A nano gap can be much brighter when blocked*

We theoretically and experimentally demonstrate that, for a one-dimensional nano gap perforated in a thin (i.e., sub-wavelength-thick) metal plate, blocking the gap with a conducting layer can lead to a boost in light transmission through the gap, meaning that the gap can be much brighter than an unblocked one. This counter-intuitive phenomenon is shown to stem from the strongly modified cavity resonance condition of the fundamental gap mode reflecting at two gap ends. Our theory predicts that the blocking layer induces an anomalous phase shift to the gap mode reflection at the blocked end, giving rise to the emergence of the cavity resonance in a deep sub-wavelength thickness and boosted light transmission. From this, we reveal a non-intuitive consequence that a complete perforation of a gap does not always yield the brightest: the brightest one can appear when partially perforated with residual metal. This prediction is verified by optical spectroscopy with a gold sample.