W. Gawlik

Resonant nonlinear magneto-optical effects in atoms

In this article, we review the history, current status, physical mechanisms, experimental methods, and applications of nonlinear magneto-optical effects in atomic vapors. We begin by describing the pioneering work of Macaluso and Corbino over a century ago on linear magneto-optical effects (in which the properties of the medium do not depend on the light power) in the vicinity of atomic resonances, and contrast these effects with various nonlinear magneto-optical phenomena that have been studied both theoretically and experimentally since the late 1960s. In recent years, the field of nonlinear magneto-optics has experienced a revival of interest that has led to a number of developments, including the observation of ultra-narrow (1-Hz) magneto-optical resonances, applications in sensitive magnetometry, nonlinear magneto-optical tomography, and the possibility of a search for parity- and time-reversal-invariance violation in atoms.

Nonlinear magneto-optical rotation with frequency-modulated light in the geophysical field range

Recent work investigating resonant nonlinear magneto-opticalrotation (NMOR) related to long-lived (tau_rel approx 1s) ground-stateatomic coherences has demonstrated potential magnetometric sensitivitiesexceeding (10-11 G Hz-1/2) for small (<1 micro G) magnetic fields. Inthe present work, NMOR using frequency-modulated light (FM NMOR) isstudied in the regime where the longitudinal magnetic field is in thegeophysical range (sim 500mG), of particular interest for manyapplications. In this regime a splitting of the FM NMOR resonancedue tothe nonlinear Zeeman effect is observed. At sufficiently high lightintensities, there is also a splitting of the FM NMOR resonances due toac Stark shifts induced by the optical field, as well as evidence ofalignment-to-orientation conversion type processes. The consequences ofthese effects for FM-NMOR-based atomic magnetometry in the geophysicalfield range are considered.

Nonlinear magneto-optical rotation with amplitude modulated light

We study the possibility of creating quantum superposition states in alkali atoms. Our methodology is based on the phenomenon of nonlinear magneto-optical rotation (NMOR). The effect of magneto-optical rotation occurs whenever the resonant light propagates through the medium immersed in a longitudinal magnetic field and manifests itself as a rotation of the polarization plane of light. The effect is strongly enhanced by the nonlinearity of light-atom interaction and allows state-of-the art atomic magnetometry. On the other hand, the nonlinear magneto-rotation provides means to controllably affect and detect atoms quantum state. We present the work on this topic and report on some characteristics of superposition quantum states such as lifetimes or efficiency of its creation in various conditions.

Time-of-flight measurement of the temperature of cold atoms for short trap-probe beam distances

We analyse the time-of-flight method of measuring the temperature of cold trapped atoms in the specific case of short distances of the probe beam from the trap centre and finite atomic cloud size. We theoretically examine the influence of the probe beam shape and its distance from the initial position of the cloud on the temperature evaluation. These results are then verified with a three-dimensional Monte Carlo simulation and applied to our experimental data to show that the proposed procedure allows accurate and reliable determination of the temperature.

Probe spectroscopy in an operating magneto-optical trap: The role of Raman transitions between discrete and continuum atomic states

We report on cw measurements of probe beam absorption and four-wave-mixing spectra in a {sup 85}Rb magneto-optical trap taken while the trap is in operation. The trapping beams are used as pump light. We concentrate on the central feature of the spectra at small pump-probe detuning and attribute its narrow resonant structures to the superposition of Raman transitions between light-shifted sublevels of the ground atomic state and to atomic recoil processes. These two contributions have different dependencies on trap parameters and we show that the former is inhomogeneously broadened. The strong dependence of the spectra on the probe-beam polarization indicates the existence of large optical anisotropy of the cold-atom sample, which is attributed to the recoil effects. We point out that the recoil-induced resonances can be isolated from other contributions, making pump-probe spectroscopy a highly sensitive diagnostic tool for atoms in a working magneto-optical trap.

Selective Addressing of High-Rank Atomic Polarization Moments

We describe a method of selective generation and study of polarization moments of up to the highest-rank kappa=2F possible for a quantum state with total angular momentum F. The technique is based on nonlinear magneto-optical rotation with frequency-modulated light. Various polarization moments are distinguished by the periodicity of light-polarization rotation induced by the atoms during Larmor precession and exhibit distinct light-intensity and frequency dependences. We apply the method to study polarization moments of 87Rb atoms contained in a vapor cell with antirelaxation coating. Distinct ultranarrow (1-Hz wide) resonances, corresponding to different multipoles, appear in the magnetic-field dependence of the optical rotation. The use of the highest-multipole resonances supported by a given system has important applications in quantum and nonlinear optics and in magnetometry.

Pump-probe nonlinear magneto-optical rotation with frequency-modulated light

Specific types of atomic coherences between Zeeman sublevels can be generated and detected using a method based on nonlinear magneto-optical rotation with frequency-modulated light. Linearly polarized, frequency-modulated light is employed to selectively generate ground-state coherences between Zeeman sublevels for which m=2 and m=4 in 85 Rb and 87 Rb atoms, and additionally m=6 in 85 Rb. The atomic coherences are detected with a separate, unmodulated probe light beam. Separation of the pump and probe beams enables independent investigation of the processes of creation and detection of the atomic coherences. With the present technique the transfer of the Zeeman coherences, including high-order coherences, from excited to ground state by spontaneous emission has been observed.

Conical emission in barium vapour

Abstract Studies of the angular structure of a pulsed dye-laser beam transmitted through a dense barium vapour are reported. In contrast to previous experiments, self-trapping of the laser beam has been excluded and precise control over the light intensity within the nonlinear medium was attained. Evidence is provided that the cone of light generated by the laser tuned to the blue side of the atomic resonance stems from the low-frequency Rabi sideband. This resolves the long-lasting ambiguity about the origin of conical emission for blue detuning.

Production and detection of atomic hexadecapole at Earth’s magnetic field

Optical magnetometers measure magnetic fields with extremely high precision and without cryogenics. However, at geomagnetic fields, important for applications from landmine removal to archaeology, they suffer from nonlinear Zeeman splitting, leading to systematic dependence on sensor orientation. We present experimental results on a method of eliminating this systematic error, using the hexadecapole atomic polarization moment. In particular, we demonstrate selective production of the atomic hexadecapole moment at Earths magnetic field and verify its immunity to nonlinear Zeeman splitting. This technique promises to eliminate directional errors in all-optical atomic magnetometers, potentially improving their measurement accuracy by several orders of magnitude.

Degenerate parametric emission in dense barium vapour

Abstract Studies of degenerate parametric interaction in laser beam propagation through dense barium vapour are reported. Various transverse instabilities have been observed when two strong, copropagating beams intersect in the medium. Most of them are attributed to parametric interaction of the intersecting beams and their main features are consistent with the existing four-wave mixing formalism. The conditions of their generation and their mutual competition are discussed.