Institute of Chemical Engineering of the Bulgarian Academy of Sciences

Abstract A frequency chain for converting the frequency of an optical clock based on ultracold 87Sr atoms is updated for its comparison with the frequency of microwave standards from the State Primary Standard of time and frequency units and the national time scale, GET 1-2018. The results of the corresponding experiments are reported and analysed. An instrumental complex for reproducing and keeping the time and frequency units and the national time scale of the primary standard is described; this complex includes an optical clock based on strontium atoms and microwave standards of new generation. The order of the atomic time scale generation with application of optical clocks is also determined.

Abstract We investigate displacement measurements of up to 17 μm on a heterodyne laser interferometer laboratory model. The measurement error for small (up to 200 nm) linear displacements is found to be 270 pm at a 10-s averaging time. The results obtained can be used for developing a space laser interferometric system for the global Earth’s gravity field mapping.

Abstract The cooling mechanisms of a Fermionic strontium-87 gas are refined in order to study its evolution under a non-Abelian gauge field. Significant attention is paid to the effect of the finite temperature of the gas on the process in question. The efficiency of the loading of atoms in a cross-dipole trap is described in detail, the quantitative performance of the evaporative cooling is calculated, and a degenerate Fermi gas is characterised using a Thomas – Fermi distribution.

Abstract Using the method of molecular dynamics, the expansion of a two-component, pulsed laser-produced ultracold plasma is directly calculated for various values of the number and density of particles and their electron temperatures. A new method is presented for generating and diagnosing a steady-state ultracold plasma formed under continuous wave laser irradiation. The performed calculations show the difference in the properties of an ultracold plasma obtained by pulsed and continuous wave laser irradiation.

Abstract Compact superluminescent diodes based on AlGaInAs/InP separate-confinement double heterostructures with strain-compensated quantum wells are studied. The characteristics of these diodes that allow their application for development of fibre-optic gyroscopes in a temperature range from – 55 °C – +70 °C are demonstrated. The devices demonstrate an acceptable reliability and a potential for further improvement.

Abstract The physical processes that form the spectra of saturated absorption resonances at atomic transitions between energy levels with angular momenta J = 1/2 and J = 1 in the field of two unidirectional linearly polarised laser waves are studied analytically and numerically. It is shown that the specific features of the resonance spectra are formed in the Λ-schemes of transitions and manifest themselves in the form of narrow coherent structures – dips due to the magnetic coherence (optical orientation) of the transition levels induced by the optical fields. In this case, the energy levels of the lower state make the main contribution, and the contribution of the transfer of magnetic coherence from the sublevels of the upper state to the lower ones in the resonance amplitude manifests itself as an additive. Conditions are found under which the nonlinear resonance is exclusively coherent. The effect of the saturating wave field on the shape of coherent resonance structures is studied.

Abstract The possibility of obtaining superfluid phases for a Fermi gas of dysprosium with a magnetic dipole – dipole interaction is discussed. The obstacles and possible solutions are shown. The required phases are similar to the A1 phase and the polar β phase in 3He. It is expected that in dysprosium, the macroscopic properties of the phase will be determined by the symmetry of the pair interactions. It is assumed to observe the kinetics of phase formation and spontaneous choice between two energy-degenerate phases with different projections of the orbital angular momentum.

Abstract This paper presents a detailed analysis of the effect of the optical lattice field on clock transition spectroscopy, as exemplified by thulium atoms. We consider the applicability of the sifting of atoms in an optical lattice by ramping down the power of the laser light that produces it. This method allows the number of filled vibrational sublevels to be reduced down to a single vibrational state, without changing the inner state of the atoms. The effectiveness of the method is illustrated by the example of the spectroscopy of a clock transition in thulium atoms in the resolved sideband regime.

Abstract Laser ablation of titanium in distilled water at high and low power densities of copper vapour laser beam in the focal spot on a target surface is experimentally studied. Ablation regimes with different pulsed energy inputs and target irradiation times are investigated. The morphology and elemental and phase compositions of micro- and nanoaggregates obtained in different irradiation regimes are studied using electron microscopy, X-ray diffraction analysis, and Raman spectroscopy. The ablation products are found to be practically totally in the X-ray amorphous state.

Abstract We present a brief review of experimental work on the investigation of stimulated low-frequency Raman scattering of light in systems of submicron and nanosized particles of various physical nature.

Abstract The work is dedicated to the further development of a compact quantum frequency standard based on a rubidium gas cell with a mixture of buffer gases. The results of frequency measurements and analysis of short-term frequency instability obtained on a laboratory prototype of a microwave rubidium atomic frequency standard (RAFS) with pulsed optical pumping (POP) are presented. The main in magnitude contributions to the overall frequency instability of the RAFS with POP are estimated. Short-term frequency instability expressed in terms of the Allan deviation and measured at averaging times τ up to several tens of seconds, σy (τ) = 2.5×10−13 τ −1/2, coincides satisfactorily with the calculated value of σy (τ) = 2.1×10−13 τ −1/2.

Abstract We report the development of a cw optical parametric oscillator (OPO) for the mid-IR range. The oscillator uses a 50-mm long periodically poled lithium niobate crystal in a ring resonator. The OPO is pumped by an ytterbium fibre laser with a wavelength of 1.064 μm. At a pump power of 10 W, the OPO generates continuous radiation with a wavelength of 3.6 μm and a power of 2.8 W. The pump power depletion in the OPO is 94 %. The idler wave frequency of the OPO is stabilised using a precision laser wavelength meter. The rms error of the idler wave frequency in the stabilisation regime is 3 MHz. A simple theoretical model for the operation of a cw OPO is proposed, based on an empirical characteristic of the OPO, i.e., the pump depletion, and on a quantum description of three interacting light waves. The power characteristics of the operating OPO are in good agreement with those predicted by the theoretical model.

Abstract We examine the possibilities of refining an asymptotic description and quantitative calculations of the effects induced by thermal blackbody radiation (BBR) of the environment on the Rydberg states of atoms. Numerical values are calculated and asymptotic expressions are proposed for simplified estimates of natural lifetimes and threshold photoionisation cross sections for Rydberg states of rubidium and caesium atoms with large values of the principal quantum number, n ⩾ 20, and small orbital momenta, l = 0, 1, 2, 3. Based on analytical expressions, we present numerical estimates for the contributions of photoionisation probabilities to the BBR-induced broadening of the Rydberg energy level, as well as the contributions of continuum integrals to thermally induced shifts in the Rydberg-state energy levels.

Abstract The temporal dynamics of three-photon 5S1/2 → 5P3/2 → 6S1/2 → 39P3/2 laser excitation of mesoscopic ensembles of cold Rb atoms to Rydberg states in a magneto-optical trap is studied using cw single-frequency lasers at each stage. The ensembles comprise N = 1 – 5 atoms and are detected by the method of selective field ionisation with postselection with respect to the number of atoms. The dependence of the excitation probability on the duration of the exciting laser pulses and the number of detected Rydberg atoms is investigated. At short interaction times, a linear increase in probabilities is observed, and at large times, the probabilities reach saturation, while each number of atoms has its own characteristic features. The experimental dependences are compared with the results of numerical calculations in the framework of a four-level model, and their good agreement is obtained. The conditions necessary for observing Rabi population oscillations are determined. The obtained results are important for the application of Rydberg atoms in quantum information.