Fabrication-friendly polarization-sensitive plasmonic grating for optimal surface-enhanced Raman spectroscopy

Abstract

Plasmonic nanostructures are widely utilized in surface-enhanced Raman spectroscopy (SERS) from ultraviolet to near-infrared applications. Periodic nanoplasmonic systems such as plasmonic gratings are of great interest as SERS-active substrates due to their strong polarization dependence and ease of fabrication. In this work, we modelled a silver grating that manifests a subradiant plasmonic resonance as a dip in its reflectivity with significant near-field enhancement only for transverse-magnetic (TM) polarization of light. We investigated the role of its fill factor, commonly defined as a ratio between the width of the grating groove and the grating period, on the SERS enhancement. We designed multiple gratings having different fill factors using finite-difference time-domain (FDTD) simulations to incorporate different degrees of spectral detunings in their reflection dips from our Raman excitation (488 nm). Our numerical studies suggested that by tuning the spectral position of the optical resonance of the grating, via modifying their fill factor, we could optimize the achievable SERS enhancement. Moreover, by changing the polarization of the excitation light from transverse-magnetic to transverse-electric, we can disable the optical resonance of the gratings resulting in negligible SERS performance. To verify this, we fabricated and optically characterized the modelled gratings and ensured the presence of the desired detunings in their optical responses. Our Raman analysis on riboflavin confirmed that the higher overlap between the grating resonance and the intended Raman excitation yields stronger Raman enhancement only for TM polarized light. Our findings provide insight on the development of fabrication-friendly plasmonic gratings for optimal intensification of the Raman signal with an extra degree of control through the polarization of the excitation light. This feature enables studying Raman signal of exactly the same molecules with and without electromagnetic SERS enhancements, just by changing the polarization of the excitation, and thereby permits detailed studies on the selection rules and the chemical enhancements possibly involved in SERS.

Effect of molecular concentration on excitonic nanostructure based refractive index sensing and near-field enhanced spectroscopy

“…Such feature enables the application of nanoplasmonic structures in the development of RI sensors. 5,[7][8][9][10][11] Plasmonic modes (SPP, LSP, and PSLR) can also provide enhancement [12][13][14][15] and confinement [16][17][18] of light at the near-field close to the structure. Consequently, metallic nanostructures are widely exploited as resonant substrates for near-field enhanced spectroscopy ranging from weak [19][20][21][22] to strong [23][24][25] light-matter coupling.…”

Polarized SERS substrates with directionality, repeatability and orderability: an anisotropic Ag nanocavity array

“…When the angle between the electric field direction of the polarized light and the gap of the grating is near 90°, the largest electromagnetic SERS enhancement effect is obtained. 27…”

“…When the angle between the electric field direction of the polarized light and the gap of the grating is near 901, the largest electromagnetic SERS enhancement effect is obtained. 27 In recent years, there has always been an obstacle in the practical application of SERS that it is difficult to observe the Raman signal in the background of complex media or field conditions. It is a common technology to remove the background signal by changing the wavelength of the Raman signal, which mainly includes shift-excited Raman differential spectroscopy (SERDS) and modulated Raman spectroscopy.…”