Department of Physics, University of Washington, Seattle, Washington 98195, USA.
Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong, Hong Kong, China.
Nat Nanotechnol. 2015 May;10(5):407-11. doi: 10.1038/nnano.2015.73. Epub 2015 Apr 20.
Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material and, so far, dynamical control of optical nonlinearities remains confined to research laboratories as a spectroscopic tool. Here, we report a mechanism to electrically control second-order optical nonlinearities in monolayer WSe₂, an atomically thin semiconductor. We show that the intensity of second-harmonic generation at the A-exciton resonance is tunable by over an order of magnitude at low temperature and nearly a factor of four at room temperature through electrostatic doping in a field-effect transistor. Such tunability arises from the strong exciton charging effects in monolayer semiconductors, which allow for exceptional control over the oscillator strengths at the exciton and trion resonances. The exciton-enhanced second-harmonic generation is counter-circularly polarized to the excitation laser due to the combination of the two-photon and one-photon valley selection rules, which have opposite helicity in the monolayer. Our study paves the way towards a new platform for chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.
非线性光学频率转换，其中光场与非线性介质相互作用产生新的场频率，在现代光子系统中无处不在。然而，产生这种现象的非线性电感应率在给定材料中通常难以调节，到目前为止，光学非线性的动态控制仍然局限于研究实验室作为光谱学工具。在这里，我们报告了一种在单层 WSe₂中电控制二阶光学非线性的机制，这是一种原子薄的半导体。我们表明，在低温下通过在场效应晶体管中静电掺杂，二阶谐波产生的强度在 A 激子共振处可调谐一个数量级以上，在室温下可调谐近四倍。这种可调谐性源于单层半导体中的强激子充电效应，这使得在激子和三体子共振处的振荡器强度能够得到极好的控制。由于双光子和单光子谷选择规则的结合，激子增强的二次谐波产生与激发激光的圆偏振相反，在单层中它们的螺旋性相反。我们的研究为基于二维半导体的片上、电可调谐非线性光学器件开辟了新的平台。
