Role of multipolar plasmon resonances during surface-enhanced Raman spectroscopy on Au micro-patches

Abstract

The enhancement of a Raman signal by multipolar plasmon resonances – as opposed to the more common practice of using dipolar resonances – is investigated. A wide range of gold stars, triangles, circles and squares with multipolar resonances in the visible region were designed and then produced by electron beam lithography. We used 633 nm excitation and Rhodamine 6G as a probe molecule to confirm that, although the dipolar resonances of these shapes lie well into the infrared, SERS in the visible can still be obtained by coupling to their ‘dark mode’ multipolar resonances. However, the magnitude of enhancement in any given shape varied significantly and stochastically. Electromagnetic simulations were used to probe this surprising phenomenon further. These revealed that the presence or absence of a multipolar plasmon resonance at an particular excitation wavelength depended critically on the size and symmetry of the shape being examined, with a good SERS response only possible if the peak of the multipolar resonance is aligned with the 633 nm laser excitation.