Search

The interfacial effect in mixed-dimensional van der Waals heterostructure can improve the performance of terahertz (THz) components, such as THz emitter and THz modulator, which highlight the development of THz technology. However, this improvement has only been demonstrated in heterostructure based on graphene or small bandgap two-dimensional materials. We present the enhancement for THz generation and modulation from Si by insulating hexagonal boron nitride (h-BN) layer coating. The surface adsorption charges and the interface trapped charges lower the Fermi energy on Si surface, leading to an enlarged built-in electric field. Consequently, THz generation from h-BN/Si demonstrated significant enhancement compared with that by bare Si surface. Furthermore, the enlarged built-in electric field separates the interface photocarriers and reduces the surface recombination rate. The resulting dense charge accumulation raises up the photoconductivity after optical illumination and the enhanced THz modulation at h-BN/Si interface is approximately ten times higher than that by Si surface. These results not only present an efficient THz generation and modulation from the h-BN/Si interface but also pave the way for the deep understanding of interfacial effect in mixed-dimensional van der Waals heterostructures.

Tunable optical properties play an important role in the high performance of optoelectronic applications based on two-dimensional (2D) transition metal carbide and nitride (MXene) materials. Herein, the optical properties of functionalized MXene monolayers Sc2CT2 (T = O and OH) are investigated by strain engineering. The strain-dependent linear optical properties of Sc2CT2 possess broadband optical response due to the geometry and orbital overlap effect. The peaks from the second-order nonlinear coefficient elements d (d15, d16, and d31) at around half the band-gap exhibit a redshift for Sc2CO2 (blueshift for Sc2C(OH)2) with the increase of strain. The strain-dependent d reveals that Sc2CO2 with -1268 pm V-1 %-1 has a larger photoelastic coefficient than that of Sc2C(OH)2 with -574 pm V-1 %-1 at 1% strain. Meanwhile, the photoelastic tensors can not only be increased but also reduced with the increase of strain due to the dispersion relation. Moreover, the azimuthal angle-dependent second harmonic generation (SHG) from strained Sc2CT2 monolayers depends highly on the strained states and the pumping photon energy. The results pave the way for the tunable, broadband, and anisotropic applications of nonlinear optoelectronic devices based on MXenes based on strain engineering.

Graphene/semiconductor heterostructures demonstrate an improvement of traditional electronic and optoelectronic devices because of their outstanding charge transport properties inside and at the interfaces. However, very limited information has been accessed from the interfacial properties by traditional measurement. Herein, we present an active THz surface emission spectroscopy for the interface build-in potential and charge detrapping time constant evaluation from the interface of graphene on SiO2/Si (Gr/SiO2/Si). The active THz generation presents an intuitive insight into the depletion case, weak inversion case, and strong inversion case at the interface in the heterostructure. By analyzing the interface electric-field-induced optical rectification (EFIOR) in a strong inversion case, the intrinsic build-in potential is identified as -0.15 V at Gr/SiO2/Si interface. The interface depletion layer presents 44% positive THz intrinsic modulation by the reverse gate voltage and 70% negative THz intrinsic modulation by the forward gate voltage. Moreover, a time-dependent THz generation measurement has been used to deduce the charge detrapping decay time constant. The investigation will not only highlight the THz surface emission spectroscopy for the graphene-based interface analysis but also demonstrate the potential for the efficient THz intrinsic modulation as well as the enhancement of THz emission by the heterostructures.

Photodoping as a nonvolatile and reversible method can be used to modify the carrier distribution at the heterojunction interface. Herein, we explore the 2D/3D van der Waals (vdW) graphene/silicon (G/Si) heterostructure in real time by terahertz (THz) emission spectroscopy. Photoinduced doping is introduced by a continuous wave laser, which leads to a screening effect to the built-in electric field at the interface. The resulting decrease in transient photocurrent reduces the THz emission amplitude from the G/Si heterostructure. The photoinduced doping effect suggests a 40% THz intrinsic modulation depth at external reverse bias. This work provides an effective way to actively control the THz emission process from the G/Si interface and paves the way for analyzing the interfacial process under photoinduced doping in vdW heterostructures.

We investigated the terahertz (THz) radiation mechanism of the layered tin sulfide (SnS2) crystal via THz emission spectroscopy. Under non-resonant excitation, THz generation from SnS2 is mainly due to the optical rectification effect, which is verified by the azimuthal angle, polarization angle, and pump fluence dependence of the THz signals. Further incident angle dependent THz signals suggest that the optical progress happened at the surface of SnS2 with both in-plane and out-of-plane photon-induced dipoles. This work puts forward THz emission spectroscopy as a noninvasive characterization tool for the in-plane and out-of-plane photon-induced dipoles in layered materials.

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide ($\mathrm{W}{\mathrm{S}}_{2}$). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of $\mathrm{Mo}{\mathrm{S}}_{2}$ based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in $\mathrm{W}{\mathrm{S}}_{2}$. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12% approximately. All these suggest the main THz mechanism from $\mathrm{W}{\mathrm{S}}_{2}$ is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of $\mathrm{W}{\mathrm{S}}_{2}$ with the maximum surface depletion field of approximate $1.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm}$. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.

Broadband terahertz (THz) impedance matching is important for both spectral resolution improvement and THz anti-radar technology. Herein, graphene-silicon hybrid structure has been proposed for active broadband THz wave impedance matching with optical tunability. The main transmission pulse measured in the time domain indicates a modulation depth as high as 92.7% totally from the graphene-silicon interface. The interface reflection from the graphene-silicon junction implies that an impedance matching condition can be actively achieved by optical doping. To reveal the mechanism, we propose a graphene-silicon heterojunction model, which gives a full consideration of both the THz conductivity of graphene and the loss in doped junction layer. The theory fits well with the experimental results. This work proves active THz wave manipulation by junction effect and paves the way for active anti-reflection coating for THz components.