Plasmons Enable Tunable IR Light Source

A nanoscale optical parametric amplifier (OPA) has been demonstrated to function as a tunable IR light source. The device functions much like a laser; but while lasers have a fixed output frequency, the output from the nanoscale OPA can be tuned over a range of frequencies, including a portion of the IR spectrum. The device boosts the output of one light by capturing and converting energy from a second light.

Rice University's light-amplifying nanoparticle consists of a 190-nm-diameter sphere of barium tin oxide surrounded by a 30-nm-thick shell of gold. Courtesy of Alejandro Manjavacas/Rice University.

Rice University researchers developed the single-nanoparticle OPA by generating a surface plasmon-enhanced difference frequency by integrating a nonlinear optical medium, BaTiO₃, in nanocrystalline form within a plasmonic nanocavity.

Study lead author Yu Zhang. Courtesy of Jeff Fitlow/Rice University.

These nanoengineered composite structures provided large enhancements of the confined fields and efficient coupling of the wavelength-converted idler radiation to the far-field. The result was a nanocomplex that worked as a nanoscale tunable IR light source.

"There are intrinsic inefficiencies in the OPA process, but we were able to make up for these by designing a surface plasmon with triple resonances at the pump, signal and idler frequencies," said researcher Yu Zhang. "The strategy allowed us to demonstrate tunable emission over a range of IR frequencies — an important potential step for further development of the technology."

Lead scientist Naomi Halas. Courtesy of Tommy LaVergne/Rice University.

Parametric amplification has been used for decades in microelectronics.

"Optical parametric amplifiers operate with light rather than electricity," said research director Naomi Halas. "In OPAs, a strong pump light dramatically amplifies a weak 'seed' signal and generates an idler light at the same time. In our case, the pump and signal frequencies are visible, and the idler is infrared."

While the pump laser in Rice's device has a fixed wavelength, both the signal and idler frequencies are tunable. The researchers believe it’s the first instance of a tunable nanoscale IR light source. Commercially available tunable IR OPA light sources cost around $100,000 and have a large footprint; the Rice device is about 400 nm in diameter. Zhang said that shrinking an infrared light source to such a small scale may open doors to new kinds of chemical sensing and molecular imaging that are not possible with today's nanoscale IR spectroscopy.

The research was published in Nano Letters (doi: 10.1021/acs.nanolett.6b01095).