Shree Shivangi, Lagarde Delphine, Lombez Laurent, Robert Cedric, Balocchi Andrea, Watanabe Kenji, Taniguchi Takashi, Marie Xavier, Gerber Iann C, Glazov Mikhail M, Golub Leonid E, Urbaszek Bernhard, Paradisanos Ioannis
Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Avenue Rangueil, 31077, Toulouse, France.
Department of Physics, University of Washington, Seattle, WA, USA.
Nat Commun. 2021 Nov 25;12(1):6894. doi: 10.1038/s41467-021-27213-8.
Second-harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS, that shows strong optical oscillator strength for the intra- but also interlayer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations. Since interlayer exciton transitions are highly tunable also by choosing twist angle and material combination our results open up new approaches for designing the SHG response of layered materials.
二次谐波产生（SHG）是一种非线性光学过程，其中两个光子相干地组合成一个能量为其两倍的光子。对于具有破缺反演对称性的晶体，如过渡金属二硫属化物单层，会发生高效的二次谐波产生。在这里，我们展示了在反演对称晶体中对非线性光学过程的调控。这种可调谐性基于双层MoS₂的独特性质，它对层内和层间激子共振都表现出很强的光学振子强度。当我们通过改变激光能量将二次谐波产生信号调谐到这些共振上时，二次谐波产生振幅增强了几个数量级。在共振情况下，双层二次谐波产生信号的振幅达到与单层非共振信号相当的水平。在施加电场时，由于层间激子固有的电偶极矩，通过斯塔克效应可以调谐层间激子能量。结果，层间激子简并被消除，双层二次谐波产生响应进一步增强了两个数量级，这与我们的模型计算结果很好地吻合。由于通过选择扭转角和材料组合也可以高度调谐层间激子跃迁，我们的结果为设计层状材料的二次谐波产生响应开辟了新途径。
