The interaction of ultrasound and light in biological tissues results in a small amount of the scattered light being shifted relative to the carrier frequency (typically 1 part in 108). We have developed an inherently efficient and low noise quantum memory based technique to selectively absorb these ‘ultrasound tagged’ photons in a pair of atomic frequency combs, and recover them delayed in time as a photon echo. In this manner we have demonstrated record ultrasoundmodulated sideband-to-carrier discrimination (49dB). Further, we confirm that the technique is compatible with highly scattering samples, and present initial acoustic pulse tracking measurements. This strongly suggests the suitability of the technique for biological tissue imaging.
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
We demonstrate the optical detection of ultrasound using spectral hole burning in a cryogenic rare earth ion
doped crystal. The dispersion due to the hole is used to perform phase to amplitude modulation conversion.
This method allows sensitive detection of ultrasonic displacements with the advantage of large étendue. This
method is also attractive as it requires only moderate absorption contrast to achieve high sensitivity. We also
describe a method for diode laser stabilisation using optical feedback through spectral holes which dramatically
reduces the laser phase noise.
Here we show that the photon echo equivalent of an NMR gradient echo is completely efficient if the sample is
optically thick, the detunings of the atoms vary linearly along the direction of propagation and the storage time
is short compared to the decay rate of the atoms. In this process the only light that interacts with the sample
of atoms during the storage and retrieval process is the light that is to be stored and then retrieved, their are
no auxiliary beams. The stored and recalled light travel in the same direction and their is no need for the phase
matching operation that is present in previous quantum memory proposals using controlled inhomogeneous
broadening. This greatly simplifies various possible implementations. We present analytical, numerical and
experimental results of this scheme. We report experimental efficiencies of up to 15% and suggest simple realizable
improvements to significantly increase the efficiency.