An international collaboration of scientists has unveiled a groundbreaking technique to excite and manipulate highly confined light-matter waves known as higher-order hyperbolic phonon polaritons (HPhPs). This novel methodology achieves unprecedented efficiency in excitation and propagation quality, significantly advancing the frontier of nanoscale photonics. Published in Nature Photonics, this breakthrough introduces an innovative two-step excitation mechanism combined with sharp boundary scattering, enabling a remarkable pseudo-birefringence effect that spatially separates and directs different polariton modes. This discovery lays the foundation for revolutionary on-chip optical devices capable of high-speed data processing and ultra-sensitive molecular detection.
In the realm of photonics, polaritons represent hybrid quasi-particles emerging from the strong coupling between photons and material excitations such as plasmons or phonons. They are celebrated for their exceptional ability to confine and manipulate light far beyond the diffraction limit, effectively compressing electromagnetic waves into dimensions significantly smaller than free-space wavelengths. Among polaritons, higher-order hyperbolic phonon polaritons stand out due to their increased confinement and extraordinary momentum characteristics, yet their practical exploitation has remained elusive due to challenges in efficient excitation.
Traditional single-step excitation strategies fall short in providing the large momentum boost required to access higher-order modes, severely limiting their propagation length and coherence. Addressing this, the research team, spearheaded by experts from Shanghai Jiao Tong University and the National Center for Nanoscience and Technology in China, in partnership with CIC nanoGUNE and ICFO in Spain, devised an ingenious two-step excitation scheme. The process initiates with a nanoscale gold antenna illuminated by light, which generates a fundamental (zero-order) hyperbolic phonon polariton on a smooth biaxial molybdenum trioxide (MoO₃) crystal layered atop a single-crystalline gold substrate.
The critical innovation emerges as this zero-order polariton wavefront propagates toward an abrupt termination of the gold substrate, where the MoO₃ crystal crystal becomes suspended in air. At this precisely engineered sharp gold-air interface, the fundamental mode undergoes scattering, which imparts the significant momentum leap necessary to spawn higher-order phonon polariton modes. This mechanism substantially enhances excitation efficiency compared to conventional methods, opening a pathway to explore the rich physics associated with these elusive modes.
Prof. Rainer Hillenbrand, co-lead author of the study, highlights, “The scattering of the zero-order polariton at the nanoscale boundary delivers the momentum augmentation critical for accessing higher-order modes, overcoming longstanding bottlenecks in polariton research.” The surpassing of excitation efficiency barriers ushers in unprecedented clarity and propagation length, as demonstrated by the observation of extraordinarily high quality factors around 45, and extended coherent propagation distances that herald potential integration into next-era photonic circuits.
Beyond sheer excitation performance, the team discovered a striking phenomenon termed “pseudo-birefringence,” a mode-selective steering effect emerging precisely at the gold-air boundary. Unlike traditional birefringence, which relies on polarization-dependent refractive index differences within anisotropic materials, this pseudo-birefringence arises from spatial mode sorting without altering the inherent polarization of the polaritons. Here, the fundamental and higher-order modes diverge into distinct propagation directions with sharply different angles, enabling a nanoscale traffic-control architecture for light.
Prof. Qing Dai, who co-led the research, describes this effect: “Our system functions as a sophisticated light routing mechanism at nanoscales, enabling selective mode division and directed propagation. The pseudo-birefringence effect here is more than an order of magnitude stronger than classical birefringence phenomena, yet operates without polarization changes.” This capability to spatially separate hyperbolic polariton modes based on their order lays a powerful foundation for mode-division multiplexing on chip-scale photonic platforms.
Mode-division multiplexing—a technique that multiplexes distinct spatial mode profiles to carry multiple data streams simultaneously—could greatly enhance the capacity of on-chip optical interconnects by leveraging the mode-sorting behavior established by pseudo-birefringence. Alongside this, the architecture opens avenues for compact photonic components including tunable optical filters and waveplates, as well as sensors with ultra-high sensitivity due to enhanced optical field confinement and mode selectivity.
Central to achieving these results was the implementation of an ultra-smooth and low-loss MoO₃ slab suspended in air, which minimized phonon scattering and non-radiative dissipation. This meticulous sample preparation ensured that the higher-order modes could sustain longer lifetimes and propagate further, critical aspects for practical device integration where signal loss is a major concern.
The implications of this work extend far beyond academic curiosity. In the context of nanophotonics and on-chip optical circuitry, mastering the excitation, propagation, and directional control of higher-order hyperbolic phonon polaritons offers unprecedented opportunities to miniaturize and enhance the speed of optical communication and processing systems. This could transform applications ranging from ultrafast computing hardware, to biosensing platforms capable of detecting trace chemical signatures with exceptional specificity.
Moreover, the methodology exemplifies the profound impact of combining sophisticated nanoscale engineering with fundamental physics of light-matter interaction. By exploiting engineered boundaries and advanced antenna design, the researchers have unlocked a new dimension in polariton-based photonics. The mode-sorted polariton waveguide approach epitomizes how nanoscale structures can be used not only to confine light but to exercise fine control over its propagation characteristics, opening a suite of design possibilities for emerging photonic technologies.
Looking forward, the team envisions exploring the versatility of their approach across different material platforms and excitation wavelengths, tailoring polariton behavior for specific functional requirements. The integration of such polaritonic circuits with complementary electronic or optoelectronic components could accelerate the development of hybrid systems capable of meeting the growing demands for speed, bandwidth, and miniaturization in information and sensing technologies.
This transformative advance in the control and excitation of hyperbolic phonon polaritons offers a compelling vision for the future of nanoscale light-matter interaction, setting the stage for a generation of photonic devices that marry outstanding optical performance with practical scalability. By harnessing pseudo-birefringence and multi-step excitation, the work redefines what is achievable at the crossroads of nanotechnology, quantum optics, and materials science.
Subject of Research: Higher-order hyperbolic phonon polariton excitation and manipulation using two-step scattering mechanisms in biaxial MoO₃ crystals on gold substrates.
Article Title: Efficient excitation and pseudo-birefringent control of higher-order hyperbolic phonon polaritons for on-chip nanophotonics.
News Publication Date: Not explicitly stated in the source; refer to DOI for publication date.
Web References: DOI: 10.1038/s41566-025-01755-5
Image Credits: Na Chen, Hanchao Teng, and Hai Hu.
