Nonlinear spectroscopy with a vertical-cavity surface-emitting laser (VCSEL)

We have evaluated the suitability of a vertical-cavity surface-emitting laser diode (VCSEL) for spectroscopic applications. Despite its low output power it is possible to observe narrow resonances in a saturated absorption spectroscopy experiment on the cesium D ₂ transition at 852 nm, limited in width by the laser linewidth of several tens of MHz. High modulation efficiency of the VCSEL allows us to create modulation sidebands at 9.2 GHz frequency via direct modulation of the laser injection current. Using the carrier and either one of the sidebands coherent population trapping (CPT) resonances in a buffered cesium vapor can be prepared with linewidths below 130 Hz. With this very compact setup we have studied the dependence of CPT resonance position and linewidth as a function of optical detuning and find evidence of the influence of the excited state hyperfine structure. Received: 30 April 1999 / Revised version: 25 June 1999 / Published online: 30 November 1999     

An all-optical, high-sensitivity magnetic gradiometer

Optical magnetometers have reached sensitivities that make them interesting candidates for the measurement of weak magnetic fields also outside physics laboratories. In order to overcome problems with stray magnetic fields a common solution with existing magnetometers is to operate a pair of them in a gradiometer configuration: one sensor measures the signal plus the stray fields, while the other one is mounted such that it is only influenced by the stray fields. In the difference signal the stray fields cancel. We have constructed such a gradiometer consisting of two sensors based on coherent population trapping (CPT) resonances in a thermal cesium vapor. Using a magnetic bias field the intrinsically scalar CPT magnetometer can be turned into a true vector magnetometer that is insensitive to magnetic fields perpendicular to a chosen measurement direction. We describe how to align and calibrate the gradiometer. Stray field suppression by more than two orders of magnitude has been achieved, limited by the sensitivity of the magnetometer. This makes possible the detection of picotesla flux density changes in a weakly shielded or even unshielded environment. Received: 3 April 2002 / Published online: 15 November 2002 RID="*" ID="*"New address: Département de Physique, Université de Fribourg, Chemin du Musée 3, 1700 Fribourg, Switzerland RID="**" ID="**"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch     

Nonlinear diffraction due to the transient polarization in a thin film of atomic gases*

Nonlinear optical response of a thin layer of rarefied atomic vapor is examined by taking into account the atomic motion as well as collisions with the cell walls. Extraordinary pattern of self-diffraction spectrum due to the transient polarization is predicted. It is shown that the spectra are Doppler free and depend strongly upon the vapor thickness. A new possibility of signal enhancement is revealed and a simple device is suggested to enhance greatly the nonlinear reflection signal from a resonant vapor layer. Received: 3 July 1997/Accepted: 5 December 1997     

Optical properties of impurity atoms in pressurized superfluid helium

We report on the optical properties of alkali atoms (Cs and Rb) in the pressurized superfluid helium. We observed excitation and emission spectra at various pressures from the saturated vapor pressure to about 25 atm. The theoretical calculations on the basis of the atomic bubble model have also been worked out. The qualitative agreement between the theoretical and experimental results with respect to the peak shift, the linewidth, and their pressure dependence is achieved in the framework of the spherical atomic bubble model. TheD ₂ excitation spectra with the double peaks are interpreted qualitatively in terms of the Jahn-Teller effect, indicating the existence of the nontotally symmetrical density distribution of the surrounding helium atoms.     

Coherent population trapping and strong electromagnetically induced transparency resonances on the D<Subscript>1</Subscript> line of potassium

In this paper we report the first experimental observation of coherent population trapping (CPT) in thermal potassium vapor in a three levels Λ scheme. We demonstrate that K presents the advantage of a reduced modulation frequency with a large resonance contrast (up to 40%), in comparison to similar approaches with other alkalis. We report also the first evidence of electromagnetically induced transparency (EIT) resonances in K in the so called Hanle configuration. We tested different kinds of cells, demonstrating strong enhancement of the resonance contrast and amplitude for antirelaxation coated and buffered cells containing K vapor: resonance contrast up to 90% (for coated cells) and 65% (for buffered cells) is achieved with a linewidth of about 13 mG, while under similar conditions, the EIT resonance contrast in Cs vapor buffered by Ar gas is about 1%. Such relevant improvement is due to the reduced optical pumping in K, because of the overlapping of the hyperfine levels Doppler profiles, which does not occur in the case of Rb and Cs vapor. For this reason, K can be considered very promising for further CPT and EIT applications, especially for those where optical pumping losses represent a major limiting factor, such as light slowing and magnetometry.     

Frequency Stability of Atomic Clocks Based on Coherent Population Trapping Resonance in ^85Rb

An atomic clock system based on coherent population trapping （CPT） resonance in 85Rb is reported, while most past works about the CPT clock are in ST Rb. A new modulation method （full-hyperfine-frequency-splitting modulation） is presented to reduce the effect of light shift to improve the frequency stability of the CPT clock in SSRb. The experimental results show that the short-term frequency stability of the CPT clock in SSRb is in the order of 10^-10/s and the long-term frequency stability can achieve 1.5 × 10^-11/80000s, which performs as well as 87 Rb in CPT resonance. This very good frequency stability performance associated with the low-cost and low-power properties of SSRb indicates that an atomic clock based on CPT in SSRb should be a promising candidate for making the chip scale atomic clock.     

Radiation trapping effects in Cu, Mn, Ca, Sr, and Ba vapor lasers

The radiation trapping effect was investigated in Cu, Mn, Ca, Sr, and Ba vapor lasers with a cylinder discharge tube. Compared with the negligible impact induced by the gas temperature, the lower level population density of radiation trapping was found to dominate the effective spontaneous emission probability. Meanwhile a tube radius scanning from 1 mm to 20 mm revealed that the threshold parameters of radiation trapping effect were affected little by the tube radius when the radius changed from 20 mm to 5 mm, but they varied visibly when the tube radius decreased from 5 mm to 1 mm. Furthermore, the threshold temperature and the lower level population density of the radiation trapping effect in six resonant lines in Cu, Mn, Ca, Sr and Ba vapor lasers were calculated, and the threshold temperature was found to be close to the initial lasing temperature of the corresponding resonance level to metastable level transition.     

Spectroscopic studies of laser induced aluminum plasma using fundamental,second and third harmonics of a Nd:YAG laser

In the present work, we have studied the spatial evolution of the aluminum plasma produced by the fundamental (1064 nm), second (532 nm) and third (355 nm) harmonics of a Q-switched pulsed Nd:YAG laser. The experimentally observed line profiles of neutral aluminum have been used to extract the excitation temperature using Boltzmann plot method whereas the electron number density has been determined from the Stark broadened profiles. Besides we have studied the variation of excitation temperature and electron number density as a function of laser irradiance at atmospheric pressure. In addition, we have performed quantitative analysis of photon absorption and vapor ionization mechanism at three laser wavelengths and estimated the inverse bremsstrahlung (IB) absorption and photoionization (PI) coefficients. The validity of the assumption of local thermodynamic equilibrium is discussed in the light of the experimental results.     

Absorption spectrum of rubidium and cesium dimers by compact computer operated spectrometer

In this paper we are presenting the visible absorption spectrum of both rubidium and cesium vapor in the 570-870 K temperature range. We used a classical absorption spectroscopy experimental scheme with several new features. The first concerns the use of modern, compact, computer operated spectrometer (Ocean Optics HR4000CG-UV-NIR), which allowed us to record spectra instantaneously resulting in higher signal-to-noise ratio. The second improvement is connected with the use of the all-sapphire cells (ASC) enabling work with a high density of alkali atoms within precisely defined vapor column. In the superheated regime (above 700 K) thermal destruction of dimer molecules clearly distinguishes triplet from singlet transitions.     

Mass transport on adsorbate multilayers studied by surface plasmon polariton wave excitation

We excited surface-plasmon polariton waves (SPPW) on Cu(111) by coupling a monochromatic optical beam with a xenon multilayer thickness grating on the metal. The SPPW excitation was detected with an angle-resolved oblique-incidence reflectivity difference technique (OI-RD). The amplitude of the resonance OI-RD signal was a quadratic function of the grating modulation depth. By monitoring the decay of the resonance OI-RD signal as a function of time and temperature, we were able to study the mass transport of xenon that plays a key role in the annealing of a “rough” Xe multilayer crystalline film.     

Simple parameterization of dark-resonance line shapes

We have studied the dispersion and absorption line shapes of coherent population trapping resonances (dark resonances) in thermal cesium vapor. Outside the time-of-flight regime the line shapes can be described by a simple phenomenological model: a sum of a dispersive and an absorptive Lorentzian of identical width and position, whose relative weights depend on the detuning of the laser frequencies from the optical resonance. This functional form can be derived from a theoretical model for the full multilevel structure of an alkali atom. The analytical expressions for the parameters of the line shape model show the same behavior as the experimental data. Received: 4 October 2002 / Revised version: 30 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch     

Atomic clocks based on coherent population trapping: a review

The paper gives an overview of the use of the coherent population trapping phenomenon (CPT) in alkali-metal atoms in the implementation of atomic frequency standards. Several avenues are examined. These include: the approach using a combination of the CPT phenomenon and the Ramsey separated interaction field technique on an atomic beam; the passive approach in a cell in which the microwave hyperfine resonance excited by the CPT phenomenon is detected directly on the transmitted radiation; the maser approach in which the same resonance is observed by means of stimulated emission in a microwave cavity-cell arrangement; and, finally, the proposed approach using pulses in a time sequence that implements the combined CPT–Ramsey separated interaction field technique in time rather than in space. A review of field and laboratory implementations using these approaches is made.     

High-resolution spectroscopy of velocity-selected atoms in a thin cell

We propose a novel technique of sub-Doppler spectroscopy using a thin vapor cell. Optical pumping in a thin cell transfers atoms with small velocity components to a specific quantum state. The resultant velocity distribution appears as a sub-Doppler structure in the absorption spectrum of a probe light. A single laser beam from a laser diode is split into two paths: one beam optically pumps Cs atoms on the D₂ line, and the other probes the absorption on the same line from a perpendicular direction. Observed hyperfine-resolved spectra and their parameter dependence are analyzed on the basis of rate equations. Received: 16 January 2002 / Revised version: 11 February 2002 / Published online: 24 April 2002     

Dark resonance of Cs atoms velocity-selected in a thin cell

We carried out experimental and theoretical research on dark Raman resonances in the absorption of the probe monochromatic wave in a thin vapor cell, whose length is much less than the diameters of probe and coupling laser beams. In this case, only the slow atoms have enough time to settle down to the dark quantum state during a free flight between the end walls of the cell. As a result, a sub-Doppler structure is observed when one sweeps both the coupling and probe laser frequencies keeping the Raman resonance condition. Second harmonic lock-in detection is used to evaluate sharpness of the dark-resonance line. The cell-length dependence of the observed spectral profile is satisfactorily reproduced by the steady-state analysis of the density matrix in which the velocity-dependent transit time effect is taken into account.     

Spectral structure of CuBr vapor laser lines

The atomic density, the buffer gas pressure, and the voltage dependence of the 510.6 nm laser line shape and its temporal evolution emitted from a Cu/CuBr vapor laser are measured. It is found that the monochromaticity of these laser lines can be improved greatly at a relative high buffer gas pressure. The temporal evolution of the line shape offers a stereoscopic view of a laser pulse. The sequence of appearance of the peaks of the line shape is governed by the intensity of the hyperfine component of the copper line.     

Stark broadening in the Sb III spectrum

The shapes of nine doubly ionized antimony (Sb III) spectral lines have been obtained in the laboratory helium plasma at 17 500 K electron temperature and electron density of . Measured line profiles are of a Voigt type. At the mentioned plasma conditions the Stark broadening has been found as the dominant mechanism in the Sb III line shapes formation. Using a deconvolution procedure the Lorentz (Stark) FWHM (Full-Width at Half of the Maximal intensity, W) have been obtained. Our measured Sb III Stark widths are the first data in the literature. The modified version of the linear, low-pressure, pulsed arc was used as a plasma source operated in helium with antimony atoms, as impurities. They were evaporated from the thin antimony layer, deposited on the silver cylindrical plates, located in the homogenous part of the discharge. This plasma source ensures good conditions for the generation of the doubly ionized antimony atoms in the helium plasma due to atomic processes concerning helium metastables.     

Investigations of optical coherence properties in an erbium-doped silicate fiber for quantum state storage

We studied optical coherence properties of the 1.53 μm telecommunication transition in an Er³⁺-doped silicate optical fiber through spectral holeburning and photon echoes. We find decoherence times of up to 3.8 μs at a magnetic field of 2.2 T and a temperature of 150 mK. A strong magnetic-field dependent optical dephasing was observed and is believed to arise from an interaction between the electronic Er³⁺ spin and the magnetic moment of tunneling systems in the glass. Furthermore, we observed fine-structure in the Erbium holeburning spectrum originating from superhyperfine interaction with ²⁷Al host nuclei. Our results show that Er³⁺-doped silicate fibers are promising material candidates for quantum state storage.     

芯片原子钟相干布居囚禁谱线特性研究

通过微型原子蒸汽室产生质量满足要求的相干布居囚禁(CPT)信号是实现芯片原子钟的关键之一.本实验通过对光源实施频率调制和对光场与⁸⁷Rb原子作用产生的信号作相敏解调获得高信噪比的CPT微分谱线, 利用CPT微分谱线研究了CPT信号随工作参数变化的规律以及信号质量对原子钟频率稳定度的影响, 所获研究结果与理论模型预期相符合, 实验结果为芯片原子钟推荐了最佳工作参数.实验所采用的方法利用芯片原子钟自身的资源就可以实施, 因此为芯片原子钟开展性能研究和实施工作参数优化提供了实用的手段.     

A simple model of laser-induced fluorescence under arbitrary optical thickness conditions at the excitation wavelength

A simple model is worked out to investigate laser-induced fluorescence in media of arbitrary optical thicknesses at the excitation wavelength. Close-form expressions are obtained for laser fluence, absorbed energy density and emitted fluorescence signal as a function of optical thickness and incident laser fluence. It is shown that in optically thick media saturation curves are not proportional to density of the species excited by the laser, except at saturation, and that the saturation fluence increases with optical thickness. Experimental saturation curves of lead in laser-produced plasmas of brass are discussed in the light of this model.