Optical imaging of strain in two-dimensional crystals

Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows to extract the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.Comment: 13 pages, 4 figure

Second harmonic generation in strained transition metal dichalcogenide monolayers: MoS2, MoSe2, WS2, and WSe2

Second-harmonic generation (SHG) is a powerful measurement technique to analyze the symmetry properties of crystals. Mechanical strain can reduce the symmetry of a crystal and even weak strain can have a considerable impact on the SHG intensity along different polarization directions. The impact of strain on the SHG can be modeled with a second-order nonlinear photoelastic tensor. In this work, we determined the photoelastic tensors at a fundamental wavelength of 800 nm for four different transition metal dichalcogenide (TMD) monolayers: MoS2, MoSe2, WS2, and WSe2. Strain is applied using a three-point bending scheme, and the polarization-resolved SHG pattern is measured in backscattering geometry. Furthermore, we connected the strain dependent SHG with the strain dependence of the A-exciton energy. With the second-order nonlinear photoelastic tensor, full strain information can be accurately extracted from polarization-resolved SHG measurements. Accordingly, uniaxial strain, induced by polydimethylsiloxan (PDMS) exfoliation and transfer, is measured. We find that TMD monolayers fabricated with PDMS are strained by ∼0.2%. With the experimentally determined nonlinear photoelastic tensors, it will be possible to optically probe arbitrary strain fields in TMD monolayers

Intrinsic donor-bound excitons in ultraclean monolayer semiconductors

© 2021, The Author(s). The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellitesʼ photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors