Dynamic local strain in graphene generated by surface acoustic waves

We experimentally demonstrate that the Raman active optical phonon modes of single layer graphene can be modulated by the dynamic local strain created by surface acoustic waves (SAWs). In particular, the dynamic strain field of the SAW is shown to induce a Raman scattering intensity variation as large as 15% and a phonon frequency shift of up to 10 cm$^{-1}$ for the G band, for instance, for an effective hydrostatic strain of 0.24% generated in a single layer graphene atop a LiNbO$_{3}$ piezoelectric substrate with a SAW resonator operating at a frequency of $\sim$ 400 MHz. Thus, we demonstrate that SAWs are powerful tools to modulate the optical and vibrational properties of supported graphene by means of the high-frequency localized deformations tailored by the acoustic transducers, which can also be extended to other 2D systems.Comment: 10 pages, 7 figure

Acoustically Driven Stark Effect in Transition Metal Dichalcogenide Monolayers

The Stark effect is one of the most efficient mechanisms to manipulate many-body states in nanostructured systems. In mono- and few-layer transition metal dichalcogenides, it has been successfully induced by optical and electric field means. Here, we tune the optical emission energies and dissociate excitonic states in MoSe2 monolayers employing the 220 MHz in-plane piezoelectric field carried by surface acoustic waves. We transfer the monolayers to high dielectric constant piezoelectric substrates, where the neutral exciton binding energy is reduced, allowing us to efficiently quench (above 90%) and red-shift the excitonic optical emissions. A model for the acoustically induced Stark effect yields neutral exciton and trion in-plane polarizabilities of 530 and 630 × 10-5 meV/(kV/cm)2, respectively, which are considerably larger than those reported for monolayers encapsulated in hexagonal boron nitride. Large in-plane polarizabilities are an attractive ingredient to manipulate and modulate multiexciton interactions in two-dimensional semiconductor nanostructures for optoelectronic applications. © 2021 The Authors. Published by American Chemical Society

In-place bonded semiconductor membranes as compliant substrates for III–V compound devices

Overcoming the critical thickness limit in pseudomorphic growth of lattice mismatched heterostructures is a fundamental challenge in heteroepitaxy. On-demand transfer of light-emitting structures to arbitrary host substrates is an important technological method for optoelectronic and photonic device implementation. The use of freestanding membranes as compliant substrates is a promising approach to address both issues. In this work, the feasibility of using released GaAs/InGaAs/GaAs membranes as virtual substrates to thin films of InGaAs alloys is investigated as a function of the indium content in the films. Growth of flat epitaxial films is demonstrated with critical thickness beyond typical values observed for growth on bulk substrates. Optically active structures are also grown on these membranes with a strong photoluminescence signal and a clear red shift for an InAlGaAs/InGaAs/InAlGaAs quantum well. The red shift is ascribed to strain reduction in the quantum well due to the use of a completely relaxed membrane as the substrate. Our results demonstrate that such membranes constitute a virtual substrate that allows further heterostructure strain engineering, which is not possible when using other post-growth methods

A Luminescent Thermometer Exhibiting Slow Relaxation of the Magnetization : Toward Self-Monitored Building Blocks for Next-Generation Optomagnetic Devices

The development and integration of Single-Molecule Magnets (SMMs) into molecular electronic devices continue to be an exciting challenge. In such potential devices, heat generation due to the electric current is a critical issue that has to be considered upon device fabrication. To read out accurately the temperature at the submicrometer spatial range, new multifunctional SMMs need to be developed. Herein, we present the first self-calibrated molecular thermometer with SMM properties, which provides an elegant avenue to address these issues. The employment of 2,2′-bipyrimidine and 1,1,1-trifluoroacetylacetonate ligands results in a dinuclear compound, [Dy2(bpm)(tfaa)6], which exhibits slow relaxation of the magnetization along with remarkable photoluminescent properties. This combination allows the gaining of fundamental insight in the electronic properties of the compound and investigation of optomagnetic cross-effects (Zeeman effect). Importantly, spectral variations stemming from two distinct thermal-dependent mechanisms taking place at the molecular level are used to perform luminescence thermometry over the 5–398 K temperature range. Overall, these properties make the proposed system a unique molecular luminescent thermometer bearing SMM properties, which preserves its temperature self-monitoring capability even under applied magnetic fields.peerReviewe

Fabrication and optical properties of Strain-free Self-assembled Mesoscopic GaAs structures

We use a combined process of Ga-assisted deoxidation and local droplet etching to fabricate unstrained mesoscopic GaAs/AlGaAs structures exhibiting a high shape anisotropy with a length up to 1.2 μm and a width of 150 nm. We demonstrate good controllability over size and morphology of the mesoscopic structures by tuning the growth parameters. Our growth method yields structures, which are coupled to a surrounding quantum well and present unique optical emission features. Microscopic and optical analysis of single structures allows us to demonstrate that single structure emission originates from two different confinement regions, which are spectrally separated and show sharp excitonic lines. Photoluminescence is detected up to room temperature making the structures the ideal candidates for strain-free light emitting/detecting devices