Appl. Phys. Lett. 101, 191112 (2012)

Ultrasound-modulated optical tomography is showing promise for the highly sensitive, deeply penetrating imaging of soft tissue. However, the sensitive detection of ultrasound from biological samples requires strong spectral filters that can discriminate between information in the carrier frequency and sideband frequencies of the modulated light. David McAuslan and co-workers from the University of Otago in New Zealand have now used quantum memory techniques to achieve extremely sensitive ultrasound detection. The researchers used a Pr3+:Y2SiO5 crystal as the quantum memory and generated two atomic frequency combs at ±1.5 MHz to store the ultrasound. They separated the combs by employing a transmission window that was fixed to the frequency of the input laser, and used a 606 nm laser beam that was phase-modulated by an acousto-optic modulator. This stored the resulting sidebands in the atomic frequency comb, resulting in the formation of an echo 6.667 s after the input pulse beam was transmitted. For a 1 mW input pulse beam, the peak level of noise in the detection window was 49 dB below the level of the input pulse beam, which is a significant improvement over other ultrasound detection methods based on rare-earth-doped crystals.