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.

Whispering Gallery Mode (WGM) optomechanical resonators are a promising technology for the simultaneous control and measurement of optical and mechanical degrees of freedom at the nanoscale. They offer potential for use across a wide range of applications such as sensors and quantum transducers. Double-disk WGM resonators, which host strongly interacting mechanical and optical modes co-localized around their circumference, are particularly attractive due to their high optomechanical coupling. Large-scale integrated fabrication of silicon double-disk WGM resonators has not previously been demonstrated. In this work, we present a process for the fabrication of double-layer silicon-on-insulator wafers, which we then use to fabricate functional optomechanical double silicon disk resonators with on-chip optical coupling. The integrated devices present experimentally observed optical quality factors of the order of 105 and a single-photon optomechanical coupling of approximately 15 kHz.

A DC non-contact method for measuring the magnetostrictive strain in thin-films is demonstrated, achieving a state-of-the-art sensitivity of 0.1 ppm. In this method, an optical profilometer is used to measure the curvature induced in a magnetostrictively coated coverslip under a DC field through phase-sensitive interferometry. From this the magnetostrictive stress and strain are calculated using Stoney's formula. This addresses limitations of conventional techniques that measure magnetostriction based on the deflection of a cantilever under an AC field, which require complex dedicated set-ups and are sensitive to vibrational noise. Further, it reveals information about the anisotropy of the film and allows for the possibility of measuring multiple samples simultaneously. The theoretical sensitivity limits are derived, predicting a shot-noise-limit of 0.01 ppm. The method is implemented to measure the magnetostrictive hysteresis and piezomagnetic coupling of thin-film galfenol. Degradation in film performance is observed above a thickness of 206 nm, alongside a change in coercivity. This prompts investigation into the growth and optimization of galfenol films for use in devices.

Moire twist angle underpins the interlayer interaction of excitons in twisted van der Waals hetero-and homostructures. The influence of twist angle on the excitonic absorption of twisted bilayer tungsten diselenide (WSe2) has been investigated using electron energy-loss spectroscopy. Atomic-resolution imaging by scanning transmission electron microscopy was used to determine key structural parameters, including the nanoscale measurement of the relative twist angle and stacking order. Detailed spectral analysis revealed a pronounced blueshift in the high-energy excitonic peak C with increasing twist angle, up to 200 meV when compared to the AA' stacking. The experimental findings have been discussed relative to first-principle calculations of the dielectric response of the AA'-stacked bilayer WSe2 as compared to monolayer WSe2 by employing the GW plus Bethe-Salpeter equation approaches, resolving the origin of higher energy spectral features from ensembles of excitonic transitions, and thus any discrepancies between previous calculations. Furthermore, the electronic structure of moire supercells spanning twist angles of -9.5-46.5 degrees calculated by density functional theory were unfolded, showing an uplifting of the conduction band minimum near the Q point and minimal change in the upper valence band concurrently. The combined experiment/theory investigation provides valuable insight into the physical origins of high-energy absorption resonances in twisted bilayers, which enables one to track the evolution of interlayer coupling from tuning of the exciton C transitions by absorption spectroscopy.

Optomechanical magnetometers enable highly sensitive magnetic field sensing. However, all such magnetometers to date have been optically excited and read-out either via free space or a tapered optical fiber. This limits their scalability and integrability, and ultimately their range of applications. Here, we present an optomechanical magnetometer that is excited and read out via a suspended optical waveguide fabricated on the same silicon chip as the magnetometer. Moreover, we demonstrate that thermomechanical noise limited sensitivity is possible using portable electronics and laser. The magnetometer employs a silica microdisk resonator selectively sputtered with a magnetostrictive film of galfenol (FeGa) which induces a resonant frequency shift in response to an external magnetic field. Experimental results reveal the retention of high quality-factor optical whispering gallery mode resonances whilst also demonstrating high sensitivity and dynamic range in ambient conditions. The use of off-the-shelf portable electronics without compromising sensor performance demonstrates promise for applications.

Optomechanical magnetometers enable highly sensitive magnetic field sensing. However, all such magnetometers to date have been optically excited and read-out either via free space or a tapered optical fiber. This limits their scalability and integrability, and ultimately their range of applications. Here, we present an optomechanical magnetometer that is excited and read-out via a suspended optical waveguide fabricated on the same silicon chip as the magnetometer. Moreover, we demonstrate that thermomechanical noise limited sensitivity is possible using portable electronics and laser. The magnetometer employs a silica microdisk resonator selectively sputtered with a magnetostrictive film of galfenol (FeGa) which induces a resonant frequency shift in response to an external magnetic field. Experimental results reveal the retention of high quality-factor optical whispering gallery mode resonances whilst also demonstrating high sensitivity and dynamic range in ambient conditions. The use of off-the-shelf portable electronics without compromising sensor performance demonstrates promise for applications.

Journal Article Disentangling Exciton Linewidth Broadening Factors in Transition Metal Dichalcogenide Monolayer with Electron Energy Loss Spectroscopy Get access Fuhui Shao, Fuhui Shao Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Steffi Y Woo, Steffi Y Woo Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Nianjheng Wu, Nianjheng Wu Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, FranceUniversité Paris-Saclay, Institut des Sciences Moléculaires d'Orsay, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Robert Schneider, Robert Schneider Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Andrew J Mayne, Andrew J Mayne Université Paris-Saclay, Institut des Sciences Moléculaires d'Orsay, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Steffen Michaelis, Steffen Michaelis Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Ashish Arora, Ashish Arora Institute of Physics and Center for Nanotechnology, University of Münster, Münster, GermanyIndian Institute of Science Education and Research, Pune, India Search for other works by this author on: Oxford Academic Google Scholar Benjamin J Carey, Benjamin J Carey Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Johann A Preuß, Johann A Preuß Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Noémie Bonnet, Noémie Bonnet Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar ... Show more Cecilia Mattevi, Cecilia Mattevi Department of Materials, Imperial College London, London, UK Search for other works by this author on: Oxford Academic Google Scholar Kenji Watanabe, Kenji Watanabe Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan Search for other works by this author on: Oxford Academic Google Scholar Takashi Taniguchi, Takashi Taniguchi International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan Search for other works by this author on: Oxford Academic Google Scholar Rudolf Bratschitsch, Rudolf Bratschitsch Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Luiz H G Tizei Luiz H G Tizei Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Corresponding author: luiz.galvao-tizei@universite-paris-saclay.fr Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 1778–1779, https://doi.org/10.1017/S1431927622007036 Published: 01 August 2022

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.

Journal Article Strain Relaxation and Excitonic Absorption of Atomically-Reconstructed WSe2 Moiré Superlattices Get access Steffi Y Woo, Steffi Y Woo Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Fuhui Shao, Fuhui Shao Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Nianjheng Wu, Nianjheng Wu Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, FranceInstitut des Sciences Moléculaires d'Orsay, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Robert Schneider, Robert Schneider Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Ashish Arora, Ashish Arora Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, GermanyIndian Institute of Science Education and Research, Dr. Homi Bhabha Road, 411008 Pune, India Search for other works by this author on: Oxford Academic Google Scholar Johann A Preuß, Johann A Preuß Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Benjamin J Carey, Benjamin J Carey Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Steffen Michaelis de Vasconcellos, Steffen Michaelis de Vasconcellos Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Andrew J Mayne, Andrew J Mayne Institut des Sciences Moléculaires d'Orsay, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Rudolf Bratschitsch, Rudolf Bratschitsch Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar ... Show more Luiz HG Tizei Luiz HG Tizei Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, France Corresponding author: luiz.galvao-tizei@universite-paris-saclay.fr Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2462–2463, https://doi.org/10.1017/S1431927622009424 Published: 01 August 2022

The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride (Si$_3$N$_4$), under different configurations as support or encapsulation material for WS$_2$ monolayers are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified in increasing order of importance by: monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron beam-induced damage on bare TMD monolayer is also revealed to be responsible for irreversible linewidth increases. \textit{h}-BN not only provides clean surfaces of TMD monolayer, and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which \textit{h}-BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.

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The original version of this Article contained errors in the author affiliations. Affiliation 1 incorrectly read ‘School of Chemical Engineering, University of New South Wales (UNSW), Sydney, NSW 2031, Australia’ and affiliation 4 incorrectly read ‘School of Engineering, RMIT University, Melbourne, VIC 3001, Australia.’ This has now been corrected in both the PDF and HTML versions of the Article.

In this work, magnetic and optical properties of magnetic nanoparticles were investigated, where the particles of iron oxide were prepared with a co-precipitation route and the component of gold was built up by reduction of AuCl4- on the surface of iron oxide to assemble nanocomposite structures in the form of an electrostatic stabilized suspension. The size of the particles obtained with TEM increased from of 8.9 ± 2.7 to 16 ± 6 nm after the procedure of hybridisation. In order to distinguish the impact of the gold on the optical properties, UV-Vis and Raman spectroscopy techniques were used. Magnetic properties were studied in the temperature range of 5-300 K and the superparamagnetic state of MNPs at room temperature was confirmed for both systems.

Atomically thin layers of Bi2O3 are isolated from liquid bismuth, allowing the development of ultrafast 2D-enabled UV photo-detectors.

Metal molybdates, such as lead molybdate (PbMoO4), are wide bandgap crystals with favourable photophysical and photocatalytic properties and also high stability. If these crystals are synthesized in two dimensional (2D) morphologies, they can offer the needed large surface-area for photo-reactions. However, this category of materials does not constitute natural stratified crystals, and hence cannot be readily formed into 2D sheets using conventional methods. Here we present a two-step synthesis approach. First, we exfoliate stratified α-MoO3 into α-MoO3-x. Subsequently, these defect rich 2D α-MoO3-x nanosheets are transformed into stable 2D PbMoO4 nanosheets by dipping-pulling. We show that the transformed 2D PbMoO4 display trap states within its bandgap, allowing its efficient performance as a photocatalyst under the visible light condition. The presented method in this work can be extended to establish a variety of highly stable defect rich 2D metal molybdates, which are otherwise challenging to achieve, for visible light region photo-reactions.

While the remarkable properties of 2D crystalline materials offer tremendous opportunities for their use in optics, electronics, energy systems, biotechnology, and catalysis, their practical implementation largely depends critically on the ability to exfoliate them from a 3D stratified bulk state. This goal nevertheless remains elusive, particularly in terms of a rapid processing method that facilitates high yield and dimension control. An ultrafast multiscale exfoliation method is reported which exploits the piezoelectricity of stratified materials that are noncentrosymmetric in nature to trigger electrically-induced mechanical failure across weak grain boundaries associated with their crystal domain planes. In particular, it is demonstrated that microfluidic nebulization using high frequency acoustic waves exposes bulk 3D piezoelectric crystals such as molybdenum disulphide (MoS2 ) and tungsten disulphide (WS2 ) to a combination of extraordinarily large mechanical acceleration (≈108 m s-2 ) and electric field (≈107 V m-1 ). This results in the layered bulk material being rapidly cleaved into pristine quasi-2D-nanosheets that predominantly comprise single layers, thus constituting a rapid and high throughput chip-scale method that opens new possibilities for scalable production and spray coating deposition.

Liquid metal reaction media provides exciting new avenues for synthesizing low-dimensional materials. Here, the synthesis of atomically thin sheets and nanofibers of boehmite (gamma-AlOOH) and their transformation into cubic alumina (gamma-Al2O3) via annealing is explored. The sheets are as thin as one orthorhombic boehmite unit cell. The addition of aluminum into a room temperature alloy of gallium, followed by exposing the melt to either liquid water or water vapor, allows growing either 2D sheets or 1D fibers, respectively. The isolated oxide hydroxides feature large surface areas, with the sheet morphologies also showing a high Young's modulus. The method is green, since the liquid metal solvent can be fully reused. The ultrathin boehmite sheets are found suitable for the development of freestanding membrane filters that enable excellent separation of heavy metal ions and oil from aqueous solutions at extraordinary filtrate flux. The developed liquid metal-based synthesis process offers a sustainable, green, and rapid method for synthesizing nanomorphologies of metal oxides which are challenging to obtain by conventional methods. The process is both sustainable and scalable and may be explored for the creation of other types of metal oxide compounds.

Two-dimensional piezotronics will benefit from the emergence of new crystals featuring high piezoelectric coefficients. Gallium phosphate (GaPO 4 ) is an archetypal piezoelectric material, which does not naturally crystallise in a stratified structure and hence cannot be exfoliated using conventional methods. Here, we report a low-temperature liquid metal-based two-dimensional printing and synthesis strategy to achieve this goal. We exfoliate and surface print the interfacial oxide layer of liquid gallium, followed by a vapour phase reaction. The method offers access to large-area, wide bandgap two-dimensional (2D) GaPO 4 nanosheets of unit cell thickness, while featuring lateral dimensions reaching centimetres. The unit cell thick nanosheets present a large effective out-of-plane piezoelectric coefficient of 7.5 ± 0.8 pm V − 1 . The developed printing process is also suitable for the synthesis of free standing GaPO 4 nanosheets. The low temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future 2D piezoelectric materials.