Bridged-bowtie and cross bridged-bowtie nanohole arrays as SERS substrates with hotspot tunability and multi-wavelength SERS response.

In this paper, we present bridged-bowtie nanohole arrays and cross bridged-bowtie nanohole arrays in a gold thin film as surface enhanced Raman scattering (SERS) substrates. These SERS substrates not only exhibit large electromagnetic enhancement of SERS but also have the SERS enhancement spread over a much larger area than what could be present in SERS substrates consisting of nanopillar arrays or nanopillar plasmonic nanoantennas. Numerical simulations based on the finite difference time domain (FDTD) method are employed to determine electric field enhancement factors (EFs) and therefore the electromagnetic SERS enhancement factor for these SERS substrates. It was observed that bridged-bowtie nanohole arrays and cross bridge-bowtie nanohole arrays exhibit a highest electromagnetic SERS enhancement factor (EF) of ~10, which is orders of magnitude higher than what has been previously reported for nanohole arrays as SERS substrates. This electromagnetic SERS EF (of ~10) is spread over a hotspot region of ~100 nm (in each periodic unit of the array), which is larger than the case of nanopillar arrays. In addition, it was observed that an electromagnetic SERS enhancement factor of at least 10 is spread over a large area (500 nm in each periodic unit of the array), thus increasing the average enhancement factor. It was observed that the bridged-bowtie nanohole arrays and the cross bridged-bowtie nanohole arrays can be employed as effective SERS substrates in both the transmission mode and the reflection mode. The resonance wavelength of these arrays of nanoholes can be tuned by altering the size of the nanoholes. The effects of varying the gold film thickness and the diameter of the bridged-bowtie nanoholes forming the arrays were also analyzed. The bridged-bowtie nanohole arrays and cross bridged-bowtie nanohole arrays exhibit very high electric field enhancement factors (EFs) at more than one wavelength, and can therefore be used to obtain a multi-wavelength SERS response. Moreover, the cross bridged-bowtie nanohole array allows the tunability of the position of the hotspot with the rotation of the direction of the polarization of incident field.