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Periodic arrays of subwavelength apertures on Ag film are used as the substrates for nanohole-enhanced Raman spectroscopy. This study presents a rectangular bull's eye structure in an array. We demonstrate the quantitative features of the pattern by the finite-difference time-domain method. This structure is used to enhance the Raman spectroscopy of four DNA bases without labeling. The enhancement factor is up to 106. A 1.5-fold enhancement against the circular pattern experimentally confirms the role of sharp corners. This structure may be used in nucleotide identification and DNA sequencing.