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Ohmic losses in plasmonic structures have always been considered as a major drawback for long-range propagation and the realization of efficient devices. Yet the possibility to control the rise of temperature with illumination parameters (wavelength, polarization) has fostered a strong interest in the NanoOptics community [1]. This photo-thermal energy conversion originates from the strong local field enhancement associated with the plasmonic resonances that goes along with an enhanced absorption in the metal. Metal nanostructures can therefore increase the temperature in their environment and be used as integrated heat nanosources [2]. This active research field, called thermoplasmonics, could potentially lead to breakthroughs in several fields like nanomedicine, nanochemistry or thermo-hydrodynamically assisted plasmonic trapping and manipulation.

Contact : christian.girard[at]cemes.fr


Figure 1 : Calculated spectra of the heat generated in metallic structures deposited on a substrate. The surrounding medium is water. Adapted from [1]. © CEMES-CNRS


(i) Metasurfaces and heat generation


Very recently, plasmonics has fostered another realm of applications in which dissipative effects are being advantageously utilized. Besides their widely used propensity to enhance and confine the near–field electromagnetic intensity, metal particles and nanostructures have revealed a great potential as local heat sources. This activity is developed both at the experimental level (including local temperature measurements through local spectroscopy) and a completely self—consistent scheme based on the Green Dyadic Functions (\sf GDF) formalism that compute the local field intensity and temperature distribution.



Figure 2 : (a) Example of thermoplasmonic metasurface able to generate strong temperature contrast. (b-e) Evolution of the temperature maps as a function of the incident polarization (represented by a double arrow) [4]. © CEMES-CNRS


This method has been applied in order to define and optimize new concepts for thermoplasmonic metasurfaces made of either gold rectangles or a mix of gold and silicon nanorods [3,4]. For each iteration, the heat distribution has been computed and weak solutions have been eliminated while strong parameter-sets have been kept. This approach has for instance led to optimized geometries for metasurfaces allowing for remote temperature increases, which can be tuned by the polarization or the wavelength.



 Figure 3 : (a) Scheme illustrating the evolutionary optimization. (b) Top : Illustration of the optimization model and problem. A chain of 20 nanorods on a glass substrate is searched in order to maximize the temperature increase at a specific remote location. Bottom : Mapping of the temperature increase along the chain, with zooms around the focal spot and the location of the ΔT probe. All temperature mappings are calculated at a height of 300 nm. White scale bars are 500 nm [4]. © CEMES-CNRS


[1] G. Baffou et al., App. Phys. Lett. 94, 153109 (2009).

[2] S. Viarbitskaya et al., ACS Photon. 2, 744 (2015).

[3] P. R. Wiecha et al., Phys. Rev. B 96 (3), 035440 (2017).

[4] C. Girard et al., J. Opt. 20, 075004 (2018).