The magnetic-resonance lines of alkali atoms are broadened considerably by the spin-exchange collisions for a radio-frequency magnetometer operating at high temperature. The resonance linewidths of cesium atoms are derived by solving the relaxation equations. When spin-exchange relaxation dominates, a light narrowing effect is predicted. For the cesium atomic radio-frequency magnetometer operates at high temperature, a remarkable narrowing of the cesium magnetic-resonance lines are observed by increasing the pump power. The Cs-Cs spin-exchange relaxation is partially suppressed by light narrowing. This study helps to expand the applications of radio-frequency magnetometers in the high-sensitivity radio-frequency magnetic-field detection.
The magnetic-resonance lines of alkali atoms are broadened considerably by the spin-exchange collisions for a radio-frequency magnetometer operating at high temperature. The resonance linewidths of cesium atoms are derived by solving the relaxation equations. When spin-exchange relaxation dominates, a light narrowing effect is predicted. For the cesium atomic radio-frequency magnetometer operates at high temperature, a remarkable narrowing of the cesium magnetic-resonance lines are observed by increasing the pump power. The Cs-Cs spin-exchange relaxation is partially suppressed by light narrowing. This study helps to expand the applications of radio-frequency magnetometers in the high-sensitivity radio-frequency magnetic-field detection.
A new and unusual phenomenon which we call light narrowing is reported and discussed in this paper. We discovered this effect in dense, spin-polarized cesium vapor optically pumped with a cw blue dye laser beam tuned to the second resonance D1 line (4593 \AA{}). We observe a significant narrowing of the radio-frequency power-broadened magnetic resonance lines (linewidths narrow by as much as a factor of 2.5) when the intensity of the circularly polarized incident dye laser beam is increased by either focusing the beam or by the removal of attenuating filters from the focused beam. The magnetic resonance linewidths in spin-polarized cesium vapor were measured over a wide range of cesium number densities (5\ifmmode\times\else\texttimes\fi{}1012 ${\mathrm{cm}}^{\ensuremath{-}3}$ \ensuremath{\le}[Cs]\ensuremath{\le}1\ifmmode\times\else\texttimes\fi{}1016 ${\mathrm{cm}}^{\ensuremath{-}3}$). This corresponds to cesium spinexchange rates of 4.5\ifmmode\times\else\texttimes\fi{}103 to 9\ifmmode\times\else\texttimes\fi{}106 ${\mathrm{sec}}^{\ensuremath{-}1}$. For low cesium number densities (5\ifmmode\times\else\texttimes\fi{}1012 [Cs]3\ifmmode\times\else\texttimes\fi{}1014 ${\mathrm{cm}}^{\ensuremath{-}3}$) the light-narrowing effect is large (a factor of 2.5) and independent of [Cs]. In the region of 3\ifmmode\times\else\texttimes\fi{}1014 to 1\ifmmode\times\else\texttimes\fi{}1015 ${\mathrm{cm}}^{\ensuremath{-}3}$ in number densities, the light-narrowing effect decreases with increasing [Cs]. At high cesium number densities ([Cs]> 1\ifmmode\times\else\texttimes\fi{}1015 ${\mathrm{cm}}^{\ensuremath{-}3}$) this light-narrowing effect almost completely disappears. In the limit of low-radio-frequency power the magnetic resonance linewidths for focused and unfocused dye laser beam are nearly the same. Experimental observations on this new effect are presented in detail. In the latter part of this paper a self-contained theoretical treatment of the light-narrowing effect is developed. Using Bloch equations in the presence of optical pumping, spin relaxation (diffusion, electron randomization), rapid spin exchange, and radio-frequency magnetic field, expressions for magnetic resonance line shapes are derived. In general, we find good agreement between our experimental results and the theory.
Abstract We present a theoretical study of the optical-radio-frequency (rf) double resonance on hyperfine ground states of alkali-metal atoms in the geophysical magnetic-field range, in which resonance circularly polarized laser light is used in the optical pumping and detection processes. The analytical expressions of the rf resonance signals corresponding to two ground states are obtained based on atomic multipole moments, where the alignment effect is considered and that is responsible for the resonance shape and linewidth especially when the optical power is relatively strong. In addition, we also obtain the analytical expressions of the linewidths of two separated resonance peaks and one presenting the competition process of the optical pumping, rf field and spin-exchange collision. Two different types of light narrowing phenomena are investigated by comparing the longitudinal relaxation rates of atomic multipole moments for two ground states. Applied to rubidium and cesium atoms, we show a particularly close agreement of our analytical results with more elaborate calculations using density-matrix theory. Our theoretical model is relevant for optimizing the sensitivity of magnetometers.
Alkali-metal-vapor magnetometers, using coherent precession of polarized atomic spins for magnetic field measurement, have become one of the most sensitive magnetic field detectors. Their application areas range from practical uses such as detections of NMR signals to fundamental physics research such as searches for permanent electric dipole moments. One of the main noise sources of atomic magnetometers comes from the light shift that depends on the frequency of the pump laser. In this work, we theoretically study the light shift, taking into account the relaxation due to the optical pumping and the collision between alkali atoms and between alkali atoms and the buffer gas. Starting from a full master equation containing both the ground and excited states, we adiabatically eliminate the excited states and obtain an effective master equation in the ground-state subspace that shows an intuitive picture and dramatically accelerates the numerical simulation. Solving this effective master equation, we find that in the light-narrowing regime, where the line width is reduced while the coherent precession signal is enhanced, the frequency-dependence of the light shift is largely reduced, which agrees with experimental observations in cesium magnetometers. Since this effective master equation is general and is easily solved, it can be applied to an extensive parameter regime, and also to study other physical problems in alkali-metal-vapor magnetometers, such as heading errors.
We demonstrate a way of operating an optically pumped magnetometer with miniaturized cesium cell using the light-narrowing effect. The magnetometer setup shows improvement of shot-noise-limited sensitivity (42fT/Hz in a cell of only 9.3mm3 volume) due to the suppression of spin-exchange relaxation to a large extent and the use of a strikingly increased fraction of alkali-metal atoms for signal generation, working even in $\ensuremath{\mu}\mathrm{T}$ magnetic fields, by using only a single high-intensity laser beam both for pumping and probing of atomic spins.
We describe an alkali-metal magnetometer for detection of weak magnetic fields in the radio-frequency (rf) range. High sensitivity is achieved by tuning the Zeeman resonance of alkali atoms to the rf frequency and partially suppressing spin-exchange collisions in the alkali-metal vapor. We demonstrate magnetic field sensitivity of 2 fT/Hz(1/2) at a frequency of 99 kHz with a resonance width of 400 Hz. We also derive a simple analytic expression for the fundamental limit on the sensitivity of the rf magnetometer and show that a sensitivity of about 0.01 fT/Hz(1/2) can be achieved in a practical system with a measurement volume of 200 cm3.
In this paper, a new type of high-sensitivity magnetometer that can surpass SQUID magnetometers operating in liquid-helium in sensitivity and spatial resolution is discussed. The sensitivity of atomic magnetometers is determined by the electron spin relaxation time and the number of alkali-metal atoms. The relaxation time is usually limited by spin-exchange collisions between atoms and drops at higher alkali-metal density. Spin-exchange collisions preserve the total spin of the atoms, but can change their hyperfine state, which reverses the direction of spin precession in the magnetic field. Thus, spin-exchange collisions usually cause spin decoherence. However, this relaxation process can be suppressed if the spin precession frequency is much smaller than the rate of spin-exchange collisions. These conditions lead to 'motional narrowing', where the spin precession angle between the collisions is so small that the atoms do not lose coherence. A magnetometer operating near zero magnetic field can simultaneously achieve high alkali-metal density and long spin-relaxation time, resulting in very high sensitivity.