Electric field sensing (electrometry) using the AC Stark effect or Autler–Townes effect

Splitting/spacing of spectral lines proportional to electric field amplitude

Atomic spectroscopy and Electromagnetically Induced Transparency (EIT)


Electric field sensing - Applications in areas of "weather, electronic devices, environmental and astronomical science."

How do you detect electromagnetic waves e.g. those emanated from a device? Specifically how do you detect the electric component (field) of an electromagnetic wave or the electric field distribution of a device?

Using an antenna e.g. dipoles and loop antennas.

It is believed that using an antenna "limits the precision at which the electric field distribution can be determined."

Haoquan F. et al 2015 J. Phys. B: At. Mol. Opt. Phys. 48 202001



The alternative is atomic spectroscopy electrometry, an "atomic antenna".

The example of radiofrequency electrometry using laser (infrared) spectroscopy and atomic transitions of Rydberg atoms


AC Stark effect or Autler–Townes effect


Based on the principle of Electromagnetic Induced Transparency (EIT)

Splitting/spacing of spectral lines proportional to electric field amplitude


[Figure 3 - lowest panel] If we consider a glass cell with Cesium atoms upon which we shine a laser tuned at the D2 transition of Cesium i.e. 852 nm, then the electrons found on the ground state (6S) will absorb the light (electromagnetic) energy and will transition to a higher energetic state (6P).


As the atoms will absorb all the light that is shined upon them, there will no light coming out of the cell. The cell will appear opaque to light as there will be full absorbance. (This would be the opposite of transparency).


The absorbance curve is demonstrated in the lowest right-hand panel of Figure 3 (the transmission represented on the y axis tends to become zero).


Electrons may decay to the ground state. As long as the ground state is populated by electrons, which can absorb light and transition to a higher energetic state, the absorbance will be sustained.


[Figure 3 - middle panel] If we simultaneously shine on the glass cell another laser tuned at 509 nm, this will mediate a transition of electrons from the previous excited state (6P) to the nD state. (We have coupled a new transition to the previous one. Also this laser light is termed "coupled laser" or "control laser").


If all electrons move to the new excited state, it is possible that there will be no electrons decaying to the 6S ground state.

In the absence of electrons on the ground state, it will no longer be possible to absorb laser light of 852 nm as there are no electrons to absorb it. 


Due to this, 852 nm light will be transmitted in the cell (light will go through the cell) and will not be absorbed. In other words, the cell will become transparent to 852 nm light. Electromagnetically Induced Transparency will take place.

This is demonstrated in the middle right-hand side panel where a peak of transmission appears.


[Figure 3 - highest panel] If an electric field is resonant with another transition as shown at the highest panel, it can induce a narrow absorption feature or in other words it can split the transmission lineshape in two peaks. 


The splitting between the transmission peaks is proportional to the electric field amplitude as shown in Figure 2.




Figure: Figure 2 and 3 from Haoquan F. et al





"Electromagnetically Induced Transparency" or EIT


It is used for sensing of electric fields (electric component of electromagnetic fields)


Lecture by Stanford Professor S. Harris | https://youtu.be/iqQCI-VfnH0?t=168


A natural case of "Electromagnetically Induced Transparency" is presented, that of neutral zinc. As two strong states decay to the same end state, we do not have a case of two Lorentzians for the absorption of the two states but instead there is a strong final destructive interference of a sharp appearance (indicated at the first figure by the laser point dot and the red arrow) with the result being not absorption but transparency.


In order to reproduce this experimentally (figure 2), first we take an absorptive transition which has a Lorentzian absorption. Then we take a transition mediated by a specific laser which has the same upper state and also has a lower state which is non-decaying or metastable.

We couple the two transitions, which means that we stress up to the same energy and we mix. (Note: the second transition is called the coupling field or control field).


The bare atom plus the laser field makes an "equivalent atom". The "equivalent atom" is similar to the neutral zinc atom presented previously which has two states which decay to the same identical state.

Note that by adding the second transition you introduce a resonance that will cause destructive interference in the system. The second transition represents the addition of a destructive interference factor for the upper state.


"One expects that midway between you will have zero absorption." "You are below one state and above another state so you expect a steep dispersive profile". This is the basic idea of EIT.






Use of an atom vapor cell and lasers to record and play back music


"#Quantum audio recording: @usnistgov researchers use an atom vapor cell and lasers to record, and play back, guitar music" http://ow.ly/7Kpz50uLyZV