Sandra S. Vianna

Temporal coherent control of a sequential transition in rubidium atoms.

An experimental investigation of the coherent interaction of femtosecond pulses with two resonant sequential transitions of Rb atoms is presented. Fluorescence from the atomic system exhibits beating at a frequency given by difference in the sequential atomic transitions. The results are interpreted in terms of quantum interference in the induced coherence and its interaction with the field that results from a cooperative emission process.

Coherent accumulation in two-level atoms excited by a train of ultrashort pulses

In this paper we consider the interaction of inhomogeneously broadened two-level atoms with a train of pulses from a femtosecond mode-locked laser. Analytical results are obtained for the case in which the atomic relaxation times are longer then the laser repetition period, leading to accumulation in both population and coherence. These results are adequate for arbitrary pulse areas, and saturation effects are discussed. We illustrate our analytical results with a comparison to a direct numerical integration of the Bloch equations.

A new approach for improving the birefringence analysis of dental enamel mineral content using polarizing microscopy

The main problem in interpreting birefringence of dental enamel under polarizing microscopy is the lack of physical constants able to allow the Wiener equation to be applied directly to the composition of such tissue. The present study introduces a new approach to circumvent this constraint. Because the nonmineral phase of enamel is heterogeneous, its refractive index can be computed in terms of its components (namely, water, which is partially replaced by the immersion medium, and organic matter), thereby providing a more acceptable refractive index to be used in the Wiener equation. Furthermore, the enamel mineral volume is ordinarily calculated on the basis of the density 3.15 g cm−3. The density 2.99 g cm−3 has been, however, reported to be more accurate for enamel hydroxyapatite, so enamel mineral volumes from selected published data were converted using such a density. The birefringence of mature enamel computed by the Wiener equation, taking into account the above refinements, matched, for the first time, published experimental birefringence values. The theoretical water and organic contents were also consistent with published experimental data. Thus, a direct application of the Wiener equation to the enamel composition has now been achieved. It is speculated that quantitative data on the mineral, the water and the organic contents of mature dental enamel can be derived from interpretation of birefringence in two immersion media (obtained before and after extraction of the organic matter) with this new approach.

Dental enamel birefringence for a wide mineral content range and for different immersion media's refractive indexes: an improved mathematical interpretation

The aim of this study is to extend, for a larger range of mineral volume (V1) and different aqueous immersion media, the model for calculating the refractive index of the non‐mineral phase of the human dental enamel proposed by Sousa et al. Published experimental birefringence data of carious and developing human enamel in air and aqueous immersion media with different refractive indexes were interpreted. For 48%≤V1≤ 95%, quantitative estimates on the non‐mineral contents consistent with the values described in the literature were derived. New polynomial relationships of the non‐mineral contents as a function of the mineral content (48%≤V1≤ 95%) are introduced, which allows computing the theoretical mean enamel birefringence in water and air for most of the points of developing, carious and mature enamel with a difference of one order of magnitude in relation to the classical model for the non‐mineral contents refractive index and closer to the experimental data. For enamel birefringence in immersion media other than water and air, a less expressive improvement was obtained. Perspectives for future studies are discussed.

Coherence induced by a train of ultrashort pulses in a Λ-type system

We present an analysis of coherent population trapping in a three-level Λ system when excited by an ultrashort pulse train near the one- and two-photon resonances. Numerical results using Bloch equations give the transient response of the system and clearly reveal a comb of electromagnetically induced transparency (EIT) lines. An analytical solution to describe the EIT lines is derived in the steady-state regime and in the weak field limit. Moreover, numerical results indicate that selective velocity spectroscopy makes possible the observation of the comb of EIT lines in Doppler broadening media.

Phase conjugation by nondegenerate four-wave mixing in sodium vapor

Abstract We studied the generate conjugate wave with a frequency different from that of the input signal in a three-level system where the contribution of both resonant one- and two-photon transitions are present. Using the nondegenerate four-wave mixing technique with high-intensity pulsed lasers in sodium vapor a conversion efficiency of 30% and the ac Stark effect were observed. The results are in good agreement with a theoretical calculation using the density matrix equations for a three-level system.

Two-photon resonant forward four-wave mixing in rubidium vapor involving Rydberg states

We report on observations of four-wave mixing in rubidium vapor via two-photon resonant excitation with nd and (n+2)s states, for n=14–20. The intensity of the generated beam is studied in a collinear configuration, and the results are consistent with the predictions of a theoretical model based on the semiclassical Maxwell–Bloch equations, considering interference effects associated with multiple excitation pathways and with internally generated fields. We have also observed spectral broadening of the two-photon resonant lines, which is well described by a linear dependence on atomic density, in accordance with collision-induced broadening.

Two- and four-beam magneto-optical trapping of neutral atoms.

We report the experimental realization of new stable three-dimensional schemes for trapping neutral cesium atoms in a vapor cell that use either two or four Gaussian laser beams. Probe-beam absorption is employed to characterize both types of trap, and densities of 2 x 10(8) and 1 x 10(10) cm(-3), respectively, are measured for the two-and four-beam traps. The two-beam trapping mechanism combines a longitudinal magneto-optical force with a transverse spontaneous radiation pressure force associated basically with the beam geometry. This theoretical analysis compares well with our experimental observations.

Coupling between cw lasers and a frequency comb in dense atomic samples

We report on a detailed investigation of the coupling between a femtosecond-laser frequency comb and a cw diode laser interacting with an atomic medium of variable density. The comb is printed on a Doppler-broadened atomic transition and the frequency-dependent transmission of the cw laser is monitored as it is scanned over the inhomogenously broadened absorption profile. The printing process and its probing are analysed, experimentally and theoretically, as a function of both laser intensities and the atomic density. The results reveal the importance of optical pumping and power broadening by both lasers, allowing us to determine various regimes of competition between them.

Influence of propagation and external phase in sequential two-photon absorption of femtosecond pulses

We study the influence of shaping the phase of the optical field when time-delayed femtosecond pulse pairs excite a sequential two-photon transition in rubidium atoms. Propagation through this optically dense medium modifies the pulse profile. When an external phase is introduced into one of the pulses, partial cancellation of the two-photon absorption signal is observed for temporal delays much greater than the pulse duration. This behavior is qualitatively explained in terms of a negative group delay.