I've been wondering about something regarding the physics behind SPR spectroscopy:
As far as I understand, SP resonance can only occur if frequency and the vector component of impulse in propagation direction for the photon and surface plasmon are identical. Taking a look at the dispersion relation for photons and surface plasmons, there is one specific impulse for each frequency and vice versa. In angular SPR the wavelength (and thus the frequency) is constant. So SP resonance should only occur at one specific angle of incidence at which the relevant vector component of impulse matches that of the surface plasmons. If this is true, shouldn't the reflectance have a sharp minimum at this specific angle instead of being a rather broad curve?
The only explanation I could come up with is, that the light source used in biacore instruments isn't a laser but an LED, so the emitted light isn't monochromatic. So I could think of the shape of the dips being a result of the spectral bandwidth of this light source. Then again in literature there are several figures of SPR dips in dependence of the wavelength and there it seems, that the broadness of the dips increases with decreasing wavelength which argues against my thought.
So here again in brief my question: Why is the SPR dips a curve and not a discrete minimum in reflectivity?
You are right, the light source in the Biacore is a near infrared LED of peak intensity wavelength of 760 nm, emitting polarized light.
This can be found in the sensor surface handbook of Biacore (available on-line) and in various publications
Thanks for the reply. I know, that the light source is an LED (sorry, I didn't state that clearly) - that is why I proposed that the spectral bandwidth (<5 nm according to Biacore) might cause the broad dips, but does anyone know, whether this is in fact the reason and whether it is the only reason for band broadening, respectively?
The optical light source band width, dispersion of the light in optical elements, spot widening and polarization rotation all have a small effect to the SPR peak shape, but the main reason is actually caused by physics of the phenomenon. The widening is coming mostly from the limited conductivity and electrical damping of the "SPR metal" used. This allows the SPP excitation to be partially matched also at a set of non-optimal conditions (peak widening). By considering the electro-optical properties of the SPR metal in terms of dielectric constants (instead of the complex RI), we get a simple relationship: The higher the conductivity (negative real part of dielectric constant) and lower the damping (imaginary part of dielectric constant), the sharper the SPR peak. This can be easily by theoretical modeling of SPR peak with dedicated softwares like Winspall, BioNavis LayerSolver, or by comparing dielectric constants from databases (i.e. refractiveindex.info).