A group of scientists from the University of Ottawa in Canada, along with colleagues and researchers from the University of Bayreuth in Germany, has shown new ways to greatly improve THz nonlinearities in graphene-based structures in a study published in Light: Science & Applications.
Schematic of the experimental configuration to generate and detect THz third harmonic generation. A multicycle driving field at the frequency w is transmitted through a lowpass filter. Inside a nonlinear graphene-based sample, this field generates a third harmonic at frequency 3ω. A highpass filter then selectively blocks the residual driving field, lowering the overall signal reaching a detection scheme (not shown) and, thus, increasing the detection sensitivity to the third harmonic signal. The nonlinear sample is a stack of two graphene layers (black honeycomb lattices) with electrodes (gold bars at top and bottom) and a polymer layer on the top surface acting as the electric gate (semi-transparent square). Image Credit: Ali Maleki, Moritz B. Heindl, Yongbao Xin et al.
The potential of nonlinear optical processes in the technologically significant terahertz (THz) spectral range to transform industries like signal processing and wireless communication has drawn more and more attention. Harmonic generation, a process that transforms optical energy into various frequencies capable of creating new communication channels and speeding up the information transfer rate, is one of the most extreme phenomena in nonlinear optics. Professor Jean-Michel Ménard led the study.
A promising material for this technology is graphene, a hexagonal layer of single carbon atoms that provides remarkable nonlinear properties and easy integration with small, scalable devices. However, a significant obstacle to practical applications is the relatively weak harmonics produced in single-layer graphene, mostly caused by an inherently short light-matter interaction length. Researchers are creating novel strategies to improve nonlinear effects and maximize graphene's special qualities to overcome this obstacle.
The researchers' multilayered graphene design increases the interaction length between a driving THz field and the nonlinear sample by stacking multiple decoupled graphene sheets. Compared to single-layer graphene, this method produced a notable increase in third harmonic generation (THG), with enhancements of more than 30 times.
It is also possible to anticipate comparable improvements in harmonic generation at higher frequencies. This frequency conversion process required the team to find the ideal balance between linear absorption and nonlinear interactions to determine the ideal number of layers.
The team experimented with multilayered designs and incorporated electrodes into these structures to adjust graphene's doping concentration and, consequently, its nonlinear response. The THG process could be further optimized up to a factor of three by controlling the free carrier density by applying a gate voltage. These experiments show that frequency conversion in real-world multilayered samples can be dynamically controlled.
In a third set of tests, the group locally enhanced the THz field inside the graphene-based devices using plasmonic metasurface substrates. By acting as resonators, these metasurfaces increased the THz driving field's intensity and the efficiency of harmonic generation. It was shown that a bandpass resonator design was the most successful of the various plasmonic metasurface types.
Interestingly, the researchers employed creative experimental techniques to investigate these nonlinear effects using a table-top THz system. To maximize detection sensitivity at the third harmonic frequency, they notably used custom lowpass and highpass filters to regulate the spectrum of the THz driving field.
These experiments show that harmonic generation efficiency can be increased by more than two orders of magnitude using a device architecture that combines electrical gating, metasurface substrates, and a multilayered design.
“This platform offers the possibility to explore a vast range of materials and potentially identify new nonlinear mechanisms,” explained the researchers.
To improve THz frequency conversion methods and eventually incorporate this technology into applications, such research and development is essential. This is especially true for the development of effective, chip-integrated nonlinear THz signal converters, which will power future communication systems.
Journal Reference:
Maleki, A., et al. (2025) Strategies to enhance THz harmonic generation combining multilayered, gated, and metamaterial-based architectures. Light: Science & Applications. doi.org/10.1038/s41377-024-01657-1