A recent study published in Engineering delves into the exciting realm of nonlinear meta-devices, exploring their potential to revolutionize the field of nanophotonics. The research, led by a team of scientists from the City University of Hong Kong and the Hong Kong University of Science and Technology, offers a comprehensive review of the latest advancements in this area.
Nonlinear optics, which emerged from the discovery of second harmonic generation in 1961, has since seen significant progress. However, it is at the nanoscale that the real magic happens. Here, the detrimental effects of phase mismatching between fundamental and harmonic waves are minimized, opening up new possibilities for enhancing nonlinear optical responses.
The researchers first explore the theoretical frameworks behind the nonlinear optical properties of meta-devices. They discuss how plasmonic and dielectric materials induce these properties. Plasmonic materials, despite their limitations such as thermal heating and high reflectivity, can enhance the field near the surface through resonance modes like surface plasmon-polaritons and localized surface plasmon resonance, thus improving the nonlinear response. On the other hand, dielectric nanostructures, with their non-inversion symmetric crystal structures in some cases, offer an alternative approach. Materials like zinc oxide (ZnO) and gallium arsenide (GaAs) are excellent candidates for second-order and third-order nonlinear processes respectively.
One of the key focuses of the study is on enhancing the nonlinear efficiency of meta-devices. By exciting strong resonant modes, these devices can boost the nonlinear efficiency at the subwavelength scale without the need for phase-matching as required in bulk crystals. For example, a hybrid metasurface integrating plasmonic meta-atoms with an epsilon-near-zero (ENZ) nanofilm achieved a 104-fold increase in second harmonic generation. Dielectric metasurfaces, with their high-Q resonances, are also promising for generating short-wavelength light, such as the ZnO-based metasurface that can produce vacuum ultraviolet light.
Another important aspect is radiation shaping. The harmonic wave generated by a nonlinear meta-device usually diffracts, leading to energy dispersion. But by shaping the radiation pattern, the device can produce directional radiation, enhancing the collection of nonlinear light energy. Strategies such as changing the pump polarization state, designing materials with specialized nonlinear tensors, and using asymmetric structures have been explored.
Nonlinear phase modulation is yet another area of interest. It combines the advantages of efficiency enhancement and radiation shaping. Meta-lenses, for instance, can generate and focus second-harmonic light, and also facilitate harmonic imaging and holographic display.
Looking ahead, the researchers identify several potential research directions. These include high-harmonic generation, which could extend the harmonic excitation to deep ultraviolet and X-ray regions; nonreciprocity, for enhanced manipulation in the spatial domain; time-varying systems for ultrafast modulation; and the integration of quantum optics, which could lead to the development of high-dimensional quantum-entangled optical devices.
The paper "Nonlinear Meta-Devices: From Plasmonic to Dielectric," authored by Rong Lin, Jin Yao, Zhihui Wang, Che Ting Chan, Din Ping Tsai. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.11.021
