Surface Plasmon Phenomena in Terahertz Electromagnetic Systems

Surface plasmon phenomena in THz electromagnetic systems underlie the confinement and guiding of terahertz waves at metal–dielectric interfaces via collective oscillations of free electrons. Unlike optical frequencies, conventional metals exhibit higher losses and a weaker plasmonic response in the THz band, prompting the design of engineered surfaces—such as metamaterials, periodic gratings and dielectric-coated films—to support surface plasmon polaritons (SPPs) with enhanced field localisation and reduced radiative losses. Core challenges include efficient coupling of free-space THz radiation into surface modes, containment of absorption losses in metallic films and precise characterisation of interfacial properties. Recent advances in fabrication of subwavelength structures and refined theoretical models have unlocked new modalities for non-destructive sensing, high-resolution imaging and on-chip THz communication. Research has focused on the trade-off between plasmon confinement and propagation length, the influence of dielectric coatings on dispersion, and novel excitation schemes capable of bridging macroscopic air gaps in plasmonic circuitry. The global significance of THz plasmonics spans chemical and biological sensing, security screening, wireless data links and the integration of active components within lab-on-a-chip platforms.

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Experimental investigations have quantified the generation efficiency of THz surface plasmon polaritons using the end-fire coupling technique on gold surfaces coated with a submicrometre dielectric layer. By varying the thickness of a zinc sulfide coating on a curved gold segment, generation efficiencies up to 60 % were achieved experimentally, with theoretical predictions indicating potential increases to 80 % through optimisation of the SPP field profile and incidence conditions. These findings demonstrate a simple yet effective route to couple free-space THz radiation into confined surface modes for plasmonic circuitry.

A novel method to extract the effective dielectric permittivity of conducting surfaces in the THz range has been developed by analysing the dispersion of surface plasmon polaritons measured via interferometric techniques. The incorporation of a thin, subwavelength dielectric layer suppresses radiative losses, enabling more accurate determination of the metal’s permittivity. This approach provides a robust tool for the design and characterisation of integrated plasmonic components and communication lines at THz frequencies.

Foundational reviews of THz surface wave sensing have consolidated amplitude- and phase-based modalities across metals, semiconductors and structured interfaces. These studies compare surface plasmon polaritons, surface phonon polaritons and spoof surface waves, emphasising the balance between field localisation and sensor sensitivity. Detailed theoretical analyses have guided the selection of natural and artificial materials, informing the development of high-performance THz sensors for spectroscopy and imaging applications.

Surface Plasmon Phenomena in Terahertz Electromagnetic Systems publication trend

The graph below shows the total number of articles in surface plasmon phenomena in terahertz electromagnetic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Surface Plasmon Polariton (SPP): A coupled oscillation of free electrons and electromagnetic fields propagating along a metal–dielectric interface, confined within a subwavelength region.

Terahertz (THz): The region of the electromagnetic spectrum with frequencies from 0.1 to 10 THz, bridging microwaves and infrared waves.

End-fire coupling: A method of exciting surface plasmons by directing focused radiation onto the edge of a conducting surface, enabling efficient SPP generation.

Effective permittivity (εm): The macroscopic dielectric constant of a material as perceived by propagating surface plasmons, accounting for surface coatings and dispersion effects.

References

1. Surface Wave Enhanced Sensing in the Terahertz Spectral Range: Modalities, Materials, and Perspectives. Sensors (2019).
2. Evaluation of the Efficiency of Generation of Terahertz Surface Plasmon Polaritons by the End-Fire Coupling Technique. Photonics (2023).
3. Obtaining the Effective Dielectric Permittivity of a Conducting Surface in the Terahertz Range via the Characteristics of Surface Plasmon Polaritons. Applied Sciences (2023).
4. Experimental investigations into capability of terahertz surface plasmons to bridge macroscopic air gaps. Optics Express (2015).