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Have you ever wondered how light operates at the nanoscale? Nanoplasmonics refers to the study of optical phenomena that occur at the nanoscale, that is, an almost infinitesimal domain of physical systems which are mainly governed by quantum mechanics. Research in this area is based on the behaviour that light presents below the diffraction limit which usually is usually half the width of the wavelength [1]. Optical phenomena at the macrorealm are generally restricted by the limit of diffraction, which prevents the focus of light at much smaller volumes than the wavelength. When using nanoparticles physicists can localize the electromagnetic radiation generated below the diffraction limit, which produces electric fields spots larger than the incident radiation [2, 3, 4]. These punctual spots have a disposition below the sub-wavelength volume, making them really sensitive to the nanometric scale. According to empirical data [5], this type of reactions are normally localised at metal surfaces, where free photons are transformed into charge-density oscillations, the so-called localised surface plasmons (LSP). Nanoplasmonics deals with how plasmons interact with their surroundings. Consequently, these experiments are typically conducted on noble metal nanoparticles, which are a concern for antenna theory designs [1]

Over the last two decades, noble metal nanoparticles have been the subject of extensive research in the frame of nanotechnology due to their unique optical properties. However, their use dates back to hundreds of centuries ago. We could take as both relevant and historic the example of the Lycurgus Cup (4th century CE) [1, 6]. This experiment demonstrated  that depending on the side where light is incident from, the surface of the cup is observed either red or green when coming from behind or in front, respectively. This is also known as dichroism, because the changing colour effect could be also explained by the macroscopic interaction of light: transmission and reflection. Thus, when light comes from behind the metal surface, containing silver and gold nanoparticles, the observer sees the transmitted light. On the other hand, when it hits from in front, the result is the effect of light reflection. 

We currently have acknowledged the potential of this branch of science thanks to recent developments and the fast-growing knowledge the field has experienced. Likewise, noble metal nanoparticles undergoing plasmonic resonances behave as nanosources of light, energetic electrons and heat. One must know that such nanoparticles emit free electron gas that creates an oscillation upon illumination in the visible part of the spectrum. The features of this resonant vibration depend on the constitutive material, the chassis and nanoparticles’ microenvironment. An enhancement of the electrical field takes place when frequency of the incoming light is equal to plasmons resonance, creating a coherent reemission of the light, as well as behaving exactly like a dipole [7].

Moreover, part of this emitted light is scattered and lost whilst a small part is condensed above the metal surface nanoparticle. The oscillation is also responsible for energy dissipation mediated by Joule’s effect, resulting in heat generation, as well as increasing the surface nanoparticle’s temperature. Although it may seem unbelievable, this little portion that remains concentrated on the nanoparticle constitutes the base of improvement of chemical reactions. One can design optical nanostructures that could work as nanotweezers, which could trap objects down to the order of a molecule, achieving higher selectivity thanks to quantum rules [7].

Among other applications, nanoplasmons can be used to create more resistance and purer colour palettes [1]. Imagine you have an ink palette of multiple colours. You would then need to mix the pigments in order to generate a unique tonality. How could we play with metals, from an engineering point of view, in order to produce a broad range of colours? The solution to that question resolves in two main components: nanoplasmons and metal nanostructures that rely on a parameter made of a triplete: size, shape and relative positioning. 

Medicine is another field that has been benefiting from nanoplasmonics in cutting-edge techniques or treatments such as minimally invasive surgical procedures, photothermal cancer therapy and drug delivery. It is really interesting the use of nanosensors [4, 8, 9] made from polymer, glass, and gold nanoparticles. They have the ability to perform diagnosis by analysing the concentration of cancer markers in the blood. Analytical platforms can be modelled in such ways that from a single drop of blood, doctors could have the same amount of information as the one coming from a blood sample thanks to the sensitivity of the nanoparticles surface. These biomedical applications allow us to provide early treatments [2].

To conclude, optical systems studied from a multiscale point of view give us a reason to scrutinise the interaction of light below the limit of diffraction supported in small quantities of matter. These nanoparticles react as if they were nanolenses. Effectively, their comprehension unravels the unique properties that the universe has hidden behind the tangible limit. 


References

[1] Nanowerk. Nanoplasmonics Available from: https://www.nanowerk.com/nanoplasmonics.php [Accessed 4th March 2015]

[2] ICFO People. Romain Quidant Plasmon Nano-Optics Research Group [Video] 2014 Available from: https://www.youtube.com/watch?v=5WtsJpRI9w0&ab_channel=ICFOPeople [Accessed 13th March 2015]

[3] NanoDiode. Romain Quidant (FR/ES) [Video] 2015 Available from: https://www.youtube.com/watch?v=0K6lzdajtDc&ab_channel=NanoDiode [Accessed 13th March 2015]

[4] Instituto Italiano di Tecnologia. Computational Nanoplasmonics Available from: https://www.iit.it/it/web/computational-nanoplasmonics [Accessed 9th March 2015]

[5] Yavas, O., Svedendahl, M., & Quidant, R. (2019). Unravelling the Role of Electric and Magnetic Dipoles in Biosensing with Si Nanoresonators. ACS nano, 13(4), 4582–4588. Available from: https://doi.org/10.1021/acsnano.9b00572

[6] Wikipedia. Lycurgus Cup  Available from: https://en.wikipedia.org/wiki/Lycurgus_Cup  [Accessed 2nd March 2015]

[7] Baffou, Guillaume & Quidant, Romain. (2014). ChemInform Abstract: Nanoplasmonics for Chemistry. ChemInform. 45. Available from: https://www.researchgate.net/publication/263670342_ChemInform_Abstract_Nanoplasmonics_for_Chemistry 

[8] Jeong, HH., Mark, A., Alarcón-Correa, M. et al. Dispersion and shape engineered plasmonic nanosensors. Nat Commun 7, 11331 (2016). Available from:  https://doi.org/10.1038/ncomms11331 

[9] and Image: Anker, J., Hall, W., Lyandres, O. et al. (2008) Biosensing with plasmonic nanosensors. Nature Mater 7, 442–453. Available from: https://doi.org/10.1038/nmat2162