We explore the zero-phonon line of single photon emitters in helium-ion treated monolayer MoS_2, which are currently understood in terms of single sulfur-site vacancies. By comparing the linewidths of the zero-phonon line as extracted directly from optical spectra with values inferred from the first-order autocorrelation function of the photoluminescence, we quantify bounds of the homogeneous broadening and of phonon-assisted contributions. The results are discussed in terms of both the independent boson model and ab-initio results as computed from GW and Bethe-Salpeter equation approximations.

We study the non-reciprocal properties of the iron oxide α-Fe_2O_3 (hematite) in the canted easy-plane antiferromagnetic phase, specifically in the GHz to THz frequency range. First, using the the microscopic spin Hamiltonian, we obtain the correct classical ground state where the canting is induced by the Dzyaloshinskii-Moriya interactions (DMI). The magnon spectrum is simulated using linear spin wave theory. We then compute the polarizability and the sub-gap optical conductivities using linear response. We find that the conductivity tensor contains frequency peaks at the zero momentum magnon gaps of order 0.1meV which can be tuned by the DMI and on-site anisotropic spin interactions. Furthermore, we show that the canting-induced net magnetic moment 𝐦 represents a measure for the effective time-reversal-symmetry breaking and non-reciprocity of the system: a finite 𝐦 results in a non-zero Hall conductivity. Finally, we discuss the prospective application of hematite in non-reciprocal circulator design, by computing the non-reciprocal circulator transmission amplitude using the conductivities as input.

Compared to bulk solids, defects in low-dimensional materials and, specifically, 2D systems are expected to have a stronger effect, detrimental or beneficial, on their properties. Owing to their geometry, defects in 2D materials can easily be formed due to the interaction with the environment or under impacts of energetic particles, such as ions and electrons. At the same time, many concepts of defect production under irradiation in bulk systems are not applicable for 2D materials or require substantial modifications. Various aspects of the physics and chemistry of defects in 2D materials have been addressed, and the results of these investigations are presented in hundreds of research papers and review articles. However, the challenges and open questions that still remain in the field have received relatively little attention. These topics were recently addressed at the symposium “Defect-mediated engineering of nanomaterials for energy and quantum applications” organized by the Beilstein-Institut. Following the discussions at the symposium, here, we present the challenges and open questions in our understanding of the behavior of defective 2D materials, interaction of energetic particles with low-dimensional targets, and defect-mediated engineering of the properties of 2D systems. We further discuss possible solutions to these problems or suggest “work-arounds”, which should accelerate the progress in the field.

We explore the optical dipole orientation of single photon emitters in monolayer MoS2 as produced by a focused helium ion beam. The single photon emitters can be understood as single sulfur vacancies. The corresponding far-field luminescence spectra reveal several photoluminescence lines below the dominating luminescence of the exciton in MoS2. These sub-bandgap emission lines were predicted by ab initio theory, but they have never been resolved in luminescence experiments because of their small amplitude. We reveal the lines by their dependence as a function of the photon energy and momentum as measured in the back focal plane of the optical circuitry. The agreement between theory and experiment suggests that the defect states interact strongly within the Brillouin zone.

We demonstrate the in situ readout of the spatial profile of suspended MoS2 monolayers hosted on substrates with nanostructured holes. As the profiles are spatially bent, the suspended MoS2 monolayers act as exciton traps with a tunable luminescence intensity and energy. The tunability is realized by controlling the environmental pressure on the monolayers, which allows one to control hundreds of suspended MoS2 monolayers on a single substrate. The in situ readout is based on Fabry-Perot interferences and a model of the corresponding reflectance contrast maps of the investigated monolayers.

Strain engineering promises to enable manipulation and control of the properties of exfoliated flakes of 2D van der Waals (vdW) ferromagnets for spintronic applications. However, while previous studies of strain effects have focused on global properties, the impact on local magnetic spin textures remains unexplored. Here, manipulation of magnetism in the 2D ferromagnet Fe3GeTe2 (FGT) is demonstrated using geometry-induced strain. Employing scanning transmission X-ray microscopy (STXM), the effects of spatially varying strain profiles on the magnetic order of FGT sheets stamped onto micropillar arrays are directly visualized. It is found that the in-plane strain components, with magnitudes <0.5%, locally elevate the Curie temperature of FGT by 10 K, stabilizing magnetic domains near the pillar corners. These domains include skyrmions and higher-order topological spin textures such as skyrmioniums and skyrmion bags. The possibility to locally seed and control topological spin textures via strain opens new avenues for future spin-based information technologies.

Polarized optical contrast spectroscopy is a simple and non-destructive approach to characterize the crystalline anisotropy and orientation of two-dimensional materials. Here, we develop a 3D-printed motorized polarization module, which is compatible with typical microscope platforms and enables to perform broadband polarization-resolved reflectance spectroscopy. As proof of principle, we investigate the in-plane birefringence of exfoliated $$\hbox {MoO}_3$$ thin films and few-layer $$\hbox {WTe}_2$$ crystals. We compare the measured spectra to a model based on a transfer matrix formalism. Compared to other polarization sensitive approaches, such as Raman or second harmonic generation spectroscopy, optical contrast measurements require orders of magnitude less excitation power densities, which is particularly advantageous to avoid degradation of delicate van der Waals layers.

We investigate the interplay between vertical tunneling and lateral transport phenomena in electrically contacted van der Waals heterostructures made from monolayer MoS2, hBN, and graphene. We compare data taken by low-temperature scanning tunneling spectroscopy to results from room-temperature conductive atomic force spectroscopy on monolayer MoS2 with sulfur vacancies and with varying hBN layers. We show that for thick hBN barrier layers, where tunneling currents into the conductive substrate are suppressed, a side-contact still enables addressing the defect states in the scanning tunneling microscopy via the lateral current flow. Few-layer hBN realizes an intermediate regime in which the competition between vertical tunneling and lateral transport needs to be considered. The latter is relevant for device structures with both a thin tunneling barrier and a side-contact to the semiconducting layers.

We demonstrate the in-situ read-out of the spatial profile of suspended MoS_2 monolayers hosted on substrates with nano-structured holes. As the profiles are spatially bent, the suspended MoS_2 monolayers act as exciton traps with tunable luminescence intensity and energy. The tunability is realized by controlling the environmental pressure on the monolayers, which allows to control hundreds of suspended MoS_2 monolayers on a single substrate. The in-situ read-out is based on Fabry-Pérot interferences and a model of the corresponding reflectance contrast maps of the investigated monolayers.

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.

We demonstrate that graphene-based two-terminal devices allow autocorrelating femtosecond mid-infrared pulses with a pulse duration of about 100 fs in the wavelength regime of 5.5-14 mu m. The results suggest that the underlying ultrafast detection principle relies on an electric field dominated autocorrelation in combination with the optoelectronic dynamics at the metal-graphene interfaces. The demonstrated scheme excels because of the ease in nanofabrication of two-terminal graphene-based optoelectronic devices and their robustness.

We report lasing of moiré trapped interlayer excitons (IXs) by integrating a pristine hBN-encapsulated MoSe 2 /WSe 2 heterobilayer into a high- Q (>10 4 ) nanophotonic cavity. We control the cavity-IX detuning using a magnetic field and measure their dipolar coupling strength to be 78 ± 4 micro–electron volts, fully consistent with the 82 micro–electron volts predicted by theory. The emission from the cavity mode shows clear threshold-like behavior as the transition is tuned into resonance with the cavity. We observe a superlinear power dependence accompanied by a narrowing of the linewidth as the distinct features of lasing. The onset and prominence of these threshold-like behaviors are pronounced at resonance while weak off-resonance. Our results show that a lasing transition can be induced in interacting moiré IXs with macroscopic coherence extending over the length scale of the cavity mode. Such systems raise interesting perspectives for low-power switching and synaptic nanophotonic devices using two-dimensional materials.

In van der Waals materials, external strain is an effective tool to manipulate and control electronic responses by changing the electronic bands upon lattice deformation. In particular, the band gap of the layered transition metal pentatelluride HfTe5 is sufficiently small to be inverted by subtle changes of the lattice parameters resulting in a strain-tunable topological phase transition. In that case, knowledge about the spatial homogeneity of electronic properties becomes crucial, especially for the microfabricated thin film circuits used in typical transport measurements. Here, we reveal the homogeneity of exfoliated HfTe5 thin films by spatially resolved Raman microscopy. Comparing the Raman spectra under applied external strain to unstrained bulk references, we pinpoint local variations of Raman signatures to inhomogeneous strain profiles in the sample. Importantly, our results demonstrate that microfabricated contacts can act as sources of significant inhomogeneities. To mitigate the impact of unintentional strain and its corresponding modifications of the electronic structure, careful Raman microscopy constitutes a valuable tool for quantifying the homogeneity of HfTe5 films and circuits fabricated thereof.

We report on the spatial and temporal coherence of interlayer exciton ensembles as photoexcited in MoSe2/WSe2 heterostructures and characterized by point-inversion Michelson-Morley interferometry.(1) Below 10 K, the measured spatial coherence length of the interlayer excitons reaches values equivalent to the lateral expansion of the exciton ensembles. In this regime, the light emission of the excitons turns out to be homogeneously broadened in energy with a high temporal coherence. At higher temperatures, both the spatial and temporal coherence lengths decrease, most likely because of thermal processes. The presented findings point towards a spatially extended, coherent many-body state of interlayer excitons at low temperature.

We show that nano-gaps formed in graphene by utilizing a focused helium ion beam can act as ultrafast photoswitches. By temperature-dependent, time-integrated, and ultrafast photocurrent measurements, we demonstrate that the optoelectronic dynamics across such nano-gaps are dominated by a space-charge limited current in combination with the ultrafast dynamics of hot electrons. The demonstrated methodology allows the creation of ultrafast photoswitches with an amplification gain exceeding the ones as formed by pristine graphene.

We report on the spatial coherence of interlayer exciton ensembles as formed in MoSe_{2}/WSe_{2} heterostructures and characterized by point-inversion Michelson-Morley interferometry. Below 10 K, the measured spatial coherence length of the interlayer excitons reaches values equivalent to the lateral expansion of the exciton ensembles. In this regime, the light emission of the excitons turns out to be homogeneously broadened in energy with a high temporal coherence. At higher temperatures, both the spatial coherence length and the temporal coherence time decrease, most likely because of thermal processes. The presented findings point towards a spatially extended, coherent many-body state of interlayer excitons at low temperature.

We report on the optical absorption characteristics of selectively positioned sulfur vacancies in monolayer MoS2, as observed by photovoltage and photocurrent experiments in an atomistic vertical tunneling circuit at cryogenic and room temperature. Charge carriers are resonantly photoexcited within the defect states before they tunnel through an hBN tunneling barrier to a graphene-based drain contact. Both photovoltage and photocurrent characteristics confirm the optical absorption spectrum as derived from ab initio GW and Bethe-Salpeter equation approximations. Our results reveal the potential of single-vacancy tunneling devices as atomic-scale photodiodes.

Single spin-defects in 2D transition-metal dichalcogenides are natural spin-photon interfaces for quantum applications. Here we report high-field magneto-photoluminescence spectroscopy from three emission lines (Q1, Q2, and Q*) of He-ion induced sulfur vacancies in monolayer MoS 2 . Analysis of the asymmetric PL lineshapes in combination with the diamagnetic shift of Q1 and Q2 yields a consistent picture of localized emitters with a wave function extent of ~3.5 nm. The distinct valley-Zeeman splitting in out-of-plane B -fields and the brightening of dark states through in-plane B -fields necessitates spin-valley selectivity of the defect states and lifted spin-degeneracy at zero field. Comparing our results to ab initio calculations identifies the nature of Q1 and Q2 and suggests that Q* is the emission from a chemically functionalized defect. Analysis of the optical degree of circular polarization reveals that the Fermi level is a parameter that enables the tunability of the emitter. These results show that defects in 2D semiconductors may be utilized for quantum technologies.

We report on the spatial coherence of interlayer exciton ensembles as formed in MoSe_{2}/WSe_{2} heterostructures and characterized by point-inversion Michelson-Morley interferometry. Below 10 K, the measured spatial coherence length of the interlayer excitons reaches values equivalent to the lateral expansion of the exciton ensembles. In this regime, the light emission of the excitons turns out to be homogeneously broadened in energy with a high temporal coherence. At higher temperatures, both the spatial coherence length and the temporal coherence time decrease, most likely because of thermal processes. The presented findings point towards a spatially extended, coherent many-body state of interlayer excitons at low temperature.

One-dimensional semiconductors are interesting materials due to their unique structural features and anisotropy, which grant them intriguing optical, dielectric and mechanical properties. In this work, we report on SnBrP, a lighter homologue of the first inorganic double helix compound SnIP. This class of compounds is characterized by intriguing mechanical and electronic properties, featuring a high flexibility without modulation of physical properties. Semiconducting SnBrP can be synthesized from red phosphorus, tin and tin(II)bromide at elevated temperatures and crystallizes as red-orange, cleavable needles. Raman measurements pointed towards a double helical building unit in SnBrP, showing similarities to the SnIP structure. After taking PL measurements, HR-TEM, and quantum chemical calculations into account, we were able to propose a sense full structure model for SnBrP.