One of the most fundamental differences between desk lamps and lasers is that the latter is coherent spatially. This means the troughs and crests of its light waves are closely correlated with each other. The desk lamp, on the other hand, emits completely unrelated waves that are known as incoherent. This is just the broad understanding, in reality even the incoherent light has a high degree of spatial coherence. However, in order to detect that kind of coherence one needs to investigate into the light at a very minute length scale which cannot be accessed with the help of customary methods.
A team of researchers working in the Domenico Pacifici lab at the Brown’s School of Engineering recently discovered a way for detecting the spatial coherence in beams of light at a scale of some hundred nanometers. This was much smaller scale as compared to the one they predicted. Drew Morrill, the lead author of this research says, “There’s a very small length scale at which light that’s often said to be incoherent behaves coherently, but we’ve lacked experimental techniques to quantify it. That degree of coherence contains meaningful information we can now access, which could be useful in characterizing light sources and potentially for new imaging and microscopy techniques.”
The customary method of examining the spatial coherence of light included a number of devices that were capable of splitting the wavefront of a given light beam. The most popular one for this purpose was the Young Interferometer that was also known as the double slit experiment. The experiment goes as the source of light aims at the detecting screen along with an opaque barrier between the two. The barrier basically has two mini sized slits that permit two rays of light to pass through those. When the light waves cross these barriers, the resulting waves are many times bent towards each other leading them to recombine with each other. The recombining waves coherent with each other often make the interference pattern – a continuous series of dark and light patches on the screen. The measurement between these two kinds of patches defines the light coherence. Morrill adds, “The interference fringes are smeared out, making it difficult to quantify the degree of coherence. But if you could get around the fundamental limitations of the double slit experiment, theoretically you should be able to see those fringes.” For solving this problem, the team used a new kind of interferometer that uses plasmonics.
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