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Plasmonics that utilizes the interaction of light with charged particles, such as electrons in metals, has been an area of interest for decades. As the dimensions of the plasmonic device have been shrunk into the nanoscale, the quantum confinement effects need to be considered within such small devices. Here, we choose the ultra-thin metallic film to study the quantum confinement effect on the nonlinear properties. Due to the quantum confinement, fruitful intersubband transitions (ISBT) exist in this metallic quantum well (MQW). The dipole transition elements associated to these ISBT are on the order of enanometer, which is much larger than those of the traditional nonlinear crystals. Therefore, giant third-order nonlinearity has been achieved in the 3 nm Au quantum well. The χ(3) reaches up to (0.49 + 2.0i) × 10-15 m2V-2, which is almost four-order of magnitude higher than the case of the bulk film. Furthermore, by using epitaxial-growth method of the TiN/Al2O3 heterostructures, coupled MQW (cMQW) has been utilized to support the large second-order susceptibility up to 1500 pm/V. With such ultrahigh nonlinearities and atomic-flat quality of the TiN MQW, TiN/Al2O3 heterostructures based hyperbolic metamaterials has been implemented for the applications of the pulse limiter. In addition, when combined with the nano-photonic waveguides, such as photonic crystals, on-chip super-continuum light sources and/or frequency comb can be realized. Last but not least, due to the ultrafast response of both χ(2) and χ(3), tunable metasurface and/or on-chip optical modulators with exceptional performance are on the horizon. Implementing the MQW opens a new regime for engineering extraordinary optical nonlinearities and novel applications.
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