Quantum Optics in TMDCs
The advent of graphene has opened up a vast plethora of research into a wide variety of other, intrinsically two-dimensional Van der Waals materials. Within the generic family of Van der Waals materials, transition metal dichalcogenides (TMDCs, MX2, M=(Mo, W); X=(S, Se)) are intrinsically direct-bandgap semiconductors, with bandgaps in the visible and NIR domain1. Due to the reduced screening of the Coulomb interactions in two dimensional materials, their optical response is dominated by extremely tightly bound excitons, giving rise to an ultrafast optical response and nearly-transform limited optical linewidths2.
In combination with strong spin-orbit interactions that give rise to valley-spin locking effects, their two-dimensional nature makes the TMDCs a unique platform for solid state quantum optics experiments. In the Lukin group, we have recently embarked on a joint effort with the groups of Profs. Eugene Demler (theory), Hongkun Park and Philip Kim (experiment) to further develop and study TMDCs for quantum optics and quantum control applications.
In one approach, we are developing full electrostatic control of both charges and charged excitons (trions, attractive polarons). The strong binding energy of such excitons in twodimensional systems prevents dissociation under strong applied fields, and therefore allows for electrostatic trapping of the charged excitons3. Fig. 1) illustrates this principle: a combination of global and local electrostatic control allows for the controllable definition of nanowires and rudimentary quantum dots, which can be probed optically. We refer to Ref. 3) for more details.
The strong spin-orbit interaction gives rise to fully spin-orbit split conduction and valence bands at the band extrema (K,K’ points in the first Brillouin zone of a hexagonal lattice – see ref. 1), with rather special spectroscopic features. Depending on the type of TMDC, the lowest conduction band is either dark (first order spin-forbidden optical transition to the valence band, z-polarized), or bright. By coupling to the z-polarized near-field of a ultra-high quality plasmonic silver film, this transition can be strongly amplified, thereby brightening an otherwise dark optical transition – we refer to Ref. 4) for more details.
In addition, monolayer Van der Waals materials can be uniquely modified by means of surface interactions. Since the material is essentially one large surface, its properties, and particularly the exciton binding energy and its emission energy, are highly sensitive to its immediate environment. Modifying this environment can give rise to the creation of excitonic potential landscapes, either by modifying the immediate dielectric environment, or by the direct Van der Waals interaction between (dis)similar TMDCs. We are currently developing such potential landscapes, which are expected to be essential ingredients for future excitonics or valleytronics devices.