Quantum-Enhanced Tunable Second-Order Optical Nonlinearity in Bilayer   Graphene
  Second order optical nonlinear processes involve the coherent mixing of two electromagnetic waves to generate a new optical frequency, which plays a central role in a variety of applications, such as ultrafast laser systems, rectifiers, modulators, and optical imaging. However, progress is limited in the mid-infrared (MIR) region due to the lack of suitable nonlinear materials. It is desirable to develop a robust system with a strong, electrically tunable second order optical nonlinearity. Here we demonstrate theoretically that AB-stacked bilayer graphene (BLG) can exhibit a giant and tunable second order nonlinear susceptibility \chi ^(2) once an in-plane electric field is applied. \chi^(2) can be electrically tuned from 0 to ~ {10^5 pm/V}, three orders of magnitude larger than the widely used nonlinear crystal AgGaSe2. We show that the unusually large \chi^(2) arises from two different quantum enhanced two-photon processes thanks to the unique electronic spectrum of BLG. The tunable electronic bandgap of BLG adds additional tunability on the resonance of \chi^(2), which corresponds to a tunable wavelength ranging from ~2.6 {\mu}m to ~3.1 {\mu}m for the up-converted photon. Combined with the high electron mobility and optical transparency of the atomically thin BLG, our scheme suggests a new regime of nonlinear photonics based on BLG. 
Sanfeng Wu, Li Mao, Aaron M. Jones, Wang Yao, Chuanwei Zhang, Xiaodong   Xu
cond-mat.mes-hall