Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

High-Performance Hybrid Electronic Devices from Layered PtSe₂ Films Grown at Low Temperature

Layered two-dimensional (2D) materials display great potential for a range of applications, particularly in electronics. We report the large-scale synthesis of thin films of platinum diselenide (PtSe₂), a thus far scarcely investigated transition metal dichalcogenide. Importantly, the synthesis by thermally assisted conversion is performed at 400 °C, representing a breakthrough for the direct integration of this material with silicon (Si) technology. Besides the thorough characterization of this 2D material, we demonstrate its promise for applications in high-performance gas sensing with extremely short response and recovery times observed due to the 2D nature of the films. Furthermore, we realized vertically stacked heterostructures of PtSe₂ on Si which act as both photodiodes and photovoltaic cells. Thus, this study establishes PtSe₂ as a potential candidate for next-generation sensors and (opto-)­electronic devices, using fabrication protocols compatible with established Si technologies

Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS₂ and MoS₂ Monolayer and Few-Layer Films

The optical nonlinearity of WS₂ and MoS₂ monolayer and few-layer films was investigated using the <i>Z</i>-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS₂ and MoS₂ films and their dependences on excitation wavelength were studied. WS₂ films with 1–3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10⁴ cm/GW. TPA saturation was observed for the WS₂ film with 1–3 layers and for the MoS₂ film with 25–27 layers. The giant nonlinearity of WS₂ and MoS₂ films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA