Large and stable thickness coupling coefficients of [001]C-oriented KNbO3 and Li-modified (K,Na)NbO3 single crystals

Large thickness coupling factors kT⩽0.7 are measured in both [001]C-oriented (45° cut in the orthorhombic ac plane), monodomain and [001]C-poled, domain-engineered single crystals of KNbO3. kT is extremely stable over a large temperature range, remaining high (⩾0.65) even upon heating and cooling through the orthorhombic-tetragonal phase transition. This is likely related to another finding that spontaneously high kT values (⩽0.68) are observed in as-grown (unpoled), [001]C-cut, polydomain KNbO3 crystals. kT is also stable upon thinning to thicknesses below 60μm, as required for high-frequency applications. Finally, large kT values (⩽0.7) are reported in [001]C-poled, polydomain, Li-doped (K0.5Na0.5)NbO3.