Nonlinear optical processes in the terahertz (THz) range are drawing interest for their potential in wireless communication and signal processing. One of the more intriguing effects in this area is harmonic generation, which converts optical energy into new frequencies that could open additional communication channels. Graphene — a single layer of carbon atoms arranged in a hexagonal lattice — has attracted attention for this purpose thanks to its strong nonlinear properties and ease of integration into compact devices. However, in its single-layer form, graphene produces relatively weak harmonics due to a short light-matter interaction length, so real-world applications remain elusive.
A multicycle driving field at frequency ω passes through a lowpass filter and excites a nonlinear graphene-based sample, generating a third harmonic at 3ω. A highpass filter then selectively removes the remaining driving field, reducing the overall signal and improving sensitivity to the third harmonic. The sample itself is made of two stacked graphene layers (shown as black honeycomb lattices) with top and bottom electrodes (gold bars) and a polymer layer on the top surface serving as an electric gate (semi-transparent square).
A new study published in Light: Science & Applications presents possible ways around this roadblock. Led by Professor Jean-Michel Ménard at the University of Ottawa and collaborators in Germany, the research team experimented with a multilayer graphene design, stacking several decoupled graphene sheets to boost the interaction length. This strategy enhanced third harmonic generation (THG) more than 30 times than single-layer graphene. The team also notes that higher-frequency harmonic generation could see similar improvements, though they caution that the optimal number of layers involves balancing nonlinear interactions with linear absorption.
They integrated electrodes into these layered designs, enabling them to fine-tune graphene’s doping concentration and refine its nonlinear response. By applying a gate voltage, the researchers increased the THG process by a factor of three. A third set of experiments used plasmonic metasurface substrates as resonators to intensify the local THz field and further raise harmonic generation. A bandpass resonator design proved the most effective among the tested approaches.
While the findings look promising, the researchers emphasize that further work is needed. They relied on a tabletop THz system with custom lowpass and highpass filters to isolate the third harmonic frequency, suggesting that these techniques might need refining before being used in commercial or industrial settings.
Still, the results point to potential device architectures — combining multiple graphene layers, electrical gating, and metasurface substrates — that could increase harmonic generation by more than two orders of magnitude. “This platform offers the possibility to explore a vast range of materials and potentially identify new nonlinear mechanisms,” the authors write on . If validated by further studies, these advancements could bring efficient, chip-integrated THz converters closer to reality, offering faster data processing options for future communication technologies.
Graphene’s promising use in electronics has been covered by R&D World over the years, including in this article.
