Terahertz generation and detection with van der Waals heterostructures
LTL Quantum Physics Seminar (Nanotalo). Speaker: Dr. D. Svintsov (Moscow Institute of Physics and Technology, Dolgoprudny, Russia).
Van der Waals heterostructures are the novel compounds comprising graphene and related layered materials, and such combinations offer many new opportunities in optoelectronics. In the present communication we review several prospects for terahertz generation and detection using these new materials.
The resonant tunneling between graphene layers separated by a thin barrier layer can be accompanied by the emission of surface plasmons . This channel of tunneling is extremely efficient due to the short localization length (~λ/100) of plasmon energy in the vicinity of graphene layers. The plasmons emitted upon resonant tunneling can exhibit radiative decay into free-space electromagnetic modes. This effect opens up the possibility to create ultra-compact voltage tunable radiation sources with frequency from THz to mid-IR ; the first demonstration of such emission was presented recently . The plasmon-assisted tunneling between graphene layers possesses several resonances inherent from quasi-relativistic nature of charge carriers – the collinear tunneling resonance and plasmaronic resonance. Both are absent in the spectra of tunneling emission from metal-insulator-metal structures and tunnel-coupled massive two-dimensional systems. We show that such resonances further increase the probability of plasmon-assisted tunneling (in addition to high confinement) and even allow for plasmon gain and spaser action under optimal selection of barrier layer .
The inverse of tunneling accompanied by photon/plasmon emission is the tunneling with absorption of electromagnetic energy. We discuss the applications of this effect for light detection with van der Waals heterostructures. Particularly, the photon absorption-aided tunneling between coupled graphene layers allows highly resonant terahertz detection . Another opportunity is the light detection exploiting the tunneling of photogenerated electrons in graphene to the conduction band of the barrier. We discuss the possibility of achieving high photoelectric gain in such photodetector, i.e. the number of electrons passing in detector circuit per absorbed photon much exceeding unity [5,6]. The origin of gain is the increase in injection current due to the formation of hole space charge in graphene layers upon illumination. We show that the gain coefficient is inversely proportional to the electron capture probability from the conduction band of the barrier to the graphene layer, and can be as high as ~50.
 D. Svintsov, Z. Devizorova, T. Otsuji, and V. Ryzhii, Phys. Rev. B 94, 115301 (2016).
 A.A. Dubinov, A. Bylinkin, V.Ya. Aleshkin, V. Ryzhii, T. Otsuji, and D. Svintsov,Optics Express 24 p. 29603 (2016)
 D. Yadav, S.B. Tombet, T. Watanabe, S. Arnold, V. Ryzhii, and T. Otsuji, 2D Mater. 3 p. 45009 (2016).
 V. Ryzhii, T. Otsuji, M. Ryzhii, V. Aleshkin, A. Dubinov, D. Svintsov, V. Mitin and M. Shur 2D Mater. 2 p. 025002 (2015)
 V. Ryzhii, M. Ryzhii, D. Svintsov, V. Leiman, V. Mitin, M. Shur, T. Otsuji, Infrared Physics and Technology, doi: 10.1016/j.infrared.2017.01.016 (article in press) (2017)
 V. Ryzhii, M. Ryzhii, D. Svintsov, V. Leiman, V. Mitin, M. S. Shur, and T. Otsuji, Optics Express 25, 5536 (2017)