How to focus light at the nanoscale

New light-focusing device may lead to radical improvements in hard drives, microscopes, and communications
December 11, 2012
Nanofocusing Device

Artist’s impression of nanofocusing device (credit: Young-Hee Lee, Caltech)

California Institute of Technology (Caltech) engineers have created a device that can focus light into a point just a few nanometers (billionths of a meter) across — an achievement they say may lead to next-generation applications in computing, communications, and imaging.

The Caltech device complements IBM’s just-announced “silicon nanophotonics,” which allows for the integration of different optical components side-by-side with electrical circuits on a single silicon chip.

Focusing light into tinier spaces can squeeze more data through optical fibers and increase bandwidth, and optical devices can also be made more compact, requiring less energy to power them.

But once you reach dimensions smaller than the wavelength of light — a few hundred nanometers in the case of visible light — you reach the “diffraction limit,” and it’s physically impossible to focus the light any further.

Bypassing the diffraction limit

Nanofocusing Device

Nanofocusing device (credit: Young-Hee Lee, Caltech)


The waveguide is made of amorphous silicon dioxide (similar to glass) covered in a thin layer of gold. Just under two microns long, the device is a rectangular box that tapers to a point at one end.


Instead of focusing the light, the new device focuses these coupled electron oscillations, called surface plasmon polaritons (SPPs). The SPPs travel through the waveguide and are focused as they go through the narrow end.

The new device is built on a semiconductor chip with standard nanofabrication techniques, says Choo, so it is easy integrate with today’s technology



Applications in imaging, computing, and communication

  • Computer hard drives that hold 50 times more memoryNormal hard drives consist of rows of tiny magnets whose north and south poles lay end to end. Data is recorded by applying a magnetic field to switch the polarity of the magnets. Smaller magnets would allow more memory to be squeezed into a disc of a given size.But the polarities of smaller magnets made of current materials are unstable at room temperature, causing the magnetic poles to spontaneously flip — and for data to be lost.Instead, more stable materials can be used — but those require heat to record data. The heat makes the magnets more susceptible to polarity reversals. So to write data, a laser is needed to heat the individual magnets, allowing a surrounding magnetic field to flip their polarities.Today’s technology, however, can’t focus a laser into a beam that is narrow enough to individually heat such tiny magnets. Indeed, current lasers can only concentrate a beam to an area 300 nanometers wide, which would heat the target magnet as well as adjacent ones — possibly spoiling other recorded data.Because the new device can focus light down to such small scales, it can heat smaller magnets individually, making it possible for hard drives to pack more magnets and therefore more memory. With current technology, discs can’t hold more than 1 terabyte (1,000 gigabytes) per square inch. A nanofocusing device, Choo says, can bump that to 50 terabytes per square inch.

