Abstract
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.
Highlights
The availability of highly intense light, made possible by the development of short-pulse lasers, has allowed for the investigation of nonlinear optical processes.1 There is a wide range of nonlinear optical effects, spanning from frequency mixing2 to the optical Kerr effect.3 The first discovered nonlinear optical effect is second-harmonic generation (SHG),1 which has, together with the related phenomena of sum- and difference-frequency generation, many important applications
The photoelastic coefficients p1 and p2 for the relevant transition metal dichalcogenide (TMD) monolayers were determined by applying varying levels of strain and measuring the polarization-resolved SHG
In order to avoid slipping of the TMD monolayer on the bent substrate, we limited the applied strain to values below 0.5%
Summary
The availability of highly intense light, made possible by the development of short-pulse lasers, has allowed for the investigation of nonlinear optical processes. There is a wide range of nonlinear optical effects, spanning from frequency mixing to the optical Kerr effect. The first discovered nonlinear optical effect is second-harmonic generation (SHG), which has, together with the related phenomena of sum- and difference-frequency generation, many important applications. The availability of highly intense light, made possible by the development of short-pulse lasers, has allowed for the investigation of nonlinear optical processes.. There is a wide range of nonlinear optical effects, spanning from frequency mixing to the optical Kerr effect.. The first discovered nonlinear optical effect is second-harmonic generation (SHG), which has, together with the related phenomena of sum- and difference-frequency generation, many important applications. These effects are used, for example, in optical parametric oscillators, in quantum optics to generate entangled photons, or to produce femtosecond light pulses by Kerr-lens mode-locking.. Odd-ordered nonlinear processes, such as third-harmonic generation (THG), are allowed in all materials
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