A recent study published in Engineering digs into the intriguing world of nonlinear meta-devices, examining their potential to transform nanophotonics. The study, led by scientists from the City University of Hong Kong and the Hong Kong University of Science and Technology, provides a comprehensive overview of the most recent advances in this field.
Nonlinear excitation can be categorized into two types: natural and artificial. The natural type arises from the intrinsic nonlinear response of the material, while the artificial type stems from the asymmetric design of nanostructures. However, relying solely on either natural characteristics or structural design is insufficient to achieve an optimal nonlinear response. Optimization can be pursued from both excitation and collection viewpoints. Efficiency enhancement focuses primarily on improving efficiency through resonances, whereas radiation shaping aims to control the radiation pattern to increase collection efficiency. Furthermore, phase manipulation in nonlinear meta-devices can integrate the advantages of these two approaches, significantly enhancing the energy density of the nonlinear signal and enabling more complex manipulation in nonlinear processes. Finally, potential directions for nonlinear meta-devices including high-harmonic generation, nonreciprocity, time-varying systems, and quantum optics are discussed. Image Credit: Rong Lin
Nonlinear optics, which emerged in 1961 with the discovery of the second harmonic generation, has subsequently made considerable advances. However, the real magic takes place at the nanoscale. The negative consequences of phase mismatching between fundamental and harmonic waves are mitigated here, opening up new avenues for improving nonlinear optical responses.
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