Near-field engineering of Fano resonances in a plasmonic assembly for maximizing CARS enhancements.

Jinna He,Pei Ding,Chunzhen Fan,Erjun Liang,Shuangmei Zhu

doi:10.1038/srep20777

Abstract

Surface enhanced coherent anti-Stokes Raman scattering (SECARS) is a sensitive tool and promising for single molecular detection and chemical selective imaging. However, the enhancement factors (EF) were only 10~100 for colloidal silver and gold nanoparticles usually used as SECARS substrates. In this paper, we present a design of SECARS substrate consisting of three asymmetric gold disks and strategies for maximizing the EF by engineering near-field properties of the plasmonic Fano nanoassembly. It is found that the E-field “hot spots” corresponding to three different frequencies involved in SECARS process can be brought to the same spatial locations by tuning incident orientations, giving rise to highly confined SECARS “hot spots” with the EF reaching single-molecule sensitivity. Besides, an even higher EF of SECARS is achieved by introducing double Fano resonances in this plasmonic nanoassembly via further enlarging the sizes of the constituent disks. These findings put an important step forward to the plasmonic substrate design for SECARS as well as for other nonlinear optical processes.

Highlights

Coherent anti-Stokes Raman scattering (CARS), a well-known tool in multiphoton imaging and nonlinear spectroscopy, has been widely used in molecular identification and vibrational bioimaging since the late nineties[1]
It is found that the E-field “hot spots” of the trimer pointing at a specific spectral position of Fano resonance could be tuned by varying the incident angle of excitation light, and the pumping, Stokes and anti-Stokes photons are in resonance in the same spatial position with the supported trimer substrate, leading to the highly confined surface enhanced CARS (SECARS) “hot spots” with the enhancement factors (EF) higher than that of no near-field optimization
When the gap distance is decreased, the amplitude of E-field “hot spots” at the gaps will be enhanced dramatically owing to the increased near-field coupling between the constituent disks[34,35], thereby resulting in a significant increase of SECARS EF

Summary

Introduction

Coherent anti-Stokes Raman scattering (CARS), a well-known tool in multiphoton imaging and nonlinear spectroscopy, has been widely used in molecular identification and vibrational bioimaging since the late nineties[1]. Double Fano resonances are demonstrated in this trimer by further enlarging the sizes of the constituent disks, which produces an even higher SECARS enhancement These findings put an important step forward to the plasmonic substrate design for SECARS as well as for other nonlinear optical processes, such as four wave mixing and stimulated Raman scattering, etc., in which multi-photons with different frequencies are involved and multi-frequency resonance and “hot spots” spatially overlapping are required

Methods

Results

Conclusion