Overcoming the Diffraction Limit in Wave Physics Using a Time-Reversal Mirror and a Novel Acoustic Sink

In recent years, time-reversal (TR) mirrors have been developed that create TR waves for ultrasonic transient fields propagating through complex media. A TR wave back propagates and refocuses exactly at its initial source. However, because of diffraction, even if the source is pointlike the wave refocuses on a spot size that cannot be smaller than half a wavelength. Here, by using a TR interpretation of this limit, we show that this latter limitation can be overcome if the source is replaced by its TR image. This new device acts as an acoustic sink that absorbs the TR wave. Here we report the first experimental result obtained with an acoustic sink where a focal spot size of less than 1/14th of one wavelength is recorded.

Figure 1

Schematic of experimental setup.

Figure 2

Signal emitted by the transducer device. The long part (between times 0 and 1.5 ms) is the TR window in order to TR the field inside the reverberating chaotic cavity. When only this part of signal is emitted, the focusing shown on Fig. 3a is recorded. The short part (around time 1750 ms) corresponds to the TR initial waveform $f(-t)$ (cf. text) in order to obtain the TR source. When the long and the short parts are emitted, the acoustic sink is obtained [Fig. 3b].

Figure 3

Time evolution recorded by the interferometer over a 20 mm by 20 mm square around the initial source during time-reversal propagation (1 ms separates each snapshot): (a) without the TR source, (b) with the TR source. On sequence (b), an acoustic sink is obtained.

Figure 4

Focal spot sections without the TR source (dashed line), with TR source (continuous line). Inside the graphic box, the focal spots normalized with respect to their maximums are plotted.