A new wave for antennas

Photo by Brooks Canaday.

From solar panels to high-​​resolution imaging, a host of advanced tech­nolo­gies relies on the manip­u­la­tion of light waves.

Engi­neers have tra­di­tion­ally bent light beams toward a desired focal point using glass lenses, according to Hos­sein Mos­al­laei, a pro­fessor of elec­trical and com­puter engi­neering at North­eastern Uni­ver­sity. But lenses are bulky and curved, lim­iting their ability to be incor­po­rated onto the minia­tur­ized, flat chips that enable today’s tiny devices.

When one thinks of antennas, on the other hand, the first thing that comes to mind is not usu­ally light trans­mis­sion. But the straight wires pro­jecting from old-​​school mobile phones and the hood of grandpa’s Cadillac are just one type in a range of antenna tech­nolo­gies. As Mos­al­laei says, an antenna is merely some­thing that can scatter an elec­tro­mag­netic wave.

Giant dish antennas dec­o­rate vast desert expanses, sending and receiving microwaves from the dis­tant reaches of the solar system. Even lenses, which reflect, transmit and bend light waves, are, tech­ni­cally speaking, antennas. Recently, Mos­al­laei has trans­formed the con­cept of array antennas, com­monly used with microwave fre­quen­cies, to enable light engi­neering in the sub­wave­length scale.

Since the size of a metallic antenna is directly pro­por­tional to the wave­length of interest, very small wires are required for the very short wave­lengths that make up the optical range. Mos­al­laei is lever­aging that fact to design nanoscale antennas embedded in mate­rials to tailor light in the optical fre­quency range.

Photo cour­tesy of Hos­sein Mosallaei.

In 2011, Mosallaei’s research team released a paper in the journal Optics Let­ters, which ended up becoming the publication’s most down­loaded article of the year. The paper laid out the first example of an array of tiny antennas arranged on a flat, thin-​​film sub­strate, called a meta­sur­face. Each pixel of the sub­strate is dotted with a sep­a­rate antenna, each of which receives an optical wave and locally bends it in a par­tic­ular direc­tion. “Now, if I shine light on it, magic hap­pens,” said Mos­al­laei, who believes this new approach will create a new par­a­digm for optics engineering.

Of course, it’s not magic, but the pre­cise tai­loring of the wave’s ampli­tude and phase. With dif­ferent antenna arrange­ments, Mosallaei’s meta­sur­face can direct optical waves along any desired path. “This will be a trans­for­ma­tive con­cept to make flat nano­ma­te­rials that can engi­neer the light in sub­wave­length scale,” said Mos­al­laei. Having inclu­sions of nanoan­tennas in a thin-​​film can fea­ture unique optical char­ac­ter­is­tics, he said, such as engi­neered reflec­tion, trans­mis­sion, or absorption.

Addi­tion­ally, since the nanoan­tennas are them­selves smaller than the wave­lengths they manip­u­late, they do a much better job of focusing light than lenses, which are lim­ited by diffraction.

If I can con­cen­trate the light in an area of 20nm, instead of 400nm, this means inter­ac­tion on smaller scales, leading to better sensing devices or higher-​​resolution imaging,” said Mos­al­laei. This feat can also help with new uncon­ven­tional nanofab­ri­ca­tion schemes.

Mosallaei’s team works on the the­o­ret­ical and com­pu­ta­tional level, but in col­lab­o­ra­tion with George White­sides, a pro­fessor of chem­istry at Har­vard Uni­ver­sity, pro­to­types of these meta­sur­faces are becoming a reality.