S. Cartaleva

Coherent effects on the Zeeman sublevels of hyperfine states in optical pumping of Rb by monomode diode laser

Experimental examination of the coherent population trapping that can be established on the D and D lines of Rb at a 12 single hyperfine transition using the Zeeman sublevels of the hyperfine states is reported here. The coherent effects have been examined in Hanle configuration, exciting the Rb vapour in a glass cell by linearly polarized monomode laser beam. Experimental evidence is presented of subnatural width resonances in the fluorescence and the transmitted through the Rb cell laser power, which are due to the process of coherent population trapping on the Zeeman sublevels. In case of degeneracy of the ground level F lower than that of the excited level F , bright peaks in the fluorescence and increased ge absorption have been observed, together with the well known dark resonances for F G F. The peaks in the fluorescence in ge

Cesium coherent population trapping magnetometer for cardiosignal detection in an unshielded environment

We present encouraging results obtained with an experimental apparatus based on coherent population trapping and aimed at detecting a biological (cardiac) magnetic field in a magnetically compensated but unshielded volume. The work includes magnetic-field and magnetic-field-gradient compensation and uses differential detection to cancel common mode magnetic noise. Synchronous data acquisition with a reference (electrocardiographic or pulse-oximetric) signal makes possible improvement of the signal-to-noise ratio in off-line averaging. The setup has the significant advantages of working at room temperature with a small-size head, and the possibility of fast adjustments of the dc bias magnetic field, which makes the sensor suitable for detecting a biomagnetic signal at any orientation with respect to the axis of the head and in any position on the patients chest, which is not the case with other kinds of magnetometers.

Saturation effects in the sub-Doppler spectroscopy of cesium vapor confined in an extremely thin cell

Saturation effects affecting absorption and fluorescence spectra of an atomic vapor confined in an extremely thin cell (cell thickness L<1 {mu}m) are investigated experimentally and theoretically. The study is performed on the D{sub 2} line ({lambda}=852 nm) of Cs and concentrates on the two situations L={lambda}/2 and L={lambda}, the most contrasted ones with respect to the length dependence of the coherent Dicke narrowing. For L={lambda}/2, the Dicke-narrowed absorption profile simply broadens and saturates in amplitude when increasing the light intensity, while for L={lambda}, sub-Doppler dips of reduced absorption at the line-center appear on the broad absorption profile. For a fluorescence detection at L={lambda}, saturation induces narrow dips, but only for hyperfine components undergoing a population loss through optical pumping. These experimental results are interpreted with the help of the various existing models and are compared with numerical calculations based upon a two-level modeling that considers both a closed and an open system.

Narrow structure in the coherent population trapping resonance in rubidium

An investigation is reported of coherent resonance in the degenerate two-level system of the (Fg = 2 --> Fe = 1) transition of the 87Rb D1 line by means of a Hanle effect configuration. In a coherent population trapping experiment, a very narrow (approximately 1-kHz) resonance superimposed upon a broader resonance (approximately 100 kHz) was observed. The dependence of the width and the amplitude of the coherent resonance on the laser power density, on additional constant magnetic fields, on the laser frequency position along the fluorescence profile, and on Rb cells with different dimensions was investigated.

Dual channel self-oscillating optical magnetometer

We report on a two-channel magnetometer based on nonlinear magneto-optical rotation in a Cs glass cell with buffer gas. The Cs atoms are optically pumped and probed by free running diode lasers tuned to the D2 line. A wide frequency modulation of the pump laser is used to produce both synchronous Zeeman optical pumping and hyperfine repumping. The magnetometer works in an unshielded environment, and a spurious signal from distant magnetic sources is rejected by means of differential measurement. In this regime the magnetometer simultaneously gives the magnetic field modulus and the field difference. Rejection of the common-mode noise allows for high-resolution magnetometry with a sensitivity of 2 pT/sqrt Hz. This sensitivity, in conjunction with long-term stability and a large bandwidth, makes it possible to detect water proton magnetization and its free induction decay in a measurement volume of 5 cm3.

Sub-Doppler spectroscopy of cesium vapor layers with nanometric and micrometric thickness

The saturation behavior of absorption and fluorescence spectra on the D2 line of Cs is presented, demonstrating a significant difference between open and closed transitions. Cs vapor is confined in an extremely thin cell (ETC) with widely tunable thickness L=(0.5-3)λ, where λ is the light wavelength. In the saturation regime, the closed transition demonstrates enhanced absorption in a narrow spectral interval due to the Dicke effect, while the open one demonstrates only a velocity-selective dip in the absorption. The fluorescence of open transitions shows reduced fluorescence dips, enhancing their contrast with ETC thickness. The closed transition exhibits only a small plateau around the optical transition center. Applying two-level theoretical modeling based on optical Bloch equations, a qualitative agreement with experimental observations is achieved. The rate of contrast enhancement with cell thickness is larger for the theoretical than for the experimental dips. In addition, for the closed transition a tiny peak in the fluorescence is theoretically predicted, with the first experimental confirmation presented. The sub-Doppler spectra of vapor layers with a thickness of several light wavelengths show potential for realization of precise frequency references and photonics sensors.

Coherent effects on the Zeeman sublevels of hyperfine states at the D1 and D2 lines of Rb

Subnatural-width resonances due to coherent effects in optical pumping at hyperfine (hf) transitions of Rb are investigated using a linearly or circularly polarized single-frequency diode laser beam. The sign and amplitude of the resonances in the fluorescence are measured by scanning the laser frequency over the hf transitions of the Rb D1 and D2 lines. The fluorescence perpendicular to the laser beam is detected as a function of magnetic field parallel to the beam propagation and scanned around zero value. In the case of linear polarization of the field, dark resonances are observed for hf transitions with Fg→Fe = Fg-1 and Fg→Fe = Fg, and bright resonances are observed for transitions with Fg→Fe = Fg + 1. This is in agreement with the recently developed theories. For all (except Fg = 2→Fe = 3, 85Rb, D1 line) hf transitions the sign of the resonances reverses when the polarization of the field changes to circular. The amplitudes of the resonances correspond to the probability of the hf transitions and to the calculated loss rate of the population to the ground hf level not excited by the laser field. The observed narrow resonances (FWHM: few tens of kHz) and their sensitivity to weak magnetic fields are interesting for high-resolution spectroscopy and for development of simple and precise magnetometers.

Ground-state magneto-optical resonances in cesium vapor confined in an extremely thin cell

Experimental and theoretical studies are presented related to the ground-state magneto-optical resonance observed in cesium vapor confined in an extremely thin cell (ETC), with thickness equal to the wavelength of the irradiating light. It is shown that utilization of the ETC allows one to examine the formation of a magneto-optical resonance on the individual hyperfine transitions, thus distinguishing processes resulting in dark (reduced absorption) or bright (enhanced absorption) resonance formation. We report experimental evidence of bright magneto-optical resonance sign reversal in Cs atoms confined in an ETC. A theoretical model is proposed based on the optical Bloch equations that involves the elastic interaction processes of atoms in the ETC with its walls, resulting in depolarization of the Cs excited state, which is polarized by the exciting radiation. This depolarization leads to the sign reversal of the bright resonance. Using the proposed model, the magneto-optical resonance amplitude and width as a function of laser power are calculated and compared with the experimental ones. The numerical results are in good agreement with those of experiment.

Frequency tunable monomode diode laser at 670 nm for high resolution spectroscopy

The frequency tuning behaviour of four diode lasers manufactured by Toshiba is investigated using an external-cavity diode-laser system with optical feedback from a diffraction grating in Littrow configuration. Continuous frequency tuning up . to 90 GHz is achieved in the spectral region around 670 nm without antireflection AR coating of the laser facets. The largest frequency tuning is observed for external cavity length equal to an integer multiple of the effective diode laser length. An explanation of the experimental results is proposed based on the steady-state solutions for the threshold gain and the lasing frequency of the grating-tuned external-feedback diode laser. The theoretical model has been expanded by considering the grating reflectivity profile. The I ro-vibrational absorption spectrum was measured around 673.1 nm. q 1998 Elsevier 2 Science B.V.

High-contrast dark resonances on the D1 line in cesium nanocell: the advantages compared with the other alkali D lines

Electromagnetically induced transparency (EIT) effect in a -system formed by Cs atoms line, enclosed in nanometric-thin cells, is studied both experimentally and theoretically for the first time. The atomic column thickness varies in the range of 50–1500 nm. It is demonstrated that when the coupling laser frequency is in exact resonance with the corresponding atomic transition, the parameters of the EIT resonance (also called dark resonance (DR)) depend weakly on , which allows us to detect DR at nm with the contrast of . The obtained DR parameters are the best as compared with those for Rb and lines and Cs line. The DR contrast and width are studied versus laser frequency detunings, coupling laser power, cell thickness, temperature, and applied external magnetic field. The well-resolved splitting of the DR resonance in a magnetic field for nm can be used for magnetometry with high spatial resolution. The theoretical model describes well the observed results.