Confocal surface plasmon microscopy with pupil function engineering

Abstract

Surface Plasmon microscopy can measure local changes of refractive index on the micron scale. Interferometric plasmon imaging delivers quantitative high spatial resolution sensitive to refractive index. In addition the so called V(z) method allows image contrast to be controlled by varying the sample defocus without substantially degrading spatial resolution. Here, we show how a confocal system provides a simpler and more stable alternative. This system, however, places greater demands on the dynamic range of the system. We therefore use a spatial light modulator to engineer the microscope pupil function to suppress light that does not contribute to the signal. ©2012 Optical Society of America

Surface plasmon microscopic sensing with beam profile modulation

“…The experimental system is similar to that used in [5], except that here a phase SLM (BNS 512*512 phase SLM) is used to replace the amplitude SLM; its function will be described at length in the paper. Figure 1(b) shows a simplified schematic of the optical system.…”

“…We have shown that the lateral resolution of SP microscopy is optimized with an interferometric arrangement and we have more recently demonstrated that similar performance can be achieved with a confocal microscope arrangement [5]. Figure 1(a) shows a conceptual diagram of SP imaging in a confocal system, as the sample is defocused there are two principal contributions to the signal arising from paths P1 and P2, which interfere with each other at the pinhole.…”

“…In reference [5] we showed that amplitude apodization was necessary since a sharp edge to the pupil function introduces phase and amplitude fluctuations in the response which are unrelated to SP propagation. Moreover, it is also useful to attenuate light incident at angles intermediate between normal incidence and angles close θ p as these angles do not contribute to the desired signal, so the relative amplitude of the plasmonic contribution is increased when these angles are eliminated.…”

“…In recent years there has been a growing interest in performing surface plasmon (SP) measurements within a microscopic environment [1][2][3][4][5][6]; this offers the possibility of obtaining localized high resolution measurements of local changes in refractive index as well as performing valuable imaging of cell attachment [7][8][9]. We have shown that the lateral resolution of SP microscopy is optimized with an interferometric arrangement and we have more recently demonstrated that similar performance can be achieved with a confocal microscope arrangement [5].…”

“…In order to obtain SP oscillations with a well defined period it is necessary to apodize the pupil function as otherwise the sharp edge of the aperture introduces oscillations that are unrelated to the SPs which beat with the SP oscillations. In reference [5] we used an amplitude only spatial light modulator (SLM) to apodize the illumination in the back focal plane, to ensure good quality oscillations. In the present paper we perform the same role with a phase only SLM, which also provides two other key functions: firstly it enables one to obtain a plot equivalent to the V(z) curve without mechanical movement, we call this V(α), for reasons that will become apparent later.…”