"Nanotechnology and nanomaterials"

Nonlinear optics of nanocomposites with bimetallic nanoparticles Ag/Au at different laser wavelengths
Tolochko A.1, Rudenko V.1, Zhulai D.1, Bugaychuk S.1, Klimusheva G.1, Yaremchuk G.2, Mirnaya T.2
1Institute of Physics of the NAS of Ukraine. Prospect Nauki, 46, Kyiv-03039, Ukraine. 
E-mail: klimush@iop.kiev.ua
2V. I. Vernadsky Institute of General and Inorganic Chemistry, NAS of Ukraine, Prospect Palladina., 32/34, Kyiv-03142 Ukraine.

The results of a study of the nonlinear optical properties of mesomorphic composites based on a liquid-crystalline cadmium alkanoate matrix with bimetallic nanoparticles (NPs) Ag/Au are presented with using Nd:YAG laser generated pulses at 532 nm and 1064 nm. Synthesis of bimetallic NPs was carried out directly in the thermotropic liquid-crystalline phase of cadmium octanoate (CdC8) by the simultaneous chemical method [1]. The total concentration of NPs was 4% mol. or varied according to the formula x Ag + (4-x) Au % mol. Structural characterization of NPs was obtained by the method of small-angle X-ray scattering. Optical and nonlinear optical measurements were performed on vitrified nanocomposites at room temperature. Bimetallic NPs actively absorb light in both the visible and infrared regions of the spectrum due to surface plasma resonance. The studies of the absorption spectra of bimetallic Ag/Au NPs in the visible range showed that the ratio of silver and gold in nanocomposite changes the position of the maximum of the surface plasma resonance band in a wide optical range. Nonlinear optical properties nanocomposites CdC8+Ag/Au (3:1); CdC8+Ag/Au (1:1); CdC8+Ag/Au (1:3) were studied by z-scan method. The nonlinear optical measurements were performed by using Nd:YAG pulsed laser, generating 9 ns pulses, with wavelength 532 nm or 1064 nm. The dependences of the cubic nonlinearity X (3) value on the wavelength of the acting laser field, as well as the plasma spectra on the composition and dimensions of the NPs were studied. We explored fast and quite high values of nonlinear optical parameters of the nanocomposites on a wavelength of 532 nm and 1064 nm. At lower input intensities there is competition of mechanisms the saturation absorption (SA) and the reverse saturation absorption (RSA) depending on the distance to the focus for our samples. The closed aperture z-scan scheme of CdC8+Ag/Au (1:1) NPs sample have shown the process of defocusing, the sign of the nonlinear refractive index is negative. In case of samples CdC8+Ag/Au (3:1) NPs and CdC8+Au/Ag (3:1) the mechanism of saturation absorption SA prevails on mechanism the reverse saturation absorption RSA. It was obtained that NPs with core – Ag/Au NPs or Au/Ag NPs and shell - Au NPs or Ag NPs not only increase the nonlinear optical characteristics of the nanocomposites CdC8 +Ag/Au (3:1) NPs and CdC8+Au/Ag (3;1), but also leads to a change in the sign of nonlinear refraction index (n2) from negative to positive, the effect of self-focusing is obtained. The nonlinear optical characteristics of nanocomposites at a laser wavelength of 1064 nm also showed relatively high values of nonlinear optical characteristics of bimetallic NPs. The excitation an optical wavelength of 1064 nm also has great interest for photoacoustic research due to low intrinsic absorption at 1064 nm wavelength, the reducing absorbance increases image depth [2]. So, our multifunctional nanocomposites with bimetallic homogeneous or gradient alloys Ag/Au NPs may be considered promising candidates for use in optoelectronic, photonic applications and photoacoustic.

1.	Mirnaya T., Yaremchuk G. / Obtaining and optical properties of nanocomposites based on liquid crystals with bimetallic Au + Ag nanoparticles of the alloy type // Ukr.Chim.Jour. 2019, 85 (5)

2.	Sara Park, Gyeongbae Park, Jeesu Kim, Wonseok Choi, Unyong Jeong, and Chulhong Kim / Bi2Se3 Nanoplates for Contrast-enhanced Photoacoustic Imaging at 1064 nm, Nanoscale, 2018, 10, 20548-20558