*Editorial* **Latest Advances in Nanoplasmonics and Use of New Tools for Plasmonic Characterization**

**Grégory Barbillon**

EPF-Ecole d'Ingénieurs, 55 Avenue du Président Wilson, 94230 Cachan, France; gregory.barbillon@epf.fr

Nanoplasmonics is a research topic that takes advantage of the light coupling to electrons in metals, and can break the diffraction limit for light confinement into subwavelength zones allowing strong field enhancements [1–4]. In the past two decades, a very significant explosion of this research topic and its applications has occurred. The applications cover a great number of fields such as plasmonic devices [5–8], plasmonic biosensing [9–14], plasmonic photocatalysis [15–19], plasmonic photovoltaics [20–23], surfaceenhanced Raman scattering (SERS) [24–29] and its derivatives as the photo-induced enhanced Raman spectroscopy [30–35], SERS effect induced by high pressure [36] and the shell-isolated nanoparticle-enhanced Raman spectroscopy [37–41], other surface-enhanced spectroscopies, such as sum frequency generation (SFG) [42,43] and second harmonic generation (SHG) [44,45]. Thus, this Special Issue is focused on recent advances and insights in the research topic of nanoplasmonics and its applications.

This Special Issue is composed of nine research articles, and three review articles. The first part of the Issue is devoted to the surface plasmon resonance (SPR) spectroscopy [46,47]. Daniyal et al. have shown the use of the SPR spectroscopy for the optical characterization of a thin film based on nanocrystalline cellulose (NCC) [46]. Andam et al. have also used this SPR spectroscopy for determining the optical properties of ultra-thin films of azo-dye-doped polymers [47]. The second part is dedicated to plasmonic devices [48,49]. Firstly, Zhang et al. have reported on a plasmonic narrowband filter based on an equilateral triangle-shaped cavity and a metal–insulator–metal waveguide [48]. Lastly, Adibzadeh et al. have investigated the performances of plasmonic InP nanowire array solar cells [49]. In the third part, Gonçalves et al. demonstrated surface plasmon and Fano resonances in titanium carbide nanoparticles in the spectral range from visible to infrared [50]. In the fourth part, the plasmonic sensing is addressed [51–53]. At first, Cardoso et al. have reported on a second-order dispersion sensor based on multi-plasmonic resonances in Dshaped photonic crystal fibers [51]. Next, Ramdzan et al. have demonstrated a plasmonic sensing of mercury ions in an aqueous medium by using as a sensitive layer, a thin film composed of NCC and poly(3,4-ethylenethiophene) (PEDOT) which is deposited on a gold plasmonic film [52]. To finish this part, Barchiesi et al. have reported on the performance of plasmonic sensors based on copper/copper oxide films [53]. In the following part, the addressed topics are focused on surface-enhanced spectroscopies [54–56]. At first, Humbert et al. have highlighted a plasmonic coupling with the vibrations of the thiophenol molecule by using two-colour sum-frequency generation spectroscopy with an enhancement factor of the intensity around two orders of magnitude from blue to green–yellow due to the presence of a significant number of hotspots between Au nanosphere aggregates [54]. Yang et al. present in a review paper the applications of the SERS effect to agriculture and food safety [55]. Barbillon introduced a review paper on the recent applications of the shell-isolated nanoparticle-enhanced Raman spectroscopy [56]. To conclude this Special Issue on the latest advances in nanoplasmonics, Barbillon exhibited a short review paper on nanoplasmonics in high pressure environments [57].

**Citation:** Barbillon, G. Latest Advances in Nanoplasmonics and Use of New Tools for Plasmonic Characterization. *Photonics* **2022**, *9*, 112. https://doi.org/10.3390/ photonics9020112

Received: 11 February 2022 Accepted: 14 February 2022 Published: 17 February 2022

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**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

To realize this Special Issue entitled "Latest Advances in Nanoplasmonics and Use of New Tools for Plasmonic Characterization", we have obtained various contributions from authors of the high standard around the world. I want to thank all these authors as well as the whole editorial office of the journal "Photonics" for their great support and help in the management process of a great number of tasks associated to manuscript submissions. Finally, I expect that you will find this special issue dedicated to nanoplasmonics and their applications useful and attractive, which is aimed to the students or researchers who are or wish to be interested in this topic.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.

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