Time Reversal of Electromagnetic Waves

In a nondissipative medium, the wave equation is time-symmetric. Therefore, for every wave diverging from a pulsed source, there exists in theory a wave, the time-reversed wave, that precisely retraces all its original paths in a reverse order and converges in synchrony at the original source as if time were going backwards. This time-symmetry exists even in a strongly heterogeneous medium where the waves are strongly reflected, refracted, or scattered. In 1989, an original method has been proposed for generating such a timereversed wave from a surface. This time-reversal technique has led to numerous applications in ultrasound and underwater acoustics such as brain therapy, lithotripsy, nondestructive testing, or underwater telecommunications [7].