Nanooptics and photonics
Impact of the peripheral nanostructures on the second and the third harmonics generation in ZnO harmonic nanoparticles
V.V. Multian1,2, V.V. Lisnyak3, V.Yu. Klymchuk3, 
Y. Mugnier2, R. Le Dantec2, V.Ya. Gayvoronsky1
1 Institute of Physics NAS of Ukraine. Pr. Nauki, 46, Kiev 03028, Ukraine 
E-mail: vlad@iop.kiev.ua
2 Université Savoie Mont Blanc, SYMME, F-74000 Annecy, France
3Chemical Faculty, Taras Shevchenko National University of Kyiv, 01060 Kyiv, Ukraine

Harmonic Nanoparticles (HNPs) belong to a new class of optic nanomaterials. They can effectively convert the frequency of the laser radiation and generate the second (SH), the third (TH), and higher optical harmonics. The HNPs have promising parameters for a wide range of biological applications. By using HNPs, one can vary the response wavelength; high imaging depth and photostability of HNPs support long-term observations. 
Here, for the first time, we report on the excitation spectra of the second and third optical harmonics and the photoluminescence (PL) of individual ZnO HNPs studied by multiphoton microscopy. Multiphoton measurements were conducted under femtosecond laser excitation, at λex= 710–1300 nm. For ZnO HNPs of ~150 nm in size, we found three excitation ranges with the most efficient characteristic response at 710 < λex<800 nm for SH, at 800 < λex <950 nm for PL, and at 950 < λex <1200 nm for TH.
Apart from the common "large" HNPs, here we present specially prepared nanostructures: "large" ZnO HNPs (~150 nm) which periphery decorated, by using gas-phase epitaxy, with "small" crystalline ZnO HNPs (<10 nm). For these nanostructures, a significant expansion into a near IR ranges is found for the excitation spectra of SH and TH. Also, a significant increase of the laser frequency conversion efficiency was registered in specific ranges. The obtained result is important for the production of highly effective HNPs for the biological and sensing applications.
Acknowledgment. V.M. acknowledges support by the NASU project (grant # 0118U002330) for research groups of young scientists.
1.	Multian V.V., Gayvoronsky V.Ya., Dantec R., et al. Averaged third-order susceptibility of ZnO nanocrystals from third harmonic generation and third harmonic scattering. Opt. Mat. 84 (2018) 579–585.
2.	Uklein A.V., Multian V.V., Gayvoronsky V.Ya., et al. Nonlinear optical response of bulk ZnO crystals with different content of intrinsic defects. Opt. Mat. 84 (2018) 738–747.