I have fabricated a periodic array of cylindrical nanoparticles and taken the transmittance spectrum. The single particle resonance dip that I am getting for the sample is getting blue shifted when the sample is coated with CdSe/ZnS quantum dots. Is it due to strong coupling of surface polaritons with the quantum dots? The absorption peak for the quantum dots are at around 613nm and florescence is at around 665nm. The dip in the transmittance spectra without quantum dot is around 650nm and with quantum dots is around 620nm.
I am not sure. Shift can appear due to change of dielectric function of the environment even if there is no coupling.
I think the blue shift comes from the change of refractive index of the coating dielectric.
The unique signature of the strong coupling is Rabi splitting. If you observe the splitting of a single particle resonance then yes you are in business. The energy of the Rabi splitting determines how strong your coupling is. Since you see only a blue shift I suspect you are in low coupling regime [if at all].
Thank you all. It looks like it is only due to refractive index change.
A way to measure the polariton coupling and/or the refractive index shift is by adsorbing the particles to a gold layer generating SPR at 613 nm. At the same time you should detect the SPR excited fluorescence at 665 nm using a fluorescence camera on top of the SPR device (with reflection at the bottom). A set-up of such instrumentation was for the first time published by Wolfgang Knoll. You can find a manuscript of prof Knoll of this setup in the Handbook of Surface Plasmon Resonance chapter 9 .Thank you
By measuring the polariton coupling with respect to refractive index shift as described in my previous answer is a way to get experimental data. By using SPR imaging in combination with fluorescence microscopy it will clarifiy that the excitation in the evanescent field of the nanoparticles will give a Stokes shift which can be measured with the shift to the longer wavelengthin using the fluorescence camera.
In the same time the refractive index shift can be measured by following the adsorption process by the angular SPR dip shift. In this way two physical phenomena can be separated from each other. However it looks in this way simple, but in the experimental setting more effects will mask these two phenomena. First the particles will adsorb and locally will change the refractive index in the evanescent field. This will give rise not only to a shift in SPR dip but also the SPR dip depth will less deep because of the inhomogeneity . At the same time it can also be caused by polariton excitation. However the Stokes shift as a function of the SPR dip angle will give definitely answers to the relation of RI and polariton coupling of different nanoparticles of different sizes, shapes and compositions. Thank you....
Dear Schasfoort,
Thanks for your detailed analysis. As you have mentioned, I have also observed a decrease in SPR dip depth. So, it is quite clear now.
Thanks,
Upkar
Since, the dielectric constant of CdSe/ZnS is greater than that of air, blue shift in plasmon band cannot be directly related to dielectric properties of CdSe/ZnS. An increase in surrounding dielectric constant will lead to red-shift in plasmon band, however, it is true only when dielectric does not significantly absorb the radiation itself. But since CdSe/ZnS will absorb at wavelengths around 650 nm, the resultant composite will exhibit spectra affected by absorption properties of both plasmonic system as well as QD system. One has to then calculate the extinction to ascertain the optical properties of composite and quantify red/blue shift.
Dear Himanshu,
I was about to put the same point that as R.I of the quantum dot is more than that of air, there should be a red shift. But as you rightly said CdSe/Zns absorbs around 650nm (613nm for the QD I have), the blue shift is essentially due to absorption by quantum dots. Thanks for you explaination. It was very helpful.
Upkar
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