Localized surface plasmon resonances (LSPRs), arising from the interaction of light with noble metal nanostructures, can be described as the collective oscillations of free conduction electrons.In the past decade, LSPRs in noble metal nanostructures have been combined with nonlinear optics that brings a new concept named “nonlinear plasmonics”.Many nonlinear processes can be boosted based on the strong local field enhancement, including Raman scattering, second- and third-harmonic generation (SHG and THG) and four-wave mixing.Asa fundamental and important nonlinear optical effect,SHGin metal nanostructures has been studied widely.Generally,the mostpopular wayto promote SHG conversion efficiency in well-designed plasmonic nanostructuresis making use ofthe resonance enhancement effect.However, SHG is limited by the non-centrosymmetry requirement of the material structure as the nature of even-order optical nonlinearity, leading to a rather weak SHG signal in centrosymmetric nanostructures even in the strong resonance condition. Since the resonance enhancement effect is usually prominent, this relatively small contribution from the asymmetry of the nanostructureis easily neglected or covered by the strong resonance enhancement factors.
The ultrafast optics group led by Prof. Peixiang Lu presents a novel approach to quantitatively extract the contribution of asymmetric local-field from the strong resonance enhancement effect by a white-light supercontinuum signal.The experimental and calculated results all reveal that the pure effectiveχ(2)increasesas the asymmetric degree of local-field distribution increase. Thisillustrates the importance of asymmetric local-field toχ(2), which provides a new evidence for the nonlinear plasmonics design in metal nanostructures.
This work is published on Opt. Express Vol. 25, No. 2,1296(2017). This work was supported bythe973 Programs (2014CB921301),National Natural Science Foundation of China (NSFC) (11204097),Doctoral fund of Ministry of Education of China (20130142110078)andFundamental Research Funds for the Central Universities (HUST: 2016YXMS015).
Figure Thelocal-fielddistribution and theSH intensityasafunction oftheasymmetric degree of thecross-shape nanohole.
