the shape of some tiny antennas within such a lens we can shift the focus? This could allow imaging components to be miniaturized like never before.
Metamaterial Wave Plates
Conventionally, rotating light’s polarization, or switching between linear and circular polarization, calls for special crystals with indices of refraction that depend on which way the incoming light is polarized. The researchers found they could mimic these effects with their antenna arrays instead. Since by switching light’s polarization one could encode data, this class of elements has potential applications in quantum optical computing and communication.
Metamaterial Lab Optics
Two common lab tools have also been remade in metamaterial form. The first is a spiral phase plate, which converts a normal laser spot into a doughnut beam of light with angular momentum. The resulting vortex beam, as it’s called, can send illuminated particles into orbit around its dark center. The second is an axicon, an optical element that generates a nearly diffractionless Bessel beam, which can be used to precisely position micron-sized particles and move them along a well-defined path.
Here’s how it works:
It turns out these optical resonators are not too different from everyday antennas.
Light’s oscillating electric field drives electrons up and down the length of each conductor in the antenna. In a receiver, such as a TV antenna, this fluctuating current is picked up and processed. But there’s a secondary effect at work: accelerated charges also radiate. And the re-radiated field will generally differ in polarization, amplitude, and phase. How much difference depends on shape, size, and orientation of each resonator, and it’s this scattered field that the scientists manipulate. Polarization describes the direction of the electric field; amplitude gives the size of the oscillations; and the phase marks where the wave is in its cycle. Together with frequency, they’re enough to fully describe a beam of light.
Conventional optical elements alter the phase and polarization of light by passing it through material with varying thickness. The thicknesses have to be carefully measured, the curvatures precisely cut, to avoid errors and defects. The antenna array is able to achieve the same by directly setting the desired polarization, amplitude, and phase of light at each point in space, as if the beam had just passed through a lens. In this way, the wave is built up pixel by pixel, almost digitally.
At a distance less than a wavelength above the array, the new beam is already fully formed. This breakthrough has no doubt brought us the most compact optics the world has ever seen. In an upcoming paper in IEEE, the researchers summarize their findings. They review the science behind their new, antenna-based metamaterial, and update us on the progress they’ve made in replacing conventional optics with these flat alternatives.
What it is: A class of optics mostly free from the distortions common to their conventional counterparts, which are easy to make by well-established printing methods, and essentially two-dimensional.
What it’s not: These antennas aren’t broadly useful for dealing with visible wavelengths of light. Nor are these components polarization-maintaining, since different aspects of the wave (polarization, phase, amplitude) are manipulated together, which limits applications. Another problem is inefficiency, due to their reliance on the mechanism of scattering.
But even with such limitation, as the examples above suggest, there’s enough reason to be excited.