laboratoire pierre aigrain
électronique et photonique quantiques
laboratoire pierre aigrain

Seminar, 11th January 2017 (13h30 L363-365)

Stéphane Albon Boubanga Tombet - RIEC (Japan) and LPMC (Amiens)
Emission and Detection of Terahertz Radiation in Graphene/hBN Heterostructures

This communication reviews recent advances in terahertz (THz) emission and detection in graphene based heterostructures. The first part will focus on double-graphene-layered van der Waals (vdW) heterostructure. The photon-assisted inter-graphene layer resonant tunneling (RT) radiative transitions will be discussed and its application in graphene vdW heterostructure for THz emiters [1] and photodetectors (PDs) [2]. The first experimental observation of THz emission and detection in those structures will also be discussed. Our fabricated devices consist of two independently contacted graphene layers (GLs) separated by the thin transparent hexagonal Boron Nitride (h-BN) tunnel-barrier layer. The bias voltage applied between the GL’s contacts induces the electron and hole gases in the opposing GLs. The band-offset energy (‘Δ’) between the Dirac points of the GLs determines the energies of the photons emitted or absorbed by the structure in the RT inter-GL transitions. The inter-GL population inversion and photon assisted RT transitions are explored for new types of THz devices. We conducted THz emission experiments using a Fourier-transform far-infrared (FTIR) spectrometer to observe the spontaneous emissions associated with the photon assisted RT transitions. For the band offset (Δ) conditions where the energy of the final state is lower than the initial state, the tunneling is associated with emission of the electromagnetic photon. Our experimental results show THz
emission exclusively in this case. When the energy of the final state is higher, the tunneling is preceded by absorption of photons. Room temperature detection results at 1 THz was observed in this case. Those results give important insights of carrier dynamics in graphene-based vdW heterostructures and pave the way towards new THz graphene based devices. The second part will focus on plasmonic structures based on hBN/graphene/h-BN heterostructure featuring an interdigitated dual-grating-gate (DGG). Current driven plasmon generation and instabilities in those graphene plasmonic structures will be discussed. It will be shown that terahertz absorption spectra in the graphene DGG devices exhibit gate tunable pronounced peak associated with excitation of the plasmons in the structure. We also show that direct current flow along the graphene channel induces red shift of the plasmonic resonance with a significant reduction of THz absorption up to a transparency regime. Above this
regime, the current induces amplification of the incoming THz radiation and a blue shift of the resonances. The absorption completely vanishes, meaning that our structure becomes totally transparent to the incoming THz beam from a threshold region where the energy transfer from the electromagnetic wave to the plasmonic
system stops as the current induced wave growth takes over their decay and the plasmon radiative damping becomes zero. For values of drain current above this threshold, the energy transfer goes from the plasmonic system to the electromagnetic wave as the incoming THz beam is being amplified. The additional energy
supply is likely coming from the current flow via the applied drainto-source voltage, which is first transferred to the plasma wave through different processes such as reflection at asymmetric boundaries [3], an electron transit time based mechanism [4], and/or lowering plasmonic velocity in the adjacent cavity [5]. This energy transfer drives the plasma waves in an instability regime when the instability increment takes
over the decrement related to the plasmon damping [6]. The energy stored in the plasmon field is then released and transferred to the electromagnetic radiation with the help of the grating. This finding opens pathways to use graphene for THz emitters and amplifiers.

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