Nano-Crystals for Quadratic Nonlinear Imaging : Characterization and Applications

Nonlinear optics is a well established field today, covering a large and rich range of applications which expands every year. Its extension to sub-wavelength scale processes is however highly non trivial, and nano-scale nonlinear optics is only in its infancy. Downscaling high order light-matter optical interaction is however nowadays accessible using new technological tools such as near field techniques, high resolution microscopy and pulsed femtosecond lasers. As a consequence, a large amount of effort has been recently invested into the invention and engineering of nano-structures that exhibit nonlinear optical properties and lead to new optical functions. Among the promising routes followed by nonlinear nano-optics, the development of new nano-sources originating from frequency mixing processes is particularly successful. In this chapter, we describe how nano-crystals have been advantageously developed and used in Second Harmonic Generation (SHG) (or by extension Sum Frequency Generation : SFG), which is the lowest order nonlinear process, dependent on the square of the incident field (Boyd, 1992). Recent developments have shown that these nano-structures are potentially key elements in various fields, such as new nano-probes for bio-imaging, or nano-scale optical fields probing in the ultra-short pulses regime. They also combine the interesting properties of frequency mixing processes with the nano-scale regime: unlike the resonant fluorescence process, coherent harmonic generation is active in the non-resonant regime and therefore free from photo-bleaching which is otherwise generally considered a major drawback from fluorescent nano-probes today. The coherent nature of frequency mixing also allows generating different wavelengths and therefore creating nano-sources of a large range of possible frequencies. An additional advantage of these nano-sources is to avoid phase-matching constraints since their size is well below the coherent length of the underlying nonlinear process, at which nonlinear propagation suffers from destructive interferences between the propagating and induced nonlinear waves. The availability of a large emission frequency range finally allows imaging 11