Optical Orientation and Atomic Radio Spectroscopy

INTRODUCTION TO THE OPTICAL PUMPING AND DOUBLE RADIOOPTICAL RESONANCE

Very fast evolution of the atomic radiospectroscopy science started in 1950's due to two remarkable events:
1. invention of magnetic resonance optical detection by F.Bitter [1], and
2. the almost simultaneous discovery of the principle of optical pumping of atomic ground state sublevels by A.Kastler [2,3].

Further development of these ideas resulted in creation of a family of quantum optically pumped magnetometers (QOPM). These instruments allowed to measure magnetic field with extremely high accuracy and sensitivity (see, for example, reviews [4,5,6,7,8,9]). Having no match in absolute accuracy, QOPMs can surpass even superconducting quantum interference devices (SQUID) in terms of sensitivity [10].

The essence of the optical pumping method is selective optical excitation of magnetic and/or hyperfine levels of excited state of an atom. If the ground state of the atom has an energetic structure, violation of thermal equilibrium may arise as a result of repeated cycles of optical excitement and spontaneous emission. The kind of optical pumping that results in the rise of non-zero dipolar magnetic moment in the media is called optical orientation. There are also methods of creating macroscopic quadrupole moment (aligment), as well as higher momenta.

The physical basics of the optical pumping process are described in numerous publications, starting from the 1950's; reviews [11,12,13,14,15] must especially be mentioned, and, as the most informative and comprehensive, review [16].

The double radiooptical resonance (DRR) method is normally applied together with the optical pumping method. The essence of DRR method is combination of resonant optical excitation of the atoms with inducing of radiofrequency transitions between excited or ground state sublevels, so each event of radiofrequency photon absorption or stimulated emission is accompanied with absorption or spontaneous emission of optical photon. It allows to increase the sensitivity of magnetic resonance detection by many orders of magnitude compared to the standard radiospectroscopic methods, since the energy of optical photon is many orders of magnitude higher than the energy of radiofrequency one.

Some methods of stimulated combinational (Raman) scattering, not implying direct application of radiofrequency field to atoms, can also be considered as belonging to a variety of DRR methods. The typical example is the so-called Λ (lambda) scheme, which consists of three levels: two bottom levels 1 and 2 are connected to each other through upper level 3 with two coherent optical harmonics whose frequencies differ by the value corresponding to the radiofrequency transition 1-2.

In 1949 French physicists A.Kastler and J.Brossel proposed the DRR method as a tool for radiospectroscopy of short-living excited atomic states[17], and later they applied this method to the study of excited 6³P₁ state of mercury [18]. Combining DDR with optical pumping method made it possible to spread DDR to a ground atomic state.

Fig.1. (a) Optical pumping in three-level system;
(b) The radiooptical resonance

Let us examine this method in details. Assume that there is the ground state, inclusive of two sublevels 1 and 2, and excited state 3 (Fig. 1). If one provides selective excitement of sublevel 1 of ground state, then - on condition that the probability of transition between sublevels 1 and 2 is low as compared with optical excitation probability √ it is possible to increase population of sublevel 2 at the expense of sublevel 1. A.Kastler proposed to detect resonance observing the changes in intensity and polarization of light emitted by atoms. The success of this method mainly depended on the possibility of achieving low enough relaxation rate between sublevels 1 and 2. For magneto-dipole transitions the probability of spontaneous relaxation is negligibly small, but relaxation caused by other reasons, particularly by inter-atomic collisions, may be quite fast. In order to eliminate ground state relaxation, A.Kastler applied the atomic beam method in his first experiment, because atoms in the beam practically do not collide along the length of the entire trajectory. In the atomic beam experiment the time of atomic ground state relaxation was approximately equal to the the atom flight time (~10^-4 s).

Fig.2. The scheme of optical pumping of alkali atom ground state nS_1/2 with resonant D₁-line

The next step was to use the gas cells instead of atomic beam. It was experimentally discovered that relaxation caused by collisions of atoms with the container walls can be prevented by filling the container with a special buffer gas. This gas must slow down the diffusion of oriented atoms towards the walls, but collisions with gas molecules must not destroy the orientation. It was found that many gases, above all inert ones, satisfy these requirements. The reason for this is that electronic state S of an atom does not possess orbital moment, and therefore its spin state is not sensitive to collisions with atoms and molecules which do not possess electronic spin. Fig. 2 illustrates the process of optical pumping of alkali atom ground state nS_1/2 with resonant D₁-line.

Similar, and even better than with the gas cells, results may be obtained by coating the inner surface of the cell with paraffin or polysilaxan layer - by analogy with technology of Teflon coating used in hydrogen masers. These coatings are characterized by anomalously low energy of absorption of alkali atoms √ the time they spend on the wall (about 10^-10 s) is too short compared to the electron spin relaxation time.

These two methods of slowing down the spin relaxation are characterized by different pumping processes. An atom in a coated cell spends most of the time in free flight, and therefore the probability that it will suffer any disturbance in excited state is very low. In this case the process of optical pumping with circularly polarized light is ruled by the law of conservation of angular moment projection in axisymmetric system. Indeed, each act of absorption of a polarized photon introduces to the atomic system a unit of angular moment projection to the direction of the light propagation. Spontaneous emission is caused by the interaction of the atom with the isotropic field of vacuum fluctuations, and therefore angular moment brought to atom by light is, on average, preserved. This type of optical pumping is called re-pumping.

On the contrary, in the case of buffer gas cells, an atom undergoes several collisions while in excited state. Unlike ground state, which possesses only spin moment, the excited state also possess orbital moment, which is quite sensitive to any collisions √ its re-orientation cross-section is about 10 ¹⁴ - 10 ¹⁵ cm². Therefore, even at pressure of several torrs (higher pressures of tens and hundreds of torrs are typically used) angular moment of ground state becomes completely randomized, so in the spontaneous emission process all levels of the ground state are populated with equal probability. In this case, the optical pumping process is totally determined by the light absorption character, and therefore it is called depopulating pumping.

The difference between these two pumping types becomes most apparent in the case of pumping alkali metals with one resonant D₂ line: in a coated cell the level with maximal angular moment projection is mostly populated, though its excitation probability is the highest among ground state sublevels; but in buffer gas cells this level is the least populated. On the contrary, pumping with resonant D₁ line in both coated and gas cells results in similar population distributions, since the level with maximal angular moment projection does not absorb light and therefore it is mostly populated in both cases (Fig.2).

In the first ground-state optical pumping experiments the population distribution change was detected (as in [18]) by measuring the variation in polarization of emitted light. But later it was found that the registration of the population distribution changes by measuring the light absorption is much (to the extent of the efficiency of collecting the light passed through the cell) more efficient. This registration method was first suggested by H.G.Dehmelt [19].Registration of atomic polarization by off-resonant light polarization rotation (paramagnetic Faraday effect [20]) is also possible.

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