As optical components continue to replace electronics in ultrafast signal processing applications, a growing interest in
further miniaturization and integration of photonic devices on a single chip is observed. Therefore, optical waveguides of
high refractive index contrast of core and cladding materials are developed since a couple of years. They can have a very
small cross section and also bending radius, enabling the development of ultra-compact photonic integrated devices and
circuits. Silicon-On-Insulator (SOI) waveguides ("photonic wires") and devices are the most prominent examples.
A corresponding technology for Lithium Niobate-On-Insulator (LNOI) waveguides is still in its infancy, though LN
offers - in contrast to SOI - excellent electro-optic, acousto-optic, and nonlinear optical properties. Moreover, it can be
easily doped with rare-earth ions to get a laser active material. Therefore, LNOI photonic wires will enable the
development of a wide range of extremely compact, active integrated devices, including electro-optical modulators,
tunable filters, nonlinear (periodically poled) wavelength converters, and amplifiers and lasers of different types.
The state-of-the-art of LNOI films as platform for high-density integrated optics is reviewed. Using a full-wafer
technology (3" diameter), sub-micrometer thin LN films are obtained by high-dose He+ ion implantations,
crystal-bonding to a low-index substrate (preferably SiO2) and cleaving by a special annealing step ("ion-beam-slicing").
Various LNOI structures, also combined with metallic layers, are presented. Based on such platforms, photonic wires
and micro-photonic devices are developed using different micro- and nano-structuring techniques. To be specific, the
fabrication and characterization of LNOI photonic wires with cross-section < 1 μm2, and periodically poled LNOI
photonic wires for second harmonic generation are reported in detail.