Scientists Steer Infrared Light Through an Invisible Waveguide

Researchers at the University of Stuttgart and the Istituto Italiano di Tecnologia (IIT) in Milan have demonstrated that placing a nanoscale gold antenna on a molybdenum oxy-dichloride (MoOCl₂) crystal can direct infrared light along a single, narrow route without requiring any physically carved waveguides, according to findings published in Nature Nanotechnology.

How Natural Crystal Anisotropy Directs Light Without Waveguides

Light typically spreads outward in circular or spherical wavefronts from a source, forcing conventional optical systems to rely on manufactured waveguides like glass fibers or lithographically etched nanostructures to guide beams efficiently. To bypass these expensive manufacturing steps, Professor Harald Giessen of the 4th Physics Institute at the University of Stuttgart and Dr. Antonio Ambrosio of the IIT led a team investigating the natural properties of the two-dimensional material MoOCl₂.

According to the researchers, MoOCl₂ is a biaxial material with optical properties that differ sharply along separate crystal axes. Along one axis, the material behaves like a metal and supports surface plasmons—collective electron oscillations that transport electromagnetic energy. Along the perpendicular axis, it acts as a dielectric and restricts plasmon movement. This extreme directional difference, or anisotropy, confines plasmonic waves into a narrow path that researchers term plasmon canalization.

Mapping Plasmon Canalization Using Near-Field Optical Microscopy

To capture and map this behavior, doctoral researcher Farid Aghashirinov from the University of Stuttgart and postdoctoral fellow Andrea Mancini from the IIT used scattering-type scanning near-field optical microscopy (SNOM). The team positioned a nanoscale gold antenna on the MoOCl₂ crystal surface and illuminated it with an infrared laser to generate tightly confined optical waves.

The experiments utilized a tunable infrared laser developed by Stuttgart Instruments GmbH, a spin-off company from the University of Stuttgart. The team recorded the resulting near-field optical distributions using a SNOM microscope produced by attocube systems AG, another technology company with academic roots.

Controlling Optical Propagation via Wavelength Shifting

The shape and trajectory of the light waves change substantially depending on the infrared excitation wavelength, according to the experimental data. At approximately 4 µm, the team observed strongly directional canalized propagation where the light remained strictly confined within an invisible, natural channel.

When the wavelength increased to 5 µm, the wavefront returned to a conventional ring-like form resembling ripples moving evenly across water. At 3 µm, the wave developed an open hyperbolic shape instead. The researchers explain that the canalized state functions as an elliptical wavefront stretched so far that its focal points effectively move to infinity, allowing the system to transition between open and closed forms simply by tuning the excitation wavelength.

Implications for Integrated Photonics and Quantum Technologies

The demonstration of intrinsic plasmon canalization could provide a new foundation for integrated photonics, according to the study authors. By transporting light directionally without lithographically manufactured waveguides, the approach may simplify the production of photonic chips.

The mechanism could also broaden available designs for optical interconnects, nanoscale photonic circuits, and integrated quantum technologies. Co-authors Lin Nan, Giacomo Venturi, and Bettina Frank contributed to the research alongside Giessen, Ambrosio, Aghashirinov, and Mancini.

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