Once a topic of fundamental interest, nanofabrication has finally reached a level of development at which it is becoming highly practical. Indeed, it presently has hundreds of applications in a very diverse range of fields, including medicine, defense, construction business and others. Previously regarded as too expensive, the fabrication of nanostructures has now become affordable and even routine to a certain extent. Researchers all over the world currently enjoy the opportunity of making new discoveries based on nanostructures and nanodevices that one could not have dreamed of some years ago. Among such exotic discoveries during the past decade are negative refractive index materials, epsilon-near-zero metamaterials, optical cloaking devices and, on a more practical side, integrated photonic circuits for biosensing, lab-on-a-chip devices, and optical communication networks.
The importance of this paper lies in the experimental demonstration of the competition between second- and third-harmonic generation in ZnO nanorods.. The light generated as a result of this competition spans the entire visible spectrum, from red to blue, as the colors of the second- and third-harmonic light mix, and in this way a part of the RGB color code used in displays can be reproduced. It is possible to control the color that results from the harmonics competition process by changing the intensity of the fundamental-frequency radiation used to generate them. The intensities of the second- and third-harmonic beams scale as the second and third power of the fundamental-frequency intensity, respectively. The second-harmonic generation is the dominant effect at the lower values of the fundamental intensity, while the third-harmonic generation dominates at significantly higher values of the intensity. There is thus a range of intensity values where both processes are present and comparable. Before the work of Dai and co-authors, it had not been possible to reach this intensity range because two-photon absorption present in ZnO led to material damage. The authors managed to avoid this parasitic effect through a proper choice of the fundamental beam’s wavelength, enabling the demonstration of the competition of optical harmonics without optical damage, hence opening the way for a possible new color display technology.
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