Metasurface, IDT, Nanoparticles Yield Acoustic-Driven GHz-Rate Tuning of Light

What you’ll learn:

How a Stanford University research team used acoustic waves to manipulate light.
Device implementation involving gold nanoparticles placed in a particle-on-mirror configuration with a compressible polymer spacer.
Adding an interdigitated transducer (IDT) as an ultrasonic speaker to produce high-frequency sound waves.

The convergence of interactions among optics, acoustics, and electronics leads to some fascinating research advances. A good example of this interplay is seen in recent work at Stanford University, where a team devised a novel way to use acoustic waves to manipulate light that has been confined to gaps only a few nanometers across. It resulted in detailed control over the color and intensity of light via mechanical means.

(They also acknowledge that this new device isn’t the first to manipulate light with sound, but it’s smaller and potentially more practical and powerful than conventional methods.)

In brief, the team placed gold nanoparticles in a particle-on-mirror configuration with a few-nanometers-thick, compressible polymer spacer. Surface acoustic waves (SAWs) driven by an interdigital transducer (IDT) were then used to tune light scattering at speeds approaching the gigahertz region.

The SAWs produced mechanical deformations in the polymer, and the resulting nonlinear mechanical dynamics led to unexpectedly large levels of strain and spectral tuning. Their approach provides a design strategy for electrically driven dynamic metasurfaces and fundamental explorations of high-frequency, polymer dynamics in ultra-confined geometries.

SAWs + Gap Plasmons to Manipulate Light

Of course, the full story is more complicated than that relatively simple description. Their objective was to devise ways to electrically manipulate the optical resonances of metallic nanostructures in nanophotonics and do so at a high speed.

To achieve this, they used electrically driven SAWs and the extreme light concentration offered by gap plasmons. The SAWs produced mechanical deformations in the polymer, and the resulting ensuing nonlinear mechanical dynamics led to unexpectedly large levels of strain and thus large amplitude spectral tuning.

What is a Plasmon?

The new device is deceptively simple, with gold nanoparticles placed in a particle-on-mirror configuration with a compressible polymer spacer (Fig. 1). Specifically, a thin gold mirror is coated with an ultra-thin layer of a rubbery, silicone-based polymer only a few nanometers thick. The research team could fabricate the silicone layer to desired thicknesses, anywhere between 2 and 10 nm (for comparison, the mid-band wavelength of light is around 550 nm).

1. Acoustic modulation of Au nanoparticles on an Au mirror. (A) Schematic of the proposed surface-acoustic-wave (SAW) device capable of acoustically modulating gap plasmons. A chirped interdigitated electrode transducer (IDT) on a LiNbO₃ substrate (gray) launches SAWs that can modulate the thickness of a polydimethylsiloxane layer (PDMS, blue) placed in the gap of an Au nanoparticle-on-mirror (NPoM system). The resulting changes in the plasmon resonance can be monitored in a dark-field microscope. (B) Drawings of the states where the SAW is inactive (top) and active (bottom). The gap size of the NPoM is expected to change when the SAW is launched. (C) Optical dark-field images show clear color changes of the NPoMs in response to the SAW. (D) Electromagnetic simulations of a 100-nm-diameter NPoM showing the change in scattering cross-section spectra for various gap sizes. Calculations are performed for s-polarized illumination of an NPoM with a 40-nm-diameter facet. (E) Vertical amplitude of the scattered electric field for each of the two observed gap plasmon modes with a 4-nm gap size. The S₁₁ field profile is taken at 670 nm and the S₁₂ field is at 530 nm, as indicated by the red stars in (D). (F) Spectra of the scattering cross section taken from panel (D) for selected PDMS thicknesses. An increase of the gap from ~4 to ~6 nm is consistent with the experimentally observed color changes of the NPoMs depicted in panel (C).

The researchers deposited an array of 100-nm gold nanoparticles across the silicone. Some analogies help here: The nanoparticles “float” like golden beach balls on an ocean of polymer atop a mirrored sea floor. Light is gathered by the nanoparticles and mirror and focused into the silicone between, shrinking the light to the nanoscale.

But that’s only part of the story. They attached an interdigitated transducer as an ultrasonic speaker to the side and sent high-frequency sound waves rippling across the film at nearly a gigahertz (Fig. 2). The SAWs surf along the surface of the gold mirror beneath the nanoparticles. The elastic polymer acts like a spring, stretching and compressing as the nanoparticles bob up and down while the sound waves pass by.

2. Surface-acoustic-wave transduction: (A) Photograph of the experimental setup, showing the device under a microscope mounted on a printed circuit board. Two watts of RF energy is coupled in via a coaxial cable. The rainbow-colored path illustrates the s-polarized dark-field illumination from the optics on the right. (B) Simulated mode shape of the SAW from an electromechanical simulation, showing the total displacement distribution through the thickness of the substrate. The color bar represents the magnitude of the total displacement. (C) Vertical displacement of the SAW under the NPoM structures on the Au film, with both simulated and representative measured results shown. (D) Optical image of the device, with insets showing an optical image of the IDT fingers and a micrograph of the NPoM atop the metal film to the right of the IDT. (E) SAW simulation at 600 MHz showing the vertical displacement amplitude *u_y* through the thickness of the LiNbO₃ substrate. A SAW is observed as a guided wave propagating along the surface of the substrate, while a bulk acoustic wave (BAW) is a parasitic mode emitted from the IDT into the bulk of the substrate.

Squeezing the Light

When the researchers shined light into the system, this light gets “squeezed” into the oscillating gaps between the gold nanoparticles and the gold film. Although the gaps change in size by just the width of a few atoms, it’s enough to produce an outsized effect on the light. The size of these gaps determines the color of the light resonating from each nanoparticle. The researchers controlled the gaps by modulating the acoustic wave and therefore controlled the color and intensity.

The result is a series of flickering, multicolored nanoparticles against a black background, like stars twinkling in the night sky. Any light that doesn’t strike a nanoparticle is bounced out of the field of view by the mirror, and only the light that’s scattered by the particles is directed outward toward the human eye. Thus, the gold mirror appears black, and each gold nanoparticle shines like a star.

One obvious question is what applications might use this arrangement. Short answer: Not sure, it’s all highly speculative. Possible, applications include ultra-thin screens, 3D holographic imagery, optical communications, and ultra-fast light-based neural networks. You just never know when it comes to these sorts of advances and techniques.

The work is detailed in their paper “Acoustic wave modulation of gap plasmon cavities” published in Science. While that paper is behind a paywall, it does include open links to three short videos as well as supplementary material. Fortunately, the “student version” of the entire paper as published is also posted at Arvix, but without the video links.