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Scientists have been interested in the optical properties of metal nanoparticles since the 1800s when Faraday conducted explorations into the properties of colloidal gold, and this fascination has only increased.
Technology for Investigating Nanoparticles
Today, we have many different technologies used to investigate nanoparticles in detail, and research shows that they produce a wide variety of phenomena. These unique properties make nanoparticles of interest to almost all industries and a wide range of applications.
The unique optical properties of metal nanoparticles are one specific area of interest. We have already seen revolutionary applications that take advantage of special interactions between light and nanoparticles. For example, modern buildings are now being built with self-cleaning glass through the addition of metal nanoparticle coatings, which react with the light and cause a chemical reaction. This reaction then breaks down the dirt on the glass.
What are Metal Nanoparticles?
Metal nanoparticles have a unique spectral response; they interact differently with each given color of light. This interaction depends on the material, shape, size, and dielectric properties of the metal nanoparticle.
They are also incredibly efficient light absorbers; better than their bulk material counterparts. This means that they can be used in applications that take advantage of their enhanced absorption properties.
Conversely, as well as being efficient at absorbing light, metallic nanoparticles are very inefficient at emitting light. This results in almost all of the light energy that they absorb being converted into heat. Metallic nanoparticles can, therefore, be heated selectively by tuning the light color in line with each nanoparticle’s specific attributes.
Metallic nanoparticles present an enhanced light scattering cross-section. This characteristic is dependant on the size of the nanoparticle, and studies have found that in particles that are smaller than 15nm, absorption overtakes scattering; whereas this effect is the opposite in larger nanoparticles.
The different plasmon modes of nanoparticles generate equally specific electric fields around the particle. This gives the particle two further specific characteristics. The first is that the particle is made evanescent, meaning that the intensity is at its maximum at the surface of the nanoparticle, and this decreases into the surrounding area. Secondly, this creates hotspots where the intensity of the field is multiple times what it is in the incident field.
Lastly, lightwaves interact with the plasmon mode in a specific manner, generating a unique, angular light pattern. The light is absorbed or scattered, and the direction determines the efficiency of this.
Optical Properties of Metallic Nanoparticles
Optical properties of metallic nanoparticles are dependant on the specific particle, which means that further exploration of the different metals is required.
Current research is making important discoveries around these properties, with a team at Rice University recently discovering that gold nanoparticles display a fundamentally different form of light-matter interaction. The researchers ran experiments showing that if the particles were excited in a specific way, they produced an almost perfect modulation of light scatter. Experts believe that this discovery will be relevant to the innovation of new optical components for both computers and antennas.
Conclusion
The applications of the specific optical properties of metal nanoparticles have yet to be fully explored, and while we have seen some innovations from laboratories entering the real world, the full potential of metallic nanoparticles is yet to be fulfilled.
Over the coming years, we can expect to see an increase in the use of metallic nanoparticles in applications that utilize these special optical characteristics.
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