In this communication, we demonstrate a new use of the micrometric (700 μm) thick, high-quality optical cell filled with Cs vapor that intrinsically contains extremely small concentration of Rb atoms. When heated to temperature about 200 °C such alkali mixture consists of Cs2 dimers and Cs atoms together with very small number of Rb atoms. We use the Rb atomic spectrum as a reference one, in order to measure spectral profiles and frequency positions of the observed spectral lines of Cs2 molecules within the B1Пu←X1Σg+ absorption band in the spectral region around the D2 resonance line of Rb with wavelength λ = 780.24 nm.
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.
The fluorescence and non-linear absorption spectra of Cs133 vapour in an extremely thin cell were calculated by using the perturbation theory with respect to the pumping field intensity. The problem of the atoms’ non-linear polarization was solved for arbitrary values of the total momenta of the resonance levels pumped by a linearly-polarised laser field. It was demonstrated that the spontaneous emission from the upper level affects the amplitude and sign of the lower level longitudinal alignment and results in a change of the amplitude and sign of the non-linear absorption resonance at the closed transition. The comparison of the numerical calculations with the experimental data for the D2 line of 133Cs shows a very good agreement.
A narrowband R-type resonance is formed in a Λ-system, on the D1 line of Rb atomic vapor using two continuous diode lasers with λ=795 nm. A 8mm- long cell filled with the Rb vapor and 20 Torr neon gas has been used. We have shown that use of an additional (3rd) laser which is resonant with the Rb D2 line (λ=780 nm) makes it possible to control the amplitude and sign of the R -type resonance, i.e. to convert a resonance which demonstrates increase in absorption into one which demonstrates reduction in absorption. 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. A description in terms of double-Λ systems allows us to explain the experimental results in a simple manner.
We report simultaneous observation of the N- and EIT- resonances formation using multi-region (MR) cell, which contains micrometric –thin region with the thickness L varying in the range of 1μm - 90μm. MR cell is filled with natural Rb and 150 Torr neon gas. To form N- and EIT- resonances in Λ-system, two lasers are used: the probe (with tunable frequency), and the coupling (with the fixed frequency) with λ ≈ 795 nm wavelength and 1MHz- line-width. Although, the best parameters of N-resonance could be obtained for a cell thickness about 1cm, the use of MR cell with the thickness 30-40 μm still allows to obtain a good contrast and a narrow line-width. The N-resonance demonstrates subnatural increase of the probe absorption and is formed when probe frequency νP1 is in resonance with the 85Rb transition Fg=2 →5P1/2, while for the coupling frequency the condition υC = υP1 + Δ1 (Δ1=3036 MHz) is fulfilled. The EITresonance is formed when νP2 scans the transition 87Rb, Fg=2 →5P1/2 and for the same coupling frequency the condition υC = υP2 +Δ2 (Δ2=6835 MHz) should fulfill. The frequency separation between N- and EIT-resonances is equal to Δ2-Δ1≈ 3.8 GHz. We also detected the low-frequency beating signal when the condition υC ≈ νP' is fulfilled, which allows a directly determine the coupling frequency υC. Splitting of N-resonance in the external longitudinal magnetic field into five components is detected. Possible applications are addressed.
We present here the behavior of Electromagnetically Induced Transparency (EIT), Velocity Selective Optical Pumping (VSOP) resonances and Velocity Selective Excitation (VSE) resonances observed in Cs vapor confined in а micrometric optical cell (MC) with thickness L = 6λ, λ = 852nm. For comparison of behavior of VSE resonance another conventional optical cell with thickness L=2.5 cm is used. Cells are irradiated in orthogonal to their windows directions by probe beam scanned on the Fg = 4 → Fe= 3, 4, 5 set of transitions and pump beam fixed at the Fg = 3 → Fe = 4 transition, on the D2 line of Cs. The enhanced absorption (fluorescence) narrow VSOP resonance at the closed transition transforms into reduced absorption (fluorescence) one with small increase of atomic concentration or light intensity. A striking difference appears between the VSE resonance broadening in L = 6λ and conventional L = 2.5cm cells.
The so-called "λ/2-Zeeman technique" (HLZT) for studies of individual optical transition between Zeeman sublevels of
atomic hyperfine structure in an external magnetic field of B = 10 - 2500 G is presented. Particularly, implementation of
HLZT allows one to realize a direct determination of frequency shift and a strong modification of an optical transition
probability between Zeeman components in a B-field. The main advantages of the method compared to "λ-Zeeman
technique" (LZT) is that it allows one to study weak transitions. Particularly, with the help of fluorescence on 87Rb, D2
line, Fg = 1 → Fe = 3 transitions, three "forbidden" transitions in magnetic field B are detected and studied. Also, on 87Rb
D1 line, Fg = 1, mF = 0 → Fe = 1, mF = 0 "forbidden" transition is detected when B ~ 400 G. A strong modification of
the probability for these "forbidden" transitions is revealed. The theoretical model well describes the observed results.
Comparison of absorption and fluorescence in a nano-cell containing Rb vapor with other Rb nano-cells with addition of
neon gas is presented. It is shown that the effect of collapse and revival of Dicke-type narrowing occurs for Rb nanocells
containing N2 as buffer gas under 6 and 20 Torr pressure for the thickness L = λ /2 and L = where λ is the resonant λ,
laser wavelength 794 nm (D1 line). Particularly for 6 Torr the line-width of the transmission spectrum for the thickness L
=λ/2 is 2 times narrower than that for L = λ. For an ordinary Rb cell with L = 0.1 - 10 cm with addition of buffer gas, the
velocity selective optical pumping/saturation (VSOP) resonances in saturated absorption spectra are fully suppressed
when the buffer gas pressure > 0.5 Torr. A spectacular difference is that for L = λ, VSOP resonances located at the
atomic transitions are still observable even when Ne pressure is ≥ 6 Torr. Narrowband fluorescence spectra of a nano-cell
with L = λ/2 can be used as a convenient tool for online buffer gas pressure monitoring for the conditions when ordinary
pressure gauges are unusable. Comparison of electromagnetically induced transparency (EIT) effect in a nano-cell filled
with pure (without a buffer gas) Rb with another nano-cell, where buffer gas nitrogen is added, is presented. The use of
N2 gas inside Rb nano-cells strongly extends the range of coupling laser detunings in which it is still possible to form
EIT resonance.
In this communication we report the first observation of a narrow, reduced fluorescence dip in the profile of the
completely closed transition on the D2 line of 133Cs vapor, confined in Extremely Thin Cell with nanometric thickness.
The theoretical modeling of the fluorescence based on the Optical Bloch Equation for two-level atomic system, shows
that the narrow dip in the fluorescence could be attributed to a very small loss in the excitation process of the examined
degenerate transition. While the population in the atomic system remains constant, the depolarization of the excited level
can lead to some loss in the efficiency of the optical transition excitation. Under the conditions of our experiment, no dip
in the fluorescence is registered for the cell thickness where a well pronounced Dicke peak in the absorption takes place.
For the cells with nanometric thickness, the previous investigations demonstrate that the fluorescence profiles of optical
transitions differ significantly from the absorption profiles. However, our experiment shows that some traces of the
coherent Dicke process contributing to the absorption line still remain in the fluorescence, which is result of non-coherent
processes.
Recently developed thin cells containing atomic vapor of micrometric column thickness L allow one to study
peculiarities of Electromagnetically Induced Transparency (EIT) phenomenon, along with the accompanying velocity
selective optical pumping/saturation (VSOP) resonances for the case when < 100 &mgr;m. The micrometric thin cells (MTC)
are filled with pure Rb and neither buffer gas nor paraffin-coated walls were used. The Λ-systems on D2 line of 85Rb
have been studied experimentally with the use of bi-chromatic radiation of two separate diode lasers (λ≈780 nm, γL&≈ 5MHz). It is demonstrated that when L~60 &mgr;m it is still possible to form the EIT resonance with the sub-natural
linewidth ~ 5 MHz. The EIT resonance linewidth increases up to 10 MHz , when column thickness L is reduced down to
~ 2 &mgr;m. Six VSOP resonances are detected in the fluorescence and absorption spectra when the thickness L ~ 10 &mgr;m.
Dependence of the EIT resonance linewidth as a function of the atomic vapor column thickness L is presented.
Electromagnetically induced transparency (EIT) resonance, along with peculiarities of the accompanying velocity selective optical pumping/saturation (VSOP) resonances, have been studied using a thin cell with the thickness of Rb vapor column varying in L = 10-700 &mgr;m range, as well as using an extremely thin cell (ETC) with the thickness varying in the range of radiation wavelength &lgr; = 780 nm (L = 0.5&lgr; - 2.5&lgr;). The &Lgr;-systems on D2 line of 85Rb and 87Rb have been studied experimentally with the use of bichromatic radiation of two separate diode lasers. It is demonstrated that size-conditioned strongly anisotropic contribution of atoms with different velocities in ETC results in several dramatic distinctions in formation of EIT and VSOP resonances in ETC, as compared with ordinary cm-size cell. The boundary between these two regimes falls on L ~ 10 &mgr;m. Theoretical model taking into account the peculiarities of the transmission spectra as a function of the ratio L/&lgr; is developed. The experimental transmission spectra are well described by the theoretical model.
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