The researchers started by looking at the theoretical frameworks that underpin meta-devices’ nonlinear optical features. They explain how plasmonic and dielectric materials cause these behaviors. Despite their limitations, such as thermal heating and high reflectivity, plasmonic materials can improve the field near the surface via resonance modes such as surface plasmon-polaritons and localized surface plasmon resonance, hence increasing nonlinear responses.
On the other hand, dielectric nanostructures, which have non-inversion symmetric crystal structures in some situations, provide an alternative method. Zinc oxide (ZnO) and gallium arsenide (GaAs) are ideal for second- and third-order nonlinear processes, respectively.
One of the primary goals of the research is to improve the nonlinear efficiency of meta-devices. By stimulating strong resonant modes, these devices can increase nonlinear efficiency at the subwavelength scale without the need for phase-matching, which is necessary in bulk crystals.
For example, a hybrid metasurface that combined plasmonic meta-atoms with an epsilon-near-zero (ENZ) nanofilm increased second harmonic generation by 104 times. Dielectric metasurfaces, with their high-Q resonances, are also intriguing for generating short-wavelength light, such as the ZnO-based metasurface capable of producing vacuum ultraviolet light.
Another crucial consideration is radiation shaping. The harmonic wave produced by a nonlinear meta-device typically diffracts, resulting in energy dispersion. However, by controlling the radiation pattern, the device can generate directional radiation, which improves the collection of nonlinear light energy. Strategies such as varying the pump polarization state, developing materials with particular nonlinear tensors, and employing asymmetric structures have been investigated.
Nonlinear phase modulation is another topic of interest. It combines the benefits of efficiency enhancement with radiation shaping. Meta-lenses, for example, may produce and focus second-harmonic light while also enabling harmonic imaging and holographic display.
The researchers list a number of possible future lines of inquiry. These include time-varying systems for ultrafast modulation; nonreciprocity for improved manipulation in the spatial domain; high-harmonic generation, which could extend the harmonic excitation to deep ultraviolet and X-ray regions; and the integration of quantum optics, which could result in the creation of high-dimensional quantum-entangled optical devices.
Journal Reference:
Lin, R. et. al. (2025) Nonlinear Meta-Devices: From Plasmonic to Dielectric. Engineering. doi.org/10.1016/j.eng.2024.11.021