The cooperative resonance fluorescence radiation from two atoms separated by an arbitrary distance and driven by a coherent laser field is studied, taking into account the dipole-dipole near-field interaction. The steady-state total scattering intensity, as a function of laser frequency and field strength, differs considerably from the single-atom one for sufficiently small distances. Photon bunching and antibunching occur in the scattered radiation depending on the laser frequency. Practically a full antibunching effect is found, especially if the laser is tuned to resonance with a transition frequency of the two-atom system. This is in contrast to a recent result of Agarwal et al., who studied the two-atom system but neglected the frequency splitting due to the dipole-dipole near-field interaction.
The photon-number statistics from resonance fluorescence of a two-level atom near a metal nanosphere driven by a laser field with finite bandwidth is studied theoretically. Our analysis shows that all interesting physics here takes place in a small area around the nanosphere where the near field and the atom-nanosphere coupling essentially affect the radiative properties of the atom. Computer modeling estimates this area roughly as $r\ensuremath{\le}2a$
(r is the distance from the center of the nanosphere to the atom), with
a being the radius of the nanosphere. At the larger distances, the influence of the nanoparticle vanishes and the atom tends to behave similarly to that in free space. It is shown that the distribution function
p(n,T) of the emission probability of
n photons in a given time interval
T in steady-state resonance fluorescence drastically depends on the atom location around the nanosphere for $r\ensuremath{\le}2a$, featuring a characteristic twist in the ridgelike dependence and a convergence time of up to $9\ensuremath{\mu}\text{s}$, two orders of magnitude slower than for the atom in free space. At large distances, the distribution converges to a Gaussian one, as for the atom in free space. The typical convergence time scale at large distances
r>2a tends to the convergence time of the atom in free space. There are also two areas symmetrical around the nanosphere in which $\ensuremath{\Omega}\ensuremath{\sim}\ensuremath{\gamma}$ and the convergence time goes to zero. This behavior is determined by the interplay of the radiative and nonradiative decay rates of the atom due to the coupling with the metal nanosphere and by the near-field intensity. Additional parameters are the normalized laser frequency detuning from the atomic resonance and the bandwidth of the incoming laser field.
Modification of the resonance fluorescence spec-trum of a two-level atom driving by a lasermonochromatic field in the close proximity of aplasmonic nanostructure (metal sphere) is studiedin detail. It is shown that one can control thisspectrum varying four key parameters: distancebetween the atom and the nanosphere, atom’s lo-cation around the nanosphere, the radius of thenanosphere, and polarization of the incident ra-diation. These parameters affect the local fieldenchancement, transition frequency shift and themodification of the radiative decay rate of the atominteracting with the nanosphere, which lead tomodification of the resonance fluorescence spec-trum of the atom (frequency shift of the satellitelines in the Mollow-type triplet, widths of the lines,the spectrum intensity) by contrast with that onein free space. The permittivity of the metal thenanosphere is made of is also an additional param-eter, which defines the nonradiative decay. The lat-ter in combination with other parameters allows tocontinuously control the transition from resonancefluorescence enhancement to its quenching. Thecalculation results are generalized to the case ofN two-level atoms, distributed around the nanopar-ticle in the close proximity of its surface. Thecalculations were performed for different positionsof the detector relative to the system nanoparticle-atom(s).
The authors have studied the fluorescence spectrum of a two-level atom interacting with a resonant smooth laser pulse. Assuming a filter width larger than the Fourier width of the pulse, an approximate analytic closed form for the power spectrum valid for any pulse shape is obtained. In these conditions the power spectrum exhibits a time-developing three-peak structure, while the energy spectrum presents a single line, unless the pulse is near rectangular. Theoretical energy spectra are in qualitative agreement with measurements of the resonance fluorescence emitted by strontium atoms irradiated by short laser pulses.
A cascade transfer of light energy to a resonance atom situated near a spherical nanoparticle and then, by a nonradiative mechanism, to the nanoparticle itself is considered. It is established that the efficiency of the cascade transfer essentially depends on the frequency and polarization of light, on the distance between the atom and the particle, on the optical properties of the particle, and on the time conditions of radiation. The rate of light absorption by a metal nanoparticle via cascade energy transfer may be 10 4 –10 5 times higher than the direct absorption of light by a nanoparticle. For a fixed frequency of light, the cascade transfer of energy is a sharply selective function of the distance between the atom and the particle (the resonance width is about 10 −2 of the particle radius). Atomic fluorescence exhibits similar behavior. This feature can form the basis for a new method of optical scanning microscopy and location and localization of atoms near the surface of a particle.