A phenomenon called "evanescent coupling" could allow electronic gadgets to start charging themselves as soon as their owner walks into their home or office.
Researchers have been looking for a way to make a wireless charger for some time. One idea is to use electromagnetic induction - passing an electric current through a coil to create a magnetic field that induces a current in a neighbouring coil.
This is the way devices like electric toothbrushes are charged, and has been proposed as the basis of a universal recharger pad before (see One charger pad could power up all gadgets).
The snag as far as mobile devices are concerned is that the charger and device must be in close contact with each other for it to work. Alternative schemes - for instance, transmitting electromagnetic waves in all directions to reach any device in a room - would be hugely wasteful.
Trapped at source
Instead, Marin Soljacic at the Massachusetts Institute of Technology wants to use evanescent coupling, which allows electromagnetic energy "trapped" in a charging device to be tapped by a "drain" mobile device if the two have the same resonant frequency.
"The energy is trapped at source, until I bring a device that has the same resonant frequency close to it. Only then can the energy 'tunnel through'," says Soljacic. Crucially, the "charger" only starts powering another device when a compatible gadget comes within range.
Soljacic and colleagues Aristeidis Karalis and John Joannopoulos have carried out numerous computer simulations to see if the idea will work. They discovered that a small circuit, consisting of an inductor loop and a capacitor, could be made to resonate at a frequency of 3 to 4 megahertz, allowing it to trap electromagnetic energy without emitting radio waves to its surroundings.
In the wireless charger design, alternating current from the mains is converted to this resonant frequency and sent into the circuit. The current travels round the circuit, generating a magnetic field as it passes through the inductor loop and an electric field as it passes through the capacitor. This pulsing magnetic field extends up to 5 metres around the device.
The magnetic field created by the wireless charger is relatively weak, meaning it consumes little power. However, if a mobile gadget fitted with a similar circuit, with the same resonant frequency, is brought into the room, the charger's magnetic field induces an electric current in the gadget's inductor loop.
This current travels round the mobile device's circuit, constantly switching between electrical and magnetic states, just as in the charger's circuit. As a result, the two circuits start to "resonate" together. This increases the transmission of electromagnetic energy via induction and that energy is used to charge up the gadget.
Placing one of these wireless chargers in each room of a home or office could provide coverage throughout the building. Soljacic presented the results at the American Institute of Physics Industrial Physics Forum in San Francisco on 14 November. The team is now trying to develop a prototype device.
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