Periodic Reporting for period 1 - BlochTG (Bloch Oscillations, Wannier-Stark Localisation and Coherent Terahertz Emission in Twisted Graphene Superlattices)
Periodo di rendicontazione: 2020-07-01 al 2022-06-30
BlochTG aims to study the fundamental light-mater interactions underlying the physics of Bloch oscillations, one of the oldest known quantum transport phenomena that has remained elusive in condensed matter physics experiments. The project focused on studying the opto-electronic properties of high electric field induced out-of-equilibrium electrons in graphene-based moire superlattice heterostructures, a novel class of quantum materials that has recently emerged in solid-state physics. Aside from fundamental interest, the high field phenomena studied in the project could lead to a new generation of terahertz opto-electronic applications thanks to the highly tuneable and unique properties of more superlattices. BlochTG combines the latest techniques in experimental optics and quantum transport to study novel types of light matter interactions that can be induced by strong electric fields and leverage those phenomena for novel opto-electronic devices operating in the mid-infrared to terahertz wavelength range.
A combination of high field quantum transport experiments, near-field and far-field photocurrent experiments and were performed in graphene superlattices and twisted graphene heterostructures during the project. First, a strong out-of-equilibrium electron distribution was identified generic to all graphene-based superlattices caused by Zener-tunnelling processes analogous to the Schwinger effect1. The effect demonstrated that physics related to Bloch oscillations is prominent in graphene superlattices. Specifically, the work shows that electrons in graphene superlattices can be accelerated through the entire Brillouin zone and their Fermi surface shifted to a strongly out-of-equilibrium electronic state. Second, scanning probe near-field photocurrent experiments were used to image the local structure of twisted graphene superlattices2. Using a scattering-type near-field optical microscope, the opto-electronic properties of the superlattice structure was mapped out with 50 nm spatial resolution and a strong photocurrent pattern was shown to emerge reflecting microscopic variations in the Seebeck coefficient. Third, a giant photoconductive response was demonstrated in twist-decoupled double bilayer graphene layers. The heterostructure was shown to exhibit a giant broadband response extending from visible to terahertz wavelengths with high internal quantum efficiencies thanks to strong enhancement in carrier mobilities caused by interlayer screening of Coulomb scattering.
1. A. Berdyugin, R. Krishna Kumar et al Science 375, 66579, 430-433 (2002)
2. N. C. Hesp, R. Krishna Kumar et al Nat Comm 12, 1640 (2021)
1. A. Berdyugin, R. Krishna Kumar et al Science 375, 66579, 430-433 (2002)
2. N. C. Hesp, R. Krishna Kumar et al Nat Comm 12, 1640 (2021)
The project has explored the linear and high electric field induced opto-electronic properties of graphene superlattices and twisted graphene heterostructures. A new highly out-of-equilibrium electronic phase has been identified to occur in graphene superlattices. The result underlies the fundamental non-linear electronic properties of graphene superlattices and provides a novel route towards exploring the electron-hole Dirac plasma. On the other hand, the work on photoconductivity studies in large angle twisted double bilayer graphene has had significant impact on 2D material-based photodetector applications. The work has led to the design of a broadband photoconductor spanning multiple spectral regimes from visible to terahertz wavelengths, making it an ideal candidate for applications that require hyper spectral imaging, for example, in observational astronomy.