Mode-controllable waveguide fabricated by laser-induced phase transition in KTN.

We report the fabrication of a hexagonal cladding waveguide by femtosecond laser direct writing (FLDW) in a potassium tantalate niobate (KTN) crystal with a large electric-optical effect. Confocal micro-Raman results show the laser-induced phase transition occurs in the filament areas during the waveguide fabrication. The small filaments can strongly confine the polar nanoregions especially in its ferroelectric state to enhance the waveguide birefringence, enabling excellent polarization maintaining features for both TE and TM-polarized light propagations. The temperature-dependent phase transition allows for an active control of waveguide polarization modes as well as a switchable polarization-maintaining feature.