A polarization-insensitive plasmonic SECARS substrate with multiple hot spots

A plasmonic substrate providing high, reproducible and stable Raman signals should be highly desirable for the development of surface enhanced coherent anti-Stokes Raman spectroscopy (SECARS). In this work, we theoretically present a design of SECARS substrate consisting of five different-sized gold nanodisks and investigate its enhancement properties under different excitation polarizations by using finite element method. The numerical results reveal that the pentamer supports a polarization-independent Fano-resonant scattering spectrum due to its symmetric geometrical arrangement. Multiple electromagnetic hot spots produced by the Fano resonance are overlapped spatially at three characteristic frequencies involved in SECARS process. Consequently, the theoretically estimated overall enhancement factor (EF) of SECARS nearly keeps the same order of magnitude up to ～10¹⁴ for any horizontally excitation polarizations, and the relative root mean square error of the logarithm of the overall EF (Log₁₀EF) is less than 2%. Giant and polarization-insensitive SECARS enhancements enable the pentamer structure to be promising for plasmonic substrates in SECARS as well as other enhanced nonlinear optical process.