A highly reproducible SERS substrate is designed and built based on two-dimensional Ag grating structures. The periodic substrate gives ultrahigh SERS enhancement due to coupling between localized and extended plasmons, while its uniform surface minimizes signal variability. Moreover, the two-dimensional structure eliminates the polarization dependency of the incident light, ensuring polarization-independent SERS, hence higher reproducibility. The substrate is designed using the finite element method (FEM) to achieve similar enhancement for both TE and TM polarized light. The chip is fabricated using electron beam lithography and characterized for surface-enhanced fluorescence (SEF) and SERS using R6G as the target indicator molecule. The results show an eight-fold enhancement compared to a bare Ag thin film for SEF and a staggering enhancement of 2.5x106 times for SERS. To evaluate the chemical sensing performance of the sensor surface, we characterize the chip for picric acid (PA) detection in a concentration range from nM to mM. The minimum detection limit for PA is found to be 3.04 nM,
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