Imaging magnetic scalar potentials by laser-induced fluorescence from bright and dark atoms

We present a spectroscopic method for mapping two-dimensional distributions of magnetic field strengths (magnetic scalar potential lines) using charge-coupled device (CCD) recordings of the fluorescence patterns emitted by spin-polarized Cs vapour in a buffer gas exposed to inhomogeneous magnetic fields. The method relies on the position-selective destruction of spin polarization by magnetic resonances induced by multi-component oscillating magnetic fields, such that magnetic potential lines can be directly detected by the CCD camera. We also present a generic algebraic model allowing for the calculation of the fluorescence patterns and find excellent agreement with the experimental observations for three distinct inhomogeneous field topologies. The spatial resolution obtained with these proof-of-principle experiments is of the order of 1 mm. A substantial increase of spatial and magnetic field resolution is expected by deploying the method in a magnetically shielded environment

Cesium alignment produced by pumping with unpolarized light

We demonstrate optical pumping on the four hyperfine components of the Cs D1 transition by unpolarized (UPL) resonant laser light. The evidence is based on the reduction of the absorption coefficients κ0 with increasing light power P in an uncoated Cs vapor cell with isotropic spin relaxation. For comparison we perform the same quantitative κ0(P) measurements with linearly-polarized light (LPL) and circularly-polarized light (CPL). We find that our previously published algebraic expressions give an excellent description of all experimentally recorded induced transparency signals. Based on this we can make reliable absolute predictions for the power dependence of the spin orientation and alignment produced by pumping with LPL, CPL and UPL

Spectroscopy of Mn atoms isolated in solid ⁴He

We present an experimental study of the laser-induced luminescence spectra of Mn atoms in solid helium matrices. We observe transitions of the valence electron and of inner-shell electrons. We find that the Mn-He interaction perturbs the inner-shell transitions to a lesser extent than the valence-electron transitions. The observed lineshapes of the inner-shell transitions of Mn are similar to those of an inner-shell transition in Ba studied earlier. At the same time, they are more strongly perturbed than the corresponding transitions in Au and Cu under the same conditions. We suggest a qualitative explanation of these observations based on the atomic bubble model. Our results also suggest that the inner-shell transitions of Mn in solid He are more strongly perturbed than the same lines of Mn isolated in solid Ar or Kr matrices

An arbitrary-function light power controller

We describe the design, applications, and performance of a simple light power controller. The device is built on a fiber-coupled electro-optic modulator with an active electronic feedback. It can be used to actively stabilize laser power or to impress an arbitrary waveform onto the power. The bandwidth of the device is ∼70 kHz

Comment on: Magnetic field measurements in Rb vapor by splitting Hanle resonances under the presence of a perpendicular scanning magnetic field

In a recent article in this journal Grewal and Pattabiraman reported on the splitting of ground state Hanle resonances (recorded with linearly polarized light) by a transverse field. They claimed a “linearly proportional” dependence on the transverse field strength and supported this observation with results from numerical simulations. In this comment we argue that the splitting occurs only beyond a certain threshold field value and that it has a strong non-linearity near threshold. We base this claim on our previously published algebraic expressions for the line shapes and support this by experimental evidence. Graphical abstractOpen image in new windo

Measurement of the scalar third-order electric polarizability of the Cs ground state using coherent-population-trapping spectroscopy in Ramsey geometry

The ac-Stark shift induced by blackbody radiation is a major source of systematic uncertainty in present-day cesium microwave frequency standards. The shift is parametrized in terms of a third-order electric polarizability α(3)0 that can be inferred from the static electric-field displacement of the clock transition resonance. In this paper, we report on an all-optical coherent-population-trapping pump-probe experiment measuring the differential polarizability Δα(3)0=α(3)0(F=4)−α(3)0(F=3) on a thermal Cs atomic beam, from which we infer α(3)0(F=4)=2.023(6)stat(9)systHz/(kV/cm)2, which corresponds to a scalar Stark shift parameter ks=−2.312(7)stat(10)systHz/(kV/cm)2. The result agrees within two standard deviations with a recent measurement in an atomic fountain, and rules out another recent result obtained in a Cs vapor cell

CPT-pump-probe measurement of the Cs clock transition DC Stark shift

We report progress in measuring the third order electric polarizability of the Cs ground states using a Ramsey pump-probe experiment on coherent population trapped (CPT) atoms in a thermal atomic beam. We give a short description of the apparatus as well as the Fourier transform method used to monitor the phase and frequency of the Ramsey signal. Analysis of a typical data set is shown, proving the consistency of the method. We show that the motional magnetic field phase shift can be used to test the reliability of the electric field modeling. Finally, we give a preliminary value for the Cs ground state polarizability and compare it to previous published values of the DC Stark shift

Development of an optical cardio-magnetometer

Diese Arbeit beschreibt die Entwicklung eines neuen Typs von Kardiomagnetometer, d.h. eines Geräts mit dem man das Magnetfeld des schlagenden menschlichen Herzens messen kann. Diese ausserordentlich schwachen Magnetfelder tragen medizinisch relevante Informationen und können nichtinvasiv an der Oberfläche des Körpers gemessen werden. Bisher wurden solche Messungen ausschliesslich mit supraleitenden Magnetometern (SQUID) durchgeführt, welche die notwendige magnetometrische Empfindlichkeit (< 1 pT) haben, um Feldänderungen, die fünfzig Millionen Mal kleiner als das Erdmagnetfeld sind, nachzuweisen. SQUIDs sind allerdings auf eine aufwändige Kühlung mit verflüssigten Gasen angewiesen. Die mit der Kühlung verbundenen Kosten sind einer der Gründe dafür, dass sich die Kardiomagnetometrie bis jetzt nicht in der medizinischen Praxis etablieren konnte, trotz der erwiesenen Vorteile der Methode bei der oft schwierigen Diagnose von Herzkrankheiten. Der neue Typ eines Kardiomagnetometers, der in dieser Arbeit beschrieben wird, basiert auf einer völlig anderen Technik, der optisch detektierten Magnetresonanz (ODMR), welche ohne Kühlung auskommt. Die ODMR kombiniert Laserspektroskopie und Magnetresonanz in einem Dampf von paramagnetischen Atomen — in diesem Fall Cäsium — in einer evakuierten Glaszelle. Der Drehimpuls (Spin) der Valenzelektronen der Atome bestimmt die optischen Eigenschaften des Dampfs. Da der Spin Über das assoziierte magnetische Moment auch mit Magnetfeldern wechselwirkt, kann ein äusseres Magnetfeld die optischen Eigenschaften des Mediums beeinflussen. Somit kann aus einer Messung dieser Eigenschaften, wie z.B. dem Absorptionskoeffizienten, das Magnetfeld bestimmt werden. Die Methode ist seit den sechziger Jahren bekannt und in kommerziellen Geräten verfügbar, die von Archäologen und Geologen eingesetzt werden, um Variationen des Erdmagnetfeldes zu messen. Im Gegensatz zu diesen Geräten, in denen als Lichtquelle Spektrallampen verwendet werden, benutzt unser Magnetometer einen frequenzstabilisierten Diodenlaser als Lichtquelle. Unter optimierten Bedingungen erreicht das Magnetometer eine Auflösung von 63 fT=Hz1=2 und eine Bandbreite von 140 Hz mit einem Zellenvolumen von 6 cm3. Um Störfelder im Labor zu unterdrücken, betrieben wir zwei solcher Sensoren in einer gradiometrischen Anordnung. Dabei misst ein Sensor so nah wie möglich am Herz das Herzmagnetfeld zusammen mit den Störungen. Der zweite Sensor etwas weiter entfernt vom Herz misst nur noch die Störungen, da das Herzmagnetfeld schnell mit wachsender Entfernung abfällt. Im Differenzsignal fallen dann die homogenen Störungen heraus, was es uns ermöglicht hat, Herzmagnetfelder in einer schwach magnetisch abgeschirmten Umgebung zu messen. Durch die Messung des Herzmagnetfeldes an verschiedenen Stellen Über der Brust lassen sich Magnetfeldkarten des Herzen erzeugen. Dank EKG-getriggerter Mittelung gelang es, solche Karten für jeden Zeitpunkt des Herzzyklus zu messen. Die Dynamik der Herzmagnetfeldkarten von vier gesunden Probanden haben wir mit SQUID generierten Referenzdaten verglichen. Ein direkter Vergleich der Magnetfeldkarten von zwei Probanden gemessen mit einem kommerziellen SQUID Magnetometer und unserem optischen Magnetometer wurde an der Universität Rom durchgeführt. Die Ergebnisse zeigen, dass die Daten kompatibel sind, was uns zuversichtlich stimmt, dass optische Herzmagnetometrie eines Tages als medizinisches Standardverfahren etabliert werden könnte.This thesis describes the development of a new type of cardiomagnetometer, a device that can measure the magnetic field generated by the beating human heart. Such exceedingly weak magnetic fields carry diagnostically relevant information and can be measured noninvasively at the surface of the body. Cardiomagnetic measurements were previously performed using superconducting magnetometers (SQUID) that have the required sensitivity (< 1 pT) to measure field changes fifty million times smaller than the earth's magnetic field. However, SQUIDs need to be cooled using liquified gases. The cost associated with that cooling is one of the reasons that prevented the widespread use of magnetocardiometry (MCG) in medical practice, despite the fact that MCG measurements have proven to be beneficial in the diagnosis of heart diseases. The new type of cardiomagnetometer described herein is based on a completely different technology, optically detected magnetic resonance (ODMR), that does not need expensive cooling. ODMR combines laser spectroscopy and magnetic resonance in a vapor of paramagnetic atoms — Cs in our case — sealed in a glass cell. The angular momenta (spins) of the atomic valence electrons determine the optical properties of the medium. Magnetic fields can change those optical properties because of the coupling between the spin and the field mediated by the magnetic moment. This allows us to determine the magnetic field from a measurement of the alteration of optical properties such as the absorption coefficient. The method is known since the 1960's and commercial lamp-based devices are used by archaeologists and geologists to measure variations of the earth's magnetic field. In contrast to those devices our magnetometer uses a frequency-stabilized diode-laser as light source. Under optimal conditions the magnetometer has a resolution of 63 fT/Hz1=2 and a detection bandwidth of 140 Hz using a cell of 6 cm3 volume. We used two such magnetometers in a gradiometric configuration to suppress interfering magnetic fields. One sensor was mounted close to the heart measuring both the heart field and the interfering field. The second sensor, some distance away, measured only the interfering field since the heart field drops rapidly with increasing distance. Homogeneous interfering fields cancel in the differential signal from both sensors. This setup allowed us to measure heart magnetic fields in weakly shielded environments. By measuring the field at different positions in a plane above the chest a map of the heart magnetic field can be obtained. Using ECG-triggered averaging we could measure such maps for all times in the cardiac cycle. The dynamics of the magnetic field maps from four healthy volunteers were compared to SQUID-generated reference data. A direct comparison of magnetic field maps recorded with a commercial SQUID magnetometer and with our optical magnetometer was performed at the University of Rome. The results shows that the data are compatible and makes us confident that optical cardiomagnetometry may be used one day as a standard medical technique