We propose quantitative investigations of the Fabry-Perot resonance condition for the field enhancement by an isolated
subwavelength metallic groove. The resonance condition is obtained from a simple Fabry-Perot model. Our calculations
show that the electric field near the groove mouth that contacts the specimen is drastically enhanced under the resonance
condition but is depressed out of the resonance condition. Giant electric-field enhancement can be obtained for narrow
groove, which reveals nearly 108 magnification of Raman signals. A weak dependence of the electromagnetic
enhancement on the metal type is presented and explained.