Graphene-gold THz Metasurfaces Achieve 3.2% Third-Harmonic Generation Efficiency with Tailored Resonant Structures

Terahertz technology stands to benefit from materials exhibiting strong nonlinear optical properties, and researchers are actively seeking ways to enhance these effects for practical applications. Anna Theodosi, Isaac Appiah Otoo, and Anastasios D. Koulouklidis, working with colleagues from institutions including the Foundation for Research and Technology Hellas and Aalto University, now demonstrate a novel metasurface design that significantly boosts nonlinear responses at terahertz frequencies. Their work centres on combining graphene with gold structures, creating a readily fabricated surface that concentrates light and amplifies nonlinear interactions. The team predicts and experimentally confirms substantial third-harmonic generation efficiencies and observes large frequency shifts driven by self-phase modulation, paving the way for more efficient and compact terahertz photonic devices.

Graphene Metasurfaces for Terahertz Harmonic Generation

Researchers are developing innovative metasurfaces, artificial materials engineered to manipulate light, using graphene to generate terahertz radiation more efficiently. These structures, composed of tiny gold components integrated with graphene, enhance nonlinear optical effects, allowing for the creation of new frequencies of light. This research focuses on designing metasurfaces that maximize conversion efficiency by carefully controlling the structure and leveraging the unique properties of graphene. The design relies on achieving a ‘double resonance’ effect, where the metasurface responds strongly to both the original frequency of light and its tripled harmonic.

Graphene plays a crucial role due to its exceptional ability to interact with light in a nonlinear way. Scientists use computer simulations and theoretical models to understand and optimize the design, predicting how the metasurface will behave under different conditions. These advanced metasurfaces offer a pathway towards creating compact and efficient devices for manipulating terahertz radiation, unlocking new possibilities in terahertz photonics. This work builds upon existing research in metamaterials, graphene physics, and terahertz optics, paving the way for future advancements in the field.

Graphene-Enhanced Metasurface for Terahertz Nonlinearities

Scientists have engineered a hybrid metasurface, combining gold and graphene, to significantly enhance nonlinear optical properties at terahertz frequencies. The fabrication process involves precisely patterning gold structures onto a substrate using electron-beam lithography, followed by the deposition of a graphene layer. This structure supports localized resonances, amplifying the nonlinear interactions necessary for efficient frequency conversion. Terahertz time-domain spectroscopy revealed pronounced nonlinear frequency shifts of up to 0. 5 terahertz, representing a 12. 5% fractional change in frequency, directly linked to self-phase modulation. This meticulous approach validates the theoretical design and demonstrates the potential of this technology for advanced terahertz photonics, offering a promising route towards creating compact and efficient nonlinear devices for terahertz applications.

Graphene Metasurface Boosts Terahertz Nonlinearity

Researchers have developed a hybrid metasurface combining gold patches and a uniform graphene layer to significantly enhance nonlinear optical properties at terahertz frequencies. This design offers a fabrication-friendly approach, avoiding the need for complex graphene patterning while still achieving strong resonant effects. Theoretical analysis predicts third harmonic generation efficiencies reaching 3. 2% under continuous-wave excitation, surpassing previously reported values for patterned graphene metasurfaces. The team fabricated the metasurface and characterized it using terahertz time-domain spectroscopy under pulsed excitation.

Experiments revealed pronounced nonlinear frequency shifts of up to 0. 5 terahertz, driven by self-phase modulation, which aligns with simulation results. This 0. 5 terahertz shift represents a 12. 5% fractional decrease in resonant frequency, demonstrating a substantial nonlinear response.

The design strategically positions two resonances at harmonically related frequencies to maximize the efficiency of third harmonic generation. Researchers achieved these results by integrating gold patches onto a graphene layer supported by a gold back-reflector. This configuration excites graphene surface plasmons, creating two well-defined resonances within the 0-14 terahertz frequency range. By carefully controlling the dimensions of the gold patches, the team precisely tuned the resonant frequencies to align with the fundamental and third harmonic frequencies, enabling enhanced nonlinear processes.

Terahertz Nonlinear Metasurface Demonstrates Efficient Harmonics

This work demonstrates a hybrid metasurface combining gold patches with graphene, offering a practical route to creating nonlinear optical devices at terahertz frequencies. The researchers successfully fabricated and characterized this structure, predicting and observing strong nonlinear responses, specifically third harmonic generation efficiencies reaching 3. 2% under continuous-wave excitation at relatively low intensities. Experimental measurements reveal significant nonlinear spectral shifts of up to 0. 5 terahertz, attributed to self-phase modulation, and these results align closely with theoretical simulations. Researchers acknowledge that the metasurface’s performance is affected by the angle of incidence, particularly for TM polarization, and deviations from normal incidence can reduce efficiency. Nevertheless, these findings highlight the potential of carefully designed graphene-based metasurfaces as a versatile platform for developing advanced nonlinear photonic devices.