Take for example a 20 nm Ag NP with its LSPR in air at 400 nm. Now when this nanoparticle will be in water, light will refract from air to water to interact with it. Since the wavelength of all the light will now be changed as : Wavelength/1.33. So 532 nm wavelength in air now will be converted to 400 nm in air (400 nm in air will be converted to 300 nm). So as of now, if we exclude the effect of dielectric medium on the plasmons, then the LSPR should be at 532 (because the nanoparticle is actually experiencing 400 nm wavelength).
But then of course, the effect of the dielectric medium on the polarizability comes into the picture. And we have an LSPR at around 420 (Not 532!).
So it looks like, the dielectric medium actually blue shifted the LSPR from 532 to 420. In the absence of the effect of the dielectric medium on the plasmonics, the peak would actually be at 532.
Is this argument correct?
Thanks a lot in advance for your response!
The index of refraction decreases the speed of light, not the peak position wavelength of the material...
Thank you for your answer.
Yes, refraction changes the speed of light (the frequency remains constant) and hence the wavelength changes too. 400 nm (LSPR wavelength for the NP) light in air will become 300 nm and will now be too small to be in resonance with the plasmons in the nanoparticle. My argument is: 532 nm will be 400 nm and 400 nm should resonate with the plasmons (assuming other effects to be absent).
Indeed the frequency of the light remains constant and so does the energy (Planck's constant multiplied by frequency). When you plot your spectral intensity vs. the wavelength, you should always keep in mind that this is the vacuum wavelength independent of what system you are investigating, so that it is interchangeable with frequency and energy (or wavenumber). So energy remains constant, and since you assume "to exclude the effect of dielectric medium on the plasmons" the frequency and the energy of the plasmon will also not change. It is the equality of energy of the light and the plasmon (phonon etc.), i.e. the energy difference between two quantum states that counts. If your argumentation would be correct, then e.g. absorbance spectroscopy of solutions would reveal strong band shifts of the solute compared to the pure substance, which simply do not occur.
Thank you for the answer. Indeed, if we think in terms of frequency or photon energy, then it is unambiguous as there is no change of frequency/photon energy during refraction. So does it mean that the fact that the wavelength changes has no effect at all on the plasmon spectra?
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