Paving the way for third harmonic generation in hollow-core photonic   bandgap fibers
  We present two novel hybrid photonic structures made of silica that possess two well-separated frequency bandgaps. The addition of interstitial air holes in a precise location and size allows these bandgaps to open up with a ratio of ${\sim3}$ between their central frequencies at the air line ${ck_z/w=1}$, thus fulfilling the basic guidance condition for third harmonic generation in hollow-core fibers. In addition, these designs may serve as high-power laser delivery of two well-separated wavelengths, such as visible and near infrared. 
Z. Montz, and A. A. Ishaaya
physics.optics