Abstract The linear absorption spectrum of excitons in transition metal dichalcogenides monolayers under the influence of an in-plane magnetic field is theoretically studied. We demonstrate that in-plane magnetic fields induce a hybridization between spin-bright and spin-dark exciton transitions, resulting in a brightening of spin-dark excitons. We analytically investigate spectral features including resonance energy shifts, broadening and amplitudes ratios. In particular, for a MoSe 2 monolayer with linewidths dominated by reradiation, we find a complex interplay of dark-bright splitting and linewidth difference of both involved spin-bright and spin-dark excitons.

Transition metal complexes are well-known for their efficient light emission and are promising for applications ranging from bioimaging to light-emitting diodes. In solution, interactions between the metal centers of two complexes become possible and drastically change the photophysical properties. For real-world devices, solid-state materials consisting of these molecules are preferable. Recently, the ligand-controlled aggregation of platinum(II) and palladium(II) complexes into molecular single crystals and the controlled formation of metal-metal contacts have been demonstrated. Here, we show how the metal-metal distance can be tuned in a controlled way by exerting pressure on the molecular crystal. Using optical spectroscopy inside a diamond anvil cell, we find strong and reversible piezochromism up to 18 GPa. Using time-dependent density functional theory, we attribute the wavelength shift to a reduction in the metal-metal distance and enhanced π orbital overlap in the dimers.

The linear absorption spectrum of excitons in TMDC monolayers under the influence of an in-plane magnetic field is theoretically studied. We demonstrate that in-plane magnetic fields induce a hybridization between spin-bright and spin-dark exciton transitions, resulting in a brightening of spin-dark excitons. We analytically investigate spectral features including resonance energy shifts, broadening and amplitudes ratios. In particular, for a MoSe_2 monolayer with radiatively-limited linewidth, we find a complex interplay of dark-bright splitting and linewidth difference of both involved spin-bright and spin-dark excitons.

AbstractFaraday rotation is a fundamental effect in the magneto-optical response of solids, liquids and gases. Materials with a large Verdet constant find applications in optical modulators, sensors and non-reciprocal devices, such as optical isolators. Here, we demonstrate that the plane of polarization of light exhibits a giant Faraday rotation of several degrees around the A exciton transition in hBN-encapsulated monolayers of WSe2 and MoSe2 under moderate magnetic fields. This results in the highest known Verdet constant of -1.9 × 107 deg T−1 cm−1 for any material in the visible regime. Additionally, interlayer excitons in hBN-encapsulated bilayer MoS2 exhibit a large Verdet constant (VIL ≈ +2 × 105 deg T−1 cm−2) of opposite sign compared to A excitons in monolayers. The giant Faraday rotation is due to the giant oscillator strength and high g-factor of the excitons in atomically thin semiconducting transition metal dichalcogenides. We deduce the complete in-plane complex dielectric tensor of hBN-encapsulated WSe2 and MoSe2 monolayers, which is vital for the prediction of Kerr, Faraday and magneto-circular dichroism spectra of 2D heterostructures. Our results pose a crucial advance in the potential usage of two-dimensional materials in ultrathin optical polarization devices.

Two-dimensional inorganic-organic hybrid perovskites are in the limelight due to their potential applications in photonics and optoelectronics. They are environmentally stable, and their various chemical compositions offer a wide range of bandgap energies. Alternatively, crystal deformation enables in situ control over their optical properties. Here, we investigate (C6H9C2H4NH3)2PbI4, a hybrid perovskite whose organic linkers are 2-(1-cyclohexenyl)ethylammonium. Pressure-dependent optical absorption and emission spectroscopy reveal a hysteretic piezochromism that was not reported for other lead iodide-based 2D perovskites. We combine our optical studies with high-pressure X-ray diffraction experiments and first-principles calculations to demonstrate that the deformation of the inorganic lead iodide layers is the main reason for the observed changes in the optical bandgap.

The optical and electronic properties of multilayer transition metal dichalcogenides differ significantly from their monolayer counterparts due to interlayer interactions. The separation of individual layers can be tuned in a controlled way by applying pressure. Here, we use a diamond anvil cell to compress bilayers of 2H-MoS2 in the gigapascal range. By measuring optical transmission spectra, we find that increasing pressure leads to a decrease in the energy splitting between the A and the interlayer exciton. Comparing our experimental findings with ab initio calculations, we conclude that the observed changes are not due to the commonly assumed hydrostatic compression. This effect is attributed to the MoS2 bilayer adhering to the diamond, which reduces the in-plane compression. Moreover, we demonstrate that the distinct real-space distributions and resulting contributions from the valence band account for the different pressure dependencies of the inter- and intralayer excitons in compressed MoS2 bilayers.

A Faraday rotation spectroscopy (FRS) technique is presented for measurements on the micrometer scale. Spectral acquisition speeds of about two orders of magnitude faster than state-of-the-art modulation spectroscopy setups are demonstrated. The experimental method is based on charge-coupled-device detection, avoiding speed-limiting components, such as polarization modulators with lock-in amplifiers. At the same time, FRS spectra are obtained with a sensitivity of 20 µrad ( 0.001 ° \[0.001{\bm{^\circ }}\] ) over a broad spectral range (525-800 nm), which is on par with state-of-the-art polarization-modulation techniques. The new measurement and analysis technique also automatically cancels unwanted Faraday rotation backgrounds. Using the setup, Faraday rotation spectroscopy of excitons is performed in a hexagonal boron nitride-encapsulated atomically thin semiconductor WS2 under magnetic fields of up to 1.4 T at room temperature and liquid helium temperature. An exciton g-factor of -4.4 ± 0.3 is determined at room temperature, and -4.2 ± 0.2 at liquid helium temperature. In addition, FRS and hysteresis loop measurements are performed on a 20 nm thick film of an amorphous magnetic Tb20 Fe80 alloy.

We study experimentally the propagation of spin waves in waveguide structures consisting of two submicrometer-width yttrium iron garnet waveguides intersecting at a right angle. We show that, despite the significant spatial variations of the internal static magnet field and the in-plane anisotropy of the dispersion characteristics, the incident spin wave can efficiently pass through the microscopic cross-shaped structure and be transmitted into all its arms. This process depends strongly on the frequency of the wave and the orientation of the static magnetic field. By varying these parameters, one can achieve a controllable uniform or preferential transmission of the wave into different arms of the cross. Our results create the basis for the implementation of nanoscale magnonic networks to be used for the realization of complex non-Boolean data-processing schemes, including neuromorphic computing.

Mechanical strain is a powerful tool to tune the optical and optoelectronic properties of atomically thin semiconductors. Inhomogeneous strain plays an important role in exciton funneling and the activation of single-photon emitters in 2D materials. Here, we create an inhomogeneous strain profile in a 2D semiconductor on a micrometer scale by a nanoimprint process. We present a nanoimprint setup, where a mold is used to apply pressure in a controlled way to a WS2 monolayer on a heated polymer layer. After printing, the strain created in the 2D semiconductor is verified by hyperspectral optical imaging. The developed nanoimprint technique is scalable and could be transferred to commercial nanoimprint machines.

We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe2, WS2,MoSe2, and MoS2 monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, X-D(-), 19 meV below the lowest bright trion, X-1(-), in WSe2 and WS2. In MoSe2, X-D(-) lies 6 meV above X-1(-), while X-D(-) and X-1(-) almost coincide in MoS2. Our results agree with GW-Bethe-Salpeter equation (GW-BSE) ab initio calculations and quantitatively explain the optical response of doped monolayers with temperature.

Defect centers in hexagonal boron nitride represent room-temperature single-photon sources in a layered van der Waals material. These light emitters appear with a wide range of transition energies ranging over the entire visible spectrum, which renders the identification of the underlying atomic structure challenging. In addition to their eminent properties as quantum light emitters, the coupling to phonons is remarkable. Their photoluminescence exhibits significant side band emission well separated from the zero phonon line (ZPL) and an asymmetric broadening of the ZPL itself. In this combined theoretical and experimental study we show that the phonon side bands can be well described in terms of the coupling to bulk longitudinal optical (LO) phonons. To describe the ZPL asymmetry we show that in addition to the coupling to longitudinal acoustic (LA) phonons also the coupling to local mode oscillations of the defect center with respect to the entire host crystal has to be considered. By studying the influence of the emitter's wave function dimensions on the phonon side bands we find reasonable values for the size of the wave function and the deformation potentials. We perform photoluminescence excitation measurements to demonstrate that the excitation of the emitters is most efficient by LO-phonon assisted absorption.