When two-dimensional electron gases (2DEGs) are exposed to magnetic field, they resonantly absorb electromagnetic radiation via electronic transitions between Landau levels (LLs) 1 . In 2DEGs with a Dirac spectrum, such as graphene, theory predicts an exceptionally high infrared magneto-absorption, even at zero doping 2-5 . However, the measured LL magneto-optical effects in graphene have been much weaker than expected 2,6-12 because of imperfections in the samples available so far for such experiments. Here we measure magneto-transmission and Faraday rotation in high-mobility encapsulated monolayer graphene using a custom designed setup for magneto-infrared microspectroscopy. Our results show a strongly enhanced magneto-optical activity in the infrared and terahertz ranges characterized by a maximum allowed (50%) absorption of light, a 100% magnetic circular dichroism as well as a record high Faraday rotation. Considering that sizeable effects have been already observed at routinely achievable magnetic fields, our findings demonstrate a new potential of magnetic tuning in 2D Dirac materials for long-wavelength optoelectronics and plasmonics. arXiv:1905.07159v1 [cond-mat.str-el]

We demonstrate that giant Faraday rotation in graphene in the terahertz range due to the cyclotron resonance is further increased by constructive Fabry-Perot interference in the supporting substrate. Simultaneously, an enhanced total transmission is achieved, making this effect doubly advantageous for graphene-based magneto-optical applications. As an example, we present far-infrared spectra of epitaxial multilayer graphene grown on the C-face of 6H-SiC, where the interference fringes are spectrally resolved and a Faraday rotation up to 0.15 radians (9°) is attained. Further, we discuss and compare other ways to increase the Faraday rotation using the principle of an optical cavity.

We describe a simple magneto-optical experiment and introduce a magneto-optical KramersKronig analysis (MOKKA) that together allow extracting the complex dielectric function for leftand right-handed circular polarizations in a broad range of frequencies without actually generating circularly polarized light. The experiment consists of measuring reflectivity and Kerr rotation, or alternatively transmission and Faraday rotation, at normal incidence using only standard broadband polarizers without retarders or quarter-wave plates. In a common case, where the magneto-optical rotation is small (below ∼ 0.2 rad), a fast measurement protocol can be realized, where the polarizers are fixed at 45 • with respect to each other. Apart from the time-effectiveness, the advantage of this protocol is that it can be implemented at ultra-high magnetic fields and in other situations, where an in-situ polarizer rotation is difficult. Overall, the proposed technique can be regarded as a magnetooptical generalization of the conventional Kramers-Kronig analysis of reflectivity on bulk samples and the Kramers-Kronig constrained variational analysis of more complex types of spectral data. We demonstrate the application of this method to the textbook semimetals bismuth and graphite and also use it to obtain handedness-resolved magneto-absorption spectra of graphene on SiC.

We have measured the far-infrared reflectivity and Kerr angle spectra on a high-quality crystal of pure semimetallic bismuth as a function of magnetic field, from which we extract the conductivity for left-and right handed circular polarisations. The high spectral resolution allows us to separate the intraband Landau level transitions for electrons and holes. The hole transition exhibits 100% magnetic circular dichroism, it appears only for one polarisation as expected for a circular cyclotron orbit. However the dichroism for electron transitions is reduced to only 13 ± 1%, which is quantitatively explained by the large effective mass anisotropy of the electron pockets of the Fermi surface. This observation is a signature of the mismatch between the metric experienced by the photons and the electrons. It allows for a contactless measurement of the effective mass anisotropy and provides a direction towards valley polarised magneto-optical pumping with elliptically polarised light.Circular dichroism, the property of materials to interact differently with left and right circularly polarised light is associated with symmetry breaking due to e.g. molecular chirality, spontaneous magnetisation or an external magnetic field. A charged particle in a magnetic field moves in a circular orbit perpendicular to that field. Radiation propagating parallel to the field is only absorbed by that particle for right or left circular polarisation, leading to 100% magnetic circular dichroism. In condensed matter, the behaviour of charge carriers in a lattice is described using an effective mass, which can be very different from the free-electron mass, but also strongly anisotropic [1]. In the latter case the cyclotron orbits become elliptical which, as we will show, strongly reduces the magnetic circular dichroism. The reduction of magnetic circular dichroism can thus be regarded as a manifestation of the mismatch between the metric experienced by photons (isotropic) and the electrons (anisotropic), which becomes apparent when they interact [2-4]. As a consequence, elliptically polarised light with the same ellipticity as an electron pocket can cause a 100% valley polarised magneto-optical absorption.Bismuth is a canonical semimetal which possesses a rich electronic structure with strong spin-orbit interaction, low carrier density, long mean-free path and small cyclotron mass [5][6][7][8] (Fig. 1a). A hole pocket is oriented along the trigonal axis, where the bands are almost parabolic.Three electron pockets are tilted 6• from the plane perpendicular to the trigonal axis and have a Dirac-like band dispersion. In this plane the dielectric function is isotropic. A magnetic field parallel to the trigonal axis combined with light propagating along the same axis therefore presents an ideal case to measure the dichroism in relation to the effective mass. For holes the mass is isotropic in the plane whereas for electrons it is strongly anisotropic (a factor > 200 [18]), leading to a strong dichroism contrast between electron and hole transi...

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