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In 1873, German physicist Ernst Abbe published a law that limits the width of light beams.On the basis of this law, the diameter of a spot of light, obtained by focusing a light beam through a lens, cannot be smaller than half its wavelength – around 500 nanometres (500 billionths of a metre) for visible light.And while this law plays a huge role in modern optical microscopy, it also sets up a barrier for any efforts from researchers to produce extremely small dots – in the nanometre region – to use as binary bits.In our study, we showed how to break this fundamental limit by using a two-light-beam method, with different colours, for recording onto discs instead of the conventional single-light-beam method.Both beams must abide by Abbe’s law, so they cannot produce smaller dots individually. But we gave the two beams different functions: The first beam (red, in the figure right) has a round shape, and is used to activate the recording. We called it the writing beam The second beam – the purple donut-shape – plays an anti-recording function, inhibiting the function of the writing beamThe two beams were then overlapped. As the second beam cancelled out the first in its donut ring, the recording process was tightly confined to the centre of the writing beam.This new technique produces an effective focal spot of nine nanometres – or one ten thousandth the diameter of a human hair.
