|LIGHTPIPE SPR SENSORS|
The lightpipe SPR sensor was developed at the University of Washington and has a number of advantages over traditional prism-based SPR sensors. A lightpipe sensor uses a non-coherent waveguide to transmit light to and from the active sensor region. Because the lightpipe is thick relative to the wavelength, simple geometric ray optics techniques may be used to predict the path of the light. A key advantage of the SPR lightpipe is the ability to use multiple SPR operating angles. The lightpipe splits incoming light into a series of bands at the output based on the angle of the light, which affects the number of reflections that the light makes within the lightpipe. If there are an odd number of reflections, then the light will exit the lightpipe in a direction (up or down) opposite the incoming light. Since sequential bands are reflected in opposite directions, there is good separation between bands, making the design of the optical detection system easier.
The figure above shows two bands of light traveling through the lightpipe ("folded"). Above the folded lightpipe, the light path has been unfolded to show the bands more clearly. The blue band (band 4) has an even number of reflections, so that the light exits the lightpipe going in an upward direction. In contrast, the red band (band 3) exits the lightpipe in a downward direction. In this example, the gold-coated active area of the SPR sensor is positioned over the first reflection on the top of the lightpipe. Note that it is therefore possible to interrogate the surface plasmon wave with two different angles, so that resonance will occur at two different wavelengths of light. By providing additional information about the surface plasmon wave, a sensor using more than one simultaneous operating angle can be used to provide additional information about the thin film being detected; for instance, simultaneous measurement of the thickness and the refractive index of the film.
The lightpipe SPR sensor has other advantages. If the lightpipe is a replaceable substrate, then the SPR sensor element may be replaced without having to index-match the substrate to the rest of the optics. In traditional prism-based SPR sensors, an index-match material or fluid must be used between the prism and the substrate to allow the light to transmit between the two. The index matching is required because the angle of the light is beyond the critical angle for light for a glass-air interface, so that light would be totally reflected and would be unable to transmit through the small air gap into the substrate. Since the angle of the light at the ends of the lightpipe are a complimentary angle to the SPR operating angle, light can enter the lightpipe from the end without index matching.
The figure above shows the light path through an SPR lightpipe sensor developed at the University of Washington to detect biological toxins. Prisms are used to bend the light at right angles to the sensor surface, so that the optics may be placed below the substrate, keeping the SPR sensor surface clear for a flow cell.