Optical spectroscopy involves splitting light up into its component wavelengths, exactly like how a glass prism splits sunlight up into a rainbow (or 'spectrum') of colours. While this may, at first, just appear to be aesthetically pleasing, the rainbow of colours does include some very useful information about the light it came from. For example, if one were to look very carefully at the orange part of the spectrum of the sun (roughly around the colour corresponding to old orange street lights), one would find a pair of dark lines where the light appears to be missing. This corresponds to absorption due to sodium, which tells us that there is sodium in the upper atmosphere (or 'chromosphere') of the sun; we have, in effect, learned part of what the sun is made of, from 93 million miles away, using nothing more than a glass prism. This is the power of optical spectroscopy.
Optical spectroscopy has many other uses; it can investigate the chemical composition of forensic samples, help locate a tumour, identify chemical weapons from a distance, monitor deforestation from orbit, authenticate artwork, and many more besides. Nevertheless, if we try to take pictures like we would do with a camera, there's a problem; a camera can only capture 2D information, and if we have a spectrum at each pixel, we either need to illuminate one line at a time (and use the other axis of the camera to measure the spectrum) or use a sequence of filters to get the data one wavelength at a time. In fact, if we know what spectrum we're looking for, the filter approach is much faster, but that requires carrying a stack of filters for each thing you might want to measure. That might be OK for a few things, but for portable or space-based applications, or if there are a lot of potential analytes, that can become infeasible. The alternative is to make a new filter each time, using a photorefractive polymer.
This new system can create any filter that the user might want, writing it into a material similar to that used to make holograms. This is unlike normal tunable filters, which are typically only capable of tuning a single transmission band's width and center wavelength. This new approach can create any filter profile the user might want, including multiple independent bands and different band shapes. It works by recording an interference pattern in a holographic plate, using two laser beams. The angle between the laser beams is changed, and another interference pattern is recorded. After doing this many times, the filter profile is recorded in the hologram; this entire process takes less than a second. Once the filter isn't needed any more, it can be rewritten, and a new pattern created; any number of analytes can be searched for, including those which the system has never seen before; they need only be programmed in by the control computer.
The holographic material is a new type known as a photorefractive polymer. Unlike normal holograms, this material is rewritable; it can be erased, and a new pattern written into it. While this has been used to create rewritable holographic displays before, this is the first example where the material will be used to create a holographic filter. Nevertheless, synthesizing it is not difficult; it requires just two components to be made from scratch, and these are both easy synthetic procedures with high yields.
Overall, this project offers to create a kind of 'Instagram for spectroscopy'; rather than being limited to a small selection of physical camera filters, a user can digitally apply any one that might be needed, including programming a new one from scratch if necessary. This makes the spectroscopic imaging process faster, and more efficient, allowing the user to gather data over larger areas, and with more precision, than ever before.
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