# Autler–Townes effect

In spectroscopy, the **Autler–Townes effect**, named after American physicists Stanley Autler and Charles Townes, is a type of the dynamical (also known as AC) Stark effect, corresponding to the case when an oscillating electric field (e.g., that of a laser) is tuned in resonance (or close) to the transition frequency of a given spectral line, and resulting in a change of the shape of the absorption/emission spectra of that spectral line.

It is the AC equivalent of the Stark effect which splits the spectral lines of atoms and molecules in a constant electric field. Compared to its DC counterpart, the fields in the AC effect are typically much larger, and the effects are harder to predict.[1]

While generally referring to atomic spectral shifts due to AC fields at any (single) frequency, the effect is more pronounced when the field is tuned to the frequency of a natural two level transition.[2] In this case, the alternating field has the effect of splitting the two bare transition states into doublets or "dressed states" that are separated by the Rabi frequency.[3] This is commonly achieved by a laser tuned to (or near) the desired transition.

This splitting results in a Rabi cycle or Rabi oscillation between bare states which are no longer energy eigenstates of the atom-field Hamiltonian.[4] The resulting fluorescence spectrum of an atom is known as a Mollow triplet. The AC stark splitting is integral to several other phenomena in quantum optics, such as Electromagnetically induced transparency and Sisyphus cooling. Vacuum Rabi oscillations have also been described as manifestation of the AC Stark effect from atomic coupling to the vacuum field.[3]

## History

The AC stark effect was discovered in 1955 by American physicists Stanley Autler and Charles Townes while at Columbia University and Lincoln Labs at the Massachusetts Institute of Technology. Before the availability of lasers, the AC Stark effect was observed with radio frequency sources. Autler and Townes' original observation of the effect used a radio frequency source tuned to 12.78 and 38.28 MHz, corresponding to the separation between two doublet microwave absorption lines of OCS.[5]

The notion of quasi-energy in treating the general AC Stark effect was later developed by Nikishov and Ritis in 1964 and onward.[6][7][8] This more general method of approaching the problem developed into the "dressed atom" model describing the interaction between lasers and atoms[4]

Prior to the 1970s there were various conflicting predictions concerning the fluorescence spectra of atoms due to the AC Stark effect at optical frequencies. In 1974 the observation of Mollow triplets verified the form of the AC Stark effect using visible light.[2]

## General semiclassical approach

In a semiclassical model where the electromagnetic field is treated classically, a system of charges in a monochromatic electromagnetic field has a Hamiltonian that can be written as:

where , , and are respectively the position, momentum, mass, and charge of the -th particle, and is the speed of light. The vector potential of the field, , satisfies

.

The Hamiltonian is thus also periodic:

Now, the Schrödinger equation, under a periodic Hamiltonian is a linear homogeneous differential equation with periodic coefficients,

where here represents all coordinates. Floquet's theorem guarantees that the solutions to an equation of this form can be written as

Here, is the "bare" energy for no coupling to the electromagnetic field, and has the same time-periodicity as the Hamiltonian,

or

with the angular frequency of the field.

Because of its periodicity, it is often further useful to expand in a Fourier series, obtaining

or

where is the frequency of the laser field.

The solution for the joint particle-field system is therefore a linear combination of stationary states of energy , which is known as a *quasi-energy* state and the new set of energies are called the *spectrum of quasi-harmonics*.[8]

Unlike the DC Stark effect, where perturbation theory is useful in a general case of atoms with infinite bound states, obtaining even a limited spectrum of shifted energies for the AC Stark effect is difficult in all but simple models, although calculations for systems such as the hydrogen atom have been done.[9]

## Examples

General expressions for AC Stark shifts must usually be calculated numerically and tend to provide little insight.[1] However, there are important individual examples of the effect that are informative.

### Two level atom dressing

An atom driven by an electric field with frequency close to an atomic transition (consider detuning ) can be approximated as a two level quantum system since the off resonance states have low occupation probability.[3] The Hamiltonian can be divided into the bare atom term plus a term for the interaction with the field as:

In an appropriate rotating frame, and making the rotating wave approximation, reduces to

Where is the Rabi frequency, and are the strongly coupled bare atom states. The energy eigenvalues are

and for small detuning,

The eigenstates of the atom-field system or dressed states are dubbed and .

The result of the AC field on the atom is thus to shift the strongly coupled bare atom energy eigenstates into two states and which are now separated by . Evidence of this shift is apparent in the atom's absorption spectrum, which shows two peaks around the bare transition frequency, separated by (Autler-Townes splitting). The modified absorption spectrum can be obtained by a pump-probe experiment, wherein a strong *pump* laser drives the bare transition while a weaker *probe* laser sweeps for a second transition between a third atomic state and the dressed states[10].

Another consequence of the AC Stark splitting here is the appearance of Mollow triplets, a triple peaked fluorescence profile. Historically an important confirmation of Rabi flopping, they were first predicted by Mollow in 1969[11] and confirmed in the 1970s experimentally.[3]

### Adiabatic elimination

In quantum system with three (or more) states, where a transition from one level, to another can be driven by an AC field, but only decays to states other than , the dissipative influence of the spontaneous decay can be eliminated. This is achieved by increasing the AC Stark shift on through large detuning and raising intensity of the driving field. Adiabatic elimination has been used to create comparatively stable effective two level systems in Rydberg atoms, which are of interest for qubit manipulations in quantum computing.[12][13][14]

### Electromagnetically induced transparency

Electromagnetically induced transparency (EIT), which gives some materials a small transparent area within an absorption line, can be thought of as a combination of Autler-Townes splitting and Fano interference, although the distinction may be difficult to determine experimentally. While both Autler-Townes splitting and EIT can produce a transparent window in an absorption band, EIT refers to a window that maintains transparency in a weak pump field, and thus requires Fano interference. Because Autler-Townes splitting will wash out Fano interference at stronger fields, a smooth transition between the two effects is evident in materials exhibiting EIT.[15]

## See also

## References

- Delone, Nikolai Borisovich, and Vladimir Pavlovich Krainov. "AC Stark shift of atomic energy levels." Physics-Uspekhi 42.7 (1999): 669-687.
- Schuda, F., C. R. Stroud Jr, and M. Hercher. "Observation of the resonant Stark effect at optical frequencies." Journal of Physics B: Atomic and Molecular Physics 7.7 (1974): L198.
- Fox, Mark. Quantum Optics: An Introduction: An Introduction. Vol. 15. Oxford university press, 2006.
- Barnett, Stephen, and Paul M. Radmore. Methods in theoretical quantum optics. Vol. 15. Oxford University Press, 2002.
- Autler, S. H; Charles Hard Townes (1955). "Stark Effect in Rapidly Varying Fields".
*Physical Review*. American Physical Society.**100**(2): 703–722. Bibcode:1955PhRv..100..703A. doi:10.1103/PhysRev.100.703. - Nikishov, A. I., and V. I. Ritus. Quantum Processes in the Field of a Plane Electromagnetic Wave and in a Constant Field. PART I. Lebedev Inst. of Physics, Moscow, 1964.
- Ritus, V. I. "Shift and splitting of atomic energy levels by the field of an electromagnetic wave." Sov. Phys. JETP 24 (1967): 1041-1044.
- Zel'Dovich, Ya B. "Scattering and emission of a quantum system in a strong electromagnetic wave." Physics-Uspekhi 16.3 (1973): 427-433.
- Crance, Michèle. "Nonperturbative ac Stark shifts in hydrogen atoms." JOSA B 7.4 (1990): 449-455.
- Four-wave mixing in dressed cold cesium atoms GC Cardoso, JWR Tabosa Optics communications 185 (4-6), 353-358
- B.R. Mollow, Phys. Rev. 188, 1969
- Brion, E; Pedersen, L H; Mølmer, K (2007-01-17). "Adiabatic elimination in a lambda system".
*Journal of Physics A: Mathematical and Theoretical*.**40**(5): 1033–1043. arXiv:quant-ph/0610056. Bibcode:2007JPhA...40.1033B. doi:10.1088/1751-8113/40/5/011. ISSN 1751-8113. - Radmore, P M; Knight, P L (1982-02-28). "Population trapping and dispersion in a three-level system".
*Journal of Physics B: Atomic and Molecular Physics*.**15**(4): 561–573. Bibcode:1982JPhB...15..561R. doi:10.1088/0022-3700/15/4/009. ISSN 0022-3700. - Linskens, A. F.; Holleman, I.; Dam, N.; Reuss, J. (1996-12-01). "Two-photon Rabi oscillations".
*Physical Review A*.**54**(6): 4854–4862. Bibcode:1996PhRvA..54.4854L. doi:10.1103/PhysRevA.54.4854. hdl:2066/27687. ISSN 1050-2947. PMID 9914052. - Anisimov, Petr M.; Dowling, Jonathan P.; Sanders, Barry C. (2011). "Autler-Townes Splitting vs. Electromagnetically Induced Transparency: Objective Criterion to Discern Between Them in any Experiment".
*Physical Review Letters*.**107**(16): 163604. arXiv:1102.0546. Bibcode:2011PhRvL.107p3604A. doi:10.1103/PhysRevLett.107.163604. PMID 22107383.

## Further reading

- Cohen-Tannoudji
*et al.*, Quantum Mechanics, Vol 2, p 1358, trans. S. R. Hemley*et al.*, Hermann, Paris 1977