Forty years after the first dark resonance experiment: an overview of the COSMA project results

TL;DR: COSMA: Coherent Optics Sensors for Medical Application as mentioned in this paper was an European Marie Curie Project running from 2012 to March 2016, with the participation of 10 teams from Armenia, Bulgaria, India, Israel, Italy, Poland, Russia, UK, USA.

Abstract: COSMA: Coherent Optics Sensors for Medical Application is an European Marie Curie Project running from 2012 to March 2016, with the participation of 10 teams from Armenia, Bulgaria, India, Israel, Italy, Poland, Russia, UK, USA. The main objective was to focus theoretical and experimental research on biomagnetism phenomena, with the specific aim to develop all-optical sensors dedicated to their detection and suitable for applications in clinical diagnostics. The paper presents some of the most recent results obtained during the exchange visits of the involved scientists, after an introduction about the phenomenon which is the pillar of this kind of research and of many other new fields in laser spectroscopy, atomic physics, and quantum optics: the dark resonance.

Summary

Invited Paper

• 19th International Conference and School on Quantum Electronics: Laser Physics and Applications, edited by Tanja Dreischuh, Sanka Gateva, Albena Daskalova, Alexandros Serafetinides, Proc. of SPIE Vol.
• The development of more and more advanced atomic clocks, towards outstanding precision limits, able to measure general relativity effect at very small distances, has been favored by using narrow and 100% contrast dark line signal as a reference7.
• The most awarded experiments of the last 20 years, regarding laser cooling and trapping, have got benefit by dark resonances, for example via the Velocity Selective Coherent Population Trapping mechanism10.
• Going to even more complicated physical systems, CPT has been demonstrated in a semiconductor, for a hole spin state in a quantum dot12.

2.1 Study of fundamental processes

• One of the possible goal of a fundamental quest of the CPT related processes is the search for new efficient excitation schemes, as well as the comprehension and control of the phenomena involved.
• The paper shows that the light shift for a fixed optical power, when the optical pumping is negligible, is independent of the size of the laser beam, with the exception of small systematic contributions.
• These results are important for modern magnetic sensors that make use of auxiliary fictitious fields and can be extended to other spatially averaged quantities in cells with long coherence times, for example, to the averaging of the scalar and tensor light shifts.
• In an alkali dilute gas (for example, Rb and Cs) in presence of a buffer gas, when the exciting laser radiation is tuned to the half-sum frequency of transitions from the two hyperfine sublevels of the ground state, a "bright" resonance can be detected17.

2.2 Tests of coatings

• The analysis is focused on the case of EIT in spectroscopy cells coated with anti-relaxation polymers.
• This provides a formal tool to indirectly evaluate the properties of a coating and thus a quantitative criterion to evaluate its suitability for critical applications, such as ultra-sensitive atomic magnetometry.
• The magneto-optical resonances, detected in two orthogonal polarizations are measured.
• Numerical calculations with parameters, close to the experimental ones are performed by using a program, which is based on the irreducible tensor operator formalism.

2.3 Miniaturization of sensors – the micro- and nano-cells systems and the related new physics

• One of the most intriguing tools of the last 20 years of investigation in the field of high resolution laser spectroscopy is the so – called nanocell, i.e. a special vessel of centimeter size in diameter, but with a thickness limited to lengths ranging from some hundreds of nanometers (the optical radiation wavelength order of magnitude) to micrometric scale.
• The influence of alignment strongly depends on the configuration of the coupling and probe frequencies as well as on the configuration of the magnetic field.
• The advantages and disadvantages of a particular type of cell are discussed along with the possible practical applications.

Development of new Magnetometers

• UCL designed and developed an alternative experimental approach, based on Magnetic Induction Tomography (MIT), for applications of OAMs in biomedical diagnostics and imaging.
• The main goal is the imaging of the heart and, in particular, the diagnosis of atrial fibrillation.
• The all-optical scheme can be more easily used in arrays and in magnetically sensitive fundamental physics experiments, e.g., searches for a permanent electric dipole moment of the neutron.
• If the main interest is not an optimized sensitivity, but a wide field measurement and an experimental simplicity, the two axis optical magnetometer of ref. 34 can be a good solution, especially for geophysical and material science studies.

Figures

Figure 1. Left: three level scheme for the Sodium excitation; right: picture of the effect and calculation (taken by ref.2) of the transition profile when one of the two laser frequencies is scanned across the resonance.

Full text

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Proceedings Volume 10226, 19th International Conference and School on Quantum Electronics: Laser Physics and
Applications
Cronfa URL for this paper:
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_____________________________________________________________
Conference contribution :
Mariotti, E., Bevilacqua, G., Biancalana, V., Cecchi, R., Dancheva, Y., Khanbekyan, A., Marinelli, C., Moi, L., Stiaccini,
L., Cartaleva, S., Andreeva, C., Alipieva, E., Gateva, S., Krasteva, A., Slavov, D., Taskova, E., Taslakov, M., Todorov,
P., Tsvetkov, S., Wilson Gordon, A., Margalit, L., Gawlik, W., Pustelny, S., Stabrawa, A., Sudyka, J., Wojciechowski,
A., Renzoni, F., Deans, C., Hussain, S., Marmugi, L., Rassi, D., Ozun, O., Sarkisyan, D., Azizbekyan, H., Drampyan,
R., Khanbekyan, A., Mirzoyan, R., Papoyan, A., Sargsyan, A., Shmavonyan, S., Tonoyan, A., Ghosh, P., Dey, S.,
Mitra, S., Ray, B., Nasyrov, K., Chapovsky, P., Entin, V., Nikolov, N., Petrov, N., Budker, D., Patton, B., Wickenbrock,
A., Zhivun, E. & Gozzini, S. (2017). Forty years after the first dark resonance experiment: an overview of the COSMA
project results. Proceedings Volume 10226, 19th International Conference and School on Quantum Electronics: Laser
Physics and Applications, (pp. 102260K Sozopol, Bulgaria: 19th International Conference and School on Quantum
Electronics: Laser Physics and Applications.
http://dx.doi.org/10.1117/12.2264896
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I
Forty years after the first dark resonance experiment:
an overview of the COSMA project results
E.Mariotti
a
, G.Bevilacqua
a
, V.Biancalana
a
, R.Cecchi
a
, Y.Dancheva
a
, Alen Khanbekyan
a
, C.Marinelli
a
, L.Moi
a
, L.
Stiaccini
a
, S. Cartaleva
b
, C. Andreeva
b
, E. Alipieva
b
, S. Gateva
b
, A. Krasteva
b
, D. Slavov
b
, E.T. Taskova
b
, M.
Taslakov
b
, P. Todorov
b
, S. Tsvetkov
b
, A.Wilson Gordon
c
, L. Margalit
c
, W. Gawlik
d
, S. Pustelny
d
, A. Stabrawa
d
, J.
Sudyka
d
, A. Wojciechowski
d
, F. Renzoni
e
, C. Deans
e
, S. Hussain
e
, L. Marmugi
e,a
, D. Rassi
f
, O. Ozun
f
, D.
Sarkisyan
g
, H. Azizbekyan
g
, R. Drampyan
g
, Alek. Khanbekyan
g
, R. Mirzoyan
g
, A. Papoyan
g
, A. Sargsyan
g
, S.
Shmavonyan
g
, A. Tonoyan
g
, P.N. Ghosh
h
, S. Dey
h
, S. Mitra
h
, B. Ray
h
, K.A. Nasyrov
i
, P. Chapovsky
i
, V. Entin
i
, N.
Nikolov
i
, N. Petrov
i
, D. Budker
j,k
, B. Patton
j
, A. Wickenbrock
j,e,k
, L. Zhivun
j
, S.Gozzini
l
a
UniSiena, via Roma 56, Siena, Italia;
b
IEBAS, Sofia, Bulgaria;
c
Dept. of Chemistry, BIU, Israel;
d
Faculty of
Physics, Astronomy, and Applied Computer Science, JU, Krakow, Poland;
e
Dept. of Physics and Astronomy,
UCL, London, UK;
f
College of Human and Health Science, UniSwansea, UK;
g
Institute for Physical Research,
NAS RA, Ashtarak, Armenia;
h
Dept. of Physics, Kolkata Univ., India;
i
IAE, Siberian branch of RAS, Novosibirsk,
Russia;
j
Dept. of Physics, UCB, USA;
k
Helmholtz Institute, JGU Mainz;
l
INO – CNR, Pisa
ABSTRACT
COSMA – Coherent Optics Sensors for Medical Application is an European Marie Curie Project running from 2012 to
March 2016, with the participation of 10 teams from Armenia, Bulgaria, India, Israel, Italy, Poland, Russia, UK, USA.
The main objective was to focus theoretical and experimental research on biomagnetism phenomena, with the specific
aim to develop all-optical sensors dedicated to their detection and suitable for applications in clinical diagnostics. The
paper presents some of the most recent results obtained during the exchange visits of the involved scientists, after an
introduction about the phenomenon which is the pillar of this kind of research and of many other new fields in laser
spectroscopy, atomic physics, and quantum optics: the dark resonance.
Keywords: Coherent Population Trapping, Electromagnetically Induced Transparency, Optical Magnetometry,
Nanocell, Coatings, Imaging, All-optical sensors
1. INTRODUCTION
The experiment of Alzetta, Gozzini, Moi, and Orriols
1
was the first experimental observation of the so – called “black
line”, later indicated as Coherent Population Trapping (CPT), a quantum phenomenon where the destructive interference
of the transition amplitudes, when the two laser fields are tuned in resonance with the transitions connecting the
hyperfine ground state levels to the excited one, is able to cancel the fluorescence, with a very tight resonance condition.
In this way, by the application of both an inhomogeneous magnetic field and multimode, circularly polarized laser
radiation to a sodium vapor sealed cell, the authors could directly see by eye one or more thin dark volumes along the
dye-laser beam. These dark regions were located in the positions where the frequency distance of two laser modes
coincided with the Zeeman shifted distance of atomic levels, to form a lambda system, as shown in fig 1.
Figure 1. Left: three level scheme for the Sodium excitation; right: picture of the effect and calculation (taken by ref.2) of
the transition profile when one of the two laser frequencies is scanned across the resonance.
∆νHFS
Fg=2
Fg=1
Fe=1
ν1
ν1-ν2 = ∆νHFS
ν2
ν
Fl
Invited Paper
19th International Conference and School on Quantum Electronics: Laser Physics and Applications,
edited by Tanja Dreischuh, Sanka Gateva, Albena Daskalova, Alexandros Serafetinides, Proc. of SPIE
Vol. 10226, 102260K · © 2017 SPIE · CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2264896
Proc. of SPIE Vol. 10226 102260K-1
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The interpretation of the ultra – narrow resonant phenomenon, whose reduced bandwidth is related to the very long
lifetime of the ground states, was given by Arimondo and Orriols
2
, who solved the optical density matrix correlated
problem in an analytic way, giving the plot with the typical narrow dip in a much broader structure, obtainable when
scanning one of the two laser frequencies across the resonance (upper – right insert of fig.1)
This very clear demonstration of a new effect paved the way for a large variety of applications, still growing in number
and importance. We want to briefly give here a few examples of the fantastic consequences this seminal article has left
as a legacy.
A particular case of the phenomenon is the use of one of the two lasers as a pump saturating the transition, while using
the other one as a probe. This choice has been called Electromagnetically Induced Transparency
3
, and recently it has
been applied to both a few atoms and even to a single atom stored in a high finesse cavity, merging two innovative fields
of quantum optics
4
. One of the most spectacular results of the steep resonance produced by CPT is the possibility to
reduce the light velocity to values of the order of tens of meters per second, as demonstrated in
5
. Of course, it is not
mandatory to create a three level system playing with the first excited state: in fact, ref.
6
gives a proof of a possible dark
state spectroscopy of Rydberg levels, measured as a suppression of the ionization in Neon atoms. The development of
more and more advanced atomic clocks, towards outstanding precision limits, able to measure general relativity effect at
very small distances, has been favored by using narrow and 100% contrast dark line signal as a reference
7
. It is worth
noting that there are also suggestions to implement a CPT – based atomic clock in an undergraduate lab
8
, just to
underline that both the dark resonances and their applications has become as much “popular” as well as powerful tools.
CPT is able to promote four – wave mixing, enhancing the efficiency of the generated signal, as the nonlinear absorption
competes with the linear one
9
. The most awarded experiments of the last 20 years, regarding laser cooling and trapping,
have got benefit by dark resonances, for example via the Velocity Selective Coherent Population Trapping mechanism
10
.
There are also proposals for using EIT spectroscopy in order to detect smaller density molecules in presence of a more
populated species
11
. Going to even more complicated physical systems, CPT has been demonstrated in a semiconductor,
for a hole spin state in a quantum dot
12
. At completely different energies, EIT has been put in evidence for a Mőssbauer
nuclear resonance of Iron, excited by synchrotron radiation in a low finesse cavity where two different samples are
obliged to have a superradiant and a subradiant behavior, so to get two different effective excited states, being
respectively placed in an antinode and a node of the cavity
13
. Finally, as an introduction to what is commented in the
next paragraphs, where an overview of a few of the last year results of the COSMA project will be described, we want to
cite the performances of all – optical magnetometers, where the sensitivity of fraction of fT/√Hz
14
has allowed for the
non cryogenic detection of brain signals with gradiometric configurations.
2. COSMA PROJECT RESULTS
2.1 Study of fundamental processes
One of the possible goal of a fundamental quest of the CPT related processes is the search for new efficient excitation
schemes, as well as the comprehension and control of the phenomena involved.
For the formation of high-contrast N-type resonance in a Λ-system (ground levels are F
g
=2, 3), on the D
1
line of the Rb
atoms, two continuous narrow-band diode lasers with λ=795 nm and a 8 mm- long cell filled with the Rb vapor and a
buffer gas (20 Torr neon) have been used. A new 4-level system based on an additional (3
rd
) laser radiation (we call it
“preparation” radiation) has been suggested
15
. When the frequency υ
PREP
is in resonance with rubidium on 5S
1/2
, F
g
=2→
5P
3/2
transitions (D
2
line, λ=780 nm), via optical pumping process (through 5P
3/2
level) it transfers a large number of Rb
atoms from F
g
=2 to F
g
=3 and creates the inversion condition N
3
(F
g
=3) > N
2
(F
g
=2) and if the probe frequency starts from
F
g
=3, then the amplitude of the “bright” resonance will increase. Meanwhile when frequency υ
PREP
is in resonance with
5S
1/2
, F
g
=3→ 5P
3/2
transitions it transfers atoms from F
g
=3 to F
g
=2 and creates the inversion N
2
(F
g
=2) >N
3
(F
g
=3). In this
case a Raman-type process is switched-on and the initially “bright” resonance is amplified and demonstrates increase in
transmission (in the spectrum it looks like a “dark” resonance). Thus, it is possible to control the sign of the resonance.
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The good signal/noise ratio of the observed resonance allows us to follow its behavior in an applied magnetic field from
several gauss to several hundred gauss. Since narrow-band diode–lasers operating at the wavelengths 780 nm and 795
nm are widely available, the presented system is very robust and convenient for forming controllable narrow-band
resonances for practical applications.
While the light – shift problem has been thoroughly investigated in samples confined in uncoated cells, this is not true
for the Vector Light Shift (VLS) case in coated vessels. The role of VLS in an optical, synchronously pumped
orientation-based Cs magnetometer exploiting a paraffin-coated alkali-metal vapor cell has been discussed in ref.
16
.
This kind of magnetometer is based on the resonant response of the atoms to a light modulated at a frequency matching
their Larmor-precession frequency. The magnetic field can be extracted by measuring the center frequency of the
Lorentzian shaped magnetic resonance. The paper shows that the light shift for a fixed optical power, when the optical
pumping is negligible, is independent of the size of the laser beam, with the exception of small systematic contributions.
These results are important for modern magnetic sensors that make use of auxiliary fictitious fields and can be extended
to other spatially averaged quantities in cells with long coherence times, for example, to the averaging of the scalar and
tensor light shifts.
In an alkali dilute gas (for example, Rb and Cs) in presence of a buffer gas, when the exciting laser radiation is tuned to
the half-sum frequency of transitions from the two hyperfine sublevels of the ground state, a "bright" resonance can be
detected
17
. This "intermediate resonance" is absent in pure vapor cells up to at least 50 mW radiation power, while it is
observable in buffered vapor cells already at sub-mW-range power. Due to this fact, together with the existence of a
threshold with the laser radiation intensity and the resonant nature of these spectral features, the interpretation is fully or
partly attributed to a four-wave mixing process where two mid-resonance laser photons originate two photons on the D2
line hyperfine transitions. This process is strongly assisted by the velocity changing elastic collisions of alkali atoms
with the buffer gas ones, which produces an increase of the interaction time.
2.2 Tests of coatings
There are two possible methods in order to both preserve the polarization of the sample as excited by laser light and to
eliminate the mixing of the ground state levels due to collision with either the cell walls or with atoms of the same kind:
one is to fill the sensor cell with an appropriate buffer gas, the other to coat the cell walls with anti – relaxation coatings.
COSMA collaborators have also worked a lot in this second framework.
In order to increase the contrast of the coherent resonance that is one of the main parameters determining the Optical
Magnetometer sensitivity an experimental study has been performed and a new theoretical model developed
18
. Here, the
formal description of the formation of magneto-optical resonances in alkali-metal atomic vapor is based on a density
matrix approach, treated within the formalism of the Liouville equation. The analysis is focused on the case of EIT in
spectroscopy cells coated with anti-relaxation polymers. One of the key features, of direct interest for COSMA final
goals, is the parametrization of the coating’s efficiency: an empirical coefficient ε is in fact introduced, in order to
quantify the coating’s anti-relaxation properties. The approach allowed the extension of the calculations to many
different degrees of depolarization after atom/wall collisions and, therefore, to a broad range of coatings, including
uncoated cells.
This provides a formal tool to indirectly evaluate the properties of a coating and thus a quantitative criterion to evaluate
its suitability for critical applications, such as ultra-sensitive atomic magnetometry. The investigation reported in Ref.
18
,
in particular, takes into consideration also different configurations for the EIT formation and different efficiency of the
coating characteristics and the atomic energy structure. The model is validated by investigating the EIT with degenerate
Zeeman levels in
39
K D
1
and Cs D
2
lines, which exhibit respectively an almost negligible and a relevant impact of
hyperfine optical pumping. The results are compared to experimental data, exhibiting good agreement and extending the
analysis to the
39
K D
1
line in the case of degenerate and non-degenerate EIT with amplitude-modulated light.
In this way, more insight in the dynamics of EIT in presence of anti-relaxation coating is provided. In detail, with
coatings such as paraffin or polydimethilsiloxane (PDMS), atoms can collide against the cell’s walls without any change
of their internal state. This implies that atoms, already polarized after interaction with the optical pumping beam, can
interact again with the laser beam after bouncing off the wall, without losing their orientation. As a consequence, the
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Frequently asked questions

Q1. What have the authors contributed in "Forty years after the first dark resonance experiment: an overview of the cosma project results" ?

The main objective was to focus theoretical and experimental research on biomagnetism phenomena, with the specific aim to develop all-optical sensors dedicated to their detection and suitable for applications in clinical diagnostics. The paper presents some of the most recent results obtained during the exchange visits of the involved scientists, after an introduction about the phenomenon which is the pillar of this kind of research and of many other new fields in laser spectroscopy, atomic physics, and quantum optics: the dark resonance. 

Q2. What is the influence of alignment on the atomic atoms?

The influence of alignment strongly depends on the configuration of the coupling and probe frequencies as well as on the configuration of the magnetic field. 

Q3. What is the awarded experiment of the last 20 years?

The most awarded experiments of the last 20 years, regarding laser cooling and trapping, have got benefit by dark resonances, for example via the Velocity Selective Coherent Population Trapping mechanism10. 

Q4. What is the effect of the light induction on the polarization of the laser beam?

The ground state coherence is transmitted by the laser field to the upper level; thereby polarization moments with different rank contribute to the fluorescence. 

Q5. What is the goal of the CPT related processes?

One of the possible goal of a fundamental quest of the CPT related processes is the search for new efficient excitation schemes, as well as the comprehension and control of the phenomena involved. 

Q6. What is the main interest of the two axis optical magnetometer?

If the main interest is not an optimized sensitivity, but a wide field measurement and an experimental simplicity, the two axis optical magnetometer of ref. 34 can be a good solution, especially for geophysical and material science studies. 

Q7. What is the effect of the anti-relaxation coating on the fluorescence signal?

In a coated vacuum cell the fluorescence signal has a complex form, because the anti-relaxation coating preserves the created coherence, having different relaxation rates. 

Q8. What is the effect of the light induction on the atoms?

In two-level degenerated system this resonance is due to the interference between transitions involving the Zeeman sub-levels, created by interaction of resonance linear polarized laser beam with the atoms. 

Q9. What is the main reason for the Optical Magnetometer sensitivity problem?

In order to increase the contrast of the coherent resonance that is one of the main parameters determining the Optical Magnetometer sensitivity an experimental study has been performed and a new theoretical model developed18. 

Q10. What is the effect of light in a coated cell?

The Light Induced Atomic Desorption (LIAD) effect21 strongly improves the intensities of the Saturated Absorption resonances observed in coated cell. 

Q11. What are the possible applications of the MIT technique?

other possible applications include detection and imaging of certain classes of tumors, neuropathies and other conditions where the tissues’ conductivity is affected. 

Q12. What is the way to prevent the mixing of the ground state levels?

There are two possible methods in order to both preserve the polarization of the sample as excited by laser light and to eliminate the mixing of the ground state levels due to collision with either the cell walls or with atoms of the same kind: one is to fill the sensor cell with an appropriate buffer gas, the other to coat the cell walls with anti – relaxation coatings. 

Q13. What is the purpose of paper 26?

Paper 26 is the first experimental demonstration that the use of micrometric-thin cells (in the specific case, the length is around 40 µm) allows for application of saturated absorption in order to quantitatively study both the frequency splitting and the shifts of the Rb atomic transitions in a wide range of external magnetic fields, from 0.2 up to 6 kG (20–600 mT). 

Q14. What is the potential of the OAM imaging system at UCL?

The OAM imaging system under development at UCL would directly map the heart’s conductivity, thus providing new insight of the causes of atrial fibrillation and allowing healthcare professionals a dedicated diagnostic tool and more targeted treatments, either drug-based or surgical. 

Q15. What is the effect of a coating on the atoms?

In detail, with coatings such as paraffin or polydimethilsiloxane (PDMS), atoms can collide against the cell’s walls without any change of their internal state. 

Q16. What is the reason for the unusual behavior of the EIT resonances?

Such unusual behavior of the EIT resonances (i.e. the reversal of the sign from DR to BR) is likely caused by the influence of alignment process. 

Q17. What is the effect of the polarization of atoms on the atomic energy structure?

This implies that atoms, already polarized after interaction with the optical pumping beam, can interact again with the laser beam after bouncing off the wall, without losing their orientation. 

Q18. What is the purpose of the paper?

In the paper this approach is proposed as a practical, cost- and time-effective method to assess the performance of an organic coating, without hampering the regular experimental activity.