Scientists in the United States have used high-frequency sound waves to manipulate light on the nanoscale via what are known as gap plasmons (Science, doi: 10.1126/science.adv1728). They argue that their efficient, rapid modulation scheme could be exploited in advanced video displays and holographic virtual reality.

Exploiting plasmonics

Many researchers have sought to manipulate light with sound, given that the latter can vibrate at gigahertz frequencies. However, acoustic waves generally produce miniscule displacements—on the order of nanometers, which is far smaller than the wavelength of visible light. As such, light beams usually have to pass through more than 1 mm of sound wave, leading to quite bulky devices that are too big for cutting-edge nanoscale applications.

In the latest work, Mark Brongersma, Skyler Selvin and colleagues at Stanford University show how to get around this problem by exploiting the physics of plasmonics. The basic idea is to concentrate light–matter interactions in the tiny gap between a metal nanoparticle and a metal film, combining the optical electromagnetic field and electron oscillations to confine light in a subwavelength “gap-plasmon mode.”

Specifically, the researchers placed 100-nm gold particles on a mirror made from a gold film coated with a very thin layer of silicone-based polymer. This setup concentrates incident light thanks to the nanoparticles’ electric currents interacting with their mirror images via the gold film, while distortions in the polymer convert incident sound waves into variations of the particle-to-film distance.

Manipulating light with sound

Brongersma and colleagues generated high-frequency sound waves by using a transducer to apply spatially periodic electric fields to a substrate made from lithium niobate, with the piezoelectric effect yielding what are known as surface acoustic waves. Those waves were directed at the nanoparticle-covered film, modulating the thickness of the polymer layer so that the distance between the particles and film varied by a few nanometers from rest to maximum distortion.

Shining white light from above the nanoparticle–mirror system and then measuring the light scattered from the particles using a dark-field optical microscope, the researchers recorded images showing diffraction-limited points of light on a black background. The gold film behind the particles appears black, they point out, as the light that bounces off it passes beyond the microscope’s field of view.

With the sound turned on, both peaks then shifted to shorter wavelengths, with the now brighter fundamental mode in the visible and appearing as bright red.

The scientists recorded mainly greenish points of light when they used a 4-nm-thick polymer layer with the (670 MHz) sound waves turned off. This, they say, was due to the combination of scattering spectra from two different modes of resonating gap plasmons inside the particle–mirror gap—the higher-order mode generated a peak of about 550 nm, while the fundamental mode peaked in the invisible near-infrared. With the sound turned on, both peaks then shifted to shorter wavelengths, with the now brighter fundamental mode in the visible and appearing as bright red.

Surprising results and future applications

Selvin says that he and his colleagues were surprised by the size of the modulation, having expected it to be significantly smaller than the plasmon linewidth. He explains that the effect is not due to a simple mechanical resonance but may instead involve nonlinear mechanical effects: a mass-on-a-spring mechanism featuring asymmetrical spring and viscous forces (the gold nanoparticles being the masses while the polymer acts like a parallel spring and damper).

The researchers reckon that their new high-frequency acousto-optic modulator could have numerous applications. These, they say, include extremely thin video displays and ultra-fast optical communications. Another possibility, they add, would come from the ability to vary the acoustic excitation in time and space, with the resulting variation in optical phase and amplitude leading to dynamic optical holograms.

Before such applications become reality, however, they need to better control the size, position and density of nanoparticles on the gold surface, as well as improve the way they generate their acoustic field. “Then the device may be able to find a place in consumer or industrial optical systems,” says Selvin.