Infrared Spectroscopy for Diagnosing Superlattice Minibands in Twisted Bilayer Graphene near the Magic Angle.

Twisted bilayer graphene (TBG) represents a highly tunable, strongly correlated electron system. However, understanding the single-particle band structure alone has been challenging due to a lack of spectroscopic measurements over a broad energy range. Here, we probe the band structure of TBG around the magic angle using infrared spectroscopy and reveal spectral features that originate from interband transitions.

Valley-Hybridized Gate-Tunable 1D Exciton Confinement in MoSe.

Controlling excitons at the nanoscale in semiconductor materials represents a formidable challenge in the quantum photonics and optoelectronics fields. Monolayers of transition metal dichalcogenides (TMDs) offer inherent 2D confinement and possess significant exciton binding energies, making them promising candidates for achieving electric-field-based confinement of excitons without dissociation. Exploiting the valley degree of freedom associated with these confined states further broadens the prospects for exciton engineering.

High-quality nanocavities through multimodal confinement of hyperbolic polaritons in hexagonal boron nitride.

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.

Experimental signatures of the transition from acoustic plasmon to electronic sound in graphene.

Fermi liquids respond differently to perturbations depending on whether their frequency is higher (collisionless regime) or lower (hydrodynamic regime) than the interparticle collision rate. This results in a different phase velocity between the collisionless zero sound and the hydrodynamic first sound. We performed terahertz photocurrent nanoscopy measurements on graphene devices, with a metallic gate close to the graphene layer, to probe the dispersion of propagating acoustic plasmons, the counterpart of sound modes in electronic Fermi liquids.

Transverse Hypercrystals Formed by Periodically Modulated Phonon Polaritons.

Photonic crystals and metamaterials are two overarching paradigms for manipulating light. By combining these approaches, hypercrystals can be created, which are hyperbolic dispersion metamaterials that undergo periodic modulation and mix photonic-crystal-like aspects with hyperbolic dispersion physics. Despite several attempts, there has been limited experimental realization of hypercrystals due to technical and design constraints.

Engineering high quality graphene superlattices via ion milled ultra-thin etching masks.

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.

WSe as Transparent Top Gate for Infrared Near-Field Microscopy.

Independent control of carrier density and out-of-plane displacement field is essential for accessing novel phenomena in two-dimensional (2D) material heterostructures. While this is achieved with independent top and bottom metallic gate electrodes in transport experiments, it remains a challenge for near-field optical studies as the top electrode interferes with the optical path. Here, we characterize the requirements for a material to be used as the top-gate electrode and demonstrate experimentally that few-layer WSe can be used as a transparent, ambipolar top-gate electrode in infrared near-field microscopy.

Tunable and giant valley-selective Hall effect in gapped bilayer graphene.

Berry curvature is analogous to magnetic field but in momentum space and is commonly present in materials with nontrivial quantum geometry. It endows Bloch electrons with transverse anomalous velocities to produce Hall-like currents even in the absence of a magnetic field. We report the direct observation of in situ tunable valley-selective Hall effect (VSHE), where inversion symmetry, and thus the geometric phase of electrons, is controllable by an out-of-plane electric field.

Nano-imaging photoresponse in a moiré unit cell of minimally twisted bilayer graphene.

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.

Graphene-Quantum Dot Hybrid Photodetectors with Low Dark-Current Readout.

Graphene-based photodetectors have shown responsivities up to 10 A/W and photoconductive gains up to 10 electrons per photon. These photodetectors rely on a highly absorbing layer in close proximity to graphene, which induces a shift of the graphene chemical potential upon absorption, hence modifying its channel resistance. However, due to the semimetallic nature of graphene, the readout requires dark currents of hundreds of microamperes up to milliamperes, leading to high power consumption needed for the device operation.

Mid- and Long-Wave Infrared Optoelectronics via Intraband Transitions in PbS Colloidal Quantum Dots.

Optical sensing in the mid- and long-wave infrared (MWIR, LWIR) is of paramount importance for a large spectrum of applications including environmental monitoring, gas sensing, hazard detection, food and product manufacturing inspection, and so forth. Yet, such applications to date are served by costly and complex epitaxially grown HgCdTe quantum-well and quantum-dot infrared photodetectors. The possibility of exploiting low-energy intraband transitions make colloidal quantum dots (CQD) an attractive low-cost alternative to expensive low bandgap materials for infrared applications.

Nonlinear Light Mixing by Graphene Plasmons.

Graphene is known to possess strong optical nonlinearity which turned out to be suitable for creation of efficient saturable absorbers in mode locked fiber lasers. Nonlinear response of graphene can be further enhanced by the presence of graphene plasmons. Here, we report a novel nonlinear effect observed in nanostructured graphene which comes about due to excitation of graphene plasmons.