Overbias Photon Emission from Light-Emitting Devices Based on Monolayer Transition Metal Dichalcogenides.

Tunneling light-emitting devices (LEDs) based on transition metal dichalcogenides (TMDs) and other two-dimensional (2D) materials are a new platform for on-chip optoelectronic integration. Some of the physical processes underlying this LED architecture are not fully understood, especially the emission at photon energies higher than the applied electrostatic potential, so-called overbias emission. Here we report overbias emission for potentials that are near half of the optical bandgap energy in TMD-based tunneling LEDs. We show that this emission is not thermal in nature but consistent with exciton generation via a two-electron coherent tunneling process.

THE HEALTH SYSTEM OF THE FIRST CZECHOSLOVAK REPUBLIC AND ITS ROLE IN COMBATING CONTAGIOUS DISEASES IMMEDIATELY AFTER THE FIRST WORLD WAR (THE 1920s)

A complex epidemiological situation marked the health system at the time of the establishment of the Czechoslovak Republic. Reducing the number of infectious diseases was an essential task of the State Administration of Health. It required new legislation and various steps directed at reducing infectious diseases. Serious infectious diseases, such as scarlet fever, diphtheria, typhoid, dysentery, smallpox, and malaria, were among the most significant health problems in Czechoslovakia. In 1920, Act No. 412 Coll. regarding compulsory smallpox vaccination was issued, as well as government Regulation No. 298, which describes vaccination obligations and stipulated proper isolation of patients with infectious diseases. Other steps that led to improvements included establishing the National Institute of Health and mobile disinfectant units. Conclusion: The systematic development of new legislation contributed to the new Republic's proficiency at the task and the gradual reduction in the number of infectious diseases.

Exciton-assisted electron tunnelling in van der Waals heterostructures.

The control of elastic and inelastic electron tunnelling relies on materials with well-defined interfaces. Two-dimensional van der Waals materials are an excellent platform for such studies. Signatures of acoustic phonons and defect states have been observed in current-to-voltage measurements. These features can be explained by direct electron-phonon or electron-defect interactions. Here we use a tunnelling process that involves excitons in transition metal dichalcogenides (TMDs). We study tunnel junctions consisting of graphene and gold electrodes separated by hexagonal boron nitride with an adjacent TMD monolayer and observe prominent resonant features in current-to-voltage measurements appearing at bias voltages that correspond to TMD exciton energies. By placing the TMD outside of the tunnelling pathway, we demonstrate that this tunnelling process does not require any charge injection into the TMD. The appearance of such optical modes in electrical transport introduces additional functionality towards van der Waals material-based optoelectronic devices.

WSe2 Light-Emitting Device Coupled to an h-BN Waveguide.

Optical information processing using photonic integrated circuits is a key goal in the field of nanophotonics. Extensive research efforts have led to remarkable progress in integrating active and passive device functionalities within one single photonic circuit. Still, to date, one of the central components, i.e., light sources, remain a challenge to be integrated. Here, we focus on a photonic platform that is solely based on two-dimensional materials to enable the integration of electrically contacted optoelectronic devices inside the light-confining dielectric of photonic structures. We combine light-emitting devices, based on exciton recombination in transition metal dichalcogenides, with hexagonal boron nitride photonic waveguides in a single van der Waals heterostructure. Waveguide-coupled light emission is achieved by sandwiching the light-emitting device between two hexagonal boron nitride slabs and patterning the complete van der Waals stack into a photonic structure. Our demonstration of on-chip light generation and waveguiding is a key component for future integrated van der Waals optoelectronics.

Scalable all-optical cold damping of levitated nanoparticles.

Motional control of levitated nanoparticles relies on either autonomous feedback via a cavity or measurement-based feedback via external forces. Recent demonstrations of the measurement-based ground-state cooling of a single nanoparticle employ linear velocity feedback, also called cold damping, and require the use of electrostatic forces on charged particles via external electrodes. Here we introduce an all-optical cold damping scheme based on the spatial modulation of trap position, which has the advantage of being scalable to multiple particles. The scheme relies on programmable optical tweezers to provide full independent control over the trap frequency and position of each tweezer. We show that the technique cools the centre-of-mass motion of particles along one axis down to 17 mK at a pressure of 2 × 10-6 mbar and demonstrate its scalability by simultaneously cooling the motion of two particles. Our work paves the way towards studying quantum interactions between particles; achieving three-dimensional quantum control of particle motion without cavity-based cooling, electrodes or charged particles; and probing multipartite entanglement in levitated optomechanical systems.

Molecular Encapsulation from the Liquid Phase and Graphene Nanoribbon Growth in Carbon Nanotubes.

Growing graphene nanoribbons from small organic molecules encapsulated in carbon nanotubes can result in products with uniform width and chirality. We propose a method based on encapsulation of 1,2,4-trichlorobenzene from the liquid phase and subsequent annealing. This procedure results in graphene nanoribbons several tens of nanometers long. The presence of nanoribbons was proven by Raman spectra both on macroscopic samples and on the nanoscale by tip-enhanced Raman scattering and high-resolution transmission electron microscopic images.

Encoding information in the mutual coherence of spatially separated light beams.

Coherence has been used as a resource for optical communications since its earliest days. It is widely used for the multiplexing of data, but not for the encoding of data. Here we introduce a coding scheme, which we call mutual coherence coding, to encode information in the mutual coherence of spatially separated light beams. We describe its implementation and analyze its performance by deriving the relevant figures of merit (signal-to-noise ratio, maximum bit-rate, and spectral efficiency) with respect to the number of transmitted beams. Mutual coherence coding yields a quadratic scaling of the number of transmitted signals with the number of employed light beams, which might have benefits for cryptography and data security.

Ponderomotive Squeezing of Light by a Levitated Nanoparticle in Free Space.

A mechanically compliant element can be set into motion by the interaction with light. In turn, this light-driven motion can give rise to ponderomotive correlations in the electromagnetic field. In optomechanical systems, cavities are often employed to enhance these correlations up to the point where they generate quantum squeezing of light. In free-space scenarios, where no cavity is used, observation of squeezing remains possible but challenging due to the weakness of the interaction, and has not been reported so far. Here, we measure the ponderomotively squeezed state of light scattered by a nanoparticle levitated in a free-space optical tweezer. We observe a reduction of the optical fluctuations by up to 25% below the vacuum level, in a bandwidth of about 15 kHz. Our results are explained well by a linearized dipole interaction between the nanoparticle and the electromagnetic continuum. These ponderomotive correlations open the door to quantum-enhanced sensing and metrology with levitated systems, such as force measurements below the standard quantum limit.

Moving across Borders: The Work Life Experiences of Czech Cross-border Workers during the COVID-19 Pandemic.

The experiences of cross-border workers (CBWs) and the difficulties they face during the ongoing COVID-19 pandemic have been neglected in previous research. CBWs experience various stressors under normal circumstances, where they are often subjected to unequal working conditions and forced to transition between two different societies. The measures that were introduced in response to the COVID-19 pandemic, including the implementation of physical borders, further worsened the situation for these individuals. Herein, we draw on qualitative interview data from 35 CBWs from the Czech Republic and Germany to explore their experiences of work, stress, support, and their positioning in society during the pandemic. We detail the dissatisfaction felt by CBWs regarding re-bordering, the lack of coordination in the crisis management, and the lack of support from national governments and the EU. This study provides unique insights into the difficulties and experiences of CBWs in a time of crisis.

Tip-Enhanced Stokes-Anti-Stokes Scattering from Carbyne.

Carbyne, a linear chain of carbon atoms, is the truly one-dimensional allotrope of carbon and the strongest known Raman scatterer. Here, we use tip-enhanced Raman scattering (TERS) to further enhance the Raman response of a single carbyne chain confined inside a double-walled carbon nanotube. We observe an increase of the anti-Stokes scattering by a factor of 3290 and a 22-fold enhancement of the anti-Stokes/Stokes ratio relative to far-field measurements. The power dependence of the anti-Stokes/Stokes ratio under TERS conditions is indicative of coherent Stokes-anti-Stokes scattering mediated by an excited phonon. The role of resonance effects and laser-induced heating are discussed and potential opportunities are outlined.

Freestanding and Permeable Nanoporous Gold Membranes for Surface-Enhanced Raman Scattering.

Surface-enhanced Raman spectroscopy (SERS) demands reliable, high-enhancement substrates in order to be used in different fields of application. Here we introduce freestanding porous gold membranes (PAuM) as easy-to-produce, scalable, mechanically stable, and effective SERS substrates. We fabricate large-scale sub-30 nm thick PAuM that form freestanding membranes with varying morphologies depending on the nominal gold thickness. These PAuM are mechanically stable for pressures up to more than 3 bar and exhibit surface-enhanced Raman scattering with local enhancement factors from 104 to 105, which we demonstrate by wavelength-dependent and spatially resolved Raman measurements using graphene as a local Raman probe. Numerical simulations reveal that the enhancement arises from individual, nanoscale pores in the membrane acting as optical slot antennas. Our PAuM are mechanically stable, provide robust SERS enhancement for excitation power densities up to 106 W cm-2, and may find use as a building block in SERS-based sensing applications.

Tip-enhanced Raman spectroscopy of confined carbon chains.

Long linear chains of carbon encapsulated in carbon nanotubes represent the finite realization of carbyne, the truly one-dimensional carbon allotrope. Driven by advances in the synthesis of such structures, carbyne has attracted significant interest in recent years, with numerous experimental studies exploring its remarkable properties. As for other carbon nanomaterials, Raman spectroscopy has played an important role in the characterization of carbyne. In particular, tip-enhanced Raman scattering (TERS) has enabled imaging and spectroscopy down to the single-chain level. In this article, we provide a general introduction to carbyne and discuss the principles and experimental implementation of TERS as a key technology for the investigation of this material system. Within this context, the development of optical nanoantennas as TERS probes is addressed. We then summarize the latest progress in the Raman spectroscopic characterization of confined carbyne, with a focus on the findings assisted by TERS. Finally, we discuss open questions in the field and outline how TERS can contribute to solving them in future studies.

Sub-Kelvin Feedback Cooling and Heating Dynamics of an Optically Levitated Librator.

Rotational optomechanics strives to gain quantum control over mechanical rotors by harnessing the interaction of light and matter. We optically trap a dielectric nanodumbbell in a linearly polarized laser field, where the dumbbell represents a nanomechanical librator. Using measurement-based parametric feedback control in high vacuum, we cool this librator from room temperature to 240 mK and investigate its heating dynamics when released from feedback. We exclude collisions with residual gas molecules as well as classical laser noise as sources of heating. Our findings indicate that we observe the torque fluctuations arising from the zero-point fluctuations of the electromagnetic field.

Resonant Light Emission from Graphene/Hexagonal Boron Nitride/Graphene Tunnel Junctions.

Single-layer graphene has many remarkable properties but does not lend itself as a material for light-emitting devices as a result of its lack of a band gap. This limitation can be overcome by a controlled stacking of graphene layers. Exploiting the unique Dirac cone band structure of graphene, we demonstrate twist-controlled resonant light emission from graphene/hexagonal boron nitride (h-BN)/graphene tunnel junctions. We observe light emission irrespective of the crystallographic alignment between the graphene electrodes. Nearly aligned devices exhibit pronounced resonant features in both optical and electrical characteristics that vanish rapidly for twist angles θ ≳3°. These experimental findings can be well-explained by a theoretical model in which the spectral photon emission peak is attributed to photon-assisted momentum conserving electron tunneling. The resonant peak in our aligned devices can be spectrally tuned within the near-infrared range by over 0.2 eV, making graphene/h-BN/graphene tunnel junctions potential candidates for on-chip optoelectronics.

Kovacs Memory Effect with an Optically Levitated Nanoparticle.

The understanding of the dynamics of nonequilibrium cooling and heating processes at the nanoscale is still an open problem. These processes can follow surprising relaxation paths due to, e.g., memory effects, which significantly alter the expected equilibration routes. The Kovacs effect can take place when a thermalization process is suddenly interrupted by a change of the bath temperature, leading to a nonmonotonic evolution of the energy of the system. Here, we demonstrate that the Kovacs effect can be observed in the thermalization of the center of mass motion of a levitated nanoparticle. The temperature is controlled during the experiment through an external source of white Gaussian noise that mimics an effective thermal bath at a temperature that can be changed faster than any relaxation time of the system. We describe our experiments in terms of the dynamics of a Brownian particle in a harmonic trap without any fitting parameter, suggesting that the Kovacs effect can appear in a large variety of systems.

Anti-Stokes Raman Scattering of Single Carbyne Chains.

We investigate the anti-Stokes Raman scattering of single carbyne chains confined inside double-walled carbon nanotubes. Individual chains are identified using tip-enhanced Raman scattering (TERS) and heated by resonant excitation with varying laser powers. We study the temperature dependence of carbyne's Raman spectrum and quantify the laser-induced heating based on the anti-Stokes/Stokes ratio. Due to its molecular size and its large Raman cross section, carbyne holds great promise for local temperature monitoring, with potential applications ranging from nanoelectronics to biology.