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Abstract Faraday 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 (V IL ≈ +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.

Strain engineering can manipulate the propagation of excitons in atomically thin transition metal dichalcogenides. Here, the authors observe an anti-funnelling behavior, i.e., the exciton photoluminescence moves away from high-strain regions, and attribute it to the dominating role of propagating dark excitons.

This work demonstrates spectral dependencies of reflectivity and the transversal Kerr effect of a series of permalloy magnetoplasmonic crystals based on shallow lamellar diffraction gratings with a period of 500 nm, stripe’s width of 250 nm, and diffraction stripes’ heights of 28, 43, 67, and 88 nm. The fabricated magnetoplasmonic crystals show a monotonic increase of the transversal Kerr effect and the diffraction figure-of-merit with higher diffraction stripes. The maximum achieved modulation value of the transversal Kerr effect was found to be 0.78%, which can be further tuned by the periodicity and stripes width of the magnetoplasmonic crystals.

The emission of light from correlated excitonic complexes has been recently observed in atomically thin transition metal dichalcogenides. Here, the authors report electroluminescence generated by a pulsed gate voltage from excitons, trions, and biexcitons in monolayer WSe2 and WS2 encapsulated with hBN.

In atomically thin transition metal dichalcogenides, spin- and valley-polarised states can be addressed thanks to large spin–orbit coupling and circular dichroism. Here, the authors investigate theoretically and experimentally the decay dynamics of spin and valley polarisation in transition metal dichalcogenide monolayers.

Excitons, quasi-particles of bound electron-hole pairs, are at the core of the optoelectronic properties of layered transition metal dichalcogenides. Here, the authors unveil the presence of interlayer excitons in bulk van der Waals semiconductors, arising from strong localization and spin-valley coupling of charge carriers.

Strain engineering: Tuning the bandgap of 2D materials The bandgap of two-dimensional semiconducting materials can be easily tuned in real time by stretching or compressing them. An international team of researcher led by Dr. Andres Castellanos-Gomez at IMDEA Nanoscience, Spain, studied the optical properties of single-atom thick two-dimensional semiconductors under the application of tensile or compressive biaxial strain. In order to apply the strain the researchers exploited the thermal expansion or compression of the different substrates carrying the atomically thin materials and then compared their results to atomistic simulations. This strain method can be applied in a fast and reversible way and it leads to large changes in the band structure of these semiconducting materials. Research into strain engineering two-dimensional materials may help us in fabricating novel devices like color-changing light emitters or novel and more efficient solar cells.

Monolayer transition metal dichalcogenides are promising materials for valleytronics applications. Here, the authors study WS2samples using photoluminescence spectroscopy and time-resolved Kerr-rotation measurements at low temperatures, gaining insight into the valley dynamics of excitons.

Understanding the ultrafast coupling and relaxation mechanisms between valleys in transition metal dichalcogenide semiconductors is of crucial interest for future valleytronic devices. Recent ultrafast pump-probe experiments showed an unintuitive significant bleaching at the excitonic B transition after optical excitation of the energetically lower excitonic A transition. Here, we present a possible microscopic explanation for this surprising effect. It is based on the joint action of exchange coupling and phonon-mediated thermalization into dark exciton states and does not involve a population of the B exciton. Our work demonstrates how intra- and intervalley coupling on a femtosecond timescale governs the optical valley response of 2D semiconductors.

We demonstrate the coupling of single color centers in diamond to plasmonic and dielectric photonic structures to realize novel nanophotonic devices. Nanometer spatial control in the creation of single color centers in diamond is achieved by implantation of nitrogen atoms through high-aspect-ratio channels in a mica mask. Enhanced broadband single-photon emission is demonstrated by coupling nitrogen–vacancy centers to plasmonic resonators, such as metallic nanoantennas. Improved photon-collection efficiency and directed emission is demonstrated by solid immersion lenses and micropillar cavities. Thereafter, the coupling of diamond nanocrystals to the guided modes of micropillar resonators is discussed along with experimental results. Finally, we present a gas-phase-doping approach to incorporate color centers based on nickel and tungsten, in situ into diamond using microwave-plasma-enhanced chemical vapor deposition. The fabrication of silicon–vacancy centers in nanodiamonds by microwave-plasma-enhanced chemical vapor deposition is discussed in addition.

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.

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