- Sofia, Grad Sofiya, Bulgaria
Ivan Stefanov
Sofia University "St. Kliment Ohridski", Physics, Faculty Member
The photo-excited luminescence response of solutions (in organic solvent) of eight selected organometallic coordination complexes of Iridium Ir(III) ion is studied upon irradiation with high-intensity short-pulse (nanosecond) ultraviolet... more
The photo-excited luminescence response of solutions (in organic solvent) of eight selected organometallic coordination complexes of Iridium Ir(III) ion is studied upon irradiation with high-intensity short-pulse (nanosecond) ultraviolet laser irradiation, as well as by ultra-high-intensity ultrashort (femtosecond) laser pulses in the visible (violet). The Ir-complexes are cyclometalated and contain bis-phenylbenzothiazole as a main ligands and auxiliary β-diketone ligands, and they are intended to be used as luminescent materials and sensitive nanoprobes, e.g., in organic optoelectronics, sensorics, mechatronics and laser fluorescent microscopy. The results obtained by laser spectroscopy of dilute (10–5 M) solutions of the examined Ir-complexes show that they are efficient triplet emitters whose photo-excited emission is proper for fluorometric, optoelectronic and photonic applications.
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In this work we report experimental measurement of an intentionally introduced pulse front tilt on femtosecond laser pulses by using an inverted field correlator/interferometer. The results obtained with a low-dispersion diffraction... more
In this work we report experimental measurement of an intentionally introduced pulse front tilt on femtosecond laser pulses by using an inverted field correlator/interferometer. The results obtained with a low-dispersion diffraction grating are in good qualitative agreement with the data from a previously developed analytical model and with these from an independent interferometric measurement.
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We report the first observation of excimer laser action in the near ir region in Na2 b3∑+g(v' = 0) å x3∑+u bound -free transition centered at 830 nm. It was optically excited by a pulsed dye laser. A mechanism of collisional... more
We report the first observation of excimer laser action in the near ir region in Na2 b3∑+g(v' = 0) å x3∑+u bound -free transition centered at 830 nm. It was optically excited by a pulsed dye laser. A mechanism of collisional energy transfer is proposed for the population of the Na2 upper laser state.
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We investigated laser beam reflection by optically-transparent hydrocarbon polymeric material subjected to low-energy ion implantation. In our study were measured 5 mm-thick plates of polymethylmethacrylate (PMMA) implanted with silicon... more
We investigated laser beam reflection by optically-transparent hydrocarbon polymeric material subjected to low-energy ion implantation. In our study were measured 5 mm-thick plates of polymethylmethacrylate (PMMA) implanted with silicon ions (Si+) accelerated to a relatively low energy of 50 keV at Si+ fluence of 1016 ions/cm2. Due to ion-produced destruction of the polymer network and reorganization of the organic structure in a depth of PMMA plate, a buried ion-modified layer with ~ 80 nm thickness is formed within the PMMA. The material in this ultra-thin nano-sized layer beneath the top surface of PMMA (in a depth of ~ 100 nm) is organized in carbon nanoclusters with a mean size of about 2 - 3 nm. The nonlinear gradual in-depth distribution of the complex refractive index in the nanostructured region of the polymer results in a significant modification of the reflection of coherent light by the formed structure. Also, the reflected beam undergoes a strong laser-induced photothermal interface lensing upon irradiation by CW laser ( λ= 532 nm) with an optical power above 30 mW. The presence of thermo-lenses can limit the optical and photonic applications of transparent ion-implanted polymers. Here, this negative effect is studied as dependent on both incident laser power and incidence angle of the laser beam.
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Collisional energy transfer between atoms and molecules proved to be an efficient method for excitation of new laser lines and spectroscopy of states forbidden for direct excitation or emission by parity conservation [1]. We have studied... more
Collisional energy transfer between atoms and molecules proved to be an efficient method for excitation of new laser lines and spectroscopy of states forbidden for direct excitation or emission by parity conservation [1]. We have studied a number of stimulated emissions in the near IR 800 - 850nm in Na atom via Na2X—A transitions. Two tunable dye lasers have been employed, the first one scanning the X—A molecular transition and the second one tuned to Na 3p-4d single photon resonance, Fig.1. Taking the collision rate coefficient for the reaction
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In the present work we show experimentally and by numerical calculations a substantial far-field beam reshaping by mixing square-shaped and hexagonal optical vortex (OV) lattices composed of vortices with alternatively changing... more
In the present work we show experimentally and by numerical calculations a substantial far-field beam reshaping by mixing square-shaped and hexagonal optical vortex (OV) lattices composed of vortices with alternatively changing topological charges. We show that the small-scale structure of the observed pattern results from the OV lattice with the larger array node spacing, whereas the large-scale structure stems from the OV lattice with the smaller array node spacing. In addition, we demonstrate that it is possible to host an OV, a one-dimensional, or a quasi-two-dimensional singular beam in each of the bright beams of the generated focal patterns. The detailed experimental data at different square-to-hexagonal vortex array node spacings shows that this quantity could be used as a control parameter for generating the desired focused structure. The experimental data are in excellent agreement with the numerical simulations.
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On-demand generation and reshaping of arrays of focused laser beams is highly desired in many areas of science and technology. In this work, we present a versatile approach for laser beam structuring in the focal plane of a lens by triple... more
On-demand generation and reshaping of arrays of focused laser beams is highly desired in many areas of science and technology. In this work, we present a versatile approach for laser beam structuring in the focal plane of a lens by triple mixing of square and/or hexagonal optical vortex lattices (OVLs). In the artificial far field the input Gaussian beam is reshaped into ordered arrays of bright beams with flat phase profiles. This is remarkable, since the bright focal peaks are surrounded by hundreds of OVs with their dark cores and two-dimensional phase dislocations. Numerical simulations and experimental evidences for this are shown, including a broad discussion of some of the possible scenarios for such mixing: triple mixing of square-shaped OVLs, triple mixing of hexagonal OVLs, as well as the two combined cases of mixing square-hexagonal-hexagonal and square-square-hexagonal OVLs. The particular ordering of the input phase distributions of the OV lattices on the used spatial l...
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The luminescence response of a new coordination complex of europium Eu(III) ion, namely tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono]Eu(III) 1,4-diaza-bicyclo[2.2.2]octane, applicable in microbiology, biochemistry and molecular... more
The luminescence response of a new coordination complex of europium Eu(III) ion, namely tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono]Eu(III) 1,4-diaza-bicyclo[2.2.2]octane, applicable in microbiology, biochemistry and molecular biology, is probed upon ultra-high intensity photo-excitation with high-power femtosecond laser pulses. The use of laser light source with ultrashort and high-power pulses should increase by many orders of magnitude the intensity of photoexcitation, and therefore the efficiency of
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We demonstrate an alternative approach for generating zeroth- and first-order long range non-diffracting Gauss–Bessel beams (GBBs). Starting from a Gaussian beam, the key point is the creation of a bright ring-shaped beam with a large... more
We demonstrate an alternative approach for generating zeroth- and first-order long range non-diffracting Gauss–Bessel beams (GBBs). Starting from a Gaussian beam, the key point is the creation of a bright ring-shaped beam with a large radius-to-width ratio, which is subsequently Fourier-transformed by a thin lens. The phase profile required for creating zeroth-order GBBs is flat and helical for first-order GBBs with unit topological charge (TC). Both the ring-shaped beam and the required phase profile can be realized by creating highly charged optical vortices by a spatial light modulator and annihilating them by using a second modulator of the same type. The generated long-range GBBs are proven to have negligible transverse evolution up to 2 m and can be regarded as non-diffracting. The influences of the charge state of the TCs, the propagation distance behind the focusing lens, and the GBB profiles on the relative intensities of the peak/rings are discussed. The method is much mor...
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We study the time evolution of the radial perturbation for self-gravitating soliton and black-hole solutions in a generalized Skyrme model in which a dilaton is present. The background solutions were obtained recently by some of the... more
We study the time evolution of the radial perturbation for self-gravitating soliton and black-hole solutions in a generalized Skyrme model in which a dilaton is present. The background solutions were obtained recently by some of the authors. For both the solitons and the black holes two branches of solutions exist which merge at some critical value of the corresponding parameter. The results show that, similar to the case without a scalar field, one of the branches is stable against radial perturbations and the other is unstable. The conclusions for the linear stability of the black holes in the generalized Skyrme model are also in agreement with the results from the thermodynamical stability analysis based on the turning point method.
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ABSTRACT We present experimental results on the evolution of ring dark solitary waves and optical vortices generated by computer-synthesized holograms. The detailed comparative numerical simulations show, that this approach ensures... more
ABSTRACT We present experimental results on the evolution of ring dark solitary waves and optical vortices generated by computer-synthesized holograms. The detailed comparative numerical simulations show, that this approach ensures reproduction of the correct intensity and phase portrait of the dark waves at the entrance of the nonlinear medium. The transverse dynamics at both even and odd initial conditions is studied and compared with the theory and simulations.
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ABSTRACT We study by computer simulations the initial stage of bright background beam self-focusing initiated by the energy density redistribution due to the presence of optical vortex and/or ring dark wave. Local self-focusing Kerr... more
ABSTRACT We study by computer simulations the initial stage of bright background beam self-focusing initiated by the energy density redistribution due to the presence of optical vortex and/or ring dark wave. Local self-focusing Kerr nonlinear medium is considered. When a single ring dark wave is nested on the background, ring radius-to-width ratio Δ=2 promises up to 4 times peak intensity increase at a propagation distance of 2 dark beam diffraction lengths. Δ=6 seems adequate when flat-toped super-Gaussian beam is desired. Self-focusing in bright rings of different radii and even in two coaxial rings (at Δ=3) is observed when initially optical vortex and ring dark wave are simultaneously nested on the background. The detailed numerical analysis of the evolution of azimuthal perturbations confirmed the physical intuition that self-focusing rings of small radii suffer much less (when at all) from ring filamentation, because the spatial frequency of the perturbations on the inner rings appear higher than the critical one.
A strong laser-induced thermo-lens (LITL) effect is found in optically-transparent ion-implanted polymer upon irradiation by a cw laser with a power up to 100 mW (lambda = 532 nm). The effect is observed in bulk polymethylmethacrylate... more
A strong laser-induced thermo-lens (LITL) effect is found in optically-transparent ion-implanted polymer upon irradiation by a cw laser with a power up to 100 mW (lambda = 532 nm). The effect is observed in bulk polymethylmethacrylate (PMMA) implanted with silicon ions (Si+). A series of PMMA specimens is examined, subjected to low-energy (50 keV) Si+ implantation at various dosages in
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... S G Dinev, IG Koprinkovt and IL Stefanov Department of Physics, Sofia University, BG-1126Sofia, Bulgaria ... Muller and Hertel (1981) and Wang et a1 (1984) have recently reported IR laser lines from two-photon pumping of high-lying... more
... S G Dinev, IG Koprinkovt and IL Stefanov Department of Physics, Sofia University, BG-1126Sofia, Bulgaria ... Muller and Hertel (1981) and Wang et a1 (1984) have recently reported IR laser lines from two-photon pumping of high-lying Naz and Kz states. ...
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The luminescence of four europium thenoyltrifluoroacetonate complexes containing diamine ligands is studied in solutions. The compounds are resonantly excited in the ligand absorption band by means of high-power nanosecond UV laser... more
The luminescence of four europium thenoyltrifluoroacetonate complexes containing diamine ligands is studied in solutions. The compounds are resonantly excited in the ligand absorption band by means of high-power nanosecond UV laser pulses. Additionally, a continuous light source is also used for resonant, as well as pre-resonant excitation of the complexes. The luminescence response characterizes the synthesized europium coordination complexes as emissive compounds suitable for fluorometric applications. Related to the possible usefulness of the diamine-liganded Eu complexes as luminescent labels for sensing and imaging of biological molecules, the luminescence response of these compounds upon UV excitation was probed also in aqueous media.
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A wavefront sensor has been used to measure the Kerr nonlinear focal shift of a high intensity ultrashort pulse beam in a focusing beam geometry while accounting for the effects of plasma-defocusing. It is shown that plasma-defocusing... more
A wavefront sensor has been used to measure the Kerr nonlinear focal shift of a high intensity ultrashort pulse beam in a focusing beam geometry while accounting for the effects of plasma-defocusing. It is shown that plasma-defocusing plays a major role in the nonlinear focusing dynamics and that measurements of Kerr nonlinearity and ionization are coupled. Furthermore, this coupled effect leads to a novel way that measures the laser ionization rates in air under atmospheric conditions as well as Kerr nonlinearity. The measured nonlinear index n₂ compares well with values found in the literature and the measured ionization rates could be successfully benchmarked to the model developed by Perelomov, Popov, and Terentev (PPT model) [Sov. Phys. JETP 50, 1393 (1966)].
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ABSTRACT An efficient 6S-4P and 4D-4P stimulated emission in K is reported in a two-step process, involving dipole forbidden stimulated electronic Raman scattering. The Raman gain has been used to measure the quadrupole matrix element... more
ABSTRACT An efficient 6S-4P and 4D-4P stimulated emission in K is reported in a two-step process, involving dipole forbidden stimulated electronic Raman scattering. The Raman gain has been used to measure the quadrupole matrix element mu4S,3D =5.1 X 10-4 au.
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... HETERONUCLEAR MOLECULE SG DINEV, GB HADJICHRISTOV and IL STEFANOV Department of Physics, Sofia University, BG-1126, Sofia, Bulgaria Received ... determined by Janson and Klavins [ 1 ] by measuring the cross section for excitation... more
... HETERONUCLEAR MOLECULE SG DINEV, GB HADJICHRISTOV and IL STEFANOV Department of Physics, Sofia University, BG-1126, Sofia, Bulgaria Received ... determined by Janson and Klavins [ 1 ] by measuring the cross section for excitation energy transfer from Naz(B'II ...
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A coherent emission corresponding to the K(6P-4S) transition at 344 nm has been observed under two-wavelength excitation of potassium atomic vapor. The nonlinear origin of the signal is confirmed by divergence, forward-to-backward and... more
A coherent emission corresponding to the K(6P-4S) transition at 344 nm has been observed under two-wavelength excitation of potassium atomic vapor. The nonlinear origin of the signal is confirmed by divergence, forward-to-backward and temporal measurements. The analysis shows that a five-wave mixing process is responsible, involving additional Stokes waves, generated in the medium.
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ABSTRACT Optical six-wave mixing via two forbidden transitions has been studied in the potassium atom by resonant interaction with two tunable dye lasers. Despite the two quadrupole resonances, one in excitation and one in emission, the... more
ABSTRACT Optical six-wave mixing via two forbidden transitions has been studied in the potassium atom by resonant interaction with two tunable dye lasers. Despite the two quadrupole resonances, one in excitation and one in emission, the non-linear optical susceptibility responsible is significantly enhanced. Comparison is made with other forbidden non-linear schemes, realized by the same two-wavelength resonant excitation.
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ABSTRACT The authors have studied extensively the stimulated emission at 691.1 nm and 693.9 nm identified as 6S-4P1/2, 3/2 transitions in potassium. The origin of the emission is thought to be four-wave parametric emission induced by... more
ABSTRACT The authors have studied extensively the stimulated emission at 691.1 nm and 693.9 nm identified as 6S-4P1/2, 3/2 transitions in potassium. The origin of the emission is thought to be four-wave parametric emission induced by hybrid transitions. The stimulated emission is enhanced by cascading transitions, the presence of nearby atomic states, stimulated electronic Raman scattering and molecular background signal. Excitation, absorption and emission measurements indicate that bound-bound NaK(X-B) transitions play a dominant role. The rate of energy transfer NaK to K is evaluated as kET approximately=2.3*10-9 cm3s-1.
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ABSTRACT We report on the observation of phase-sensitive Rayleigh backscattering from a single-mode fiber excited by light pulses obtained from a highly coherent single-frequency laser diode and an Erbium doped fiber amplifier (EDFA). The... more
ABSTRACT We report on the observation of phase-sensitive Rayleigh backscattering from a single-mode fiber excited by light pulses obtained from a highly coherent single-frequency laser diode and an Erbium doped fiber amplifier (EDFA). The laser diode stability was tested using a highly imbalanced fiber optic Mach-Zehnder interferometer. The CW laser light was first modulated using fiber-optic EO modulator which formed 100ns to 500 ns pulses that correspond to 20 to 100 m of pulse length in the fiber. The backscattered light power is estimated to be about −51 dB lower than the launched power at the input and about N=500 averages are needed for a sensing length of Ls = 40 km to be in the useful dynamic range.
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A stimulated emission at 1.25 micron has been studied under excitation of potassium vapor via K(4S - 5S) magnetic-dipole transition. A chain mechanism of amplification, including a magnetic-dipole Raman scattering and an electric-dipole... more
A stimulated emission at 1.25 micron has been studied under excitation of potassium vapor via K(4S - 5S) magnetic-dipole transition. A chain mechanism of amplification, including a magnetic-dipole Raman scattering and an electric-dipole four-wave parametric emission, has been introduced to model this emission.
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ABSTRACT Optical interferometric technique is applied to characterize the nonlocal response of optically transparent ion implanted polymers. The thermal nonlinearity of the ion-modified material in the near-surface region is induced by... more
ABSTRACT Optical interferometric technique is applied to characterize the nonlocal response of optically transparent ion implanted polymers. The thermal nonlinearity of the ion-modified material in the near-surface region is induced by continuous wave (cw) laser irradiation at a relatively low intensity. The interferometry approach is demonstrated for a subsurface layer of a thickness of about 100 nm formed in bulk polymethylmethacrylate (PMMA) by implantation with silicon ions at an energy of 50 keV and fluence in the range 1014–1017 cm−2. The laser-induced thermooptic effect in this layer is finely probed by interferometric imaging. The interference phase distribution in the plane of the ion implanted layer is indicative for the thermal nonlinearity of the near-surface region of ion implanted optically transparent polymeric materials.
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... SG Dinev and IL Stefanov Department of Physics, Sofia University, BG-1126 Sofia, Bulgaria ... assisted tran-sitions was reported in sodium-argon mixtures [5]. In a recent paper a parametric emission in the uv was reported by... more
... SG Dinev and IL Stefanov Department of Physics, Sofia University, BG-1126 Sofia, Bulgaria ... assisted tran-sitions was reported in sodium-argon mixtures [5]. In a recent paper a parametric emission in the uv was reported by collisional energy transfer between Naz and Na [6]. In ...
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The optical reflectivity (both specular and off-specular) of poly(methyl methacrylate) (PMMA) implanted with silicon ions (Si+) at energy of 50keV, is studied in the spectral range 0.25–25μm. The effect from the Si+ implantation on the... more
The optical reflectivity (both specular and off-specular) of poly(methyl methacrylate) (PMMA) implanted with silicon ions (Si+) at energy of 50keV, is studied in the spectral range 0.25–25μm. The effect from the Si+ implantation on the reflectivity of two PMMA materials is examined in the dose range from 1014 to 1017ions/cm2 and is linked to the structure formed in this ion
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The luminescence response of a new coordination complex of europium Eu(III) ion, namely tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono]Eu(III) 1,4-diaza-bicyclo[2.2.2]octane, applicable in microbiology, biochemistry and molecular... more
The luminescence response of a new coordination complex of europium Eu(III) ion, namely tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono]Eu(III) 1,4-diaza-bicyclo[2.2.2]octane, applicable in microbiology, biochemistry and molecular biology, is probed upon ultra-high intensity photo-excitation with high-power femtosecond laser pulses. The use of laser light source with ultrashort and high-power pulses should increase by many orders of magnitude the intensity of photoexcitation, and therefore the efficiency of
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ABSTRACT In this work, we show both experimentally and by numerical simulations that the presence and evolution of a ring dark beam and/or an on-axis optical vortex nested on a bright background beam noticeably perturb the host... more
ABSTRACT In this work, we show both experimentally and by numerical simulations that the presence and evolution of a ring dark beam and/or an on-axis optical vortex nested on a bright background beam noticeably perturb the host background. In a photorefractive nonlinear medium (crystal SBN) these perturbations can initiate self-focusing of the background. By changing the dark ring radius and the presence of an optical vortex and keeping all other experimental parameters unchanged, we can control the dynamics at the initial stage of longitudinal self-focusing and the type of self-focusing structure (single peak or bright ring of variable radius). The presented results may appear especially important in experiments that involve cascaded nonlinear frequency mixing of singular beams, in which accelerated dark beam spreading is accompanied by self-focusing of certain portions of the perturbed host beam.