Enhancement of second harmonic generation in a doubly resonant   metamaterial
  We investigate second harmonic (SH) generation in a doubly resonant metamaterial. We show that SH generation can be enhanced when the resonant condition is satisfied for the SH frequency as well as for the fundamental frequency. A unit cell of the doubly resonant metamaterial consists of two coupled resonators, one of which resonates at the fundamental frequency, whereas the other resonates around the SH frequency. We observe that the SH generation in the doubly resonant metamaterial is 4.6 times as large as that in a singly resonant metamaterial. 
Tetsuo Kanazawa, Yasuhiro Tamayama, Toshihiro Nakanishi, and Masao   Kitano
physics.optics