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Register a new userTwo-dimensional semiconductors in the regime of strong light-matter coupling
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The optical properties of transition metal dichalcogenide monolayers are widely dominated by excitons, Coulomb-bound electron-hole pairs. These quasi-particles exhibit giant oscillator strength and give rise to narrow-band, well-pronounced optical transitions, which can be brought into resonance with electromagnetic fields in microcavities and plasmonic nanostructures. Due to the atomic thinness and robustness of the monolayers, their integration in van der Waals heterostructures provides unique opportunities for engineering strong light-matter coupling. We review first results in this emerging field and outline future opportunities and challenges.
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It is well known that optical absorption saturation of intersubband transitions in doped semiconductor quantum wells is independent of the introduced doping in the absence of a cavity. When inserting the system in a resonator, we show that this remains valid only in the weak light-matter coupling regime. In the strong light-matter coupling regime instead, we demonstrate that absorption saturation is no more doping independent and it is instead tailorable. Based on this unified formalism for saturation in weak and strong coupling, we provide designs for semiconductor saturable absorption (SESAM) mirrors and bistable systems operating in the mid-infrared range of the electromagnetic spectrum and with extremely low saturation intensities. Countering intuition, we show that the most suitable region to exploit low saturation intensities is not the ultra-strong coupling regime, but is instead at the onset of strong light-matter coupling.
Coupling different physical properties is a fascinating subject of physics. Already well-known are the multiferroics which show properties of ferroelectrics and magnets. But ferroelectricity by itself also entails the bulk photovoltaic effect, a light-matter interaction which generates dc currents. Here we propose a magnetic photogalvanic effect that couples the magnetism to the light-matter interaction. This phenomenon emerges from the $mathbf{k}$ to $mathbf{-k}$ symmetry-breaking in the band structure and does not require a static polarization. It is distinct from other known bulk photovoltaic mechanisms such as the shift current. We demonstrate such phenomena in a newly discovered layered magnetic insulator CrI$_3$. A record photoconductivity response (more than 200 $mu A V^{-2} $) is generated under the irradiation of a visible light in the antiferromagnetic phase. The current can be reversed and switched by controllable magnetic phase transitions. Our work paves a new route for photovoltaic and optoelectronic devices and provides a sensitive probe for the magnetic transition.
In transition metal dichalcogenides layers of atomic scale thickness, the electron-hole Coulomb interaction potential is strongly influenced by the sharp discontinuity of the dielectric function across the layer plane. This feature results in peculiar non-hydrogenic excitonic states, in which exciton-mediated optical nonlinearities are predicted to be enhanced as compared to their hydrogenic counterpart. To demonstrate this enhancement, we performed optical transmission spectroscopy of a MoSe$_2$ monolayer placed in the strong coupling regime with the mode of an optical microcavity, and analyzed the results quantitatively with a nonlinear input-output theory. We find an enhancement of both the exciton-exciton interaction and of the excitonic fermionic saturation with respect to realistic values expected in the hydrogenic picture. Such results demonstrate that unconventional excitons in MoSe$_2$ are highly favourable for the implementation of large exciton-mediated optical nonlinearities, potentially working up to room temperature.
A detailed understanding of charged defects in two-dimensional semiconductors is needed for the development of ultrathin electronic devices. Here, we study negatively charged acceptor impurities in monolayer WS$_2$ using a combination of scanning tunnelling spectroscopy and large-scale atomistic electronic structure calculations. We observe several localized defect states of hydrogenic wave function character in the vicinity of the valence band edge. Some of these defect states are bound, while others are resonant. The resonant states result from the multi-valley valence band structure of WS$_2$, whereby localized states originating from the secondary valence band maximum at $Gamma$ hybridize with continuum states from the primary valence band maximum at K/K$^{prime}$. Resonant states have important consequences for electron transport as they can trap mobile carriers for several tens of picoseconds.
Scanning tunnelling microscopy and low energy electron diffraction show a dimerization-like reconstruction in the one-dimensional atomic chains on Bi(114) at low temperatures. While one-dimensional systems are generally unstable against such a distortion, its observation is not expected for this particular surface, since there are several factors that should prevent it: One is the particular spin texture of the Fermi surface, which resembles a one-dimensional topological state, and spin protection should hence prevent the formation of the reconstruction. The second is the very short nesting vector $2 k_F$, which is inconsistent with the observed lattice distortion. A nesting-driven mechanism of the reconstruction is indeed excluded by the absence of any changes in the electronic structure near the Fermi surface, as observed by angle-resolved photoemission spectroscopy. However, distinct changes in the electronic structure at higher binding energies are found to accompany the structural phase transition. This, as well as the observed short correlation length of the pairing distortion, suggest that the transition is of the strong coupling type and driven by phonon entropy rather than electronic entropy.
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