Nanometer-Resolved Images from Superconducting Technology
The energy deposited in a superconductor by a single photon can register a detectable signal, which is why superconductors are employed in some extremely sensitive detectors. Now researchers have shown how to use this sensitivity to create maps of the superconducting properties of a material with nanometer resolution [1]. The technique can also detect polaritons—hybrid light–matter excitations that may be useful in quantum technologies—with higher resolution than earlier methods. The researchers expect the new technique to be useful in fields as diverse as quantum information and nanophotonics.
When a superconductor held just below its critical temperature absorbs a single photon, the superconductivity can be destroyed in a small region of the material, triggering a small electrical signal. Recent advances have expanded the operating temperatures of such detectors and improved their sensitivities to photons over a wide range of frequencies, enabling many new applications. Mengkun Liu of Stony Brook University in New York and colleagues wondered if the same sensitivity might be employed to build high-resolution spatial maps of the properties of superconducting samples. “Spatial variations often influence superconducting strength and coherence, so an ability to image these properties locally would bring valuable insight,” says Stony Brook team member Ran Jing.
Current microscopy methods use electromagnetic waves scattered from a tiny needle-shaped metallic probe, or “tip,” to induce local electromagnetic responses in a sample. Researchers can use those responses to build highly detailed maps of material properties, particularly in semiconductors. But this technique often lacks the sensitivity needed to detect spatial variations in superconductors or other materials that typically respond less strongly to incident electromagnetic waves than semiconductors do.
So Liu and colleagues developed a scanning-probe-based microscopy technique tailored for superconductors. In their setup, an infrared beam scatters from a metallic tip that hovers above the sample surface. If absorbed, the light can suppress superconductivity near the tip. This suppression generates a measurable voltage or current across electrodes situated nearby. Crucially, the critical temperature varies slightly from place to place as a result of crystal defects, sample geometry, and anything else that can strengthen or weaken superconductivity. In experiments, the researchers repeat the scan at a range of temperatures while monitoring the signal that indicates a loss in superconductivity. The results can provide a spatial map of properties such as resistivity or local quantum coherence.
The team demonstrated the technique using a bow-tie-shaped sample of the superconductor niobium that had a 200-nm-wide strip, or “nanobridge,” at the center of the bow tie. The results showed that the nanobridge lost its superconductivity at a lower temperature than other regions, likely a result of its higher current density.
The nanobridge properties were the focus of this experiment, but the technique could be useful in other ways, Jing says. “The actual material under study can be the superconductor itself, or it can be another material layered on top of—or in proximity to—the nanobridge.”
Liu and colleagues also demonstrated another use for their technique: detecting polaritons, which are promising candidates for use in quantum technologies. Polaritons are hybrid particles—part light and part electric dipole oscillations—and are difficult to detect because they are small and confined to the surface. The researchers directed midinfrared laser light onto hexagonal boron nitride (h-BN), a material known to support polaritons that travel along the surface. They placed the nanobridge beneath the thin h-BN film. As the polaritons passed over the bridge, small electric-field variations from the polaritons caused changes in heat or current flow in the superconductor, producing a measurable electrical signal reflecting the polaritons’ passage. This polariton-detection approach requires 10,000 times less power than previous techniques, which allows higher-resolution imaging.
This work “opens up the possibility to produce spatially resolved images of delicate devices or materials using very weak irradiation,” says quantum matter specialist Justin Song of the Nanyang Technological University in Singapore. “It may be most useful for probing superconductors and other systems with fragile order where too much light could destroy the ordering.”
The researchers hope to eventually image a wider class of materials and devices and to study a range of polaritonic excitations. These systems may include twisted graphene devices and some new superconductors, Jing says. The technique may also be useful for characterizing superconducting devices used in qubits, where microscopic surface defects or areas of oxidation can alter superconducting properties. The ability to detect polaritons will also help in engineering quantum devices that are based on these particles, he says.
–Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Abergavenny, UK, and Notre Dame de Courson, France.
References
- R. Jing et al., “Bolometric superconducting optical nanoscopy (BOSON),” Phys. Rev. X 15, 031027 (2025).