Most materials found in nature exhibit negligible nonlinear optical behaviors. To observe them, it is necessary to increase the interaction length  (for example, using optical fibers) and/or to amplify the pump intensity with high-powered pulse lasers. It means that the third-order nonlinear optical processes, for example, stimulated Raman scattering (SRS), optical Kerr effect, to name a few, do not appear within highly confined media or from single molecules exposed to continuous-wave low-powered laser light. Nonlinear enhancement of light becomes possible due to giant local electric fields and/or changes in higher-order nonlinear susceptibility. The nonlinear optical effects were found to occur in plasmonic and/or epsilon-near-zero (ENZ) materials [1-4]. In paper [5], the authors, for the first time, have succeeded to synthesize a metal-dielectric nanocomposite exhibiting the 2-ENZ behavior in the visible and near-infrared region. In such a medium, multiple plasmon resonances at different wavelengths are available.

In this paper, we study SRS effects using a percolated 50 nm titanium oxynitride (TiON) thin film that exhibits the 2-ENZ behavior in the visible and near-infrared region. This film was fabricated using dc reactive magnetron sputtering in an argon-nitrogen environment at elevated temperature and post-oxidation in air. In order to enhance the SRS effect we have patterned the TiON thin film by making square-shaped planar nanoantennas with focused ion beam milling. Using tip-enhanced Raman scattering, we have proved that this nanocomposite film can be represented as the mixture of metallic TiN and dielectric TiO₂ nanoparticles. The underlying mechanism to observe the SRS is linked to the enhanced effective third-order susceptibility due to plasmon resonances at the ENZ wavelengths. Earlier, we have experimentally demonstrated a far-field Raman color superlensing effect by showing a sub-wavelength resolution of l/6NA (l  is the excitation wavelength, NA - numerical aperture) at different SRS overtones using multi-walled carbon nanotubes of 40 nm in diameter directly dispersed on the TiON thin film [6]. This allows one to use this material for developing a multi-resonant meta-lens pushing a spatial resolution beyond the diffraction limit without post-recovery. The meta-lens serves as a SERS substrate that not only enhances a scattered light but provides the sub-wavelength resolution. The metal-dielectric 2-ENZ nanocomposite film can be used as a broadband perfect absorber for thermophotovoltaic cells.     

[1] Reshef O., De Leon I., Alam M. Z., Boyd R. W. Nat. Rev. Mater. 4, 535 (2019).

[2] Caspani, Kaipurath R. P. M., Clerici M.,et al.,  PRL 116, 233901 (2016)

[3] Kharintsev S.S., Kharitonov A.V., Saikin S.K., Alekseev A.M., Kazarian S. G.  Nano Lett. 17, 5533 (2017).

[4] Kharintsev S.S., Kharitonov A.V., Alekseev A.M., Kazarian S. G. Nanoscale 11, 7710 (2019).

[5] Braic L., Vasilantonakis N., Mihai A.,et al., ACS Appl. Mater. Interfaces 9, 29857 (2017).

[6] Kharintsev S.S.  Opt. Lett. 44 (24), 5909-5912 (2019).

Tyugaev M.D., Kharitinov A.V., Gazizov A.R., Fishman A.I., Salakhov M.Kh., Dedkova A.A., Alekseev A.M., Shelaev A.V., Kharintsev S.S.

JETP Letters 110, issue 12 (2019)