Creating “one-way lanes” for light in plasmonic structures

A new paper by ECE Assistant Professor Francesco Monticone explains the conditions necessary to realize unidirectional propagation of light in plasmonic structures.

The paper, titled “A truly one-way lane for surface plasmon polaritons,” is published in Nature Photonics, the premiere journal of optoelectronics, laser science, imaging and communications in the field of photonics. 

While conventional photonic devices are typically made with dielectric materials such as glass, plasmonic devices take advantage of the optical properties of metals. The field of plasmonics is sometimes referred to as “metal optics.” 

“The advantage of plasmonic devices,” Monticone said, “is the possibility to squeeze and confine light to dimensions much smaller than its wavelength, dimensions that start becoming comparable with the size of modern nano-electronic devices.”

The realization of ultra-small one-way waveguides carrying light in a preferential direction, without back-scattering light in the opposite direction, would open new opportunities to control and route light at the nanoscale, enhancing its interactions with new opto-electronic materials and nano-electronic devices. 

Monticone’s paper discusses the challenges that arise when attempting to create “one-way lanes” for light in plasmonic structures, in which wave propagation is allowed in one direction, but forbidden in the opposite direction.

“Light has a tendency to propagate symmetrically in opposite directions,” said Monticone. “Breaking this  symmetry, known as Lorentz reciprocity, is challenging. It requires “biasing” a system with certain external quantities, for example a static magnetic field.” 

Strong forms of nonreciprocity, in particular the unidirectionality explored in this new paper, are even more difficult to produce, but create opportunities for the development of novel devices. Some devices and functionalities can only be created with unidirectional light, such as on-chip optical isolators used to protect a light source (for example, a laser cavity) from back-reflections, and on-chip optical circulators able to route signals in a specific direction depending on the input channel. 

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