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Terahertz Tuning of Dirac Plasmons in Bi2Se3 Topological Insulator

P. Di Pietro, N. Adhlakha, F. Piccirilli, A. Di Gaspare, J. Moon, S. Oh, S. Di Mitri, S. Spampinati, A. Perucchi, and S. Lupi
Phys. Rev. Lett. 124, 226403 – Published 5 June 2020
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    Abstract

    Light can be strongly confined in subwavelength spatial regions through the interaction with plasmons, the collective electronic modes appearing in metals and semiconductors. This confinement, which is particularly important in the terahertz spectral region, amplifies light-matter interaction and provides a powerful mechanism for efficiently generating nonlinear optical phenomena. These effects are particularly relevant in graphene and topological insulators, where massless Dirac fermions show a naturally nonlinear optical behavior in the terahertz range. The strong interaction scenario has been considered so far from the point of view of light. In this Letter, we investigate instead the effect of strong interaction on the plasmon itself. In particular, we will show that Dirac plasmons in Bi2Se3 topological insulator are strongly renormalized when excited by high-intensity terahertz radiation by displaying a huge red-shift down to 60% of its characteristic frequency. This opens the road towards tunable terahertz nonlinear optical devices based on topological insulators.

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    • Received 21 November 2019
    • Accepted 15 May 2020

    DOI:https://doi.org/10.1103/PhysRevLett.124.226403

    © 2020 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Nonlinear opticsPlasmonicsTopological insulators
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    P. Di Pietro1,*, N. Adhlakha1, F. Piccirilli2, A. Di Gaspare3, J. Moon4, S. Oh4, S. Di Mitri1, S. Spampinati1, A. Perucchi1, and S. Lupi5

    • 1Elettra - Sincrotrone Trieste S.C.p.A., S.S. 14 km - 163,5 in Area Science Park, I-34149 Basovizza, Trieste, Italy
    • 2CNR-IOM, Area Science Park, I-34012 Trieste, Italy
    • 3NEST, CNRNANO and Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy
    • 4Department of Physics and Astronomy Rutgers, The State University of New Jersey, 136 Frelinghuysen Road Piscataway, New Jersey 08854-8019 USA
    • 5CNR-IOM and Dipartimento di Fisica, Sapienza Università di Roma, P.le Aldo Moro 2, I-00185 Roma, Italy

    • *Corresponding author. paola.dipietro@elettra.eu

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    Issue

    Vol. 124, Iss. 22 — 5 June 2020

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    • Figure 1
      Figure 1

      Plasmons in a topological insulator. Plasmons are collective oscillations of electrons that can be directly excited by electromagnetic radiation in the presence of an extra momentum kextra. This is achieved in the present experiment (a),(b), through ribbon arrays (width w, period 2w, and kextra=π/w), fabricated onto the surface of topological insulator Bi2Se3 films (a) with linearly dispersive electronic bands (c). Plasmons are excited after illumination with sub-ps, half-cycle THz pulses produced at the FERMI free-electron laser [8].

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    • Figure 2
      Figure 2

      Fluence-dependent THz spectra. Panels (a)–(e) display the THz extinction coefficient E(ν) of the w=4  μm ribbon array patterned film for different THz electric field values. THz light is perpendicularly polarized to the ribbons. E(ν) shows a plasmon-phonon coupled spectrum (see main text). In particular the low-frequency maxima correspond to the dressed plasmon and the high-frequency modes to the dressed α phonon. The full line is a fit according to Eq. (1). The same is shown in panels (f)–(i) for the w=20  μm patterned film. Both the dressed plasmons (see arrows) and the bare plasmon absorption [dashed lines as calculated from Eq. (1)], show an evident softening vs the THz electric field.

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    • Figure 3
      Figure 3

      Plasmon softening. Experimental plasmon frequency νp values for both patterned films are shown here as a function of the THz electric field ETHz. According to the Boltzmann equation formalism a softening of the plasmon (dashed red line) is theoretically predicted [6]. This model fails in reproducing the experimental data. On the other hand, a thermodynamic model (black dotted line), correctly catches our experimental observations (see main text).

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