We present quantitative optical images of human breast in vivo. The images were obtained by using near-infrared diffuse optical tomography (DOT) after the administration of indocyanine green (ICG) for contrast enhancement. The optical examination was performed concurrently with a magnetic resonance imaging (MRI) exam on patients scheduled for excisional biopsy or surgery so that accurate image coregistration and histopathological information of the suspicious lesions was available. The ICG-enhanced optical images coregistered accurately with Gadolinium-enhanced magnetic resonance images validating the ability of DOT to image breast tissue. In contrast to simple transillumination, we found that DOT provides for localization and quantification of exogenous tissue chromophore concentrations. Additionally our use of ICG, an albumin bound absorbing dye in plasma, demonstrates the potential to differentiate disease based on the quantified enhancement of suspicious lesions.optical properties ͉ contrast agents ͉ MRI coregistration

We have studied the temporal profile of photon-echo signals generated by combined gated cw and pulsed dye-laser excitation of the inhomogeneously broadened, 555.6-nm absorption line of (174)Yb vapor. We find that the echo profile is, after time reversal, essentially identical with that of the first excitation pulse. We give a new analysis of this effect. Since time-reversed pulse reproduction should also occur in inhomogeneously broadened solid samples, and since we observe time-reversed reproduced pulses up to 4% as intense as the input pulse, this effect may have important applications in optical signal-processing systems.

We have studied the response of atoms in a dilute collimated atomic beam to transient phasecontrolled resonant excitation fields. We demonstrate that the use of such excitation makes it possible to place the atoms into stationary states of the coupled atom-field system. We show that these states, which are easily visualized in the two-level -atom vector model, are equivalent to the dressed states of resonance fluorescence theory. Studies of atoms in pure dressed states should allow interesting tests of basic atom-field interactions.PACS numbers: 32.50. +d, 32.80. -t, 42.50. +q, 42.65. -k The interaction of simple two-level quantum systems (atoms) with nearly resonant electromagnetic excitation fields has been a subject of great interest over the years. Perhaps the most elegant description of the dynamics involved is afforded by the two-level -atom vector model. ' In the vector model, the atom and driving field are represented by vectors in a threedimensional space, and the atomic response to the driving field is simply visualized as a precession of the atomic-state vector about the instantaneous drivingfield vector. In the case of a constant driving field, two orientations of the atomic-state vector become very special: the atomic-state vector becomes motionless when it points parallel or antiparallel to the driving-field vector. This implies that there is no energy exchange between the excitation field and the atom even if the atom and the field are exactly resonant. The special atom-field states can thus be described as stationary states, and as discussed below, they correspond precisely to the "dressed" states familiar from the theory of resonance fluorescence. 3 5 As such they should have interesting spectral properties.In this Letter, we describe the first experimental observation of stationary atom-field states involving resonant optical excitation fields. These special states were generated through the use of amplitude-and phase-controlled excitation of a collimated atomic beam. We note that stationary spin-field states were studied in nuclear magnetic resonance work, where they were referred to as "spin-locked" states. ' Consider the response of a two-level atom [see Fig. 1(a)] to an oscillating electric field of the form E(t) = &ocos(cot+ g), and exp (i cp) P, + Qb, (3b) respectively, where n+ = [(p &o2j4X2+)+1] '1, and Q, and pb are eigenstates of the isolated atom. Note that the eigenvectors but not the eigenvalues depend on the instantaneous phase of the excitation field. The eigenvectors W+ are the semiclassical dressed atomic states discussed elsewhere. When field quantization is taken into account, 3 s the dressed states become linear combinations of atomic and photon states. In terms of energy, the fully quantized dressed states form an extended series of doublets. 3 s When one semiclassical dressed state is populated, one state within each doublet of fully quantized dressed states is populated. The atomic-state vector, p, used in the vector b,~Q-cu 4 J 'll the appropriate electric dipole momen...

No abstract

We have performed a photon-echo experiment on the 555.6-nm intercombination line of atomic Yb vapor which demonstrates that an intense laser field acts to inhibit the dephasing resulting from velocity-changing collisions. This finding implies that the optical Bloch equations, which assume that relaxation is independent of the strength of the driving field, cannot correctly describe the behavior of gas-phase samples in all regimes.