John Joannopoulos and a colleague pioneered the early work for wireless cell phone charging.
PHOTO: LEN RUBENSTEIN
Soljačić was weary: his wife had forgotten to charge her cell phone, and it had buzzed them awake. Again.
“Can’t this thing charge itself?” he asked Joannopoulos. The two, both MIT physicists, immediately began sketching ideas for a wireless charging solution on the blackboard.
“That was the beginning,” says Joannopoulos, Francis Wright Davis Professor of Physics and director, Institute for Soldier Nanotechnologies. The work led to the founding of WiTricity several years later, a start-up that is now building wireless charging systems for mobile devices, medical devices, and even electric vehicles.
The science behind wireless charging—the almost magical transfer of energy from one entity to another without wires—has been understood since the days of Michael Faraday. In 1831, Faraday showed that an oscillating electric current in one coil of wire creates a magnetic field that induces current in a nearby separate coil of wire. This principle of magnetic induction is used today to charge electronic toothbrushes, which sit atop a charging platform, close enough to be within range of the tiny magnetic field.
The IT physics professors wanted to do one better. They wanted wireless charging at a distance, so Soljačić’s wife’s phone can charge itself even if she abandons it on the kitchen table for the night, a demand shared by many battery-powered mobile device owners. To meet this goal Soljačić suggested that they build on the concept of coupled resonators. Resonators behave a bit like the opera singer and the exploding wine glass. The singer projects a note into a room full of wine glasses filled to varying levels. The glass that oscillates with the same resonant frequency as the note will begin to vibrate. As the singing continues, energy transfers to the glass until—crash!
Magnetic field resonators exchange energy similarly, though via a magnetic field rather than airwaves. Since coupled resonators oscillate at the same frequency, they transfer energy back and forth very efficiently. They also work at a distance, even at the weak outer reaches of the magnetic field. “The idea is not amazing to physicists,” says Joannopoulos. “What was new was figuring how to make a practical magnetic field resonator.” Aristeidis Karalis, then a graduate student and now a research scientist in the Research Laboratory of Electronics at MIT, joined the team and began making prototypes. The first demonstration of the technology used copper coils tuned to similar resonant frequencies. They wirelessly lit a 60-watt light bulb nearly seven feet away.
Joannopoulos, along with Soljačić and others in the department, is now focused on harnessing another well-known principle of physics to meet an emerging need for novel energy sources. The goal is to create thermal photovoltaics that resemble solar panels but generate electricity from heat rather than sunlight. The concept is based on the fact that objects, when heated to high temperatures, glow. “That means light is being emitted,” says Joannopoulos. “But the idea is to change how that light is emitted, make it more efficient, and then use that light instead of sunlight to create an electric current.”
The trick? Building a practical solution. If they succeed, and early prototypes suggest they might, one possible application could be the creation of a battery with a 20-year lifespan.