Confining photons in the smallest possible volume has long been an objective of the nanophotonics community. In this Letter, we propose and demonstrate a three-dimensional (3D) gap-plasmon antenna that enables extreme photon squeezing in a 3D fashion with a modal volume of 1.3 × 10(-7) λ(3) (∼4 × 10 × 10 nm(3)) and an intensity enhancement of 400 000. A three-dimensionally tapered 4 nm air-gap is formed at the center of a complementary nanodiabolo structure by ion-milling 100 nm-thick gold film along all three dimensions using proximal milling techniques. From a 4 nm-gap antenna, a nonlinear second-harmonic signal more than 27 000-times stronger than that from a 100 nm-gap antenna is observed. In addition, scanning cathodoluminescence images confirm unambiguous photon confinement in a resolution-limited area 20 × 20 nm(2) on top of the nano gap.

In the two lines immediately after Eq. ͑9͒ ␣ and ␤ should be read as ␣ϭ1ϩre i(ϩ2) , ␤ϭ1ϩre Ϫi2␦ , and in Eq. ͑10͒ ⌫ 0 should be read asThese corrections do not change the main results of the paper. We thank Stefan Rotter for pointing out the errors.PHYSICAL REVIEW B 67, 129903͑E͒ ͑2003͒

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