We present results of a novel and highly sensitive technique for the optical detection of ultrasound using the selective
storage of frequency shifted photons in an inherently highly efficient and low noise atomic frequency comb (AFC) based
quantum memory. The ultrasound ‘tagged’ optical sidebands are absorbed within a pair of symmetric AFCs, generated
via optical pumping in a Pr3+:Y2SiO5 sample (tooth separation Δ = 150 kHz, comb finesse fc ~ 2 and optical depth αL ~ 2), separated by twice the ultrasound modulation frequency (1.5 MHz) and centered on either side of a broad spectral pit (1.7 MHz width) allowing transmission of the carrier. The stored sidebands are recovered with 10-20% efficiency as a photon echo (as defined by the comb parameters), and we demonstrate a record 49 dB discrimination
between the sidebands and the carrier pulse, high discrimination being important for imaging tissues at depth. We further
demonstrate detector limited discrimination (~29 dB) using a highly scattered beam, confirming that the technique is
immune to speckle decorrelation. We show that it also remains valid in the case of optically thin samples, and thus
represents a significant improvement over other ultrasound detection methods based on rare-earth-ion-doped crystals.
These results strongly suggest the suitability of our technique for high-resolution non-contact real-time imaging of
biological tissues.
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