Gold‐ and Silver‐Coated Barium Titanate Nanocomposites as Probes for Two‐Photon Multimodal Microspectroscopy

Abstract

Improved multiphoton-excited imaging and microspectroscopy require nanoprobes that can give different nonlinear optical signals. Here, composite nanostructures with a barium titanate core and a plasmonic moiety at their surface are synthesized and characterized. It is found that the core provides a high second-order nonlinear susceptibility for sensitive second harmonic generation (SHG) imaging in living cells. As a second function in the twophoton regime, the plasmonic part yields high local fields for resonant and nonresonant surface enhanced hyper Raman scattering (SEHRS). SEHRS complements the one-photon surface enhanced Raman scattering (SERS) spectra that are also enhanced by the plasmonic shells. Barium titanate silver core-shell (Ag@BaTiO 3 ) composites are specifically suited for SEHRS and SHG excited at 1064 nm, while gold at barium titanate (Au@BaTiO 3 ) nanoparticles can be useful in a combination of SHG and SERS at lower wavelengths, here at 785 nm and 850 nm. The theoretical models show that the optical properties of the BaTiO 3 dielectric core depend on probing frequency, shape, size, and plasmonic properties of the surrounding gold nanoparticles so that they can be optimized for a particular type of experiment. These versatile, tunable probes give new opportunities for combined multiphoton probing of morphological structure and chemical properties of biosystems.very attractive in laser microspectroscopy and imaging. They include two-photonexcited fluorescence, [3] second harmonic generation (SHG), [4] stimulated Raman scattering [5] and coherent anti-Stokes Raman scattering [6] microscopies, and imaging based on spontaneous hyper Raman scattering (HRS). [7] Nonlinear processes are attractive in microscopy and spectroscopy since they can be excited with light in the near-infrared, which offers several advantages. The latter include deep tissue penetration capability, reduced photo damage due to the lower photon energy, and spatially more confined probed volume, which can result in an improved lateral resolution. [8] Nevertheless, when multiphoton-excited processes are complemented with their one-photon-excited counterparts, this can yield unprecedented structural information from a sample. As an example, the spontaneous twophoton-excited Raman process, termed HRS, is governed by different selection rules than the (conventional) spontaneous one-photon-excited RS. Both HRS and RS can be enhanced in the local fields of plasmonic nanostructures, resulting in surface enhanced hyper Raman scattering (SEHRS) [9] and surface enhanced Raman scattering (SERS) spectra, respectively. While imaging based on vibrational spectra gives detailed molecular structure, other nonlinear optical signals, such as SHG, are less specific but lead to information about the microscopic morphology of a sample. SHG is a two-photon process, where a

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Barium Titanate Nanostructures and Thin Films for Photonics

“…This hyper Raman scattering technique offers a complimentary vibrational information from molecules and can be also used to probe environments. [194] BaTiO 3 nanoparticles in combination with metallic seeds also show an enhanced third order nonlinearity, which makes them promising as contrast agents, biosensors or phase conjugate nanomirrors. [119] Another example of hybrid nanostructures are dimers consisting of a BaTiO 3 nanoparticle and a gold nanoparticle.…”

“…Reproduced with permission under the terms of a CC-BY-NC-ND license. [194] Copyright 2019, The Authors, published by Wiley-VCH. d) Capillarity-assisted particle assembly in PDMS cavities is used to assemble hybrid BaTiO 3 -Au dimers.…”

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