Elastic Gallium Phosphide Nanowire Optical Waveguides-Versatile Subwavelength Platform for Integrated Photonics.

Emerging technologies for integrated optical circuits demand novel approaches and materials. This includes a search for nanoscale waveguides that should satisfy criteria of high optical density, small cross-section, technological feasibility and structural perfection. All these criteria are met with self-assembled gallium phosphide (GaP) epitaxial nanowires. In this work, the effects of the nanowire geometry on their waveguiding properties are studied both experimentally and numerically. Cut-off wavelength dependence on the nanowire diameter is analyzed to demonstrate the pathways for fabrication of low-loss and subwavelength cross-section waveguides for visible and near-infrared (IR) ranges. Probing the waveguides with a supercontinuum laser unveils the filtering properties of the nanowires due to their resonant action. The nanowires exhibit perfect elasticity allowing fabrication of curved waveguides. It is demonstrated that for the nanowire diameters exceeding the cut-off value, the bending does not sufficiently reduce the field confinement promoting applicability of the approach for the development of nanoscale waveguides with a preassigned geometry. Optical X-coupler made of two GaP nanowires allowing for spectral separation of the signal is fabricated. The results of this work open new ways for the utilization of GaP nanowires as elements of advanced photonic logic circuits and nanoscale interferometers.

Spectral narrowing and broadening of Cr:ZnS/Se laser oscillation due to mode competition and spatial hole burning in the gain element.

In this paper, we demonstrate the laser characterization of Cr:ZnS/Se polycrystalline gain media in non-selective unpolarized, linearly polarized, and twisted mode cavities. Lasers were based on post-growth diffusion-doped, commercially available antireflective-coated Cr:ZnSe and Cr:ZnS polycrystals with a length of 9 mm. The spectral output of lasers based on these gain elements in non-selective unpolarized and linearly polarized cavities was measured to be broadened to ∼20-50 nm due to the spatial hole burning (SHB) effect. SHB alleviation in the same crystals was realized in the "twisted mode" cavity, with linewidth narrowing to ∼80-90 pm. Both broadened and narrow-line oscillations were captured by adjusting the orientation of intracavity waveplates with respect to facilitated polarization.

Nanoscale Electric Field Probing in a Single Nanowire with Raman Spectroscopy and Elastic Strain.

In this work we investigate the Raman response of extremely strained gallium phosphide nanowires. We analyze new strain-induced spectral phenomena such as 2-fold and 3-fold phonon peak splitting which arise due to nontrivial internal electric field distribution coupled with inhomogeneous strain. We show that high bending strain acts as a probe allowing us to define the electric field distribution with deep subwavelength resolution using the corresponding changes of the Raman spectra. We investigate the nature of the localization with respect to nanowire diameter, excitation spot position, and light polarization, supporting the experiment with 3D numerical modeling. Based on our findings we propose a research tool allowing to precisely localize the electric field in a certain subwavelength region of the nanophotonic resonator.

Interaction of Methylene Blue with Severe Acute Respiratory Syndrome Coronavirus 2 Envelope Revealed by Molecular Modeling.

Methylene blue has multiple antiviral properties against Severe Acute Respiratory Syndrome-related Coronavirus 2 (SARS-CoV-2). The ability of methylene blue to inhibit different stages of the virus life cycle, both in light-independent and photodynamic processes, is used in clinical practice. At the same time, the molecular aspects of the interactions of methylene blue with molecular components of coronaviruses are not fully understood. Here, we use Brownian dynamics to identify methylene blue binding sites on the SARS-CoV-2 envelope. The local lipid and protein composition of the coronavirus envelope plays a crucial role in the binding of this cationic dye. Viral structures targeted by methylene blue include the S and E proteins and negatively charged lipids. We compare the obtained results with known experimental data on the antiviral effects of methylene blue to elucidate the molecular basis of its activity against coronaviruses.

Terahertz emission from curved plasma filaments induced by two-color 2D Airy wave packets.

We experimentally demonstrate that 2D Airy wave packets can produce intense curved two-color filaments that emit terahertz (THz) radiation with unique characteristics. Due to the curvature of the plasma channel, THz waves, emitted from different longitudinal regions of the plasma, propagate in different directions resulting in non-concentric THz cones in the far-field. These cones have different cone angles and polarization which we attribute to the way the two-color 2D Airy driving fields are produced in the nonlinear crystal and then propagate to form the curved plasma filament.

Synthesis and Structure of Quasi-One-Dimensional Niobium Tetrasulfide NbS4.

The binary niobium sulfide NbS4 was synthesized as a crystalline phase. We showed that NbS4 can be formed from Nb metal, from defect niobium sulfide Nb1.14S2, or from some other niobium dichalcogenides in reactions with excess sulfur in an evacuated ampule at 440 °C. The crystal structure of NbS4 (monoclinic space group C2/c, a = 13.126(2) Å, b = 10.454(1) Å, c = 6.951(1) Å, ß = 111.939(5)°) is a packing of infinite chains {NbS4}1∞, analogous to VS4. In the chains, Nb atoms are in a tetragonal-antiprismatic coordination of sulfur atoms of disulfide groups (S2)2-; short Nb···Nb contacts (2.896 Å) alternating with longer ones (3.278 Å) appear within the chains at 150 K. According to density functional theory calculations, NbS4 is a thermodynamically stable compound, a nonmagnetic semiconductor. NbS4 is a new member of the family of quazi-one-dimensional compounds, group 5 metal polychalcogenides, well-known for their interesting electrophysical properties. The synthesis and crystal structure as well as the thermal stability and lattice dynamics of NbS4 are discussed here.

Replenishment in the Family of Rhenium Chalcobromides; Synthesis and Structure of Molecular {Re4S4}Br8(TeBr2)4, Dimeric [{Re4S4}Br8(TeBr2)3]2, and Polymeric {Re4S4}Br8 Compounds Based on the {Re4S4}8+ Tetrahedral Cluster Core.

We have obtained three new rhenium(IV) chalcobromides belonging to the homologous series {Re4S4}Br8(TeBr2)n (n = 0, 3, 4): a molecular complex {Re4S4}Br8(TeBr2)4 (1), a dimeric complex [{Re4S4}(TeBr2)3Br7(µ-Br)]2 (2), and a two-dimensional (2D) polymeric compound {Re4S4}Br8 (3). Compound 1 is isotypic to the already known {Re4Te4}(TeBr2)4Br8, while 2 and 3 exhibit a new type of binding of tetrahedral clusters via µ-Br bridges. Compounds were characterized by X-ray single-crystal diffraction, X-ray powder diffraction, and thermal and elemental analyses. In compound 2, two tetrahedral cluster cores {Re4S4}8+ are linked together forming a dimer through two Re-µ-Br-Re bridges. Calculations of the electron localization function (ELF) showed that there is no covalent interaction between rhenium atoms of neighboring clusters. In compound 3, each rhenium atom of the {Re4S4}8+ core is coordinated by three Br ligands: one terminal Br and two bridging µ-Br ligands. As a result, eight bridging bromine atoms link {Re4S4}8+ cluster cores into goffered layers. {Re4S4}Br8 is the new stable rhenium(IV) thiobromide, the first discovered in the Re-S-Br system, along with the already known octahedral rhenium(III) thiobromides Re6S4+xBr10-2x (x = 0-4).

Mathematical Simulation of Electron Transport in the Primary Photosynthetic Processes.

Summarized results of investigation of regulation of electron transport and associated processes in the photosynthetic membrane using methods of mathematical and computer modeling carried out at the Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, are presented in this review. Detailed kinetic models of processes in the thylakoid membrane were developed using the apparatus of differential equations. Fitting of the model curves to the data of spectral measurements allowed us to estimate the values of parameters that were not determined directly in experiments. The probabilistic method of agent-based Monte Carlo modeling provides ample opportunities for studying dynamics of heterogeneous systems based on the rules for the behavior of individual elements of the system. Algorithms for simplified representation of Big Data make it possible to monitor changes in the photosynthetic apparatus in the course of culture growth in a photobioreactor and for the purpose of environmental monitoring. Brownian and molecular models describe movement and interaction of individual electron carrier proteins and make it possible to study electrostatic, hydrophobic, and other interactions leading to regulation of conformational changes in the reaction complexes. Direct multiparticle models explicitly simulate Brownian diffusion of the mobile protein carriers and their electrostatic interactions with multienzyme complexes both in solution and in heterogeneous interior of a biomembrane. The combined use of methods of kinetic and Brownian multiparticle and molecular modeling makes it possible to study the mechanisms of regulation of an integral system of electron transport processes in plants and algae at molecular and subcellular levels.

Electrostatic Map of the SARS-CoV-2 Virion Specifies Binding Sites of the Antiviral Cationic Photosensitizer.

Electrostatics is an important part of virus life. Understanding the detailed distribution of charges over the surface of a virus is important to predict its interactions with host cells, antibodies, drugs, and different materials. Using a coarse-grained model of the entire viral envelope developed by D. Korkin and S.-J. Marrink's scientific groups, we created an electrostatic map of the external surface of SARS-CoV-2 and found a highly heterogeneous distribution of the electrostatic potential field of the viral envelope. Numerous negative patches originate mainly from negatively charged lipid domains in the viral membrane and negatively charged areas on the "stalks" of the spike (S) proteins. Membrane (M) and envelope (E) proteins with the total positive charge tend to colocalize with the negatively charged lipids. In the E protein pentamer exposed to the outer surface, negatively charged glutamate residues and surrounding lipids form a negative electrostatic potential ring around the channel entrance. We simulated the interaction of the antiviral octacationic photosensitizer octakis(cholinyl)zinc phthalocyanine with the surface structures of the entire model virion using the Brownian dynamics computational method implemented in ProKSim software (version r661). All mentioned negatively charged envelope components attracted the photosensitizer molecules and are thus potential targets for reactive oxygen generated in photosensitized reactions.

Optical and Material Characteristics of MoS2/Cu2O Sensor for Detection of Lung Cancer Cell Types in Hydroplegia.

In this study, n-type MoS2 monolayer flakes are grown through chemical vapor deposition (CVD), and a p-type Cu2O thin film is grown via electrochemical deposition. The crystal structure of the grown MoS2 flakes is analyzed through transmission electron microscopy. The monolayer structure of the MoS2 flakes is verified with Raman spectroscopy, multiphoton excitation microscopy, atomic force microscopy, and photoluminescence (PL) measurements. After the preliminary processing of the grown MoS2 flakes, the sample is then transferred onto a Cu2O thin film to complete a p-n heterogeneous structure. Data are confirmed via scanning electron microscopy, SHG, and Raman mapping measurements. The luminous energy gap between the two materials is examined through PL measurements. Results reveal that the thickness of the single-layer MoS2 film is 0.7 nm. PL mapping shows a micro signal generated at the 627 nm wavelength, which belongs to the B2 excitons of MoS2 and tends to increase gradually when it approaches 670 nm. Finally, the biosensor is used to detect lung cancer cell types in hydroplegia significantly reducing the current busy procedures and longer waiting time for detection. The results suggest that the fabricated sensor is highly sensitive to the change in the photocurrent with the number of each cell, the linear regression of the three cell types is as high as 99%. By measuring the slope of the photocurrent, we can identify the type of cells and the number of cells.

Room temperature, nanosecond, 60 mJ/pulse Fe:ZnSe master oscillator power amplifier system operating at 3.8-5.0†µm.

We report on a RT gain-switched Fe:ZnSe master oscillator power amplifier (MOPA) system tunable over 3.8-5.0 µm pumped by radiation of Er:YAG laser operating at 2.94 µm. The mechanically Q-switched Er:YAG laser with output energy up to 220 mJ was used as a pump source for a master oscillator and three-stage power amplifier. The maximum output energies in 200 ns pulses exceeded 60, 56, and 48 mJ at 4.4, 4.3, and 4.1 µm, respectively, under 220 mJ of pump energy. The extraction energy efficiencies were measured to be 25, 30, and 40% at the first, second, and third stages, respectively.

High energy (0.8 J) mechanically Q-switched 2.94 µm Er:YAG laser.

We report a flashlamp pumped mechanically Q-switched (MQS) 2.94 µm Er:YAG laser based on a spinning mirror with a highest output energy of 805 mJ at a pulse duration of 61 ns and 13 MW of peak power at 1 Hz repetition rate. This record output energy was achieved with the use of 300 mm long MQS Er:YAG laser cavity consisting of a 70% output coupler, 7 × 120 mm AR coated Er(50%):YAG crystal, and 4200 rad/s angular speed of the spinning mirror. The pulse jitter was also measured by using optical triggering and was smaller than 10 ns for 150 ns Q-switched pulses, which could be applicable to many laser applications where precise synchronization of pulses is required.

Kerr-lens mode-locked Cr:ZnS oscillator reaches the spectral span of an optical octave.

We report, to the best of our knowledge, the first super-octave femtosecond polycrystalline Cr:ZnS laser at the central wavelength 2.4 µm. The laser is based on a non-polarizing astigmatic X-folded resonator with normal incidence mounting of the gain element. The chromatic dispersion of the resonator is controlled with a set of dispersive mirrors within one third of an optical octave over 2.05-2.6 µm range. The resonator's optics is highly reflective in the range 1.8-2.9 µm. The components of the oscillator's output spectrum at the wavelengths 1.6 µm and 3.2 µm are detected at -60 dB with respect to the main peak. Average power of few-cycle Kerr-lens mode-locked laser is 1.4 W at the pulse repetition frequency 79 MHz. That corresponds to 22% conversion of cw radiation of Er-doped fiber laser, which we used for optical pumping of the Cr:ZnS oscillator.

Lasing in 15 atm CO2 cell optically pumped by a Fe:ZnSe laser.

10 µm lasing is studied in a compact CO2-He cell pressurized up to 15 atm when optically pumped by a ∼50 mJ Fe:ZnSe laser tunable around 4.3 µm. The optimal pump wavelength and partial pressure of CO2 for generating 10 µm pulses are found to be ∼4.4 µm and 0.75 atm, respectively. Without cavity optimization, the optical-to-optical conversion efficiency reached ∼10% at a total pressure of 7 atm. The gain lifetime is measured to be ∼1 µs at pressures above 10 atm, indicating the feasibility of using high-pressure optically pumped CO2 for the efficient amplification of picosecond 10 µm pulses.

Vanadium O-Centered Selenoiodide Complex: Synthesis and Structure of V4O(Se2)4I6·I2.

The first vanadium selenoiodide V4O(Se2)4I6·I2 was synthesized at a moderate temperature of 220 °C from V, Se, I2, and water. Its crystal structure (tetragonal space group P42/nbc, a = 11.838(1) Å, c = 18.689(1) Å) contains O-centered vanadium(IV) tetranuclear fragment [V4(µ4-O)(µ2-Se2)4(µ2-I)2I4], where the edges of the distorted tetrahedron V4 are bridged by four diselenide (Se2)2- and two iodide ligands; four terminal iodides coordinate V atoms additionally. This type of complex is known for Ti, Nb, and Ta but is new for vanadium. Magnetic susceptibility measurements of V4O(Se2)4I6·I2 showed four unpaired electrons on vanadium atoms at room temperature and drop of the effective magnetic moment at cool down, presumably due to partial electron pairing. Probability of this transition to the diamagnetic state is in accord with the calculated electronic structure.

Octave-spanning mid-infrared femtosecond OPA in a ZnGeP2 pumped by a 2.4†µm Cr:ZnSe chirped-pulse amplifier.

We report on the highly efficient, octave-spanning mid-infrared (mid-IR) optical parametric amplification (OPA) in a ZnGeP2 (ZGP) crystal, pumped by a 1 kHz, 2.4 µm, 250 fs Cr:ZnSe chirped-pulse amplifier. The full spectral coverage of 3-10 µm with the amplified signal and idler beams is demonstrated. The signal beam in the range of ∼3 - 5 µm is produced by either white light generation (WLG) in YAG or optical parametric generation (OPG) in ZGP using the common 2.4 µm pump laser. We demonstrate the pump to signal and idler combined conversion efficiency of 23% and the pulse energy of up to 130 µJ with ∼2 µJ OPG seeding, while we obtain the efficiency of 10% and the pulse energy of 55 µJ with ∼0.2 µJ WLG seeding. The OPA output energy is limited by the available pump pulse energy (0.55 mJ at ZGP crystal) and therefore further energy scaling is feasible with multi-stage OPA and higher pump pulse energy. The autocorrelation measurements based on random quasi-phase matching show that the signal pulse durations are ∼318 fs and ∼330 fs with WLG and OPG seeding, respectively. In addition, we show the spectrally filtered 30 µJ OPA output at 4.15 µm suitable for seeding a Fe:ZnSe amplifier. Our ultrabroadband femtosecond mid-IR source is attractive for various applications, such as strong-field interactions, dielectric laser electron acceleration, molecular spectroscopy, and medical surgery.

Impact of gas dynamics on laser filamentation THz sources at high repetition rates.

We experimentally demonstrate that the terahertz (THz) emission from two-color laser filaments in gases is strongly affected by the pulse repetition rate of the driving laser. We show that at repetition rates above 100 Hz, propagation of every next laser pulse in the pulse train is altered by gas density depressions produced by the preceding laser pulses. As a result, plasma channels at higher repetition rates become shorter, leading to less efficient THz generation. In particular, we observe a 50% decrease in the emitted THz energy when the repetition rate increases from 6 Hz to 6 kHz.

Mechanical properties of tubulin intra- and inter-dimer interfaces and their implications for microtubule dynamic instability.

Thirteen tubulin protofilaments, made of αß-tubulin heterodimers, interact laterally to produce cytoskeletal microtubules. Microtubules exhibit the striking property of dynamic instability, manifested in their intermittent growth and shrinkage at both ends. This behavior is key to many cellular processes, such as cell division, migration, maintenance of cell shape, etc. Although assembly and disassembly of microtubules is known to be linked to hydrolysis of a guanosine triphosphate molecule in the pocket of ß-tubulin, detailed mechanistic understanding of corresponding conformational changes is still lacking. Here we take advantage of the recent generation of in-microtubule structures of tubulin to examine the properties of protofilaments, which serve as important microtubule assembly and disassembly intermediates. We find that initially straight tubulin protofilaments, relax to similar non-radially curved and slightly twisted conformations. Our analysis further suggests that guanosine triphosphate hydrolysis primarily affects the flexibility and conformation of the inter-dimer interface, without a strong impact on the shape or flexibility of αß-heterodimer. Inter-dimer interfaces are significantly more flexible compared to intra-dimer interfaces. We argue that such a difference in flexibility could be key for distinct stability of the plus and minus microtubule ends. The higher flexibility of the inter-dimer interface may have implications for development of pulling force by curving tubulin protofilaments during microtubule disassembly, a process of major importance for chromosome motions in mitosis.

Hexamolybdenum Clusters Supported on Exfoliated h-BN Nanosheets for Photocatalytic Water Purification.

Nowadays, the development of new effective photocatalytic materials for the purification of real wastewaters and model systems containing organic molecules constitutes an important challenge. Here we present a preparation strategy for composite materials based on hexamolybdenum cluster complexes and exfoliated hexagonal boron nitride (h-BN) nanosheets. Cluster deposition on the nanosheet surface was achieved by impregnation of the matrix by a (Bu4N)2[{Mo6I8}(NO3)6]/acetone solution. Successful cluster immobilization and chemical composition of the samples were verified by inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy with elemental mapping (TEM/EDS), X-ray photoelectron spectroscopy (XPS), and optical diffuse-reflectance spectroscopy. A small amount of water in acetone initiates the hydrolysis of a molybdenum cluster precursor with labile NO3- ligands, which are absent in the final composite, according to the XPS data. Intermediate hydrolyzed cluster forms anchor to the surface of h-BN nanosheets and promote growth of the insoluble compound [{Mo6I8}(H2O)2(OH)4]·yH2O as the final hydrolysis product. TEM/EDS proves that the cluster exists at the nanosheet surface in the form of an X-ray diffraction amorphous thin film. The samples obtained show high photocatalytic activity in the degradation of a model pollutant rhodamine B under UV- and visible-light irradiation. The materials retain their initial photocatalytic efficacy during at least six cycles without the need for recovery.

Q-switched and gain-switched Fe:ZnSe lasers tunable over 3.60-5.15 µm.

We report on room temperature gain-switched and Q-switched Fe:ZnSe lasers tunable over 3.60-5.15 µm pumped by radiation of an 2.94 µm Er:YAG laser. The maximum output energy was measured to be 5 mJ under 15 mJ of pump energy in gain-switched regime. We also demonstrated a mechanically Q-switched regime of oscillation of Fe:ZnSe lasers. This approach could be attractive for the development of high-energy short-pulse solid-state mid-IR systems operating over 3.6-5.2 µm spectral range.