Explore the vast range of titles available.

Nanofabrication research pursues the miniaturization of patterned feature size. In the current state of the art, micron scale areas can be patterned with features down to ~30 nm pitch using electron beam lithography. Here, we demonstrate a nanofabrication technique which allows patterning periodic structures with a pitch down to 16 nm. It is based on focused ion beam milling of suspended membranes, with minimal proximity effects typical to standard electron beam lithography. The membranes are then transferred and used as hard etching masks. We benchmark our technique by electrostatically inducing a superlattice potential in graphene and observe bandstructure modification in electronic transport. Our technique opens the path towards the realization of very short period superlattices in 2D materials, but with the ability to control lattice symmetries and strength. This can pave the way for a versatile solid-state quantum simulator platform and the study of correlated electron phases. Focused-ion beam (FIB) lithography enables high-resolution nanopatterning of 2D materials, but usually introduces significant damage. Here, the authors report a FIB-based fabrication technique to obtain high quality graphene superlattices with 18-nm pitch, which exhibit electronic transport properties similar to those of natural moiré systems.

Compressing light into nanocavities substantially enhances light–matter interactions, which has been a major driver for nanostructured materials research. However, extreme confinement generally comes at the cost of absorption and low resonator quality factors. Here we suggest an alternative optical multimodal confinement mechanism, unlocking the potential of hyperbolic phonon polaritons in isotopically pure hexagonal boron nitride. We produce deep-subwavelength cavities and demonstrate several orders of magnitude improvement in confinement, with estimated Purcell factors exceeding 10 8 and quality factors in the 50–480 range, values approaching the intrinsic quality factor of hexagonal boron nitride polaritons. Intriguingly, the quality factors we obtain exceed the maximum predicted by impedance-mismatch considerations, indicating that confinement is boosted by higher-order modes. We expect that our multimodal approach to nanoscale polariton manipulation will have far-reaching implications for ultrastrong light–matter interactions, mid-infrared nonlinear optics and nanoscale sensors. Exploiting optical multimodal confinement, the deep-subwavelength confinement of hyperbolic phonon polaritons is demonstrated in isotopically pure hexagonal boron nitride, enabling nanoscale polariton manipulation.

Graphene-based moiré superlattices have recently emerged as a unique class of tuneable solid-state systems that exhibit significant optoelectronic activity. Local probing at length scales of the superlattice should provide deeper insight into the microscopic mechanisms of photoresponse and the exact role of the moiré lattice. Here, we employ a nanoscale probe to study photoresponse within a single moiré unit cell of minimally twisted bilayer graphene. Our measurements reveal a spatially rich photoresponse, whose sign and magnitude are governed by the fine structure of the moiré lattice and its orientation with respect to measurement contacts. This results in a strong directional effect and a striking spatial dependence of the gate-voltage response within the moiré domains. The spatial profile and carrier-density dependence of the measured photocurrent point towards a photo-thermoelectric induced response that is further corroborated by good agreement with numerical simulations. Our work shows sub-diffraction photocurrent spectroscopy is an exceptional tool for uncovering the optoelectronic properties of moiré superlattices. Here, the authors use a nanoscale probe to study the photoresponse within a single moiré unit cell of minimally twisted bilayer graphene, and observe an intricate photo-thermoelectric response attributed to the Seebeck coefficient variation at AB-BA domain boundaries.

Second-order superlattices form when moiré superlattices with similar periodicities interfere with each other, leading to larger superlattice periodicities. These crystalline structures are engineered using two-dimensional materials such as graphene and hexagonal boron nitride, and the specific alignment plays a crucial role in facilitating correlation-driven topological phases. Signatures of second-order superlattices have been identified in magnetotransport experiments; however, real-space visualization is still lacking. Here we reveal the second-order superlattice in magic-angle twisted bilayer graphene closely aligned with hexagonal boron nitride through electronic transport measurements and cryogenic nanoscale photovoltage measurements and evidenced by long-range periodic photovoltage modulations. Our results show that even minuscule strain and twist-angle variations as small as 0.01° can lead to drastic changes in the second-order superlattice structure. Our real-space observations, therefore, serve as a ‘magnifying glass’ for strain and twist angle and can elucidate the mechanisms responsible for the breaking of spatial symmetries in twisted bilayer graphene. Second-order superlattices emerging in magic-angle twisted bilayer graphene aligned with hexagonal boron nitride are visualized in real space through cryogenic nano-imaging, revealing the impact of strain and twist-angle variations.

Van der Waals heterostructures are at the forefront in materials heterostructure engineering, offering the ultimate control in layer selectivity and capability to combine virtually any material. Hexagonal-boron nitride, the most commonly used dielectric material, has proven indispensable in this field, allowing the encapsulation of active 2D materials preserving their exceptional electronic quality. However, not all device applications require full encapsulation but rather require open surfaces, or even selective patterning of hBN layers. Here, we report on a procedure to engineer top hBN layers within van der Waals heterostructures while preserving the underlying active 2D layers. Using a soft selective SF6 etching combined with a series of pre—and post-etching treatments, we demonstrate that pristine surfaces can be exposed with atomic scale flatness while preserving the active layers’ electronic quality. We benchmark our technique using graphene/hBN Hall bar devices. Using Raman spectroscopy combined with quantum transport, we show high quality can be preserved in etched regions by demonstrating low temperature carrier mobilities > 200,000 cm2V–1s−1, ballistic transport probed through magnetic focusing, and intrinsic room temperature phonon-limited mobilities. Atomic force microscopy brooming and O2 plasma cleaning are identified as key pre-etching steps for obtaining pristine open surfaces while preserving electronic quality. The technique provides a clean method for opening windows into mesoscopic van der Waals devices that can be used for local probe experiments, patterning top hBN in-situ, and exposing 2D layers to their environment for sensing applications.

The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, particularly when the two layers are rotated relative to each other by a small angle (θ ≈ 1°), owing to the moiré potential induced by the twist. Here we probe the collective excitations of TBG with a spatial resolution of 20 nm, by applying mid-infrared near-field optical microscopy. We find a propagating plasmon mode in charge-neutral TBG for θ = 1.1−1.7°, which is different from the intraband plasmon in single-layer graphene. We interpret it as an interband plasmon associated with the optical transitions between minibands originating from the moiré superlattice. The details of the plasmon dispersion are directly related to the motion of electrons in the moiré superlattice and offer an insight into the physical properties of TBG, such as band nesting between the flat band and remote band, local interlayer coupling, and losses. We find a strongly reduced interlayer coupling in the regions with AA stacking, pointing at screening due to electron–electron interactions. Optical nano-imaging of TBG allows the spatial probing of interaction effects at the nanoscale and potentially elucidates the contribution of collective excitations to many-body ground states.Moiré potentials substantially alter the electronic properties of twisted bilayer graphene at a magic twist angle. A propagating plasmon mode, which can be observed with optical nano-imaging, is associated with transitions between the moiré minibands.

Nanofabrication research pursues the miniaturization of patterned feature size. In the current state of the art, micron scale areas can be patterned with features down to ~ 30 nm pitch using electron beam lithography. Our work demonstrates a new nanofabrication technique which allows patterning periodic structures with a pitch down to 16 nm. It is based on focused ion beam milling of suspended membranes, with minimal proximity effects typical to electron beam lithography. The membranes are then transferred and used as hard etching masks. We benchmark our technique by engineering a superlattice potential in single layer graphene using a thin graphite patterned gate electrode. Our electronic transport characterization shows high quality superlattice properties and a rich Hofstadter butterfly spectrum. Our technique opens the path towards the realization of very short period superlattices in 2D materials, comparable to those in natural moire systems, but with the ability to control lattice symmetries and strength. This can pave the way for a versatile solid-state quantum simulator platform and the study of correlated electron phases.

A conventional optical cavity supports modes which are confined because they are unable to leak out of the cavity. Bound state in continuum (BIC) cavities are an unconventional alternative, where light can leak out, but is confined by multimodal destructive interference. BICs are a general wave phenomenon, of particular interest to optics, but BICs and multimodal interference have never been demonstrated at the nanoscale. Here, we demonstrate the first nanophotonic cavities based on BIC-like multimodal interference. This novel confinement mechanism for deep sub-wavelength light shows orders of magnitude improvement in several confinement metrics. Specifically, we obtain cavity volumes below 100x100x3nm^3 with quality factors about 100, with extreme cases having 23x23x3nm^3 volumes or quality factors above 400. Key to our approach, is the use of pristine crystalline hyperbolic dispersion media (HyM) which can support large momentum excitations with relatively low losses. Making a HyM cavity is complicated by the additional modes that appear in a HyM. Ordinarily, these serve as additional channels for leakage, reducing cavity performance. But, in our experiments, we find a BIC-like cavity confinement enhancement effect, which is intimately related to the ray-like nature of HyM excitations. In fact, the quality factors of our cavities exceed the maximum that is possible in the absence of higher order modes. The alliance of HyM with BICs in our work yields a radically novel way to confine light and is expected to have far reaching consequences wherever strong optical confinement is utilized, from ultra-strong light-matter interactions, to mid-IR nonlinear optics and a range of sensing applications.

Van der Waals heterostructures are at the forefront in materials heterostructure engineering, offering the ultimate control in layer selectivity and capability to combine virtually any material. Hexagonal-boron nitride, the most commonly used dielectric material, has proven indispensable in this field, allowing the encapsulation of active 2D materials preserving their exceptional electronic quality. However, not all device applications require full encapsulation but rather require open surfaces, or even selective patterning of hBN layers. Here, we report on a procedure to engineer top hBN layers within van der Waals heterostructures while preserving the underlying active 2D layers. Using a soft selective SF 6 etching combined with a series of pre—and post-etching treatments, we demonstrate that pristine surfaces can be exposed with atomic scale flatness while preserving the active layers’ electronic quality. We benchmark our technique using graphene/hBN Hall bar devices. Using Raman spectroscopy combined with quantum transport, we show high quality can be preserved in etched regions by demonstrating low temperature carrier mobilities > 200,000 cm 2 V –1 s −1 , ballistic transport probed through magnetic focusing, and intrinsic room temperature phonon-limited mobilities. Atomic force microscopy brooming and O 2 plasma cleaning are identified as key pre-etching steps for obtaining pristine open surfaces while preserving electronic quality. The technique provides a clean method for opening windows into mesoscopic van der Waals devices that can be used for local probe experiments, patterning top hBN in-situ , and exposing 2D layers to their environment for sensing applications.