In the year 1955, American physicists Stanley Autler and Charles Townes both showed that a microwave transition of the OCS molecule is able to split into two components when one of the two levels involved in the transition is coupled with the third level by a strong microwave field. The so formed pair of two are doublets and thus called the Autler-Townes doublet, or ‘Stark splitting’. This whole phenomenon was termed on them and called ‘Autler-Townes Effect’; also called ‘Stark Effect’.
Autler-Townes effect is mainly described as the shifting and splitting of the spectral lines of various atoms and molecules due to the presence of an external electric field. It is the electric-field where a spectral line is split into its several components due to the presence of the external magnetic field. Although the Autler-Townes effect (Stark effect) was initially coined for the static case, it is now also used in wider areas to describe the effect of electric fields dependent on time. If we look particularly, we can say that the Stark effect is responsible for the pressure broadening which can also be called the Stark broadening of the spectral lines due to the presence of some charged particles in the plasma. Thus, we see that in most of the spectral lines, the Autler-Townes effect is either linear or may be quadratic with a high amount of accuracy.
This effect can be observed for both emission and absorption lines. If we generally talk about the atomic spectral shifts taking place due to the alternating current fields at any one frequency, the effect is more correctly observed when the field is tuned to the alternating current fields at two frequencies that are a natural two-level transition. In such a case, the alternating current field has the effect of splitting the two above transition states into doublets or in pairs of two, that are separated by the Rabi frequency. Rabi frequency is a type of angular frequency. This is commonly achieved by a laser that is tuned to the desired transition levels.
EARLIER USE OF RADIOFREQUENCY
When there was no availability of laser lights at that time, the alternating current Stark effect used to be observed with radiofrequency wave sources. Autler and Towne’s original observation of the effect also used the radio waves. They used these radio frequency source tuned to approx. 12 and 38 MHz, equivalent to the separation between the two doublet microwave absorption lines of OCS. An OCS is a chemical compound called as Carbonyl sulfide whose linear formula is OCS. It is a colorless flammable gas with an unpleasant odor.
The concept of quasi-energy to treat the general alternating current Stark effect was later on developed by Nikishov and Ritis in the year of 1964. A more general method in solving the problem developed into a model of “dressed atom” that described the interaction between the lasers and atoms. In around 1970’s, the observation of Mollow triplets totally proved the form of alternating Stark effect using visible light as a source.
This effect is usually observed in the absorption spectrum of a laser as its frequency is the one that can be scanned. The Autler-Townes effect has been considered extensively for studies in atoms. In recent times, it has been used in spectroscopic applications and also for the measurements of transition dipole moments (it is an electric dipole moment that is generally associated with the transition between the two states) and lifetimes of highly excited molecular states.
Not only this but the stark effect has also been used for all-optical control of the angular momentum arrangement of nonpolar molecules (molecules that do not have any separation of charge that is it neither a negative or positive separation) for quantum control. One of the phenomena that are based on the Autler-Townes effect is Electromagnetically Induced Transparency (EIT) which is caused by the destructive quantum interference between different paths of excitation that leads to the reduction or full cancellation of transition. The Autler-Townes splitting that is caused by a laser is somewhat proportional to the Rabi frequency of that transition. The Rabi frequency in turn depends mainly on the orientation of the molecule as well as on the amplitude of the electrical field of the laser. Thus, the degree of quantum state selectivity can be extended by choosing a proper Laser scheme and proper laser field strength.
IMPACT OF EIT
EIT (Electromagnetically Induced Transparency) is something that gives some materials a small transparent area within an absorption line. It can be thought of as a combination of Autler-Townes splitting (AT effect) and Fano interference (a type of asymmetric line-shaped light scattering phenomena). Although it may be difficult to examine the difference experimentally while both Autler-Townes splitting and electromagnetically induced transparency can produce a transparent window in an absorption band, EIT is a window that maintains transparency even in a weak pump field, and thus it requires Fano interference.
There is a great use of the Autler–Townes effect such as in a quantum control mechanism of the alignment of the molecular angular momentum and also the interaction of the spin and the orbit. Here in the first case, the ability to align molecules along a fixed axis just before a chemical reaction is important to understand the allover orientations influenced reaction dynamics. And in the latter case, it is said that the Autler–Townes splitting provides a controlled mechanism to tune the spin-orbit mixing coefficients of a pair of singlet and triplet states that are weakly mixed. This mechanism can be used as a powerful tool to control the molecular valence electron spin polarization.