Plasmonic nanoparticles can be described as metallic nanoparticles with sizes smaller than visible light wavelength. When these nanoparticles are irradiated by electromagnetic waves, their free electrons oscillate in response to the applied electric field. At a suitable frequency, the oscillation becomes resonant with extreme optical response. This phenomenon is generally known as Localized Surface Plasmon Resonance. The resulting light-electron coupling confines light into small volumes and leads to extreme light scattering, local fields, and absorption.
Localized Surface Plasmon Resonance phenomena initiated in these nanoparticles mainly count on their composition, shape, geometrical distribution, size, and the refractive index of the dielectric environment used. For this reason, many efforts have been put towards controlling these structural parameters depending on the growth conditions. Silver and gold are considered suitable for plasmonic applications owing to their stability when formed into nanoparticles as well as their strong Localized Surface Plasmon Resonance absorption bands in the visible region of the spectrum.
Nanostructuring of metals presents a greater challenge concerning future Plasmonic as well as photonic gadgets. This technology definitely calls for the inception of ultrafast, high-throughput as well as low cost fabrication methods. Laser processing takes care of the above attributes, which then presents an unrivalled tool for the long-waited arrival of modules based in metallic nanostructures, with an added advantage of easy scalability. Laser nano-structuring of an ultrathin metal film or a stratified metal/dielectric multilayer film on a substrate leads respectively on surface and subsurface Plasmonic patterns with a number of applications.
Schematics of the two optothermal processes studied: surface nanostructuring of a thin metal film (up) and sub-surface nanostructuring of a stratified metal-dielectric film (bottom). Next to the schematics are corresponding SEM surface images (top) and TEM cross sections (bottom) of plasmonic nanostructures made, along with plasmonic images “printed” using the laser annealing processes.
Researchers at University of Ioannina and Aristotle University of Thessaloniki in Greece in collaboration with Cyprus University of Technology and Nottingham Trent University in the UK simulated the photo-thermal processes entailed in the surface as well as sub-surface Plasmonic Nanostructuring and compared to experimental outcomes. They presented a design process and developed functional Plasmonic nano-structures with definite morphology through tuning the annealing parameters such as the laser fluence and wavelength and structure parameters including metallic film thickness and metallic ratio of the film on a metal/dielectric composite. Their research work is published in Thin Solid Films, amongst many others.
In surface nanostructuring, utilising the ability to tune the laser’s wavelength to either match the absorption spectral profile or to be resonant with the plasma oscillation frequency of the nanostructured thin metal film allows for the utilisation of different optical absorption mechanisms that are size-selective. The authors implemented this idea into experiments, in which repeated laser treatments target different nanoparticles size groups with different laser wavelengths, driving the final surface size distribution towards a predetermined one.
The research team also fabricated functional Plasmonic templates composed of embedded nanoparticles in dielectric matrix through laser annealing of stratified metal/dielectric nano-composite. They performed detailed theoretical investigation in a bid to identify the underlying mechanism of laser induced subsurface Plasmonic Nanostructuring. The authors then developed a semi-analytical model to approximate the photo-thermal process involved.
Implementing the developed model, the authors computed the transient temperature distribution in the multilayer structure. They found that this was majorly dependent on the structure parameters such as metal volume ratio, thermal conductivity of the dielectric, and the total thickness of the multilayer film. Through adequate design of these parameters in conjunction with tuning the laser annealing conditions, the authors could arrive at Plasmonic templates with definite morphology as well as optical response.
For subsurface Plasmonic Nanostructuring, the authors implemented temperature gradients, which were generated spatially across the metal/dielectric nano-composite structure in the course of laser treatment. The temperature gradients were dependent on the nanocrystalline attribute of the dielectric member that dictated its thermal conductivity, total thickness of the film, and the composition of the ceramic/metal.
These material parameters together with laser annealing parameters can be implemented in the design of the final morphology of the subsurface Plasmonic structure. The processes can be implemented to function as a platform that will stimulate advanced process in engineering of Plasmonic devices.
D.V. Bellas, D. Toliopoulos, N. Kalfagiannis, A. Siozios, P. Nikolaou, P.C. Kelires, D.C. Koutsogeorgis, P. Patsalas, E. Lidorikis. Simulating the opto-thermal processes involved in laser induced self-assembly of surface and sub-surface plasmonic nano-structuring. Thin Solid Films, volume 630 (2017), pages 7–24.Show Affiliations
D. V. Bellas, D. Toliopoulos, A. Siozios and E. Lidorikis are affiliated with the Department of Materials Science and Engineering, University of Ioannina, GR-45110 Ioannina, Greece.
N. Kalfagiannis and D. C. Koutsogeorgis are affiliated with the School of Science and Technology, Nottingham Trent University, NG11 8NS Nottingham, United Kingdom.
P. Patsalas is affiliated with the Department of Physics, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece.
P. Nikolaou and P.C. Kelires are affiliated with the Research Unit for Nanostructured Materials Systems, Department of Mechanical Engineering and Materials Science Engineering, Cyprus University of Technology, P.O. Box 50329, 3603 Limassol, Cyprus.
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