Strong demand for plasmonic devices with an enormously enhanced electric field and desired resonance frequencies has led to extensive investigations of metallic slot structures. While strong field enhancement can be achieved by reducing the width of the slot, the effect of the gap surface plasmon limits the maximum achievable field enhancement at higher frequencies. Specifically, the effect of the gap surface plasmon becomes stronger as the gap width decreases and strongly suppresses the transmission while causing a red-shift of the resonance. Here, we overcome these issues and realize strong field enhancements at higher frequencies, by managing the metal thickness of the nanoslots. We show that, as the nanoslots become as thin as 10 nm, they show a giant electric field enhancement of up to 7600. Moreover, the resonances are strongly blue-shifted to above 1 THz from 0.33 THz. Our work provides a novel route to achieving high field enhancements at desired frequencies, as well as a means by which to characterize the slot as the gap-sensitive or substrate-sensitive type.

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