Raman spectroscopy identifies molecules based on their vibrational finger
print. However, its sensitivity is limited due to the small scattering cross-se
ction. To address this, SERS utilizes localized surface plasmons to enhance
the light-matter interaction by several orders of magnitude. By increasing
the excitation frequency to deep-UV(DUV) and introducing resonance eff
ects, the Raman signals can be significantly boosted.
Using finite difference time domain analysis, we determined the optimal d
iameter and periodicity of the Al plasmonic nanohole array to achieve LSP
R at 266 nm, matching the wavelengths of the incident laser and oligonucl
eotide absorption. The nanohole diameter was set at 100 nm for easier fabrication, while the periodicity was swept between 150 and 250 nm to opt
imize the LSPR at 266 nm. At the optimized periodicity of 200 nm, the Al p
lasmonic nanohole array exhibited a reflectance minimum at 266 nm. Imp
ortantly, this optimized design not only simplified fabrication but also res
ulted in a high density of hot spots in the laser excitation area, further enh
ancing the sensitivity and performance of the substrate.
We epitaxially grow an Al film on a sapphire(Al2O3) substrate, by using a
plasma-assisted molecular beam epitaxy(PA-MBE) method. We introduce
an optimized plasmonic nanohole array by using electron beam lithograp
hy and a reactive ion etching process. We measure the reflectance spectra
by using a micro-DUV reflectance setup to confirm the behavior of the pla
smonic resonance. Both the experiment and simulations overlap with the
reflectance dip at around 266 nm, which confirm the designated plasmoni
c resonance of the fabricated Al nanohole array.
To evaluate the substrate's performance, we detected five nucleotides(A,
T, C, G, U) using the Al plasmonic nanohole array. The substrate exhibited t
he highest enhancement factors (EF) of up to ~10^6.This showcases the p
otential of DUV-SERRS substrates for sensitive molecular analysis.