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Topological nano-photonics

Periodic Reporting for period 4 - TOPONANOP (Topological nano-photonics)

Reporting period: 2022-05-01 to 2023-05-31

Physical systems or devices where time-reversal-symmetry (TRS) is broken exhibit striking and counterintuitive phenomena. The canonical example is the quantum hall effect where a strong magnetic field breaks TRS in a two-dimensional conductor, yielding one-directional electron flow (i.e. non-reciprocal) at the edges. The phenomena can be understood in the general framework of topology and topological protection. Recently, with the advent of novel exotic quantum materials such as 2d-materials, twisted 2d-materials, and thin-film topological insulators, entirely new concepts for topological and non-reciprocal phenomena have appeared on the horizon. Instead of relying on a magnetic field, these materials possess intrinsic topological characteristics due to quantum mechanical interferences of the electrons in the crystal. These discoveries have sparked intense studies, leading to the discovery of new topological phases, new types of highly unusual material properties, and potential technological applications.

Naturally, the question arises as to whether these concepts of topology can also be applied to the electromagnetic response in these materials. This question is what has inspired project Topological Nano-photonics (TOPONANOP), aimed at bridging nano-photonics with exotic phenomena in novel quantum materials.

The key novel experimental tool for TOPONANOP was the cryogenic near-field microscope for infrared and terahertz light. Cryogenic near-field microscopy is highly challenging, as it requires highly stable AFM operation at temperatures of 4-6K while providing optical access for infrared and terahertz light. A commercial cryo-SNOM (Neaspec) was modified to probe photocurrents for infrared and terahertz light with a spatial resolution of approximately 20 nanometers. The figure below illustrates the nanoscopic probing of a moiré system, where the spatial resolution of 20 nm allows for resolving features inside the moiré unit cells.
During the course of Toponanop, twisted graphene was discovered as a fascinating topological material with interacting electrons and numerous control knobs. For this reason, we focussed a portion of the project on this material system and applied our unique experimental techniques. Since twisted graphene is gate-tunable, we were able to switch the electron interaction effects on and off while probing the local response of the system. This has provided us with unique insights into the quantum metric and topological properties of the system, as well as the interplay of these properties with the electron interaction effects.

Several studies led to often surprising results that were clearly beyond the state-of-the-art and demonstrated novel capabilities and physical insights:

• The first photocurrent mapping of a moiré system with 20-nm spatial resolution has been achieved. This technical capability has been adopted by other research groups as it can be applied to materials that are encapsulated, where other scanning-probe techniques are no longer applicable. In particular, photocurrent nanoscopy has emerged as a versatile probe for sensing a combination of properties, including correlated electron states, Bloch band quantum geometry, quantum kinetic processes, and device characteristics of quantum materials.
• By applying the novel cryo-SNOM tool to a novel topological material system, it was discovered that the degree of interaction-induced valley polarization can be probe directly AND locally. The capability for creating spatial map enables the connection of local variations in material properties (e.g. twist angle and strain) with electronic correlations.
• By twisting two layers of bilayer graphene, we demonstrated a giant ultra-broadband photoconductivity in twisted double bilayer graphene heterostructures spanning a spectral range of 2-100 μm with internal quantum efficiencies ~ 40 % at speeds of 100 kHz. The giant response originates from unique properties of twist-decoupled heterostructures including pristine, crystal field induced terahertz band gaps, parallel photoactive channels, and strong photoconductivity enhancements caused by interlayer screening of electronic interactions by respective layers acting as sub-atomic spaced proximity screening gates. The achievements introduce twist-decoupled graphene heterostructures as a viable route for engineering gapped graphene photodetectors with 3D scalability.
• Patterning of ultrasharp gold structures on hBN enabled the creation of ultra-small nanocavities with relatively high quality factor and the formation of nanophotonic lattices. By combining these two techniques, a novel nanophotonic topological state has been realised, which exhibits robustness, strong confinement and tunability. This achievement marked the first instance of extending topological nanophotonics into the deep subwavelength regime. This discovery has the potential to revolutionize the field, as it can be directly extended and hybridized with other Van der Waals materials, enabling expanded spectral coverage and compatibility with diverse electronic and excitonic systems.
These groundbreaking results have a broader impact on society.
First, topological nanophotonic systems have opened new avenues for the development of more resilient deep subwavelength optical components. This encompasses the realization of nanocavities with topologically fixed resonant frequencies or waveguides that are tolerant to fabrication disorder. The advancement of these photonic systems could lead to the development of new technologies such as ultra-fast and energy-efficient data transfer systems, sensors and active nanoscale optoelectronic components.
Secondly, the infrared and terahertz properties of twisted graphene systems, owing to its unique band structure, topological and correlated electron properties, offer new avenues for engineering detectors and emitters. These developments are relevant to various industries, including the data communications sector, industry 4.0 (e.g. inspection processes), medical imaging, space exploration, and on-chip optical circuits with active component.
In particular, the broadband photodetector can address a wide range of applications since there is no single technology that can cover such an extensive wavelength range. Simultaneously, the requirements for broadband photodetection are becoming exceedingly stringent in hyperspectral imaging. While existing technologies are constrained by their spectral range, our work has showcased a rare instance of an intrinsic ultra-broadband infrared-terahertz photoconductor that is compatible with complementary metal-oxide-semiconductor technology that can also be integrated into arrays. This introduces a viable approach to engineering gapped graphene photodetectors with 3D scalability.

Illustrations attached below (1-6)
Major achievement section