2D Attogram Surface Plasmon Resonance Imaging
5th September 2015

Differential Surface Plasmon Resonance

The second plasmon interrogation technique is an ultra-sensitive differential surface plasmon (d-SPR) ellipsometry developed by JRS. However, the equipment generally used has so far not been low voltage driven and tends to be both bulky and expensive. Also, there has been no provision in previous implementations for array sensing or multiplexing. In the last year at Exeter we have developed a liquid crystal polarization modulator, which overcomes these difficulties, clearing the way for the technique to be developed further as a flexible refractive index sensor. The Thin Film Photonics Group will develop this pixelated fast optical phase differential surface plasmon technique, which places the least stringent requirements on the camera performance.

Vertically (TM) polarized light may excite a surface plasmon resonance at a metal/dielectric interface in the Kretschmann configuration. There is a significant phase change of the reflected TM polarized light as the SP resonance is traversed, whether by changing the incident angle, the wavelength of the light or the refractive index of the bounding dielectric. If linearly polarized light consisting of both TM and TE polarizations is incident upon the SP system, then the TM polarized component undergoes a phase change, whereas the TE polarized component does not. The result of having two orthogonal components phase shifted with respect to each other is that the light reflected from the SP system becomes elliptically polarized. Because the phase changes rapidly as the SPR is traversed, the ellipticity and orientation of the polarization ellipse also changes rapidly. The azimuth of the ellipse is rotated by approximately 1o for a refractive index change of only 5 x 10-5. Therefore, all that is needed to produce a sensitive refractive index sensor is a sensitive measure of the rotation of the polarization ellipse.

If the plane of polarization of incident light upon a SP system is dithered sinusoidally and the reflected signal monitored using a phase sensitive detector with the reference set at the dither frequency, then the zeros of this differential signal correspond to the azimuth or the azimuth ±π/2rad of the output polarization ellipse. If the refractive index of the bounding dielectric medium is altered, the angular position of the zero in the differential signal also changes. Recently, we have demonstrated this same effect using a chiral hybrid aligned Liquid Crystal (LC) cell, which is low voltage driven, has low power consumption and is cheap, small, and light weight. Further, it also allows simple pixelisation, for imaging or sampling many areas simultaneously. The use of a chiral dopant in the HAN cell means that there is a twist in the director, which produces a rotation in the plane of polarization of transmitted light through the cell. Then when a voltage is applied across the cell the liquid crystal director re-orientates and untwists to an extent dictated by the voltage (without a threshold as it is a HAN cell). Therefore, the degree of polarization rotation is controlled by the applied voltage. This complete arrangement has now been tested the results being shown in the Figure 3. This has a sensitivity to changes in index of a mixture of argon and nitrogen gases of 2 x 10-7. This sensitivity corresponds to a polarization rotation resolution of only 0.02o. Improvements in the optical configuration, allowing brighter signals and also detector electronics will increase the angular sensitivity by two orders of magnitude.