Light on Boron Nitride Creates Tunable Ripples

Phonon polaritons, ripples on the surface of boron nitride, could make sharper images or communicate data in chips

2 min read
Light on Boron Nitride Creates Tunable Ripples
Illustration: Siyuan Dai

Researchers at the University of California San Diego (UCSD) have discovered that light can cause a ripple effect on the two-dimensional (2-D) material, hexagonal boron nitride, that can be maintained long enough for the waves to be usable for practical applications. This discovery could lead to the transmission of information in computer chips, better management of heat flow in nanoscale devices, or the creation higher resolution images than is possible with light, the researchers say.

The waves in this rippling effect that the researchers observed are called phonon polaritons. Polaritons are the quasiparticles produced when any type of photon strikes an object. Specifically, phonon polaritons occur when an infrared photon strikes a material.

The UCSD researchers discovered that phonon polaritons are far smaller than light waves and can be tuned to particular frequencies and amplitudes by varying the number of layers of the boron nitride. This is the feature that makes it conceivable to use them for producing images with a higher resolution than is possible with light, say the scientists.

The research, which was published in the journal Science (“Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride”), involved focusing a laser on to the tip of an atomic force microscope as it scanned across the boron nitride. The AFM was taking measurements as infrared light from the laser struck the material.  An interference patterns were created as the phonon polariton waves traveled to the edge of the material and then reflected back.

"A wave on the surface of water is the closest analogy," said Dimitri Basov, professor of physics at the University of California, San Diego, who led the project, in a press release. "You throw a stone and you launch concentric waves that move outward. This is similar. Atoms are moving. The triggering event is illumination with light."

Basov added: "You can bounce these waves off edges. You can bounce them off defects. You can play all sorts of cool tricks with them. And of course, you can design the wavelength and amplitude of these oscillations in a way that suits your purpose."

The discovery was somewhat of a surprise. It stemmed from continuing research Basov and his colleagues have been conducting with 2-D materials, such as 2012 work in which they were experimenting with focusing infrared light on the surface of graphene to control the ripples of electrons that this caused across the surface of the material.

In this case, they were working with the insulator hexagonal boron nitride, which when combined with the conductor graphene, could make complex circuits.

In their similar work with graphene, the researchers were able to generate these phonon polaritons as well, but the waves would dissipate quickly. When using the boron nitride, the waves could be maintained for a long period of time.

"Because these materials are insulators, there is no electronic dissipation. So these waves travel further," Basov said. "We didn't expect them to be long-lived, but we are pleased that they are. It's becoming kind of practical."

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