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 collagen – elastin biopolymer thin films treated by CPA Ti:Sapphire laser (Femtopower – Compact Pro) at 800nm central wavelength with 30fs and 1kHz repetition rate are investigated. A process of surface modifications and microporous scaffold creation after ultrashort laser irradiation has been observed. The single–shot (N=1) and multi–shot (N<1) ablation threshold values were estimated by studying the linear relationship between the square of the crater diameter D2 and the logarithm of the laser fluence F for determination of the threshold fluences for N=1, 2, 5, 10, 15 and 30 number of laser pulses. The incubation analysis by calculation of the incubation coefficient ξ for multi – shot fluence threshold for selected materials by power – law relationship form Fth(N)=Fth(1)Nξ-1 was also obtained. In this paper, we have also shown another consideration of the multi – shot ablation threshold calculation by logarithmic dependence of the ablation rate d on the laser fluence. The morphological surface changes of the modified regions were characterized by scanning electron microscopy to estimate the generated variations after the laser treatment.
The pump-probe spectra in a cell of micrometric thickness containing cesium vapor are reported. The line shape and nonlinear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profiles. We observed Electromagnetically Induced Transparency (EIT) and enhanced Velocity Selective Optical Pumping (VSOP) signals. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams will see much lower Doppler shift of incident frequencies and hence will lead to considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientation of atomic velocities with respect to the laser beam direction. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of the light intensity and temperature change on the non-linear features is reproduced by the simulation.
Second harmonic generation microscopy has recently become an important tool for studying materials. In this article, we use a recently developed analytical method, for second-harmonic generation microscopy, to determine the point group symmetry of micro crystals of enantiomerically pure 1,1’-bi-2-naphtol.
We report experimental and theoretical examinations of the peculiarities in Velocity Selective Optical Pumping (VSOP) resonance behavior at open and closed hyperfine transition spectra of Cs atoms (on the D2 line), confined in optical cell with thickness L = 6λ, where λ = 852 nm. For linear and circular polarizations of the irradiating light, open transitions exhibit reduced absorption (fluorescence) VSOP resonances whose contrast increases with atomic concentration and light intensity. However, in case of closed transition the situation is different, the enhanced absorption (fluorescence) VSOP resonance reverses its sign with the atomic concentration and light intensity. Theoretical analysis based on the density matrix formalism, taking into account the statistical tensors describing atomic population and longitudinal alignment, shows that the VSOP resonance sign reversal at the closed transition can be attributed to the efficiency reduction of population transfer by the spontaneous decay with atomic source temperature.
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.
We demonstrate a bi-chromatic pump-probe spectroscopy comparison between conventional cm-size Cs-filled cell and cells with thickness miniaturized down to 5.2 μm and 700 μm. In the 5.2 μm cell very-good-contrast enhancedtransparency- resonances were obtained that can be efficiently suppressed (controlled) with small about 31 MHz detuning of the pump laser frequency. The resonances do not move their spectral position with the pump frequency detuning like it is in the classical cm-size cells. Miniaturization down to 700 μm does not compromise the enhanced-transparencyresonances. In addition, at that level of miniaturization, the probe beam transmission spectra show as narrow features as in classical cm-size cells. Comparison of the obtained coherent resonances in 700 μm cell with that in 2.5 cm shows no broadening and both have resonance width limited by the emission linewidth of the used pump and probe lasers.
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.
μInterest to the narrow band gap semiconductors for example Pb1-xCdxSe is connected with problem of creation of new
sources of IR radiation. Possibility of control of the properties of polycrystalline films of the lead chemical compounds
by luminescent and Raman methods is considered.
Laser spectroscopy experiments are reported on rubidium atoms by using two or three external cavity diode lasers to
study multiple resonance transitions in Λ- and V-type systems as well as on (Λ+V)-type system. Electromagnetically
induced transparency (EIT) peaks having sub-natural line-widths are found on the Doppler broadened transmission
backgrounds with velocity selective enhanced absorption dips. In the presence of a pump laser, probe spectrum shows
enhancement of EIT signal by tuning the control laser frequency when the Λ- and V-type EIT signals are made to
overlap. Theoretical analysis is carried out by solving the optical Bloch equations for five-level atomic model under Λ
configuration. With the numerically simulated spectra, variation of EIT transmission peak with ground state decay rate
and excited state spontaneous decay rate are investigated. Effect of pump Rabi frequency on the transmission peak is
also shown.
We present the results of an experimental study of Coherent Population Trapping (CPT) in potassium, obtained by means
of modulation of laser light amplitude with kHz frequency. The radiation from an external cavity diode laser, matching
the D1 line of K, is modulated by an acousto-optical modulator. In the cell containing buffer gas, the CPT resonance
width is reduced more than three orders of magnitude as compared to the cell containing pure potassium vapor. In K this
resonance narrowing occurs with high resonance contrast; such behavior is not observed in buffered cells containing Rb
or Cs, where the optical pumping to the non-interacting with the light ground level is very effective and depletes the
population of the working ground Zeeman sublevels. The narrow CPT resonance of reduced fluorescence transforms to
the one of enhanced fluorescence with the cell temperature rising. The transformed resonance exhibits higher contrast
and lower width than those of the reduced fluorescence resonance. Hence, beside its scientific importance the resonance
sign reversal can be used for the improvement of the CPT resonance parameters.
We present the first experimental observation of Coherent Population Trapping (CPT) in Potassium, obtained with kHz-frequency
modulation of the laser light amplitude. It is performed by acousto-optical amplitude modulation of the
radiation from an external cavity diode laser, matching the D1 line of K. The CPT resonances are detected both through
K absorption and fluorescence. The resonances are studied in three kinds of K cells: i) pure-evacuated, ii)
polydimethylsiloxane (PDMS)-coated-evacuated and iii) Ne-gas buffered. In all cases CPT-resonance narrowing with
cell temperature is observed. In the pure-evacuated cell we registered the lowest contrast and the highest width of the
resonance, while in buffered/coated cells a strong enhancement of the CPT resonance contrast up to 15% is observed.
This behavior is the opposite to the one exhibited by Cs and Rb. The observed contrast enhancement in K is
accompanied by more than two orders of magnitude reduction of the resonance width. The results here presented prove
the advantage of using Potassium in CPT-based applications.
A model for description of the shapes of the coherent population trapping (CPT) resonances at different geometries of
excitation and observation is developed. The numerical calculations, based on the irreducible tensor operator formalism,
take into account the experimental geometry, the velocity distribution of the atoms, the Gaussian distribution of the laser
beam intensity and the high rank polarization moments (HRPM). The results for different laser beam diameters, aperture
of the detection, position of the photodetector etc. are compared with the experimental data. A method for description of
the experimental shapes is proposed.
Nonlinear magneto-optical rotation (NMOR) of linearly polarized light, resonant to the Fg = 2 → Fe = 1 atomic transition
of D1 line of 87Rb is investigated experimentally, by applying spatially separated laser fields in the Rb vacuum cell. In
our experiment, the probe laser beam propagates through the center of the pump laser beam, which has the shape of a
hollow cylinder. We have measured the rotation of the probe laser beam polarization with respect to the incident
polarization, as a function of the external magnetic field, parallel to the laser beam propagation. We show substantial
narrowing of the resonance if the pump beam, with the same linear polarization, is present. The opposite direction of the
probe rotation was obtained when the pump laser beam polarization is orthogonal to the probe laser beam polarization.
Absorption and fluorescent spectra are obtained in saturation regime of a single beam laser spectroscopy of Cesium D2
line, in a vapor layer with thickness close to the light wavelength. We compare experimentally and theoretically obtained
spectra distinguishing between open and closed, in terms of optical pumping, hyperfine transitions. In absorption, we
observe a persisting difference between open and closed transitions, in terms of Dicke narrowing, when increasing laser
light intensity. In fluorescence, for open transitions we note saturation dips which do not change significantly when
increasing intensity. In case of closed transition, a small feature at the fluorescence line center appears for relatively high
light intensity.
Numerical simulations are used to obtain the amplitudes and the widths of the nonlinear resonances related to the
different ranks of the tensor components &rgr;kq in dependence on the atomic system parameters, and the laser field power
(Rabi parameter - d.E/ℏ). The main attention is paid to the effect of the velocity distribution of the atoms on the
hexadecapole (k = 4) components and their influence on the fluorescence and/or absorption signals. A detailed
examination of our basic equations allows us to directly find out a relation between different rank components for a
chosen transition and to clarify their role in the observed resonances for a given geometry.
In this communication we present our results related to examination of Coherent Population Trapping resonance profile
on the D1 line of 87Rb when coupling two ground-state Zeeman sublevels belonging to different hyperfine ground-state
levels to a common excited-state one. For this, two coherent laser fields with frequency difference of about 6.8 GHz are
used. The resonance is observed as a narrow dip in the fluorescence in dependence on the modulation frequency (fm) of
the laser light, centered at 2fm = &Dgr;vhf , where &Dgr;vhf is the frequency difference between the ground-state hyperfine levels
of Rb. The resonance observation and its splitting in 7 components have been demonstrated under influence of nonshielded
laboratory magnetic field (MF). It has been shown that different components are with different sensitivity to the
MF gradients that is in agreement with the performed estimations.
Comparative studies of MO resonances and MG responses with and without resonant laser irradiation were
carried out to establish the interconnection of the effects and the influence of the self-alignment destruction on the MO
signal in positive column discharge. These studies aim to clarify the possible formation mechanisms of the neon 1 si
(2p53s) states coherences and to examine the possibility to use the galvanic resonances for estimation of the relaxation
constants of the neon 1 s5 state.
In the present work we consider theoretically the influence of an arbitrary oriented additional magnetic field on the main properties of the Coherent Population Trapping (CPT) resonances in fluorescence and/or in absorption. The exact numerical solutions are obtained for the tensor components Ρk0 (ρ = f, φ) which describe the population and the longitudinal alignment of the resonant (f) levels with quantum numbers Ff 0 and Ff =1, exited with a single frequency laser field from the ground (φ) state with Fφ=l and Fφ=2. Numerical simulations was used to obtain the width and the amplitude dependences of the resonances under different parameters and parasitic fields. The results are in qualitative agreement with resent experimental investigations.
The purpose of this communication is to summarize and compare the results obtained under the project "New all-optical systems and methods for magnetic and electromagnetic field measurement", financed by EC (contract G6RD-CT-2001-000642). The main aim of the project is investigation of the potential of different approaches based on the effect of Coherent Population Trapping (CPT) for magnetic field measurement. The investigated approaches are: (i) CPT prepared by coupling pairs of non-degenerate Zeeman sublevels belonging to the two ground-state hyperfine components of alkali atoms by means of polychromatic laser field; (ii) CPT prepared by coupling degenerate Zeeman sublevels belonging to a single ground-state hyperfine component of alkali atoms by means of single-frequency laser field of appropriate polarization at zero magnetic field (MF); and (iii) polychromatic laser field coupls pairs of non-degenerate Zeeman sublevels belonging to a single ground-state hyperfine level of alkali atoms.
The absorption and fluorescence spectra of a submicron Cs vapour layer under resonant excitation on the D2 line are measured. Sub-Doppler features centered at the resonance frequency of the hyperfine (hf) transitions have been observed. Substantial changes in the amplitude and width of the sub-Doppler resonances for individual hf transitions were recorded in dependence on the intensity of the incident laser radiation. In the fluorescence spectra narrow dips due to the effects of saturation have been observed. We also report about the observation of coherence resonances in Hanle configuration obtained at cell thickness of the order of the wavelength of the laser light.
We have characterized teh spectral properties and wavelength tuning behaviour of different types of diode lasers in order to evaluate their potential for applications in atomic spectroscopy and precision instruments. Here we report on studies of recently developed distributed feedback (DFB) and Fabry-Perot laser diodes, emitting around 780 and 795 nm, in solitary operation as well as in an extended-cavity configuration. In solitary operation both types of laser studied show continuous tuning ranges beyond 40 GHz and single-mode emission linewidths around 6 MHz, which makes them interesting candidates for use in high-precision instruments based on atomic spectroscopy such as atomic frequency standards and atomic magnetometers.
Theoretical model describing the influence of a parasitic magnetic field on CPT-resonance in Hanle configuration in a strong laser field was proposed. The exact solution of the system of equations has been obtained for the transition between a ground state with Jφ=1 and an excited state with Jf=0 and/or 1. The solutions for the tensor components f k0, describing the population and longitudinal alignment of the upper level f, are obtained, and the dependences of the resonances on the different parameters and parasitic fields were analyzed. The results are in qualitative agreement with resent experimental investigations.
In this paper we present a detailed description of a method for reduction of the light shift in laser-pumped gas-cell atomic clocks. The method consists in using a multi-frequency optical pumping obtained through frequency modulation of the laser spectrum with precisely controllable parameters. Experimental evidence of a strong reduction of the frequency dependence of the LS in an optical pumping Rb gas-cell clock is demonstrated in a large frequency interval, which is in a very good agreement with the numerical estimations reported.
We present a systematic quantitative comparison of the performance parameters of two Extended Cavity Diode Lasers (ECDL), stabilized to Doppler and sub-Doppler profiles. The experimental study is carried out on Rb atomic vapour cells. The frequency shift of the resonance under investigation with respect to a reference resonance is measured and analyzed, in dependence on the modulation amplitude, cell temperature, laser power, applied magnetic field and misalignment of the pump-probe beams angle. The Allan variance is measured for both locking methods. It shows that the stabilization to the Doppler profile results in about one order of magnitude lower stability than the one to SA resonances for the short-term measurement. However, the stability achieved using both methods is sufficient for many applications and for some of them the simplicity of the Doppler locking method can be advantageous.
A theoretical model describing magnetic resonance in a strong laser field was proposed. The exact solution of the well known system of equations was solved for the transition between a ground state with Jφ = 1 and an exited state with Jf = 0. It was shown that in the spontaneous emission intensity from the level with Jf = 0 resonant changes will be observed when the frequency ω of the altering magnetic field becomes equal to the Larmor-frequency determined by the constant magnetic field Ho.
A more detailed investigation of optogalvanic signals, induced by resonant laser excitation of Se, Ne and Ar ions in a hollow cathode discharge, is carried out in the present work. An immediate goal was to analyze and verify some hypothesis about the mechanisms, responsible for the formation of such signals, taking into account their dependence on the current characteristics, the kind and pressure of the buffer gas, as well as their radial distribution.
The lower level of laser transition on 632.8 nm (2p4) has been investigated using Hanle absorption resonance method. Advantages of the approach are discussed. Radiation width and collision (pressure and discharge current broadening are determined.
Here we report the proposed by us concept for monitoring the atmospheric and Earth Hg and Pb pollution by means of Intracavity Laser Spectroscopy (ILS). The concept profits from the extremely high sensitivity of the ILS dye laser registration by preliminary excitation of the Hg(Pb) atoms to a suitable level to induce absorption at the analytical transition corresponding to the second dye laser emission spectrum. A theoretical proof is given on the possibility for two-step optical excitation from the ground state level.
We describe (computer analysis and experimental realization) a developed Nd:YAG laser system, capable of emitting a pair of nanosecond pulses with independently controlled delay and intensity. The output parameters of such a laser in low and high flash lamp pump levels are compared. The possibilities to achieve different temporal distances between the two pulses and different energies per pulse are discussed. The build-up time and levels ratio stability are studied. A method for total output energy optimization is proposed. The behavior of the two-pulse laser output after laser amplification and frequency doubling is discussed. Pure two-pulse Nd:YAG output with pulse distance from 0 to 3 microseconds is demonstrated. The possibility to apply this technique to different laser active media have been discussed as well as the different potential areas for application of a dual pulse laser output.
In the present work we report the results of the measurement of the angular distribution of laser light reflected from a rough tooth surface and of Monte Carlo simulation of the 3-D process of photons migration in a two-layered human enamel- dentin structure. The measured reflected patterns are as a whole asymmetric and more spread than the calculated by the simulation. The measured total diffusely reflected power reaches 3 - 20% from the irradiating power. An increase in the total absorbed energy as well as in the spread of absorption is observed below the enamel-dentin interface. The scattering anisotropy affects in a different way the absorption below the tooth surface: the highly forward peaked phase function leads to decrease of absorption just below the air-enamel boundary and rise of absorption below the enamel-dentin boundary.
We report an original approach for tunable subnanosecond pulse (200 - 500 ps) generation in a ring dye laser. It is based on limitation of generation to a single `spike' from the starting transient process using for excitation a quasi- rectangular pulse with duration less or comparable with the time interval between the spikes. The pumping pulse (3 - 7 ns) is formed both by electrooptical division of a standard nanosecond (15 - 50 ns) pulse emitted by a Q-modulated laser in an original scheme and by overlapping of the obtained partial pulses which are fed bi-directionally into the active medium of the ring laser.
In this paper we report a comparative study the amplification and spectral tuning characteristics of an injection seeded Ti:Sapphire laser and multipass (4 passages) amplifiers. We show the advantages of injection seeding method to amplify input pulses of low energy levels (nJ and sub nJ). In the multi-passages amplifiers the wave competition for the two wavelength amplification is low. For high level (mJ) pulses the both systems are comparable, but in the injection locking amplifier the temporal pulse shape distortion is essentially high. The energy and power amplification levels are compared both with the change of the output width. For the reported two new multi-passages schemes we are study the dependence of the characteristics on the input polarization.
We demonstrate how a pulsed two-wavelength tunable Ti:Al2O3 laser can operate at near maximum efficiency, obtained in an optimized non-selective cavity. To achieve this efficiency we solve the intrinsic of the Passive Self- Injection Locking (PSIL) method problem of non-controlled free lasing at high pump level by combining the PSIL control with an appropriate dual-pulse pumping. Laser efficiency is increased and the background emission is limited to less than 1 percent in a region ten times larger compared with the standard PSIL control. From analysis we have found optimum conditions and show the advantages of this combined technique. The proposed approach is compared with spectral control of a Ti:Al2O3 laser, realized by injection from a coupled two-wavelength dye laser.
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