Vol 40, No 3 (2019)

We develop a theory that leads to the explanation of the atom manipulations due to active quadrupole interaction of twisted light with atoms at near-resonance frequencies. We demonstrate that the total mechanical effects of active quadrupole interaction on atoms just gives a rotational motion in the case of the configuration of a two counter-propagating beams. Typical parameters are assumed to determine both dissipative and quadrupole forces to obtain the trajectories for an atom.

Quantum key distribution (QKD), ensuring the unconditional security of information, attracts a significant deal of interest. An important task is to design QKD systems as a platform for education as well as for research and development applications and fast prototyping new QKD protocols. Here, we present a modular QKD setup driven by National Instruments (NI) cards with open source LabView code, open source Python code for post-processing procedures, and open source protocol for external applications. An important feature of the apparatus developed is its flexibility offering the possibility to modify optical schemes, as well as prototype, and verify novel QKD protocols. The other distinctive feature of the setup developed is the implementation of the decoy-state protocol, which is a standard tool for secure long-distance quantum communications. By testing the plug-and-play scheme realizing BB84 and decoy-state BB84 QKD protocols, we show that the developed QKD setup shows a high degree of robustness beyond laboratory conditions. We demonstrate the results of the use of the developed modular setup for QKD experiments in the urban environment.

We produce composite ceramic laser elements Nd³⁺:YAG/Cr⁴⁺:YAG using two different methods, i.e., (1) layer-by-layer uniaxial pressing in a metallic mold with sequential addition of appropriate portions of the powder; (2) stacking and pressing of previously uniaxially pressed tablets in a cold isostatic press. In lasers where composite ceramic Nd³⁺:YAG/Cr⁴⁺:YAG elements are used together with longitudinal diode pumping, we obtain the lasing regime for both types of active elements at a wavelength of 1,064 nm in the Q-switch mode. Lasers with ceramic composites produced by stacking and cold isostatic pressing of the previously pressed tablets demonstrate a slope efficiency (~30–33%) even higher than lasers with crystalline saturable absorber (~21%). Lasers based on ceramic composites manufactured by layer-by-layer pressing of powders have an efficiency of ~11–19%. Composites made by stacking and cold isostatic pressing of the previously pressed tablets demonstrate a generation threshold close to those in lasers with crystalline saturable absorbers.

We investigate the conductivity of a composite of PANI-PAMPSA and PANI-PAMPSA with carbon particles by impedance spectroscopy. We establish that the addition of carbon particles to the PANI-PAMPSA composite leads to a significant rise in the conductivity over the entire range of frequencies studied (from 10 Hz to 5 MHz). Such increase in conductivity is very important for using the PANI-PAMPSA composite in solar energy. We propose an equivalent electrical circuit of the samples investigated. At low frequencies, the conductivity dependence on frequency indicates the hopping mechanism of the charge carrier transport.

We designed a compact and efficient high-power laser that generates chaotic green light by perturbing a 520 nm laser diode with optical feedback. This achieved a continuous output power of 30 mW with a bandwidth of more than 1 GHz and an electrical-to-green conversion efficiency of 6.25%. All possible chaotic states were demonstrated for different diode-driving currents and feedback intensities. The FWHM and the peak side-lobe level (PSL) are affected by the injection current and feedback intensity. The FWHM increases with increasing feedback intensity, and the PSL increases with both injection current and feedback intensity. The compact chaotic laser could be used in underwater detection to suppress backscattering because of its intrinsic intensity modulation at high frequency. We finally found the optimum range of chaotic states for underwater detection, for which the ratio of input current to threshold current is 1.5 : 2.1.

We studied the Raman scattering in bulk and four-monolayer thick MoS₂ films under the resonance laser excitation of A, B, and C excitons. We demonstrate the fine structure agility of a phonon mode associated with out-of-plane vibration of sulfur atoms (A_1g) when passing from bulk to atomically thin film. The C exciton resonance excitation allows us to register the line corresponding to optical-phonon (E_1g) scattering and the set of peaks with a 660 – 740 cm⁻¹ Stokes shift associated with resonance scattering by two optical (E_u) phonons.

We studied numerically the frequency doubling of femtosecond laser pulses (up to ∼5 fs) in nonlinear photonic crystals. We investigated the impact of a number of factors (phase modulation of the primary emission pulse and spatial modulation of nonlinear susceptibility, up to the third order dispersion of a nonlinear photonic crystal) on the efficiency of nonlinear conversion and the duration of the secondharmonic pulse. We show that the efficiency of second-harmonic generation reaches η_2ω∼ 45% when ultrashort (down to 5 fs) laser pulses are used.