Periodic Reporting for period 2 - CoulENGINE (Coulomb Engineering of Quantum States in Matter)
Reporting period: 2023-10-01 to 2025-03-31
The project addresses exploration and development of alternative methods to engineer electronic energy levels in nanostructured matter. The proposed concept of dielectric engineering aims to offer a new way to control electronic bandgap, a fundamental property of any semiconductor, by changing the external dielectric environment on fast time scales and short spatial scales. Currently this property is defined using invasive methods that change core material structure and chemical composition. The latter are approaching intrinsic technological barriers in terms of speed and spatial dimensions. The proposed alternative strategy relies on designing and manipulating electronic states by exclusively taking advantage of many-particle interactions between electronic excitations mediated by Coulomb forces. The class of two-dimensional semiconductors present highly suitable platforms due to their sensitivity to the dielectric surroundings. The project aims to tune the dielectric environment on ultrafast timescales by pulsed optical injection to manipulate electronic states via proximity. The external screening will also be used to imprint spatial geometry of nanoscale objects on the electronic structure of a 2D layer via dielectric effect. Considering rapidly increasing limitations for the conventional methods to define electronic states in matter, the project will open up conceptually different pathways to design semiconductors with the opportunity to tune electronic bandgaps on ultra-fast, sub-picosecond time-scales. This would be many orders of magnitude faster than currently achievable using electronics alone, that are now limited to the nanosecond regime. In turn, this means that optical methods will be increasingly useful to control electronic states without perturbing the material itself, but only its immediate environment. This will allow to create novel, versatile experimental platforms for the fundamental solid state research and establish an alternative approach for electronic structure engineering on the nanoscale. Importantly, the proposed approach will apply to a variety of electronic systems to substantially advance our capabilities to manipulate matter, which is of core importance for the technological progress of the society. It should advance the development of well-defined quantum platforms for information and computation technologies and introduce dielectric engineering methods to guide energy transport for nanoscale detectors or photovoltaics.
In the first period the main activities were focused on (1) Establishing experimental tools for dielectric engineering in the time-domain, (2) Realizing appropriate quantum material platforms, and (3) Demonstrating the impact of strong excitation by low-energy photons on the optical properties of ultrathin materials. Regarding (1), several experimental setups were realized, including a dedicated ultrafast microscopy setup combining optical microscopy with time-resolved, 100 fs detection in pump-probe geometry. Using whitelight super continuum generated by intense laser excitation allowed for addressing a broad spectral range to detect characteristic resonances in two-dimensional semiconductors. The setup was extensively tested and is currently being extended to liquid helium temperatures, while maintaining high spatial and temporal resolution. In addition, an alternative experimental scheme involved combination of sub-bandgap optical excitation and ultra-short-lived photoluminescence was successfully tested. Both setups were combined with precise structural analysis via atomic force microscopy and a combination of optical and THz excitation was tested in an external user facility. To achieve (2), we employed heterostructures featuring combinations of two-dimensional semiconductors with one and few-layer graphene, insulating boron-nitride and titanium oxide quantum dots to realize temporally- and spatially-varying dielectric quantum confinement. Using these structures and the setups from (1) we showed transient bandgap engineering by proximity effect in the neighbouring semiconductor while making graphene conductive for extremely short time. Without perturbing the semiconductor itself, the bandgap was tuned on a sub-picosecond time-scale via transient dielectric screening effect. Using spatially-defined dielectric confinement we demonstrated the impact of quantum dots on the light emission of the proximate two-dimensional material using nanocrystals with extremely large bandgaps in the ultra-violet range. Finally, as a test system for (3) in an extreme limit, we explored the influence of strong excitation via THz photons, showing transient tuning of optical excitations and the required absence of multi-photon injection. The combined results are highly encouraging to achieve substantial changes of the electronic structure at higher temperatures, as well as to merge spatially- and temporally-defined bandgap engineering in the second phase of the project.
The demonstration of transient bandgap change on ultrafast, sub-picosecond timescales by proximity screening alone constitutes one of the main results beyond state-of-the-art. Currently, external dielectric screening is used almost exclusively in the spectral domain in static experimental geometries. As a major advance of the project we showed that it can be optically tuned with high detection fidelity in several model systems. Moreover, we demonstrated the viability of an alternative detection scheme based on observing light emission instead of absorption combined with time-delayed sub-bandgap pumping, by taking advantage of short-lived optical excitations in semiconductor/graphene heterostructures. This method proved to be equally accessible and also easier to integrate in a typical optical laboratory setting to detect photo-induced bandgap changes by dielectric proximity screening. Moreover, it highlights the sensitivity of monitoring luminescence down to very low intensities. Extending it to the available single-photon detection capabilities could allow to sense the impact of only a few 10’s of electrons in the proximate graphene layer, possibly to be extended even to the ultimate single-electron limit. Combined with the promising results related to spatial dielectric confinement this provides clear future directions to exploit transient dielectric bandgap engineering for a variety of increasingly complex and sophisticated scenarios. Most importantly, it will offer viable tools to use it in the context of different material combinations based on the fundamental nature of the studied phenomenon. This can include electronically distinct materials for dynamical screening, with both relativistic and conventional electron dispersions, long lived excitations in semiconductor/graphene heterostructures with a hBN spacer layer, and the additional use of external fields to demonstrate broader applicability space of the proposed approach. The expected results until the end of the project involve demonstration of substantially larger transient changes of the electronic bandgaps up to 100’s of meV, possibility to apply the method up to room temperature, and introduce additional material platforms for ultrafast dielectric bandgap tuning on the nanoscale. The latter should highlight the ultimate limit of using many-body phenomena for non-invasive engineering of the electronic states and offer viable pathways towards technological implementations in ultrathin devices.