We describe a scheme capable of localizing an ensemble of interacting two-level atoms. The atoms are assumed to
be bunched together in a volume much smaller than an emission wavelength, and they interact with a standing
wave laser field. Due to the laser-pumping of the atomic sample, it collectively emits fluorescence light with
properties depending on the ensemble position in the standing wave. This relation can be described by a
fluorescence intensity profile, which depends on the standing wave field parameters, the ensemble properties, and
which is modified due to collective effects in the ensemble of nearby particles. We demonstrate that the intensity
profile can be tailored to suit different localization setups.
Light scattered by a regular structure of atoms exhibits spatial interference signatures, similar to Young's classical
double-slit experiment. The first-order interferences, however, are known to vanish for strong light intensities,
where the incoherently fluctuating part of the emitted light dominates. Here, we show how to overcome these
limitations to quantum interference in stronger laser fields, and how to recover the first-order interference in strong
fields by a tailored electromagnetic vacuum with a suitable frequency dependence. We also discuss higher-order
correlation functions of the scattered light, with applications, e.g., to lithography. In the second part, we study
light propagation of a probe field pulse in closed-loop atomic systems. The closed interaction loop induces a
sensitivity to the relative driving field phase, but in general prohibits a stationary steady state. In particular,
the finite frequency width of the short probe pulse requires a time-dependent analysis beyond the so-called
multiphoton resonance assumption. Using a Floquet decomposition, we identify the different contributions to
the medium response, and demonstrate sub- and superluminal light propagation with small absorption or even
gain, where a coupling field Rabi frequency allows to switch between sub- and superluminal light propagation.
We discuss various aspects of the incoherent spontaneous emission in atomic few-level systems arising from the coupling of the atom to the surrounding vacuum. First, we consider systems where the decoherence due to spontaneous emission acts as a limiting factor. Here, we combine collective effects in larger samples of atoms with control mechanisms known from single-atom schemes, or modify the system dynamics by externally inducing multiphoton quantum interference effects. In the second part, we discuss ground-state laser cooling of trapped atoms and ions. Here, the momentum transfer in the spontaneous emission events is required to cool the particles, but needs to be controlled in order to achieve a low cooling limit. In our scheme, we make use of double electromagnetically induced transparency in order to design the absorption spectrum of the trapped particle. In the final part, we show that the incoherent part of the resonance fluorescence spectrum of a two-level system may serve as an interesting candidate for high-precision spectroscopy. For this, we discuss relativistic and radiative corrections to the resonance fluorescence spectra of laser-driven few-level systems.
The cooperative two-photon spontaneous decay of an excited atomic system in a micro-cavity is investigated. We demonstrate that the presence of a small number of thermalized photons in the microcavity mode stimulate the cooperative generation rate of the coherent entangled photon pairs.
We analyze the two-photon cooperative emission of excited atoms in microcavities with dimensions of the order of the emission wavelength. Here the two-photon dipole-forbidden transitions between the upper and the ground states of the three-level system are possible through the intermediate level that is off resonance with microcavity modes. In this situation one obtains the powerful pulse of two-photon highly correlated light. The superbunching phenomenon in radiated field is also discussed.
Due to the special interest in applications on radiotherapy, radiobiology, analysis and material testing, in the last years the Bucharest U-120 classical variable energy Cyclotron was employed as an intense source of fast neutrons produced by 13.5 MeV deuterons on a thick beryllium target, mainly for radiobiological and archaeometrical studies. Energy spectra and yields, average energy and irradiation doses were determined using time-of-flight, multiple foils and thermoluminescent detector methods. Studies on liver chromatin structure modifications and on some archaeological Greek objects; composition using FNAA are presented.