Abstract
We report on the experimental observation of "focus splitting" when light is tightly focused into a uniaxial lithium niobate crystal along its optical axis. This effect consists in the focal spot being split into two major sub-peaks along the axial direction. For the microfabrication applications such as three-dimensional photonic crystal fabrication and waveguide writing, this effect is highly undesired since it can lead to the generation of multiple distinct voxels in the vicinity of the focus. The splitting is caused by different birefringence induced aberrations for the ordinary and extraordinary polarization eigenmodes. We present numerical simulations which support our observations and suggest methods to avoid this effect.
Highlights
Lithium niobate (LiNbO3) is an extremely versatile ferroelectric nonlinear crystal material that has been widely used for integrated photonic applications [1,2,3] such as fast switches, interconnects, electro-optically-controlled multiplexers and demultiplexers, and nonlinear optical waveguides
We report on the experimental observation of “focus splitting” when light is tightly focused into a uniaxial lithium niobate crystal along its optical axis
If the fabrication direction must correspond to the optic axis of the crystal, one could use radially or azimuthally polarized light to remove the focus splitting, this will affect the intensity distribution of the focus
Summary
Lithium niobate (LiNbO3) is an extremely versatile ferroelectric nonlinear crystal material that has been widely used for integrated photonic applications [1,2,3] such as fast switches, interconnects, electro-optically-controlled multiplexers and demultiplexers, and nonlinear optical waveguides. Rare-earth ions can be incorporated to activate LiNbO3 for exploiting the radiation controlling of emitter by photonic crystals, opening the way to unique functional integrated laser systems Its nonlinearity makes it interesting for various optical applications such as second harmonic generation [4,5], optical parametric oscillation [6,7] and electro-optical modulation [8]. In previous work [9], these anisotropic properties were observed when direct-laser-writing (DLW) was performed along different directions in LiNbO3, and it was found that the fabrication is problematic when the beam is focused parallel to the optical axis of the crystal In this case the focus quality degrades more rapidly while increasing the focusing depth and we observed for the first time, to the best of our knowledge, a novel “focus splitting” phenomenon but could not give detailed explanation on the mechanism of the splitting effect. Our investigations include the direct femtosecond laser fabrication of voxels along different crystal orientations together with numerical simulations
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