Those laying the foundations for future generations of electronics have long had a fondness for photons. But until recently, there was a catch: photons would limit the miniaturization now achieved with silicon chips because they need space—at least half their wavelength—to move around. So structures on photonic chips would have to be at least a few hundred nanometers wide.
But this catch now comes with its own caveat: Researchers expect that dealing with plasmons (electron waves on a metal surface generated by light) instead of photons themselves will allow the creation of photonic chips with structures comparable in size to those on the most advanced silicon chips. Indeed, new plasmonic nanolasers that can focus light in spots much smaller than their wavelength might play a key role in future photonic circuits.
Plasmonic lasers aren’t just important because of their size; they’re also amazingly fast. Their speed was the big news reported by researchers at Imperial College London and the University of Jena in Germany last week in Nature Physics. They’ve created an ultrafast plasmonic semiconductor laser that will ultimately be able to switch its power on and and off a thousand billion times per second. This new terahertz laser is a thousand times as fast as today’s speediest lasers.
The laser features zinc oxide semiconductor nanowires as the lasing medium. The nanowires, which are a few hundred nanometers hick and are about 10 micrometers long on average, are deposited on a 10-nm-thick insulating layer that sits atop a silver film.
Image: Imperial College London
Like most experimental lasers, this one needed light from another laser to get it going. Pulses from a pump laser hit the nanowire 800,000 times per second, resulting in laser pulses inside the nanowire that lasted 800 femtoseconds.
The light is further amplified by the presence of plasmons in the 10-nm area between the metallic film and the nanowire itself. “The surface plasmons can confine the light much more strongly to the interface between the metal and the dielectric,“ Themis Sidiropoulos, a physicist at Imperial College London who worked on the nanowire laser’s development said in the Nature Physics paper. “And if you place the nanowires on the metal substrate, you get strong confinement of the optical mode, and you also get spontaneous enhanced emission—the Purcell effect.” This spontaneous emission basically makes the nanowire laser faster, says Sidiropoulos.
The fact that the nanowire is optically pumped is still an impediment to it reaching its full potential. Sidiropoulos says that for applications in photonics and communications, optical pumping will have to be replaced by electrical pumping. "This is still very tricky, and there are a lot of people working on it; this is still ongoing work," says Sidiropoulos.