the deposited gold layer on cover glass slid in 50nm thickness has a particles size 35 nm..the excitation laser is 633 nm was used in prism based SPR ...is this enough to generate LSPR ? or I need frequency matching also... what is the boundary line split between SPR and LSPR I am confused>> if they ask me in viva.
Take it easy, it is quite simple. Surface Plasmon Resonance (SPR) is a phenomenon observed on thin metal films. Important is that it appears on continuous films. So it is propagation of the electromagnetic wave (or we can say that it is propagation of surface polarization, therefore it is substituted by quasi-particles called "surface plasmons"). To observe this effect you need continuous metal layer illuminated at certain wavelength and angle of incidence. For each wavelength is there some angle of incidence where it is observable (of course, for selected metals). It should be noted that the film thickness determines the SPR quality. For very thin film the layer is not continuous. For very thick films you do not have the evanescent wave so you do not observe SPR.
On the other hand, the Localized Surface Plasmon Resonance (LSPR) is a phenomenon related to the interaction of the light with single metal nanoparticle. If the nanoparticle is greatly smaller than the ligh wavelength, we can use elecctrostatic approximation. In other words, the nanoparticle is like in the electrostatic field andit is polarized (i.e. there is an induced dipole inside the nanoparticle). At certain conditions (depending on the refrctrive index of the metal and the ambient environment) this dipole generation goes to the resonance and the effect is observable. It should be noted that this phenomenon is not related to the angle of incidence (it works for any angle of incidence), but it is observaable only at specific wavelength. Keep in mind that in experiment you have not only single nanoparticle, but you are measuring large number of nanoparticles. Since the resonance wavelength is depending also on the nanoparticle size, the broad histogram of nanoparticle sizes will cause broad peak of LSPR. Furthermore, there are other efffeccts (coupling between nnoparticles, nanoparticle size, surface oxides, etc) that can influence the LSPR peak position.
In conclusion SPR is an effect related to the continuous films, while LSPR is related to the isolated nanoparticles (nanoislands, or any nano-objects). For gold layers of right film thickness is observable SPR at certain angle of incidence only. For gold nanoparticles the LSPR is independent on angle of incidence, but it is at specific resonance wavelength only.
The last but not least should be mentioned optical properties. There is very simple way how to evaluate your gold layer. If it is semitransparent color film (e.g. semitransparent green or blue piece of glass) you can be sure that you have gold nanoparticles. If the layer exhibits gold-like color and it is not transpaarent you can hope that you will see SPR.
Good luck.
Martin Weis, thanks for your help ...I will keep your notes ...easy and simple
Very good explanation by Martin generally SPR and LSPR differ by the width of metal layer. Moreover for less than 50 nm = LSPR while greater than 50 nm = SPR for quick revision
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