Active magneto-optical control of spontaneous emission in graphene

We investigate the spontaneous emission rate of a two-level quantum emitter near a graphene-coated substrate under the influence of an external magnetic field. We demonstr ate that the application of the magnetic field can substantially increase or decrease the decay rate. We show that a suppression as large as 99% in the Purcell factor is achieved even for moderate magnetic fields. The emi tter’s lifetime is a discontinuous function of |B|, which is a direct consequence of the occurrence of discrete Landau levels in graphene. We demonstrate that, in the near-field regime, the magnetic field enables an unprec edented control of the decay pathways into which the photon/polariton can be emitted. Our findings strongly s uggest that a magnetic field could act as an efficient agent for on-demand, active control of light-matter intera ctions in graphene at the quantum level. The possibility of tailoring light-matter interactions at a quantum level has been a sought-after goal in optics since the pioneer work of Purcell [1], where it was first shown that the environment can strongly modify the spontaneous emission (SE) rate of a quantum emitter. To achieve such objective, several approaches have been proposed so far. One of them is to investigate SE in different system geometries [2‐11]. Advances in nanofabrication techniques have not only allowed the increase of the spectroscopic resolution of molecules in complex environments [12], but have also led to the use of nanometric objects, such as antennas and tips, to modify the lifetime, and enhance the fluorescence of single molecules [13‐16]. The presence of metamaterials may also strongly affect quantum emitters’ radiative processes. Fo r instance, the impact of negative refraction and of the hyperbolic dispersion of some metamaterials on the SE have been investigated [17‐19]. Also, the influence of cloaking devices on t he SE of atoms has been recently addressed [20]. Progress in plasmonics has also allowed for an unprecedented control of radiative properties of quantum emitters [21‐24]. However, the metallic structures usually employed as plasmonic materials are hardly tunable, limiting their application in photonic devices. To circumvent these limitations, graphene has emerged as an alternative plasmonic material due to its extraordinary electronic and optical pr operties [25‐30]. Indeed, graphene hosts extremely confined pla smons, facilitating strong light-matter interactions [27‐ 30]. In addition, the plasmon spectrum in doped graphene is highly tunable through electrical or chemical modification of the charge carrier density. Due to these properties, graphene i s a promising material platform for several photonic applications, specially in the THz frequency range [29]. At the quantum level, the spatial confinement of surface plasmons in graphene has been shown to modify the SE rate [31, 32]. The electromagnetic (EM) field pattern excited by quantum emitters near a graphene sheet [33] further demonstrates the huge field enhancement due to the excitation of surface plasmons. A graphene sheet has also been shown to mediate suband super-radiance between two quantum emitters [34]. Recently, the electrical control of the relaxation pathways a nd SE rate in graphene has been observed [35]. Despite all these advances, the achieved modification in the emitter’s decay rate remains modest so far. Most of the proposed schemes consider emitters whose transition frequencies are in the o ptical/near infrared range, usually far from graphene’s intra band transitions. As a consequence, the effects of graphene on the SE rate are only relevant when the emitter is no more than a few dozen nanometers apart. Here, we propose an alternative mechanism to actively tune the lifetime of a THz quantum emitter near a graphene sheet by exploiting its extraordinary magneto-optical properti es. We show that the application of a magnetic field B allows for an unprecedented control of the SE rate for emitter-graphene distances in the micrometer range. This is in contrast to pre vious proposals, in which the modification of the SE rate was achieved by electrically or chemically altering graphene’s doping level. The fact that we consider a low-frequency emitter enables us to probe the effects of intraband transitions in graphene on the decay rate, which have also been unexplored so far. In summary, our key results are (i) a striking 99% reduction of the emitter SE rate compared to the case where B = 0; (ii) a new distance-scaling law ∝ d −4 e −1/d that is valid for a broad range of distances and magnetic fields; (iii ) a highly non-monotonic behavior of the SE rate as a function of |B|, with sharp discontinuities in the regime of low temperatures; and (iv) the possibility of tailoring the decay chann els into which the photon can be emitted. These findings can be physically explained in terms of the interplay among the different EM modes and of electronic intraband transitions between discrete Landau energy levels in graphene.