One key challenge for researchers has been to reconcile the length scales typical of optical and electronics circuits. Optical circuits are traditionally larger than their electronic counterparts because the diffraction limit restricts the shrinking of feature sizes below the optical wavelength. Recently, however, researchers have exploited plasmons – quantized oscillations of electrons —to confine light to subwavelength dimensions, which allows the size of optical circuits to be reduced to sizes that match with nanoscale electronics.

"We expect that the demonstration of electrically driven surface plasmonic nanocircuits will be a meaningful step for interfacing high-speed photonic devices and ultra-compact electronic components," explains Min-Kyo Seo, a researcher at Stanford University and the Korea Advanced Institute of Science and Technology.

Current contenders for nanoscale light sources

While plasmonic devices have enabled researchers to create much smaller electronic and photonic devices, there is still an acute need for techniques to electrically drive nanoscale optical circuits. Researchers have previously attempted to use nanolasers as light sources capable of electrically driving optical nanocircuits, but these have a number of limitations.

"One of the key challenges with deep-subwavelength nanolasers is that they feature a lossy metallic (i.e. plasmonic) cavity and this makes it hard to operate such a laser at lower power and at room temperature," explains Mark Brongersma, also from Stanford University. "Our nano-LEDs capitalize on a physical effect known as the Purcell effect to effectively and quickly couple the light out of the metallic cavity into an optical waveguide before too much energy is dissipated in the metal."

Enhanced directional nano-LED solution

Seo, Brongersma and colleagues fabricated a nano-LED from a 130 nm tall 60 nm wide ridge LED containing an InGaAs/GaAs quantum well emitting at 970 nm. Emission from the nano-LED is then coupled to an 80 nm wide slot in a gold slab at the output facet of the LED, exciting plasmons that that can be routed through nanoscale optical circuit elements.

As a result of the Purcell effect, observed by Edward Mills Purcell in the 1940s, spontaneous emission is enhanced within the nano-LED’s extremely small metal-clad active region. This directs the emission into the gap plasmon mode of the waveguide.

"At the moment, the Purcell factor of our nano-LEDs is estimated to be around 2," says Min-Kyo Seo. "But the Purcell enhancement could reach a factor of 10 by reducing the device width. This would allow for modulation at a speed of more than 10 GHz with an intrinsic radiative lifetime of the semiconductor active medium of less than 1 ns."

The researchers demonstrated that the plasmons could be routed through a T-shaped beam splitter coupled to the nano-LED. They also investigated coupling the plasmon in the slot waveguide to a second waveguide and found that optimal coupling could be achieved when the separation between the waveguide centres was 160 nm.

Lighting up future devices

The researchers highlight a number of applications that could benefit from the integrated nano-LED, including nanoscale sensors and functional elements, such as splitters and couplers. They could also be used in future quantum plasmonic circuitry in which the propagating surface plasmon waves can interact with quantum objects – a quantum dot, molecule, or nitrogen vacancy centre – to make unique quantum devices. Other potential applications are optical switches capable of operating at the level of a single photon, and for transporting light on and off computer chips.

Next steps

The current work demonstrates the essential physics of the Purcell effect that makes these systems possible. However, the researchers point out that further development is needed to strengthen the Purcell enhancement and make the sources power efficient. "One of the most important next steps will be the development of ultra-compact and power-efficient surface plasmonic circuit elements, such as modulators and detectors, besides sources, and the dense integration of the elements on a chip," Seo adds.

Details of the current work can be found at Nature Photonics doi:10.1038/nphoton.2014.2.

Further reading

Intercalation tunes plasmonic properties (Dec 2013)
Controlling emission from one dimensional photonic crystals for improved detection (Feb 2014)
New integrated quantum circuit is most complex ever (Jan 2014)
Deterministic ion implantation: fabricating arrays of single dopant atoms (May 2013)