Nonlinear photonic crystals (NPC) are a class of metamaterials that have constant refractive index, but spatially modulated 2nd order nonlinear susceptibility. Their development has revolutionized the field of nonlinear optics because they not only offer a versatile way to realize efficient nonlinear optical interactions, but even more importantly allow the exploration of novel classes of light-matter interactions. While simple photolithography followed by electrical field poling, have proven very efficient for domain inversion (and thus modulation of the second order nonlinear coefficient) in ferroelectric materials such as LiNbO3, new all-optical poling methods provide more flexibility and enable the fabrication of 3D NPC. A major obstacle for optimization of the fabrication methods and the design on new classes of NPC is the difficulty to visualize the ferroelectric domains, in particular in the bulk of the crystals.
We have developed a method called Cherenkov SHG Microscopy that allows for 3-dimensional visualization of fine details of the ferroelectric domain pattern in NPC with a submicron accuracy. The method is based on the strong emission of Cherenkov second harmonic signal when an infrared laser beam is tightly focused on the boundary between antiparallel ferroelectric domains.
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| (a) Schematic of the Cherenkov SHG Microscopy setup; Images of inverted domain pattern of a 2D quasi-periodic NPC obtained via (b) Cherenkov SHG inside the NPC and (c) optical microscopy after selective etching of the NPC surface. | Up: Cherencov SHG Microscopy images of domain structure, taken with the focal plane 10 microns inside the corresponding NPC. Down: (b) 3D images of domain walls and defect formations inside NPC prepared by electric field polling of congruent LiNbO3 crystal. | |||||
 Cherenkov SHG Microscopy visualization of a section of square pattern of inverted domains formed in a LiNbO3 crystal by focused infrared femtosecond pulses polling. (a) The first 15 µm-deep layer of the pattern (seen from the Z surface) illustrating good quality of the inverted domains. (b) Degradation of the domains structure at greater depths.  Cherenkov SHG Microscopy image of the 3D ferroelectric domain pattern, fabricated in ferroelectric BCT crystal by focused infrared femtosecond pulses polling. | ||||||
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