Periodic Reporting for period 4 - LASSO (Layered semiconductors and hybrid systems for quantum optics and opto-valleytronics)
Reporting period: 2023-07-01 to 2023-12-31
Semiconducting layered transition metal dichalcogenides (TMDs) and heterostructures represent a new material platform for fundamental condensed matter research and quantum science and technology. By virtue of strong spin-orbit and Berry curvature effects, the non-centrosymmetric crystals and their van der Walls heterostructures provide a quantum opto-valleytronic interface between spin- and valley-polarized electrons and circularly polarized photons. These valley-contrasting optical selection rules in turn establish means to address the multi-valley quantum resource all-optically. At this interface, where light meets valley quantum states of matter, the project aims at developing and mastering electron-hole-pair excitations and their coupling to photons in layered TMD semiconductors, heterostructures and hybrid light-matter systems. To this end, TMD monolayers are subject to versatile assemblies into van der Waals heterostructures with tailored moiré interference effects and correlated phenomena of excitons, charges and spins, or coupling to optical micro-cavities to from exciton-polaritons with mutual interactions, opto-valleytronic features or topological protection. By engineering excitons and exciton-polaritons in TMD monolayers and related van der Walls heterostructures, the project will expand the realm of linear, non-linear, chiral and topological quantum optical components for applications in quantum science and technology.
In the scope of the project, the main objectives of the individual work packages (WPs) have been achieved and additional unexpected results have been established. In WP I with focus on moiré interference effects, chemical vapor deposition (CVD) synthesis of laterally extended two-dimensional semiconductors in the form of monolayer, bilayer and heterobilayer TMD crystals has been established and protocols have been developed to fabricate van der Waals heterostructures and devices via exfoliation and dry-transfer methods. High quality of monolayers and van der Waals heterostructures embedded in hexagonal boron nitride have been fabricated, as confirmed by complementary analysis techniques and employed in various experimental settings. The main results of WP II with focus on exciton circuitry have been addressed in experiment and theory, and substantial progress has been achieved in the understanding of light-matter interactions mediated by excitons in layered TMD semiconductors. In particular, magneto-optical signatures of excitons in different spin, valley, material and layer configurations observed experimentally on representative TMD monolayers, homobilayers and heterostructures have been substantiated by theoretical modeling of exciton Landé factors from first-principles. Moreover, theoretical understanding of atomic lattice reconstruction on mesoscopic length scales in near-commensurate small-twist heterostructures has been developed. For heterostructures of non-commensurate layers with canonical moiré interference effects, experimental evidence and theoretical modeling have been developed to explain the complexity of optical phenomena on the basis of combined effects of moiré potential and interlayer hybridization. Most remarkably, moiré heterostructures have been identified to support a plethora of correlated many-body phenomena of excitons, charges and spins ordered on triangular moiré lattices. Finally, in WP III with focus on hybrid TMD-cavity systems, formation of exciton-polaritons in the regime of strong coupling has been realized for TMD monolayers at room and cryogenic temperatures with open micro-cavities and nanoplasmonic elements. Moreover, using cryogenic open-cavities, Purcell enhancement in the photoluminescence has been demonstrated for interlayer excitons in TMD heterobilayers, and the strong-coupling regime has been established for moiré excitons subject to voltage-controlled doping. For electron-charged moiré exciton-polaritons signatures of correlation-induced magnetism have been demonstrated. The successful achievements of individual and interrelated milestones of all three WPs, as disseminated in scientific publications as well as at national and international conferences and workshops, represent cumulatively the overall success of the project towards developments of novel quantum optical and opto-valleytronic elements based on layered TMD semiconductors and heterostructures.
Progress beyond the state of the art and significant achievements of different quality, scope and impact have been made in the scope of the project. Although not explicitly planned, all scientific achievements and technological breakthroughs were crucially enabled by the ERC grant. Outstanding examples of novel methodologies developed and discoveries made beyond the state-of-the-art include the synthesis of lateral and vertical van der Waals heterobilayer systems with and without moiré effects, their experimental signatures and theoretical description, and the development of ab initio calculations of Landé factors for excitons in different spin, valley and layer configurations from density functional theory. Moreover, moiré heterostructures have been identified to order excitons, charges and spin on triangular moiré lattices, which exhibit rich correlation phenomena when subjected to controlled charge doping or light-matter coupling in tunable open micro-cavities in closed-cycle cryostats.