The surface plasmon is a particular kind of electromagnetic wave which propagates along the surface of a metal (H. Raether, “Surface plasmons on smooth and rough surface and on gratings”, Springer-Verlag ISBN 3-540-1760-3, Berlin, 1998). Optical excitation of the surface plasmon can be achieved if a p-polarized, collimated light beam undergoes total reflection on the surface of glass substrate (for example a prism) coated with a thin metal film (so-called Kretschmann configuration). The momentum of photons should match the surface plasmons on the opposite surface of the metal film in order to make this possible. This occurs for a certain wavelength at a critical angle of incidence of light. The phenomenon is observed as a sharp minimum in the intensity of the reflected light when the angle of the incidence (the angle between the surface of the glass substrate and the light) is varied. The angle or wavelength at which this dip occurs depends decisively on the properties of the surface layer on the top of the metal film, and therefore the phenomenon can be used to monitor changes on this surface layer caused e.g. by a specific chemical or biological reaction or by the change of concentration of some substance in the immediate vicinity of this surface.
FIG. 1 shows the principle of an arrangement for surface plasmon resonance measurement. In FIG. 1 is a beam 1 of electromagnetic radiation (e.g. a laser beam) produced by a source 2 for electromagnetic radiation (e.g. a laser) directed in an angle (α1; α2) of incidence in relation to the surface 4 through a part 3 transparent for said radiation, a semi-circular prism 3, onto a metal film 5 on the surface 4 of the prism 3. The beam 1 of electromagnetic radiation is reflected on the surface 4 of the prism 3. When the beam 1 of electromagnetic radiation is reflected on the surface 4 of the prism 3, the surface 4 produces and directs a beam 6 of reflected electromagnetic radiation at an angle (α1; α2) of reflection (which is equally large as the angle (α1; α2) of incidence in relation to the surface 4 through the prism 3 and further to a detector 7 for detecting the intensity of the beam 6 of reflected electromagnetic radiation. Surface plasmons are excited on the opposite surface of the material layer 5 by electromagnetic radiation undergoing total internal reflection (TIR) at the surface 4. Material layer 5 and possible additional layers are inside the influence zone of the evanescent field associated with the TIR.
One of the problems associated with the above arrangement is that if the prism 3 and with it the surface 4 and material layer 5 is rotated an angle β in relation to the source 2 of electromagnetic radiation, the detector 7 for collecting the beam of reflected electromagnetic radiation should be rotated an angle 65 in relation to the surface 4, which is equal twice the angle β of rotation of the prism 3 itself. In other words, when prism 3 is rotated an angle β, the surface 4 of the prism 3 is also rotated an angle β, which leads to that the old angle α1 of incidence between the beam 1 of electromagnetic radiation and the surface 4 and material layer 5 changes to a new angle α2 of incidence between the beam 1 of electromagnetic radiation and the surface 4 and correspondingly to that the old angle α1 of reflection between the beam 6 of reflected electromagnetic radiation and the surface 4 changes to a new angle α2 of reflection between the beam 6 of reflected electromagnetic radiation and the surface 4. This leads to that the angle (not marked with a reference numeral) between the beam 1 of electromagnetic radiation and the beam 6 of reflected electromagnetic radiation changes. In order to collect a beam 1 of electromagnetic radiation produced by the source 2 and reflected as an beam 6 of electromagnetic radiation by the surface 4, the detector 7 has therefore to be rotated an angle γ, which is twice the angle β of the rotation of the prism itself in the arrangement shown in FIG. 1.
In the example in FIG. 1 this means that if the prism is rotated anti-clockwise 20 degrees about an axis of rotation 12 (the source of electromagnetic radiation is not rotated) the beam 1 of electromagnetic radiation from the source 4 enters the prism and strikes the material layer 5 on the surface 4 at an angle of incidence rotated 20 degrees clockwise compared to the non-rotated state and this leads to that the reflected beam exists the prism at an angle of reflection, which is rotated 40 degrees anti-clockwise compared to the non-rotated state. In the example in FIG. 1, the new angle α2 of incidence is 20 degrees sharper than the old angle α1 of incidence and correspondingly the new angle α2 of reflection is 20 degrees sharper than the old angle α1 of reflection flection. The angle between the new angle α2 of incidence and the new angle of α2 of reflection is therefore 40 degrees larger than the angle between the old angle α1 of incidence and the old angle of α1 of reflection. This is why the detector has to be rotated 40 degrees (twice as much as the angle of rotation of the prism 3) in relation to the source 1.
A solution to this problem is to have a rotating arrangement which, when the angle of the source is rotated rotates the detector 7 an angle, which is twice the angle of the rotation of the source 4. This solution is mechanically complex.