An Illuminating Discovery Paves Way For Optics Revolution

NTU Engineers show even humble crystals can support high-precision light control, upending decades of assumptions in photonics.

From room-sized computers to smartphones, miniaturisation has transformed electronics. Guided for decades by Moore’s Law, the number of transistors on a chip roughly doubled every two years, resulting in exponential progress and changing the way humans live. That momentum has now slowed.

But thanks to a new discovery from Nanyang Technological University (NTU) Engineering, optics could be poised to take over and drive the next radical technology leap.

A discovery published in Nature on July 16th, titled Long-range hyperbolic polaritons on a non-hyperbolic crystal surface, upends decade-long beliefs about the kinds of materials capable of supporting strong light–matter interactions at the nanoscale.

The discovery makes the interactions, which are necessary for resolution at the atomic scale, more accessible and unlocks new frontiers in imaging, sensing, and computing.

A new understanding
The work centres on controlling light and atomic-level vibrations at the nanoscale in crystals. Until now, it was widely believed that only rare, specially structured materials, so-called hyperbolic crystals, could support the light-matter waves needed to confine, steer, or manipulate light in ultra-precise ways.

The researchers lay out how they’ve broken through those beliefs about material limits, showing that the type of wave formed when light couples with the lattice vibrations, known as hyperbolic phonon polaritons, can emerge in a far more accessible crystal, and can even be tuned simply by changing temperature.

Asking questions
The breakthrough began with a fundamental question: Could the team create something previously thought impossible?

More specifically, researcher and professor Guangwei Hu, working both with NTU CoE and international collaborators, asked: Could hyperbolic properties emerge in a material that wasn't inherently hyperbolic?

For a crystal to be hyperbolic, its ability to respond to electric fields, known as its permittivity, must differ so much along different directions that sometimes it allows and sometimes it resists electric field. Scientists define this as having both positive and negative permittivity.

This rare condition creates hyperbolic dispersion, meaning light can travel in unusual, tightly confined paths that resemble the shape of a hyperbola.

Travelling in this type of path enables phenomena that create “hot spots” of light. Those hot spots in turn can interact with objects much smaller than usual, allowing imaging systems to detect details below what can usually be detected.

What Hu’s team found was remarkable. Using a simple crystal costing less than $100 known as yttrium vanadate (YVO₄), the researchers showed that while the bulk of the crystal is not hyperbolic, its surface could be.

In other words, at the interface between the crystal and air, the material could support hyperbolic surface phonon polaritons- that desired mix of atomic light and vibration- without requiring the inside of the crystal to be hyperbolic.

The team used a combination of theoretical modelling and scanning near-field optical microscopy (SNOM) nanoimaging experiments to demonstrate this phenomenon.

Next up: control the light
Then they went a step further. By precisely heating the off-the-shelf crystal, they tweaked the permittivity just enough to shift the frequency range of these surface waves, causing the surface to switch in and out of hyperbolic behaviour.

In doing so, the researchers were able to confine light into regions as small as 20 nanometres, an amount 10 to 20 times smaller than traditional optical methods achieve.

Hu and team weren’t just observing physics anymore, they were choreographing it.

“We've essentially broken the physical rules people thought were fixed," explained Hu. "We've shown that surface properties can both be dramatically different from a material's bulk and also have its properties manipulated.”

Imagine unprecedented clarity
The implications are sweeping. This technique could revolutionize observation and interaction with structures at the nanoscale and pave the way for further advancements in semiconductor chip inspection and early disease diagnosis, like those for cancer and neurological conditions.

“In electronics, we've seen how miniaturization leads to more powerful and cheaper devices. We're applying the same principle to optical technologies,” offered co-researcher and professor Qijie Wang.

“Imagine medical imaging that can see cellular structures with unprecedented clarity, or semiconductor chips that can be inspected at near-atomic scales,” asserted Wang. “That's the potential of this research.”