Twist Optoelectronics

The past decade has witnessed a paradigm change in the creation of designer electronic materials with the possibility to stack together 2D crystalline layers with nearly perfect interfaces to form van der Waals (vdW) heterostructures¹. The range of possible electronic phenomena and quantum phases readily accessible is virtually unlimited. The nature of the correlated states and emergent phenomena in general has not been understood. A comprehensive physical understanding of the emergent physics and in particular the interplay between topology and interactions is essential for the design of quantum material system with tailored properties, and to design higher temperature magnets, superconductors etc.

Emerging as a versatile probe photocurrent can sense a combination of properties, including electronic states, Bloch band quantum geometry, quantum kinetic processes, and device characteristics of quantum materials. 

Quantum technologies

The exotic nature of the interaction between light and the moiré materials systems offers unique opportunities for designing and creation of new quantum technologies:

• novel ways to detect light and even single photons, e.g., by driving moiré systems strongly out-of-equillibrium.
• Novel tuneable quantum light generation mechanisms by exploiting Bloch oscillations.
• Ultra-broadband photodetectors with twisted materials²

Cryogenic near-field microscopy

Optical probing, in particular with infrared and  THz frequencies, has been one of the most commonly used techniques for unravelling the properties of correlated electron systems. However, the lack of nanoscale spatial resolution has inhibited access to intrinsic spatial variations that are prototypical in correlated systems such as phase separation, domain formation, collective modes, with characteristic length-scales of tens to hundreds of nanometres. We solve this problem by developing a unique optical nano-imaging instrument with unprecedented capabilities: probe down to 6K the THz spectral response with nanometre-scale spatial resolution, and offer access to higher-momentum excitations.

Recently, we observed interband plasmons in twisted graphene³, following earlier works on (acoustic) plasmons in graphene^4–7.

Photocurrent nanoscopy

We employ photocurrent nanoscopy to study photoresponse within twisted 2D materials^8–10. The tool reveals a spatially rich photoresponse as an exceptional tool for uncovering the optoelectronic properties of moiré superlattices. Moreover, in the naturally occurring interfaces of AB- and BA-stacked bilayer graphene the chirality in the valleys is opposite, leading to topologically protected valley states.

References:

1. Andrei, E. Y. et al. The marvels of moiré materials. Nat Rev Mater 6, 201–206 (2021).
2. Agarwal, H. et al. Ultra-broadband photoconductivity in twisted graphene heterostructures with large responsivity. Nat. Photon. (2023) doi:10.1038/s41566-023-01291-0.
3. Hesp, N. C. H. et al. Observation of interband collective excitations in twisted bilayer graphene. Nature Physics 17, 1162–1168 (2021).
4. Woessner, A. et al. Highly confined low-loss plasmons in graphene-boron nitride heterostructures. Nature Materials 14, 421–425 (2015).
5. Woessner, A. et al. Propagating Plasmons in a Charge-Neutral Quantum Tunneling Transistor. ACS Photonics 4, 3012–3017 (2017).
6. Chen, J. et al. Optical nano-imaging of gate-tunable graphene plasmons. Nature 487, 77–81 (2012).
7. Lundeberg, M. B. et al. Tuning quantum nonlocal effects in graphene plasmonics. Science 357, 187–191 (2017).
8. Woessner, A. et al. Near-field photocurrent nanoscopy on bare and encapsulated graphene. Nature communications 7, 1–7 (2016).
9. Lundeberg, M. B. et al. Thermoelectric detection and imaging of propagating graphene plasmons. Nature materials 16, 204–207 (2017).
10. Hesp, N. C. et al. Nano-imaging photoresponse in a moiré unit cell of minimally twisted bilayer graphene. Nature communications 12, 1–8 (2021).