Nano-antenna fashions charge from light

Belle Dumé writing in reports that a new device that collects and focuses light before converting it into a current of electrons has been developed by researchers at Rice University in the US. The nano-optical antenna and photodiode – the first device of its kind – could potentially be used in a variety of applications such as photosensing, energy harvesting and imaging.

Conventional antennas, which are widely used to transmit radio or TV signals, can be used at optical frequencies as long as the device is shrunk to the nanoscale. Such optical nano-antennas work by exploiting “plasmonic modes”, which increase the coupling between light emitted by neighbouring molecules and the antenna.

Naomi Halas and colleagues have now taken advantage of these plasmonic modes to make the first optical nano-antenna that also works as a photodiode – a type of photodetector capable of converting light into either current or voltage. Halas’s team made its device by growing rod-like arrays of gold nano-antennas directly onto a silicon surface – so creating a metal–semiconductor (or Schottky) barrier formed at the antenna–semiconductor interface.

When light hits the antenna, it excites oscillating waves of electrons, known as surface plasmons (so-called because they travel near the surface of the metal). These energetic or “hot” electrons are then injected into the semiconductor over the Schottky barrier, thus creating a detectable photocurrent without the need for an applied voltage.

The resonators made by the researchers, who report their work inScience, have heights and widths of 30 nm and 50 nm, respectively, and are between 110 nm and 158 nm long. Each 15 × 20 array consists of 300 devices with a spacing of 250 nm between the antennas. The structure is surrounded by an insulating later of silicon dioxide and the ensemble is then electrically connected through an electrode made of indium tin oxide.

Applications galore!

One advantage of the device is that the photocurrent generated is no longer limited to photons with energies above the band gap of the semiconductor, but instead to photon energies above the height of the Schottky barrier. The device can thus detect light below the band gap of the semiconductor, and at room temperature to boot. “The result is important because it enables a new way to capture and detect infrared photons using cost-effective, sustainable semiconductor materials such as silicon,” Halas

As the plasmon resonance wavelengths in the device are in the near-infrared part of the electromagnetic spectrum, with shorter nanorods giving shorter resonance wavelengths, applications for the device could include silicon-based solar cells that would work in the infrared as well as in the visible parts of the spectrum. The fact that the devices work in the broad-infrared also means that they could be used to make low-cost silicon infrared-imaging detectors that might replace costly indium-gallium-arsenide detectors that work in the same spectral range.

“The range of potential applications for this device is extremely diverse,” says Halas. “For example, as it is capable of detecting sub-band-gap photons, it could find widespread use in on-chip silicon photonics that would no longer need to integrate additional semiconductor materials as detectors into chip designs – something that would also lower fabrication costs.” Halas adds that such nano-antennas could also be used in “unforeseen applications”, such as photosensing, energy harvesting, imaging and light-detection technologies.

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