This invention comprises an interrogation and readout differential method for chemical sensors based on Surface Plasmon Resonances (SPR). The integration of the SPR sensing unit (chip or other), as intermediate reflecting element of a Fabry-Perot (FP) optical resonator, is the starting point for the application of this method. A Surface Plasmon Resonance can be only coupled by radiation p-polarized, referencing to the chip incidence plane, while no coupling is possible by s-polarized radiation. Two different modal systems of the FP, corresponding to the two orthogonal polarizations, are non-degenerate in frequency because of the different SPR coupling behavior. A p-polarized field can be considered as a probe and an s-polarized field as a reference, once they are simultaneously resonating in the cavity. Starting from this, an optical heterodyne readout system is implemented based on the detection of the beat note of the intra-cavity resonating radiation fields.
Surface Plasmon Resonance based Sensors are suitable in detection of molecular targets dispersed in liquid phase at ultra-low concentration. The state-of-the-art methods for the interrogation and readout of the sensors are limited in performances by the technical noise. The radiation source maps its amplitude noise into the final measurement, while the optical apparatus pickups the acoustical vibrations from the environment. Differential and interferometric architectures, based on phase-detection, have been demonstrated as the most promising for high performances apparatuses. The main limit of these approaches stands in the delicate setups involved. The sensitivity depends on the interferometric arms length, i.e. it scales with the size of the apparatus. A new differential readout approach is demonstrated here, based on a direct frequency readout obtained by a purely optical down-conversion process to radiofrequency range. The method is particularly suited for optical integration in a waveguide setup. This opens the way to the devising of high performances compact SPR chemical sensors.
US and Europe