image: Figure 1 | Principle and experimental results of long-propagation ghost polariton wave. (a) Schematic diagram showing how the polariton can be excited using a gold antenna. An incident laser beam, after hitting the edge of the gold antenna, gets transformed into ghost polariton that travels along the calcite surface. (b) Propagation lengths of ghost polariton modes, visualized in momentum space. Based on our theoretical prediction, the longest propagation length can easily exceed 100 microns. (c) Electric field distribution of the g-HP mode highlighted in (b). While the electric field extends deep inside calcite crystal, the energy flow goes along the interface between calcite and air. (d)(e) s-SNOM measurements using (d) circular disk antenna, and (e) triangular antenna. The linecuts, as well as curve-fitting results, are also plotted. Utilizing a triangular antenna leads to selective excitation of certain long-propagating modes, thus showing a much longer propagation length compared with a round disk antenna.
Credit: Manuka Suriyage et al.
Controlling light at the nanoscale is crucial for next-generation technologies such as faster computing and ultra-sensitive sensors. Scientists are keen to harness unique hybrid waves called “phonon polaritons” (micrometer-scale waves produced when light couples with crystal vibrations), generated when light interacts with specific crystalline materials. However, fully leveraging their potential demands precise control over their propagation as well as overcoming inherent high losses that restrict them to very short distances, a challenge that has limited their practical use.
A new paper published in Light: Science & Applications details a collaboration between scientists at the Australian National University and the University of Wisconsin–Madison, who have developed a novel method to control these exotic light–matter waves. The study focuses on ghost hyperbolic phonon polaritons—unusual waves that propagate in ray-like paths within natural crystals such as calcite, a transparent material known for its birefringence (where incoming light splits into two beams traveling at different speeds). While these ghost waves can extend deep into the calcite crystal, their energy is propagating along the calcite–air interface. The breakthrough involves using specially shaped gold nano-antenna smaller than the width of a human hair, fabricated directly onto the calcite surface to launch ghost polariton waves. These nano-antennas function like miniature mirrors, selectively exciting only the long-lived modes and overcoming previous design limitations.
Similar to how large antennas direct radio signals, these gold antennas serve as launchpads to shape and control light at the microscale. The researchers found that while a disk-shaped antenna scatters waves randomly, redesigning it into an asymmetric triangle provides exceptional control. Its sharp edges act as precisely angled mirrors, reflecting incident laser light and converting it into ghost polariton waves that travel in focused beams. This process, comparable to beamforming of RF signal, helps solving a key challenge in directing energy at the nanoscale.
Beyond steering, the team achieved another milestone by significantly extending the travel distance of ghost polariton waves. Typically, phonon polaritons dissipate quickly, with recent ghost polariton demonstrations limited to about 20 micrometers. By carefully designing their triangular antennas, the scientists tuned the launch process to selectively excite modes with lower losses, thereby greatly reducing energy waste. To verify this breakthrough, the team used advanced imaging with a scattering-type Scanning Near-field Optical Microscopy (s-SNOM) system. This technique employs an extremely fine tip to detect electromagnetic waves on the sample surface, achieving an ultra-high resolution far beyond the diffraction limit of conventional optical microscopes. Using this method, they confirmed that ghost polariton waves could propagate over 80 micrometers, a four-fold improvement over previous results.
This breakthrough opens exciting avenues for future technologies. For example, in high-performance computing environments, such as large-scale GPU clusters where effective heat management is critical, phonon polariton-assisted heat transport could channel thermal energy away from chip hotspots more efficiently than conventional methods, whose performance degrades at high temperatures. Other promising applications include quantum information systems, where reliably guiding quantum states over long distances is essential, as well as molecular sensors, where extended propagation enhances interactions with target molecules and improves detection sensitivity.
“Our work shows that by carefully engineering the launching nano-antenna, much like designing mirrors at the nanoscale, we can overcome previous limitations and unlock the potential of these unique ghost polaritons,” says Professor Yuerui Lu. “The principles we’ve demonstrated aren’t limited to calcite: they offer a general strategy that could be applied to other anisotropic materials, making them very promising for a new generation of nanophotonic devices.”
Journal
Light Science & Applications
Article Title
Long-propagating ghost phonon polaritons enabled by selective mode excitation