title: Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy.
abstract: Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far-only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.
authors: CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.|Departamento de Física, Universidad de Oviedo, 33007 Oviedo, Spain.
authors: Pablo
authors: P
authors: Alonso-González
authors: CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.|IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
authors: Alexey Y
authors: 0000-0002-2327-0164
authors: AY
authors: Nikitin
authors: Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA.
authors: Yuanda
authors: 0000-0002-1604-3174
authors: Y
authors: Gao
authors: ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain.
authors: Achim
authors: 0000-0002-5024-4201
authors: A
authors: Woessner
authors: ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain.
authors: Mark B
authors: MB
authors: Lundeberg
authors: Radboud University, Institute for Molecules and Materials, 6525 AJ Nijmegen, The Netherlands.
authors: Alessandro
authors: A
authors: Principi
authors: Department of Physics, Imperial College London, London SW7 2AZ, UK.
authors: Nicolò
authors: N
authors: Forcellini
authors: CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.
authors: Wenjing
authors: 0000-0003-4176-195X
authors: W
authors: Yan
authors: CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.
authors: Saül
authors: S
authors: Vélez
authors: Neaspec GmbH, 82152 Martinsried, Germany.
authors: Andreas J
authors: AJ
authors: Huber
authors: National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
authors: Kenji
authors: 0000-0003-3701-8119
authors: K
authors: Watanabe
authors: National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
authors: Takashi
authors: T
authors: Taniguchi
authors: CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.|IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
authors: Félix
authors: F
authors: Casanova
authors: CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.|IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
authors: Luis E
authors: 0000-0002-7918-8047
authors: LE
authors: Hueso
authors: Istituto Italiano di Tecnologia, Graphene labs, Via Morego 30, 16163 Genova, Italy.
authors: Marco
authors: M
authors: Polini
authors: Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA.
authors: James
authors: J
authors: Hone
authors: ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain.|ICREA - Institució Catalana de Recerça i Estudis Avancats, 08010 Barcelona, Spain.
authors: Frank H L
authors: FH
authors: Koppens
authors: IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.|CIC NanoGUNE and UPV/EHU, 20018 Donostia-San Sebastián, Spain.
authors: Rainer
authors: R
authors: Hillenbrand