Two-step excitation method unleashes higher-order hyperbolic phonon polaritons for ultracompact nanophotonics

Higher-order hyperbolic phonon polaritons (HoHPhPs) show promise for ultracompact light-based chips because they can confine light to extreme nanoscale dimensions—but higher-order polaritons are notoriously difficult to excite.

A two-step excitation method recently developed by researchers from Shanghai Jiao Tong University and the National Center for Nanoscience and Technology in China, with CIC nanoGUNE and the Institute of Photonic Sciences (ICFO) in Spain, provides a larger momentum boost than single-step excitation methods can deliver.

“The main difficulty is these modes possess much larger wavevectors (momentum) than free-space light or even the fundamental polariton modes,” explains Qing Dai, a professor at Shanghai Jiao Tong University in Shanghai, China. “Conventional resonant-antenna approaches can supply only a modest momentum boost—far too small to bridge the gap to higher-order modes. As a result, the unique properties and potential applications of HoHPhPs are largely unexplored. Our work was driven by the critical need to develop a more efficient way to generate these elusive polaritons so their potential can finally be studied and harnessed.”

First, background basics

Polaritons are hybrid quasiparticles that form when photons strongly couple with another type of exciton within a material, such as phonons (lattice vibrations). This coupling allows researchers to squeeze light into dimensions much smaller than its wavelength in free space.

Hyperbolic phonon polaritons (HpHPs) “are a specific type of polariton found within certain anisotropic materials like the biaxial α-MoO₃ crystal we used,” says Hai Hu, a professor at the National Center for Nanoscience and Technology in Beijing, China. “Their name comes from the shape of their isofrequency contours (IFCs), which are open hyperbolas. This unique property allows them to carry large momentum.” 

Higher-order HPhPs (HoHPhPs) are different modes of HPhPs that exist within the same material. Compared to the fundamental (zero) mode, higher-order modes have even larger wavevectors and confine light even more tightly. Despite their promise, higher-order modes have remained extraordinarily difficult to excite.

Pseudo-birefringence “is a  phenomenon we observed where polaritons of different orders (fundamental vs. first-order) refract in different directions when they cross a boundary at an angle, mimicking a birefringence effect,” explains Hu. “But unlike traditional birefringence, the polarization state of the light is preserved in our experiment, so we refer to it as pseudo-birefringence. It was extraordinarily pronounced in our experiment, with an equivalent birefringence up to 41.8, which is 1 to 2 orders of magnitude higher than in natural anisotropic crystals. Such extreme pseudo-birefringence has no counterpart in natural materials and offers an entirely new degree of freedom for polariton transport.”

Why two-step excitation?

It’s all about the momentum boost. The team’s two-step excitation mechanism uses a specially designed substrate to provide the necessary momentum boost.

For the first excitation, a tiny light-illuminated gold antenna provides an initial push, which creates a fundamental (zero-order) hyperbolic phonon polariton (FPhP) mode on a smooth biaxial α-MoO₃ crystal slab placed onto a single-crystalline gold substrate.

During the second excitation, the FPhPs travel to the edge of the gold, where the substrate abruptly ends and the crystal is suspended within air. This abrupt discontinuity acts as a powerful scatterer and injects the extra momentum required to upconvert the fundamental mode into multiple higher-order branches.

“As it crosses this abrupt boundary, the wave is scattered, which provides a substantial amount of extra momentum—enough to convert the incoming fundamental polaritons (FPhPs) into multiple higher-order modes (HoHPhPs) within the suspended region of the α-MoO₃ slab,” says Hu.

The team’s process resulted in a more than six-fold increase in the excitation efficiency of HoHPhPs compared to conventional single-step methods.

Pseudo-birefringence effect

The team’s setup is essentially the sample they fabricated—a silicon dioxide (SiO₂)-on-silicon substrate with square holes etched into it. “We precisely placed a thin, atomically smooth single-crystal gold flake next to the holes to create the sharp boundary,” says Hu. “An exfoliated α-MoO₃ slab was then transferred over this structure, so it was partially supported by gold and partially suspended over the holes. And, finally, we fabricated a gold antenna on the gold-supported section.”

To see the polaritons, they use a scattering type of scanning near-field optical microscopy (s-SNOM), which involves a sharp metallic tip scanning over the sample surface and recording the light it scatters to map out the nanoscale fields of the polaritons in real-space.

“The coolest and most surprising part of our work was observing the pseudo-birefringence effect,” says Hu. “Seeing the different polariton orders physically split and travel in completely different directions was visually striking. Quantifying this effect and finding an equivalent birefringence of up to 41.8—so much larger than anything within natural crystals—was a genuine ‘wow’ moment. It immediately suggested practical applications in mode-sorting devices.”

Nanofabrication precision

Nanofabrication of the team’s device was a challenge because precision “was absolutely critical,” says Hu.

Their process first involves creating the gold/suspended hybrid substrate, which requires multiple steps of high-resolution electron-beam lithography and reactive ion etching to pattern the holes. “The biggest challenge is alignment,” Hu adds. “We had to transfer a single-crystal gold flake and precisely align its sharp edge with the boundary of the etched square hole. It was a delicate, deterministic transfer process done under a microscope. Such precision was essential because even slight misalignment or surface roughness could completely suppress higher-order mode conversion.”

Next, a thin slab gets transferred to perfectly bridge the gold and the suspended region, where any roughness or misalignment could ruin the effect, so the researchers used atomically smooth single-crystal gold flakes to minimize scattering losses.

A versatile platform for nanophotonics of the future

What does the team’s work mean for the optics and photonics world? By providing an efficient way to launch and control HoHPhPs, it establishes them as a versatile platform for new nanophotonic technologies. Being able to efficiently launch and route higher-order hyperbolic polaritons could reshape on-chip photonics and unleash unprecedented levels of mode-division multiplexing, ultracompact routing, and high-density signal processing.

“The key applications we envision involve creating ultracompact devices that can manipulate light on a chip,” says Hu.

And the strong pseudo-birefringence can be used to spatially separate different polariton modes—akin to a traffic controller for light at the nanoscale. “This ability to sort different orders of hyperbolic polaritons is a new tool for designing ultracompact photonic circuits,” Hu says. “It’s a crucial function for mode-division multiplexing to send different streams of information along the same pathway using different modes. Our findings provide a new strategy for designing devices like mode routers, splitters, and highly efficient polariton launchers for integrated nanophotonic circuits.”

What’s next?

Next up, the team plans the explore these systems at low temperatures. “Cooling the device should reduce scattering from thermal vibrations (phonons), which could lead to a significant enhancement in the propagation distance and quality factor of the HoHPhPs,” says Hu.

Their immediate next step is to “use our findings to design and demonstrate specific nanophotonics components,” Hu adds. “This includes creating prototypes for on-chip mode-division multiplexers and demultiplexers, which are crucial for increasing the information capacity of photonic circuits. Our work essentially provides a blueprint for these future devices. While it’s difficult to give a precise timeline for commercial use, the path forward for research is quite clear.”

FURTHER READING

N. Chen et al., Nat. Photon., 19, 1225–1232 (2025); https://doi.org/10.1038/s41566-025-01755-5.