BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:69d4b43044288 DTSTART:20241211T093000Z SEQUENCE:0 TRANSP:OPAQUE LOCATION:Elements Room and Online (Teams) SUMMARY:ICFO | KRYSTIAN NOWAKOWSKI CLASS:PUBLIC DESCRIPTION:In the last two decades\, two-dimensional (2D) materials have c aptivated the scientific community thanks to their surprising characterist ics and technological potential. Significant advancements in the understan ding of their properties have opened up promises of applications in the el ectronics\, photonics\, sensing\, and energy sectors.\nOne of the most unp recedented prospects is the ability to freely combine various 2D materials into heterostructures. The 2D layers can be twisted with respect to each other\, enabling a novel tuning mechanism in the solid-state physics toolb ox. Introducing a few-degree angle between the layers can create \; a superlattice structure because of the moiré\; effect\, allowing one to tune the material band structure. Graphene-based moiré\; superlat tices have become a focal point of research in recent years\, facilitating the design of many exotic phenomena and effectively bridging the fields o f strongly correlated electrons\, 2D materials\, and topological physics. Optical measurements of these materials were non-existent at the inception of this thesis\, prompting the investigation presented herein.\nIn the fi rst part of this thesis\, the initial goal was to understand the photoresp onse mechanisms of these materials using optoelectronic techniques in the mid-IR wavelength range. Our study reveals a nontrivial polarization-depen dent photocurrent response in graphene-based materials. This response can significantly affect the measured signals and exhibit complex spatial patt erns. A novel approach was developed to extract spatial maps of polarizati on-dependent components.\nOne potential responsible mechanism is the bulk photovoltage effect (BPVE) response. BPVEs are second-order photocurrent g eneration mechanisms that are intricately connected to the quantum geometr ic tensor\, reflecting the phase of the electron wavefunctions and distanc es between quantum states. Measuring BPVEs can provide insights into the u nderlying quantum nature of electronic states in moiré\; materials. We discuss role BPVEs in our results and the potential artifacts that can mimic it as well as the methodology and insights that are fundamental for discerning these different contributions.\nIn the second part of this thes is\, we report the discovery of negative differential conductance (NDC) in the high-bias regime in bilayer graphene aligned with hexagonal boron nit ride. The NDC probably arises from Bloch oscillation-like mechanisms enabl ed by a folded band structure of the \\moire superlattice. From the NDC we engineer a bi-stable state and demonstrate its sensitivity to single phot ons. This moire single photon detector (MSPD) can operate at wavelengths f rom 675 nm to 11.3 um (and beyond) and up to temperatures of 20 K\, a comb ination that remains elusive in the single-photon detector field. The desi gn is compact\, CMOS-compatible\, and array-integrable\, presenting exciti ng opportunities for upscaling.\nThis is the first observation of Bloch os cillations in a 2D system\, enabling the entire arsenal of highly versatil e experimental methods suited to these material platforms. We introduce sp atially resolved photocurrent measurements as a pioneering technique for v isualizing the Bloch oscillation regions. This advancement could lead to n ew high-frequency electronics and optoelectronic applications in addition to single-photon detection.\nThis thesis lays the groundwork for further e xploration of BPVE effects and high-bias phenomena in moiré\; superl attices and promising breakthroughs in Bloch oscillations\, 2D optoelectro nics\, and photodetector technologies. These findings significantly advanc e our understanding of both the applied and fundamental physics in these s ystems\, while also introducing crucial methodological innovations for fut ure research in optoelectronics and photodetection.\n \;\nWednesday De cember 11\, 10:30 h. ICFO Elements room and Online (Teams)\nThesis Directo r: Prof Dr. Frank Koppens DTSTAMP:20260407T073720Z END:VEVENT END:VCALENDAR