Andrea Tomadin

The question of whether or not passive subwavelength cavities can alter the properties of quantum materials is currently attracting a great deal of attention. In this article, we show that the Fermi liquid parameters of a two-dimensional metal are modified by cavity polariton modes, and that these changes can be monitored by measuring a paradigmatic magneto-transport phenomenon, Shubnikov–de Haas oscillations in a weak perpendicular magnetic field. This effect is intrinsic, and totally unrelated to disorder. As an illustrative example, we carry out explicit calculations of the quasiparticle velocity of graphene in a planar van der Waals cavity formed by natural hyperbolic crystals and metal gates. The largest effects of the cavity occur when the phonon polariton modes of the former match energetically the graphene plasmon. For typical graphene carrier densities this occurs in the Terahertz spectral range.

Graphene is a unique platform for tunable opto‐electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light‐matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all‐optical fashion, but each approach involves a trade‐off between speed and modulation depth. Here, lattice temperature, electron doping, and all‐optical tuning of third‐harmonic generation are combined in a hexagonal boron nitride‐encapsulated graphene opto‐electronic device and demonstrate up to 85% modulation depth along with gate‐tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out‐of‐equilibrium chemical potential. The work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors, and frequency converters.

Thermoelectric phenomena in photoexcited graphene have been the topic of several theoretical and experimental studies because of their potential usefulness in optoelectronic applications. However, available theoretical descriptions of the thermoelectric effect in terms of the Seebeck coefficient do not take into account the role of the photoexcited electron density. In this work, we adopt the concept of effective Seebeck coefficient [G. D. Mahan, J. Appl. Phys. 87, 7326 (2000)] and extend it to the case of a photoexcited two-dimensional (2D) electron gas. We calculate the effective Seebeck coefficient for photoexcited graphene, we compare it to the commonly used “phenomenological” Seebeck coefficient, and we show how it depends on the photoexcited electron density and temperature. Our results are necessary inputs for any quantitative microscopic theory of thermoelectric effects in graphene and related 2D materials.

Recently developed plasmonic nanostructures are able to generate intense and localized magnetic hot spots in a large spectral range from the terahertz to the visible. However, a direct measurement of the magnetic field at the hot spot has not been performed yet, due to the absence of magnetic field detectors that work at those high frequencies and that fit the hot-spot area. We propose to place a graphene ribbon in the hot spot of a plasmonic nanostructure driven by a laser beam, such that a current is generated due to both the magnetic field at the hot spot and the electric field of the laser. We demonstrate that a nonlinear Hall voltage, which can be measured by standard electrical means, builds up across the ribbon, making it possible to directly probe the magnetic field at the hot spot.

The electron gas hosted in a two-dimensional solid-state matrix, such as a quantum well or a two-dimensional van der Waals heterostructure, supports the propagation of plasma waves. Nonlinear interactions between plasma waves, due to charge conservation and current convection, generate a constant density gradient which can be detected as a DC potential signal at the boundaries of the system. This phenomenon is at the heart of a plasma-wave photodetection scheme, which was first introduced by Dyakonov and Shur for electronic systems with a parabolic dispersion and then extended to the massless Dirac fermions in graphene. In this work, we develop the theory of plasma-wave photodetection in bilayer graphene, which has the peculiarity that the dispersion relation depends locally and dynamically on the intensity of the plasma wave. In our analysis, we show how quantum capacitance effects, arising from the local fluctuations of the electronic dispersion, modify the intensity of the photodetection signal. An external electrical bias, e.g., induced by top and bottom gates, can be used to control the strength of the quantum capacitance corrections, and thus the photoresponse.

It is by now well established that high-quality graphene enables a gate-tunable low-loss plasmonic platform for the efficient confinement, enhancement, and manipulation of optical fields spanning a broad range of frequencies, from the midinfrared to the terahertz domain. While all-electrical detection of graphene plasmons has been demonstrated, electrical plasmon injection, which is crucial to operate nanoplasmonic devices without the encumbrance of a far-field optical apparatus, remains elusive. In this work, we present a theory of electrical plasmon injection in double-layer graphene, where a vertical tunnel current excites acoustic and optical plasmon modes. We first calculate the power delivered by the applied interlayer voltage bias into these collective modes. We then show that this system works also as a spectrally resolved molecular sensor.

Van der Waals heterostructures employing graphene and hexagonal boron nitride (hBN) crystals have emerged as a promising platform for plasmonics thanks to the tunability of their collective modes with carrier density and record values for plasmonics figures of merit. In this paper we investigate theoretically the role of moiré-pattern superlattices in nearly aligned graphene on hBN by using a continuum-model Hamiltonian derived from ab initio calculations. We calculate the system's energy-loss function for a range of chemical potential values that are accessible in gated devices. Our calculations reveal that the electron-hole asymmetry of the moiré bands leads to a remarkable asymmetry of the plasmon dispersion between positive and negative chemical potentials, showcasing the intricate band structure and rich absorption spectrum across the secondary Dirac point gap for the hole bands.

Efficient and scalable production of two-dimensional (2D) materials is required to overcome technological hurdles towards the creation of a 2D-material-based industry. Here, we present a novel approach developed for the exfoliation of layered crystals, i.e., graphite, hexagonal-boron nitride and transition metal dichalcogenides. The process is based on high-pressure wet-jet-milling (WJM), resulting in a 2 L h⁻¹ production of 10 g L⁻¹ of single- and few-layer 2D crystal flakes in dispersion making the scaling-up more affordable. The WJM process enables the production of defect-free and high quality 2D-crystal dispersions on a large scale, opening the way for their full exploitation in different commercial applications, e.g., as anode active material in lithium ion batteries, as reinforcement in polymer–graphene composites, and as conductive inks, as we demonstrate in this report.