Two researchers at Université Paris VII have reported that they have used an acoustic time-reversal mirror to achieve focal spots of less than λ/14. Although the experiment involved sound waves, the scientists are investigating the suitability of the technique for electromagnetic waves. A full report of the experiment appears in the Sept. 16 issue of Physical Review Letters.

The team employed a laser interferometer to record the wave patterns created in a 1.9-mm-thick glass plate by an incident 5-µs pulse of sound. Applying time-reversed versions of the signal to the plate eliminated all but a 0.3-mm focusing wave.