Abstract
Spatial phase modulation in an imaging interferometer is utilized in surface plasmon resonance (SPR) based sensing of liquid analytes. In the interferometer, a collimated light beam from a laser diode irradiating at 637.1 nm is passing through a polarizer and is reflected from a plasmonic structure of SF10/Cr/Au attached to a prism in the Kretschmann configuration. The beam passes through a combination of a Wollaston prism, a polarizer and a lens, and forms an interference pattern on a CCD sensor of a color camera. Interference patterns obtained for different liquid analytes are acquired and transferred to the computer for data processing. The sensing concept is based on the detection of a refractive index change, which is transformed via the SPR phenomenon into an interference fringe phase shift. By calculating the phase shift for the plasmonic structure of SF10/Cr/Au of known parameters we demonstrate that this technique can detect different weight concentrations of ethanol diluted in water, or equivalently, different changes in the refractive index. The sensitivity to the refractive index and the detection limit obtained are −278 rad/refractive-index-unit (RIU) and 3.6 RIU, respectively. The technique is demonstrated in experiments with the same liquid analytes as in the theory. Applying an original approach in retrieving the fringe phase shift, we revealed good agreement between experiment and theory, and the measured sensitivity to the refractive index and the detection limit reached −226 rad/RIU and 4.4 RIU, respectively. These results suggest that the SPR interferometer with the detection of a fringe phase shift is particularly useful in applications that require measuring refractive index changes with high sensitivity.
Highlights
The surface plasmon resonance (SPR) based sensing, which utilizes the interaction of light with free electrons at a metal-dielectric interface [1] and is very sensitive to a large variety of physical/chemical processes at the interface, is the heart of a mature technology with a number of applications in physics [2,3], chemistry [4], biology [5,6], and other fields [7,8,9]
−226 rad/RIU and 4.4 × 10−6 RIU, respectively. These results suggest that the SPR interferometer with the detection of a fringe phase shift is useful in applications that require measuring refractive index changes with high sensitivity
The SPR phenomenon is accompanied by the collective oscillations of free electrons—Surface plasmons (SPs)—That can be excited at the interface by the attenuated total reflection (ATR)
Summary
The surface plasmon resonance (SPR) based sensing, which utilizes the interaction of light with free electrons at a metal-dielectric interface [1] and is very sensitive to a large variety of physical/chemical processes at the interface, is the heart of a mature technology with a number of applications in physics [2,3], chemistry [4], biology [5,6], and other fields [7,8,9]. The most efficient way for fulfilling the resonance and generating the SPs provides the Kretschmann configuration [2], in which a prism of high refractive index is coated on its base with a thin metal film or a high refractive index glass slide with a thin metal film is attached to the prism. In both cases, the SPs are excited in the metal film by the ATR mechanism, and the field of SPs decays. Optical phase detection techniques [21,22,23] provide a large number of approaches, including heterodyne interferometry [24,25], interferometry with a Mach–Zehnder [12,26,27,28] or imaging interferometer [13], phase quadrature interferometry [29], a phase-shifted polarimetric scheme [30], schemes with a photo-elastic [14] or electro-optic [15] modulator, and a rotating analyzer method [17]
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