Nanophotonics, where light control is achieved by means of subwavelength-structured media, are so far mostly fabricated by surface lithographic techniques. Inevitably, this dictates that all nanophotonic elements have planar geometry, and lay at (or close to) the surface of dissimilar material substrates. Planarity is an eminent design restriction against the three-dimensional nature of light propagation, and the use of diverse materials imposes limitations to device robustness against environmental factors, such as temperature changes, vibrations and stress. Regardless of these limitations, two “killer” applications of planar nanophotonics, photonic integrated circuits (PIC) (based on sub-µm waveguides and 2D photonic bandgap crystals) and optical metasurfaces (i.e. wavefront shaping devices based on subwavelength-structured wavelength-thick surfaces) are already revolutionizing the communications and sensing industries. Such devices are typically based on silicon (or silicon nitride) on lower index silicon dioxide, thus providing good compatibility with the semiconductor CMOS industry. Applying surface lithography or other methods to directly nanostructure dielectric optical materials in the three dimensions has however been out of reach, so far. As a result, the general fields of solid-state lasers and crystal optics still have a very weak connection to the field of nanophotonics. In fact, the use of crystals for either laser light generation or non-linear frequency conversion is fundamentally equal today as it was in 1960: a homogenous crystal is put inside an optical setup.
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