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For heterointerfaces at micro/nano scales, extremely-space-confined second harmonic generation (SHG) of transition metal dichalcogenide (TMD) monolayers can facilitate the extraction of TMD monolayers’ 3D dielectric environment. Here, we construct SiO2 microsphere/MoS2 monolayer (3D/2D) heterostructures and spatially resolve their SHG distributions of via pixel-to-pixel spatial mapping. Asymmetric 3D refractive index distribution of the microsphere is successfully projected to 2D plane of TMD monolayer and visualized by corresponding 2D image of SHG intensity distribution, which presents a crescent-shaped shadow. Out results open up new possibilities for 3D interfacial sensing and imaging with SHG of 2D monolayers.

Observing interfacial molecular adsorption and desorption dynamics in a label-free manner is fundamentally important for understanding spatiotemporal transports of matter and energy across interfaces. Here, we report a label-free real-time sensing technique utilizing strong optical second harmonic generation of monolayer 2D semiconductors. BSA molecule adsorption and desorption dynamics on the surface of monolayer MoS2 in liquid environments have been all-optically observed through time-resolved second harmonic generation (SHG) measurements. The proposed SHG detection scheme is not only interface specific but also expected to be widely applicable, which, in principle, undertakes a nanometer-scale spatial resolution across interfaces.

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of the medium. Over the past few decades, various methods have been applied to achieve the modulation of lasing, such as building distributed Bragg reflection (DBR) [1] or distributed feedback (DFB) [2] structures, changing the cavity size, [3] and utilizing self-absorption effects, [4,5] Burstein-Moss (BM) effect [6] or Vernier effect. [7] The dynamic modulation of lasing by using piezoelectric [8,9] and electrooptic effects [10] were used to modulate the refractive index of the crystal. However, the electrical methods are more conducive to miniaturizing and integrating than the mechanical methods, which requires additional stressing devices.Electro-optic effect, as a phenomenon of electric field induced refractive index variation of electro-optic crystal, including 1st and 2nd order electro-optical effects (so-called the Pockels effect and Kerr electro-optic effect), has been commonly used to modulate the lasing modes. However, in general, lasing is resonated and amplified in a gain cavity and the modes are regulated externally in different materials and spaces. In the other word, the modulation is usually operated passively. It will be great favorable for optoelectronic integration if the gain medium itself has electrooptic effect so that plays both roles as the optical microcavity for resonance and electro-optic response for lasing mode modulation. ZnO with a non-centrosymmetric hexagonal structure has been demonstrated nonlinear optics in films. [11,12] Meanwhile, it has attracted considerable attention as an excellent gain media for UV lasing due to its wide direct bandgap and lager exciton binding energy. [13][14][15] This indicates a good candidate for functional integration of lasing generation and dynamic modulation in the ZnO microcavity.In this paper, an individual ZnO hexagonal microrod with high crystalline quality and smooth surface was utilized as a UV lasing microcavity, while an electric field was applied perpendicularly and parallelly to the C-axis of the microcrystal, respectively. The optically pumped lasing modes were modulated dynamically by changing the applied voltage. Corresponding to perpendicular and parallel direction of the electric field, the refractive index variation presents a linear and quadratic increase tendency with the electric field strength, resulted from Pockels and Kerr electro-optic effect, respectively. Based on the systematic analysis, the 1st and 2nd order electrooptical coefficients and modulation speed were estimated. This