Surface plasmon resonance (SPR) slides contain a glass slide with 50nm thick gold coating on top of it. Instead of using the Kretschmann setup with a bulky prism and having to worry about alignment, why do we not use the SPR slides as a waveguide and just shine led at the edge of the slide and have a spectrometer at the other end? Then you place some solution on the gold film and there should be a change in the spectrum. Ensuring everything is covered up so there is not ambient light, what are some issues with this idea or considerations?
As opposed to my previous answer, I think the light can be edge-coupled to the SPP waveguide as long as the transverse component of the electric field (perpendicular to the metal-glass interface) exists. But the mode mismatch might be a big problem as you are trying to butt coupled a kind of plane wave (assuming input from a laser) to the SPP mode. For edge/butt coupling, the mode matching is essential for efficient coupling. Even the edge/butt coupling to a SOI waveguide is a bit difficult. In case of SPP waveguide it should be even more difficult. Hence, even if you are able to obtain a small coupling to the SPP waveguide this way, the coupling efficiency is gonna be very small.
Also, I don't think you are gonna get a response similar to the free space coupling. For free space coupling, for example with Kretschmann configuration, you obtain a sharp peak in the transmission spectrum corresponding to a wavelength (for which the wave vector of the incoming light in the propagation direction matches with the propagation constant of the SPP mode). However, this is not true for the edge coupling, which is not that sensitive to the wavelength.
I have answered this as per my basic understanding about the modal analysis of the optical waveguide. I am not an expert in this area.
Nikhil Dhingra If I shine white light which emits rays in all directions using an LED at the edge of the SPR slide, won't some of the rays couple with the gold film and show a dip in intensity at the spectrometer readings?
Please consider my naivete of the field. I may be wrong , but shouldn't the idea of waveguiding need a high-indexed layer, rather than just glass and a gold coating? In bulky Kretschmann configuration, the principle of TIR, creates the evanescent field, that is used for sensing. For a simple gold film on glass, one may not be able to produce the evanescent wave, just by shining light on the edge. Rather, it would be easy to have a thin slab waveguide placed below, and trying to launch light within the high-indexed waveguide using high NA lens, given that it becomes extremely tough to precisely adjust a macroscopic object to a nanoscopic dimension. For convinence, people have used grating grooves at one-end (above the waveguide) for easy/macroscopic coupling of light into the wavweguide that creates evanescent fields for sensing. Even if there is no waveguide, light can be coupled through a grating (either dielectric or metal) into the glass/metal or glass/air interface by the grating 'evanescent orders'. A simple analysis provide in the following article mighty be helpful.
https://pubs.acs.org/doi/10.1021/acsami.0c12525?goto=supporting-info
I was imagining a setup like this from the paper:
Article Surface Plasmon Resonance Optical Sensor: A Review on Light ...
Although it is a review so it did not mention about the design requirements.
Article Kretschmann-Based Optical Sensor via Thermally Tunable Refra...
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