Exciton-Sensitized Second-Harmonic Generation in 2D Heterostructures

The efficient optical second-harmonic generation (SHG) of two-dimensional (2D) crystals, coupled with their atomic thickness that circumvents the phase-match problem, has garnered considerable attention. While various 2D heterostructures have shown promising applications in photodetectors, switching electronics, and photovoltaics, the modulation of nonlinear optical properties in such hetero-systems remains unexplored. In this study, we investigate exciton sensitized SHG in heterobilayers of transition metal dichalcogenides (TMDs), where photoexcitation of one donor layer enhances the SHG response of the other as an acceptor. We utilize polarization-resolved interferometry to detect the SHG intensity and phase of each individual layer, revealing the energetic match between the excitonic resonances of donors and the SHG enhancement of acceptors for four TMD combinations. Our results also uncover the dynamic nature of interlayer coupling, as evidenced by the dependence of sensitization on interlayer gap spacing and the average power of the fundamental beam. This work provides insights into how interlayer coupling of two different layers can modify nonlinear optical phenomena in 2D heterostructures

Exciton-Sensitized Second-Harmonic Generation in 2D Heterostructures

The efficient optical second-harmonic generation (SHG) of two-dimensional (2D) crystals, coupled with their atomic thickness, which circumvents the phase-match problem, has garnered considerable attention. While various 2D heterostructures have shown promising applications in photodetectors, switching electronics, and photovoltaics, the modulation of nonlinear optical properties in such heterosystems remains unexplored. In this study, we investigate exciton-sensitized SHG in heterobilayers of transition metal dichalcogenides (TMDs), where photoexcitation of one donor layer enhances the SHG response of the other as an acceptor. We utilize polarization-resolved interferometry to detect the SHG intensity and phase of each individual layer, revealing the energetic match between the excitonic resonances of donors and the SHG enhancement of acceptors for four TMD combinations. Our results also uncover the dynamic nature of interlayer coupling, as made evident by the dependence of sensitization on interlayer gap spacing and the average power of the fundamental beam. This work provides insights into how the interlayer coupling of two different layers can modify nonlinear optical phenomena in 2D heterostructures

Stacking-Specific Reversible Oxidation of Bilayer Graphene

We report, for the first time, that the oxidation of bilayer graphene (BLG) can be reversibly and stacking-specifically controlled. The infrared (IR) absorption, IR nanoscopy, and Raman spectroscopy measurements on BLG consistently show reversible changes in the spectra and images upon exposure to O-2 and H-2 at elevated temperatures. We also obtain spectroscopic and theoretical evidence that stacking orders of graphene layers have a profound influence on the oxide structures: AB-BLG reacting with singlet and triplet oxygen results in endoperoxides (-C-O-O-C-), whereas AA'-BLG reacting with oxygen generates both the epoxides (singlet, -C-O-C-) and endoperoxides (triplet). We believe that our result provides deeper understanding on the layer-dependent catalytic activities of graphene, which is crucial for the design of high-performance graphene-based catalysts needed for various electrochemical, biological, and environmental applications.11Nsciescopu

Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy

Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external electric field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that infrared (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter-and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielectric substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG

Role of Local Conductivities in the Plasmon Reflections at the Edges and Stacking Domain Boundaries of Trilayer Graphene

We employed infrared scattering-type scanning near-field optical microscopy (IR-sSNOM) to study surface plasmon polaritons (SPPs) in trilayer graphene (TLG). Our study reveals systematic differences in near-field IR spectra and SPP wavelengths between Bernal (ABA) and rhombohedral (ABC) TLG domains on SiO2, which can be explained by stacking-dependent intraband conductivities. We also observed that the SPP reflection profiles at ABA-ABC boundaries could be mostly accounted for by an idealized domain boundary defined by the conductivity discontinuity. However, we identified distinct shapes in the SPP profiles at the edges of the ABA and ABC TLG, which cannot be solely attributed to idealized edges with stacking-dependent conductivities. Instead, this can be explained by the presence of various edge structures with local conductivities differing from those of bulk TLGs. Our findings unveil a new structural element that can control SPP, and provide insights into the structures and electronic states of the edges of few-layer graphene

Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy

Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external electric field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that infrared (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter-and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielectric substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.N