Abstract
Annealing of micro-structured lithium niobate substrates at temperatures close to, but below the melting point, allows surface tension to reshape preferentially melted surface zones of the crystal. The reshaped surface re-crystallizes upon cooling to form a single crystal again as it is seeded by the bulk which remains solid throughout the process. This procedure yields ultra-smooth single crystal superstructures suitable for the fabrication of photonic micro-components with low scattering loss.
Highlights
Dense integration of optical waveguide circuits requires large dielectric contrast between the waveguide core and the cladding material which is typically achieved by fabricating devices which consist of super-structures (e.g. ridge waveguides and whispering gallery mode (WGM) resonators) where the optical confinement is high due to the high refractive index contrast between the optical material and the surrounding air
In order to demonstrate the potential of this method for the fabrication of photonic structures we have applied it to surface microstructured lithium niobate, a nonlinear optical ferroelectric crystal which is widely used in the photonics industry
The melting temperature for congruently melting lithium niobate crystals is 1257°C and the Curie temperature which marks the ferroelectric to paraelectric phase transition is 1142°C, this value does depend on the exact composition of the crystal and changes substantially with the lithium content
Summary
Dense integration of optical waveguide circuits requires large dielectric contrast between the waveguide core and the cladding material which is typically achieved by fabricating devices which consist of super-structures (e.g. ridge waveguides and whispering gallery mode (WGM) resonators) where the optical confinement is high due to the high refractive index contrast between the optical material and the surrounding air. WGM resonators which are fabricated in this way usually exhibit very low scattering loss and high Q factors [1,2] Such a method can be applied to glass, which is an amorphous material, without undue problems it is rather different if applied to single crystal materials mainly because the single crystal properties are not generally preserved after the melting and cooling cycles [3]. Light propagating in such a poly-crystalline material will experience scattering at the boundaries between adjacent crystallites introducing significant optical loss. In order to demonstrate the potential of this method for the fabrication of photonic structures we have applied it to surface microstructured lithium niobate, a nonlinear optical ferroelectric crystal which is widely used in the photonics industry
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