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The recently discovered two-dimensional materials can be used to fabricate multilayer heterostructures. Interlayer charge transfer is a key process in such heterostructures as it can enable emergent optoelectronic properties. Efficient interlayer charge transfer in van der Waals heterostructures has been observed by femtosecond transient absorption and steady-state optical spectroscopy measurements, based on measuring the interlayer carrier distribution. Here, we show that a second harmonic generation process allows direct probing of the electric field induced by the charge transfer. An ultrashort laser pulse was used to excite electrons and holes in a MoS2/WS2 heterostructure. The separation of the electrons and holes from the two monolayers generates an electric field, which enables the generation of the second harmonic of an incident fundamental pulse. We further studied the time evolution of this electric field by measuring the second harmonic signal as a function of the time delay between the pump and the fundamental pulses. The result agrees well with the dynamics revealed by a transient absorption measurement. These results provide direct evidence of interlayer charge transfer and demonstrate an all-optical method of studying charge transfer and induced electric fields in two-dimensional materials. Furthermore, this effect, if large enough, could be utilized in optical devices based on 2D heterostructures with nonlinear optical responses controllable by interlayer charge transfer.

As an elementary process of light-matter interaction in solids, upconversion photoluminescence has been extensively studied in rare-earth-doped materials and found applications in biological imaging, infrared light detection, and laser cooling. More recently, it has been shown that upconversion photoluminescence can be achieved in two-dimensional semiconductors by utilizing the strong coupling between charge carriers. Here, we show that the interlayer charge transfer, which has been widely observed in van der Waals heterostructures, can be utilized for upconversion photoluminescence. Using a MoSe2/WS2/MoS2 trilayer as an example, we show that by exciting the MoSe2 and MoS2 layers with a low-energy 670-nm laser beam, photoluminescence of 620 nm can be obtained. The upconversion photoluminescence originates from the transfer of electrons and holes from MoSe2 and MoS2, respectively, to the middle WS2 layer, where they recombine. The results illustrate an unexplored physical mechanism for upconversion photoluminescence in solids and introduce van der Waals heterostructures as materials to achieve upconversion photoluminescence.

Al2O3/NbAlO/Al2O3 sandwich dielectric films were grown on InP substrate and annealed. X-ray reflectivity measurements suggested that 1.0 nm interfacial layer existed at InP interface, x-ray diffraction and high resolution transmission electron microscopy indicated the films were crystallized. X-ray photoelectron spectra indicated the oxidization of InP substrate, and the valence-band offset between the dielectric film and InP interface was calculated to be 3.1 eV. The electrical measurements indicated that the leakage current density was 40 mA/cm(2) at gate bias of 1 V, and the equivalent oxide thickness and the dielectric constant were 1.7 and 20 nm, respectively.