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This is an author produced version of a paper published in:
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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