Efficient upconversion of sub-THz radiation in a high-Q whispering   gallery resonator
  We demonstrate efficient upconversion of sub-THz radiation into the optical domain in a high-Q whispering gallery mode resonator with quadratic optical nonlinearity. The 5x10^{-3} power conversion efficiency of continuous wave 100GHz signal is achieved with only 16 mW of optical pump. 
D. V. Strekalov, A. A. Savchenkov, A. B. Matsko and N. Yu