title: Squeezing Photons into a Point-Like Space.
abstract: 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.
authors.affiliation: †Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea.
authors.forename: Myung-Ki
authors.initials: MK
authors.lastname: Kim
authors.affiliation: †Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea.
authors.forename: Hongchul
authors.initials: H
authors.lastname: Sim
authors.affiliation: †Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea.
authors.forename: Seung Ju
authors.initials: SJ
authors.lastname: Yoon
authors.affiliation: †Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea.
authors.forename: Su-Hyun
authors.initials: SH
authors.lastname: Gong
authors.affiliation: ‡National Nanofab Center, 291 Deahak-ro, Yuseong-gu, Daejeon 305-806, South Korea.
authors.forename: Chi Won
authors.initials: CW
authors.lastname: Ahn
authors.affiliation: †Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea.
authors.forename: Yong-Hoon
authors.initials: YH
authors.lastname: Cho
authors.affiliation: †Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea.
authors.forename: Yong-Hee
authors.initials: YH
authors.lastname: Lee
keywords: 3D gap-plasmon antennas; 3D nanofabrications; Plasmonics; nano-optics