Tuning Gold Nanoparticle Self-Assembly for Optimum Coherent Anti-Stokes Raman Scattering and Second Harmonic Generation Response

The research and development of new substrates for use in surface-enhanced spectroscopy is primarily motivated by the ability to tune such substrates to provide maximum signal enhancement and therefore lower detection limits. We examined a series of multilayer nanoparticle (NP) arrays with between 1 and 17 NP layers using two nonlinear optical (NLO) techniques: Coherent anti-Stokes Raman scattering (CARS) and second harmonic generation (SHG). The CARS signal of oxazine 720 was monitored at 1600 cm(-1) using a 709-nm pump beam and an 800-nm stokes beam. Maximum signal was observed for 11 NP layers and is attributed to the matching of the CARS signal and the substrate surface plasmon excitation at 637 nm. The CARS signal for the 3000-cm(-1) C-H stretching vibration showed maximum enhancement at 13 NP layers. The maximum SHG signal enhancement occurred at 13 NP layers, with a 50-fold overall enhancement of the SHG signal. We demonstrate that the NP-containing substrates can be tuned to provide maximum NLO response based on the number of NP depositions and the wavelength(s) involved in the NLO experiments. These multilayer NP arrays yield a stable and modular spectroscopic substrate advantageous for a variety of surface-enhanced spectroscopic techniques.